# Repellor vehicles/Shielding

It's a car called a space ship. Credit: Ben.{{free media}}
In the 1960's U.S. Government laboratories, under Project Orion, investigated a pulsed nuclear fission propulsion system. Credit: NASA.{{free media}}

Shielding is needed for a person or vehicle whenever local conditions are outside fair weather and pleasant circumstances.

In cold weather or cold climates this may take the form of a coat, hat, gloves, and boots.

Vehicle shielding may be for the vehicle itself, its components, or its passengers and driver or operator.

From 1998 to the present, the nuclear engineering department at Pennsylvania State University has been developing two improved versions of project Orion known as ICAN-II (Project ICAN) and Project AIMStar using compact antimatter catalyzed nuclear pulse propulsion units,[1] rather than the large inertial confinement fusion ignition systems proposed in Project Daedalus and Longshot.[2]

"The straightahead approximation, ie, the approximation that the secondary particles from nucleon-nucleus collisions are emitted in the direction of the incident nucleon, is often used in space-vehicle shielding studies. [...] the approximation is sufficiently accurate to justify its use in obtaining estimates of the secondary-particle contribution to the dose behind thin shields."[3]

Def. a broad piece of metal or another suitable material used as a protection against blows or missiles is called shielding.

## Meteors

Arcs rise above an active region on the surface of the Sun in this series of images taken by the STEREO (Behind) spacecraft. Credit: Images courtesy of the NASA STEREO Science Center.

"The impact shielding presently incorporated into space platform design may not be adequate under Leonid storm conditions."[4]

"A magnetic cloud is a transient event observed in the solar wind. It was defined in 1981 by Burlaga et al. 1981 as a region of enhanced magnetic field strength, smooth rotation of the magnetic field vector and low proton temperature [5]. Magnetic clouds are a possible manifestation of a Coronal Mass Ejection (CME). The association between CMEs and magnetic clouds was made by Burlaga et al. in 1982 when a magnetic cloud was observed by Helios-1 two days after being observed by SMM[6]. However, because observations near Earth are usually done by a single spacecraft, many CMEs are not seen as being associated with magnetic clouds. The typical structure observed for a fast CME by a satellite such as ACE is a fast-mode shock wave followed by a dense (and hot) sheath of plasma (the downstream region of the shock) and a magnetic cloud.

## Extragalactic cosmic rays

The flux of cosmic-ray particles is a function of their energy. Credit: Sven Lafebre, after Swordy.[7]

Cosmic rays are energetic charged subatomic particles, originating in outer space.

At right is an image indicating the range of cosmic-ray energies. The flux for the lowest energies (yellow zone) is mainly attributed to solar cosmic rays, intermediate energies (blue) to galactic cosmic rays, and highest energies (purple) to extragalactic cosmic rays.[7]

Cosmic ray astronomy attempts to identify and study the sources of ultrahigh energy cosmic rays. It is unique in its reliance on charged particles as the information carriers.”[8]

The Oh-My-God particle was observed on the evening of 15 October 1991 over Dugway Proving Ground, Utah. Its observation was a shock to astrophysicists, who estimated its energy to be approximately 3×1020
eV
[9](50 joules)—in other words, a subatomic particle with kinetic energy equal to that of a baseball (142 g or 5 oz) traveling at 100 km/h (60 mph).

It was most probably a proton with a speed very close to the speed of light (approximately 0.9999999999999999999999951c), so close that in a year-long race between light and the cosmic ray, the ray would fall behind only 46 nanometers (5 x 10-24 light-years), or 0.15 femtoseconds (1.5 x 10-16 s).[10]

“The energy spectrum of cosmic rays extends to ~1020 eV (and smoothly to 1019).”[11]

Notation: let the symbol Z stand for atomic number.

let the symbol PeV stand for 1015 electron volts.

"The most dominant group is the iron group (Z = 25 − 27), at energies around 70 PeV more than 50% of the all-particle flux consists of these elements."[12]

## Galactic cosmic rays

Space weather conditions are associated with solar activity. Credit: Daniel Wilkinson.{{free media}}
Galactic cosmic rays (GCR) are displayed from 1951 to 2006. Credit: Jbo166.

The "effect of time-variations in galactic cosmic rays on the rate of production of neutrons in the atmosphere [was studied using] a series of balloon and airplane observations of the [fast neutron] flux and spectrum of 1-10 MeV neutrons, in flights at high geomagnetic latitude, during [quiet times as well as during Forbush decreases, which are rapid decreases in the observed galactic cosmic rays following a coronal mass ejection (CME), and solar particle events for] the period of increasing solar modulation, 1965-1969. It also included latitude surveys in 1964-1965 and in 1968."[13]

In the image on the right for Forbush decreases, data include GOES-15 X-rays, energetic particles, and magnetometer. Cosmic Rays from the Moscow station show a Forbush Decrease.

"Cosmic rays arise from galactic source accelerators."[14] In the graph on the right, the black line is cosmic-ray data and the red line is temperature. Ulysses data is included.

The Charge, Element, and Isotope Analysis System (CELIAS) aboard SOHO "continuously samples the solar wind and energetic ions of solar, interplanetary and interstellar origin, as they sweep past SOHO. It analyzes the density and composition of particles present in this solar wind."[15]

"The element 22Ne appears as a tracer closely related to the existence of WC stars, because a large fraction of the existing 22Ne seems to result from such stars. Thus, peculiar abundances of 22Ne as observed in galactic cosmic rays (cf. Mewaldt, 1981) and meteorites (cf. Eberhardt et al., 1981) may indicate material originating from WC stars. Moreover, as there is a strong gradient in the distribution of WC stars in the Galaxy, a similar important gradient of 22Ne should also exist in the Galaxy."[16]

## Galactic and solar cosmic ray interplay

Cosmic Ray Intensity (blue) and Sunspot Number (green) is shown from 1951 to 2006 Credit: University of New Hampshire.{{fairuse}}

Here's a quote from Bowman's "Radiocarbon Dating" book from 1990, p. 19: "High sunspot activity increases the weak magnetic field that exists between the planets, and at such times there is a greater deflection of cosmic rays and hence 14C decreases."[17]

"Cosmic rays originate from the Sun as well as from galactic sources."[18]

Here's a quote from Aitken's "Radiocarbon Dating" article from 2000, "Cosmic-ray variations are associated with changes in the strength of the Earth's magnetic field. A weak field allows more cosmic radiation to reach the upper atmosphere, and the production of carbon-14 is consequently enhanced--causing raw radiocarbon ages to be underestimates of calendar ages. The short-term wiggles mentioned above are associated with sunspot activity."[19]

"Direct observations of cosmic rays within the heliosphere over several decades have revealed a great deal of information about the acceleration and propagation of cosmic radiation through the interstellar space and the heliosphere. We now know that the cosmic radiation incident at the top of the earth’s atmosphere comes to us through several “filters”:

1. Galactic magnetic fields,
2. Interstellar magnetic fields,
3. Solar magnetic plasma within the heliosphere, regulated by solar activity, and finally,
4. the Terrestrial geomagnetic field."[20]

"Additionally, cosmic ray particles are frequently accelerated by the sun, and sometimes in a nearby supernova to make an appreciable difference in the total cosmic ray flux at the earth!"[20]

"Since fairly extensive cosmic-ray data on primary and secondary cosmic rays are available for more than the past five decades, covering five solar cycles, it is fairly easy to make reliable calculations of the magnitude of variations in cosmogenic production rates in terrestrial solids due to solar modulation of galactic cosmic-ray flux. This exercise is based on a study of relative changes in the primary cosmic-ray flux at the top of the atmosphere, and flux of low energy neutrons as measured by neutron monitors. Solar modulation of galactic cosmic-ray flux is conveniently described in terms of a modulation potential, ∅, which is a phase-lagged function of solar activity (see Castagnoli and Lal 1980; Lal 1988b, 2000 and references therein). Continuous data are available for several neutron monitors at sea-level and mountain altitudes located at different latitudes, and these data have been analyzed in terms of transfer functions relating changes in the secondary nucleon fluxes in the atmosphere to those in the primary cosmic-ray spectra (cf. Webber and Lockwood 1988; Nagashima et al. 1989). For a recent discussion on changes in cosmic-ray fluxes as measured on spacecrafts and in neutron monitor counting rates, the reader is referred to Lal (2000). The manner in which the primary and secondary cosmic-ray flux changes occur with the march of solar activity is described in detail by Lal and Peters (1967), who also estimate the changes in the isotope production rates as a function of altitude and latitude during 1956 (a period of solar minimum) and 1958 (a period of unusually high solar activity). Using this approach, and using the neutron monitor data available to date, one can improve on the earlier estimates of solar temporal variations in cosmogenic nuclide production rates at sea level and at mountain altitudes. We must mention here that several direct experiments are also being made at present by exposing targets to cosmic radiation at different altitudes and latitudes (cf. Lal 2000)."[20] The graph on the right shows an inverse correlation between sunspot numbers (solar activity) and neutron production from galactic cosmic rays.

## Anomalous cosmic rays

A mechanism is suggested for anomalous cosmic rays (ACRs) of the acceleration of pick-up ions at the solar wind termination shock. Credit: Eric R. Christian.{{fairuse}}

"While interstellar plasma is kept outside the heliosphere by an interplanetary magnetic field, the interstellar neutral gas flows through the solar system like an interstellar wind, at a speed of 25 km/sec. When closer to the Sun, these atoms undergo the loss of one electron in photo-ionization or by charge exchange. Photo-ionization is when an electron is knocked off by a solar ultra-violet photon, and charge exchange involves giving up an electron to an ionized solar wind atom. Once these particles are charged, the Sun's magnetic field picks them up and carries them outward to the solar wind termination shock. They are called pickup ions during this part of their trip."[21]

"The ions repeatedly collide with the termination shock, gaining energy in the process. This continues until they escape from the shock and diffuse toward the inner heliosphere. Those that are accelerated are then known as anomalous cosmic rays."[21]

"ACRs [may] represent a sample of the very local interstellar medium. They are not thought to have experienced such violent processes as GCRs, and they have a lower speed and energy. ACRs include large quantities of helium, oxygen, neon, and other elements with high ionization potentials, that is, they require a great deal of energy to ionize, or form ions. ACRs are a tool for studying the movement of energetic particles within the solar system, for learning the general properties of the heliosphere, and for studying the nature of interstellar material itself."[21]

## Solar energetic particles

Mean Fe charge states as a function of energy for the same event (in red) with overall mean charge state and test result for null-hypothesis (i.e. random distribution around mean). Credit: Zhangbo Guo, Eberhard Moebius, and Mark Popecki.{{fairuse}}
Charge state of Fe is a function of energy for an impulsive event in September 2000 in comparison with that for a CME-related event in June 1999 and the charge state of adjacent solar wind. Credit: Berndt Klecker and Eberhard Moebius.{{fairuse}}

"Earlier observations with ACE/SEPICA, SAMPEX/LICA, and SOHO/STOF have shown that highly ionized Fe in solar energetic particle (SEP) events (mean QFe > 14) is usually coupled with an increase of the mean charge state with energy in the range from 0.01 to 1 MeV/amu [...]. At the lowest energies the mean charge state of Fe is typically found to be well below QFe = 14. Recently, this has been demonstrated for all impulsive SEP events that were observed with SEPICA (DiFabio et al., ApJ, Nov 2008), indicating that the greater degree of ionization at higher energies was established by electron stripping in the low corona (e.g. Kartavykh et al., ApJ, 671, 947, 2007). However, observations of solar wind charge states have shown a widespread presence of QFe ≥ 16, associated with a hot plasma environment in solar wind from active regions and in interplanetary [Coronal Mass Ejections] CMEs (e.g. Lepri et al., JGR, 106, 29231, 2001; ACE News #52)."[22]

"Mean Fe charge states [in the figure on the right are] a function of energy for the same event (in red) with overall mean charge state and test result for null-hypothesis (i.e. random distribution around mean). Shown for comparison is an impulsive [Solar Energetic Particle] SEP event from June 2000 (in blue)."[22]

"Impulsive solar energetic particle events are well known for their dramatic over-abundances in 3He and heavy ions. ACE observations have extended these composition peculiarities to overabundances in the heavy isotopes of Ne and Mg."[23]

"The first charge-state measurements of impulsive events, averaged over all such events observed during one year with ISEE ULEZEQ, suggested that impulsive events feature rather high charge states with Q ≈ 20 for Fe and all elements up to Mg essentially fully stripped. These high charge states appeared to be well separated from the group of large, CME-related events with Q ≈ 14 for Fe."[23]

"With ACE SEPICA we have found that solar energetic particle events generally show a wide variety of mean charge states for Fe ranging from Q ≈ 10 continuously up to Q ≈ 20. Also, element abundance ratios appear to correlate with the ionic charge states (see ACE News #33). These two results seemed to present a puzzle, as the highest overabundances of heavy ions were observed for events with essentially fully-ionized ions up to Mg, which would not lend itself to an M/Q-based explanation for the observed fractionation. Therefore, it was suggested that fractionation and acceleration occur among lower charge state ions, with the final high charge states attained through stripping. This idea appears to be corroborated now by the observation of a very strong energy dependence of the iron charge states from 0.2 to 0.5 MeV/nuc with ACE SEPICA, a pattern that is even more pronounced when extended to ~0.01 MeV/nuc with the SOHO CELIAS STOF instrument."[23]

The "charge state of Fe [in the second figure down on the right is] a function of energy for an impulsive event in September 2000 in comparison with that for a CME-related event in June 1999 and the charge state of adjacent solar wind. Whereas the CME-related event shows Q ≈ 10 over the entire energy range, commensurate with that of the solar wind, in the impulsive event the charge state increases from Q ≈ 12 at low energies up to Q ≈ 17 at 0.5 MeV/nuc. This observation suggests that the original source material which is accelerated in these events has a much lower temperature than previously thought and is only partially ionized, thereby lending itself to M/Q fractionation. The sharp increase of the charge state with energy can be explained by electron stripping that increases with energy. This also implies that the acceleration in impulsive events occurs in the lower corona."[23]

## Ultra-heavy element nuclei

Absolute flux Φ0Z of cosmic–ray elements at E0 = 1 TeV/nucleus is plotted versus nuclear charge. Credit: Jörg R. Hörandel.{{fairuse}}

"The iron group and the ultra–heavy elements are more pronounced in cosmic rays as compared to the solar system. Especially the r–process elements beyond xenon (Z=54) are enhanced, partly due to spallation products of the platinum and lead nuclei (Z=78, 82). For the latter direct measurements at low energies around 1 GeV/n yield about a factor two more abundance as compared to the solar system and a factor of four for the actinides thorium and uranium (Z=90, 92) [66]. This has been attributed to the hypothesis that cosmic rays are accelerated out of supernova ejecta–enriched matter [67]."[24]

## Heavier element nuclei

The distribution of galactic cosmic-ray (GCR) particles is shown in atomic number (charge) and energy. Credit: W. Schimmerling, J. W. Wilson, F. Cucinotta, and M-H Y. Kim.{{fairuse}}

"These charged particles are hydrogen nuclei (protons), helium nuclei (α particles), and the nuclei of heavier elements such as iron and nickel."[25]

"Primary cosmic radiation mainly consists of the nuclei of atoms which have lost their electrons due to their extremely high velocity; these charged particles are hydrogen nuclei (protons), helium nuclei (alpha particles) and the nuclei of heavier elements such as iron and nickel; there are also some electrons (1%) and positrons (1‰)."[25]

"The relative abundances of GCR particles (9) are shown in [the figure on the right] (a), and typical energy spectra (10), are shown in [...] (b). The GCR particles of interest for radiation protection of crews engaged in space exploration range from protons (nuclei of hydrogen) to nuclei of iron; the abundances of heavier elements are orders of magnitude lower."[26]

Heavier element nuclei consist primarily of Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co and Ni.

"The two groups of elements Li, Be, B and Sc, Ti, V, Cr, Mn are many orders of magnitude more abundant in the cosmic radiation than in solar system material."[27]

## Neon nuclei

The ACE-CRIS measurements of the ratios 22Ne/20Ne and 21Ne/20Ne are plotted as a function of energy. Credit: W.R. Binns, M.E. Wiedenbeck, M. Arnould, A.C. Cummings, J.S. George, S. Goriely, M.H. Israel, R.A. Leske, R.A. Mewaldt, G. Meynet, L. M. Scott, E.C. Stone, and T.T. von Rosenvinge.{{fairuse}}

On the right, "the ACE-CRIS measurements of the ratios 22Ne/20Ne and 21Ne/20Ne are plotted as a function of energy. Abundances measured by other experiments (Wiedenbeck & Greiner 1981 [ISEE-3]; Lukasiak et al. 1994 [Voyager]; Connell & Simpson 1997 [Ulysses]; DuVernois et al. 1996 [CRRES]) are plotted as open symbols and the energy intervals for their measurements are shown as horizontal bars at the bottom of the figure."[28]

## Oxygen nuclei

Oxygen fluences were observed by the Advanced Composition Explorer (ACE). Credit: Richard Mewaldt, Caltech.

The fluences of oxygens in the galactic cosmic rays (GCRs) are plotted on the graph at right using data from the Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE). The fluences of solar 'cosmic rays' add to the GCRs at lower energy.

## Nitrogen nuclei

"For cosmic rays the low abundance ”valleys” in the solar system composition around Z=4, 21, 46, and 70 are not present. This is usually believed to be the result of spallation of heavier nuclei during their propagation through the galaxy. Hydrogen, helium, and the CNO–group are suppressed in cosmic rays. This has been explained by the high first ionization potential of these atoms [63] or by the high volatility of these elements which do not condense on interstellar grains [64]. Which property is the right descriptor of cosmic–ray abundances has proved elusive, however, the volatility seems to become the more accepted solution [65]."[24]

## Carbon nuclei

These "are nevertheless present in the cosmic radiation as spallation products of the abundant nuclei of carbon and oxygen (Li,Be,B) and of iron (Sc,Ti,V,Cr,Mn)."[27]

## Boron nuclei

Absolute boron and carbon fluxes multiplied by E2.7 as measured by PAMELA. Credit: O. Adriani, G. C. Barbarino, G. A. Bazilevskaya, R. Bellotti, M. Boezio, E. A. Bogomolov, M. Bongi, V. Bonvicini, S. Bottai, A. Bruno, F. Cafagna, D. Campana, R. Carbone, P. Carlson, M. Casolino, G. Castellini, I. A. Danilchenko, C. De Donato1, C. De Santis, N. De Simone, V. Di Felice, V. Formato, A. M. Galper, A. V. Karelin, S. V. Koldashov, S. Koldobskiy, S. Y. Krutkov, A. N. Kvashnin, A. Leonov, V. Malakhov, L. Marcelli, M. Martucci, A. G. Mayorov, W. Menn, M. Mergé, V. V. Mikhailov, E. Mocchiutti, A. Monaco, N. Mori, R. Munini, G. Osteria, F. Palma, B. Panico, P. Papini, M. Pearce, P. Picozza, C. Pizzolotto, M. Ricci, S. B. Ricciarini, L. Rossetto, R. Sarkar, V. Scotti, M. Simon, R. Sparvoli, P. Spillantini, Y. I. Stozhkov, A. Vacchi, E. Vannuccini, G. I. Vasilyev, S. A. Voronov, Y. T. Yurkin, G. Zampa, N. Zampa, and V. G. Zverev.{{fairuse}}

"In cosmic rays, both the isotopes 10B and 11B are present in comparable quantities."[29]

In the figure on the right are absolute boron and carbon fluxes multiplied by E2.7 as measured by PAMELA, together with results from other experiments (AMS02 Oliva et al. (2013), CREAM Ahn et al. (2008), TRACER Obermeier et al. (2011), ATIC-2 Panov et al. (2007), HEAO Engelmann et al. (1990), AMS01 Aguilar et al. (2010), CRN Swordy et al. (1990)) and a theoretical calculation based on GALPROP, as functions of kinetic energy per nucleon.

## Berylliums

The "presence in ... cosmic radiation [is] of a much greater proportion of "secondary" nuclei, such as lithium, beryllium and boron, than is found generally in the universe."[27]

## Lithium nuclei

The "evidence for the overwhelming majority of the Li-atoms in photospheres has its origin not only in nuclear synthesis near the stellar centers, but also by active processes in stellar atmospheres. [...] the lithium [resonance] line [is] near 478 keV."[30]

"Approximately 90% of lithium atoms originate from α - α reactions for the typical spectra of an accelerated particles on the Sun [...] During impulsive flares, interaction between the accelerated particles and the ambient medium occurs mainly at low altitudes, i.e., close to the footprints of loops."[30]

## Alpha particles

About 89% of cosmic rays are simple protons or hydrogen nuclei, 10% are helium nuclei or alpha particles, and 1% are the nuclei of heavier elements. Solitary electrons constitute much of the remaining 1%.

An "analysis of the energy-loss distributions in the GRS HEM during the impulsive phase of this event indicates that γ-rays from the decay of π0 mesons were detected [...] The production of pions, which is accompanied (on average) by neutrons, has an energy threshold of ~290 MeV for p-p and ~180 MeV for p-α interactions, giving, therefore, a lower limit to the maximum energy of the particles accelerated at the Sun."[31]

## Helions

Def. a "nucleus of a helium-3 atom"[32] is called a helion.

## Tritons

Energetic deuterons and tritons have been detected in solar flares.[33]

## Deuterons

"The flux [of deuterons in cosmic rays at a geomagnetic latitude of 7.6°N] is found to be 4 ± 1.3 M-2 sec-1 sterad-1".[34]

## Baryons

In "dense nuclear matter, such as neutron stars [it] has recently been discovered that kaon condensation in nuclear matter at a density of a few times normal nuclear matter may significantly reduce the upper mass limit of neutron stars [...] This clearly has an impact on astronomical observations. By exploiting the electron fermi level, we are able to predict kaon production at reasonable baryon number densities [...] Experimental detection of [dibaryons, hyperons] is a subtle matter [...] there is strong theoretical evidence that such states [as the dibaryon] do exist in nature. [...] the lightest dibaryon [...] is energetically stable against strong decay to [ΛΛ baryons] by 88 MeV. [The H dibaryon] is bound by 250 MeV."[35]

## Solar cosmic rays

This image shows an overview of the space weather conditions over several solar cycles including the relationship between sunspot numbers and cosmic rays. Credit: Daniel Wilkinson.
Comparison shows the observed (solar irradiance and sunspot number, symbols) and modeled (solid line) total magnetic flux Credit: Luis Eduardo A. Vieira and Sami K. Solanki.{{fairuse}}

Def. low energy cosmic rays associated with solar flares are called solar cosmic rays.

"A persistent problem of solar cosmic-ray research has been the lack of observations bearing on the timing and conditions in which protons that escape to the interplanetary medium are first accelerated in the corona."[36]

"For solar cosmic-rays, the apparent lack of proton acceleration in the corona seems justified, in contrast to the electrons, proton bremsstrahlung and gyrosynchrotron emission are negligible. This suggests a transit time anomaly, ΔTA, defined as follows:

ΔTA = ΔTonset - 11 min,

where ΔTonset is the deduced Sun-Earth transit time for the first arriving relativistic protons and 11 min is the nominal transit time for a ~2 GeV proton traversing a 1.3 AU Archimedes spiral path."[36]

"The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. ΔTA may have values from "7-19 min for a small sample of well-connected ... cosmic-ray flares."[36] The transit time anomaly may be explained by a rise time associated with the ground-level events (GLEs). "The average GLE rise time ... for well-connected ... events ... defined to be the time from event onset to maximum as measured by the neutron monitor station showing the largest increase and whose asymptotic cone of acceptance ... includes the nominal direction of the Archimedean spiral path, is 21.3 min."[36]

"Data from an extensive air shower detector of ultrahigh-energy cosmic rays shows shadowing of the cosmic-ray flux by the Moon and the Sun with significance of 4.9 standard deviations. This is the first observation of such shadowing."[37]

"The ... solar proton flare on 20 April 1998 at W 90° and S 43° (9:38 UT) was measured by the GOES-9-satellite (Solar Geophysical Data 1998), as well as by other experiments on WIND ... and GEOTAIL. Protons were accelerated up to energies > 110 MeV and are therefore able to hit the surface of Mercury."[38] {{clear}]

## Solar winds

The Solar wind dynamic pressure was detected by Ulysses-SWOOPS. Credit: Dave McComas, Ulysses, EIT-SOHO; LASCO-C2-SOHO; MLSO.
Ulysses (spacecraft) measures the variable speed of the solar wind. Credit: NASA – Marshall Space Flight Center.

"Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids."[39]

The solar wind is divided into two components, respectively termed the slow solar wind and the fast solar wind. The slow solar wind has a velocity of about 400 km/s, a temperature of 1.4–1.6×106 K and a composition that is a close match to the corona. By contrast, the fast solar wind has a typical velocity of 750 km/s, a temperature of 8×105 K and it nearly matches the composition of the Sun's photosphere.[40] The slow solar wind is twice as dense and more variable in intensity than the fast solar wind. The slow wind also has a more complex structure, with turbulent regions and large-scale structures.[41][42]

The slow solar wind appears to originate from a region around the Sun's equatorial belt that is known as the "streamer belt". Coronal streamers extend outward from this region, carrying plasma from the interior along closed magnetic loops.[43][44] Observations of the Sun between 1996 and 2001 showed that emission of the slow solar wind occurred between latitudes of 30–35° around the equator during the solar minimum (the period of lowest solar activity), then expanded toward the poles as the minimum waned. By the time of the solar maximum, the poles were also emitting a slow solar wind.[45]

The fast solar wind is thought to originate from coronal holes, which are funnel-like regions of open field lines in the Sun's magnetic field.[46] Such open lines are particularly prevalent around the Sun's magnetic poles. The plasma source is small magnetic fields created by convection cells in the solar atmosphere. These fields confine the plasma and transport it into the narrow necks of the coronal funnels, which are located only 20,000 kilometers above the photosphere. The plasma is released into the funnel when these magnetic field lines reconnect.[47]

The diagram on the right describes the Solar wind dynamic pressure as detected by Ulysses-SWOOPS.

"The average pressure of the solar wind has dropped more than 20% since the mid-1990s. This is the weakest it's been since we began monitoring solar wind almost 50 years ago."[48]

"Curiously, the speed of the million mph solar wind hasn't decreased much—only 3%. The change in pressure comes mainly from reductions in temperature and density. The solar wind is 13% cooler and 20% less dense."[49]

"Global measurements of solar wind pressure by Ulysses [are shown in the diagram on the right]. Green curves trace the solar wind in 1992-1998, while blue curves denote lower pressure winds in 2004-2008."[49]

"What we're seeing is a long term trend, a steady decrease in pressure that began sometime in the mid-1990s."[50]

"It's hard to say [how unusual this event is]. We've only been monitoring solar wind since the early years of the Space Age—from the early 60s to the present. Over that period of time, it's unique. How the event stands out over centuries or millennia, however, is anybody's guess. We don't have data going back that far."[50]

"Ulysses also finds that the sun's underlying magnetic field has weakened by more than 30% since the mid-1990s."[50]

"Unpublished Ulysses cosmic ray data show that, indeed, high energy (GeV) electrons, a minor but telltale component of cosmic rays around Earth, have jumped in number by about 20%."[49]

"The solar wind streams off of the Sun in all directions at speeds of about 400 km/s (about 1 million miles per hour). The source of the solar wind is the Sun's hot corona. The temperature of the corona is so high that the Sun's gravity cannot hold on to it. Although we understand why this happens we do not understand the details about how and where the coronal gases are accelerated to these high velocities. This question is related to the question of coronal heating."[51]

"The solar wind is not uniform. Although it is always directed away from the Sun, it changes speed and carries with it magnetic clouds, interacting regions where high speed wind catches up with slow speed wind, and composition variations. The solar wind speed is high (800 km/s) over coronal holes and low (300 km/s) over streamers. These high and low speed streams interact with each other and alternately pass by the Earth as the Sun rotates. These wind speed variations buffet the Earth's magnetic field and can produce storms in the Earth's magnetosphere."[51]

"The Ulysses spacecraft completed two orbits through the solar system during which it passed over the Sun's south and north poles. Its measurements of the solar wind speed, magnetic field strength and direction, and composition have provided us with a new view of the solar wind. Ulysses was retired on June 30, 2009."[51]

The second image down on the right shows the results of Ulysses spacecraft measurements of the solar wind speed.

## Neutrals

This image shows the IBEX (photo cells forward) being surrounded by its protective nose cone. Credit: NASA (John F. Kennedy Space Center).
A hot plasma ion 'steals' charge from a cold neutral atom to become an Energetic Neutral Atom (ENA).[52] Credit Mike Gruntman.
The ENA leaves the charge exchange in a straight line with the velocity of the original plasma ion.[52] Credit: Mike Gruntman.
This image is an all-sky map of neutral atoms streaming in from the interstellar boundary. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.

The Solar Wind Anisotropies (SWAN) aboard SOHO "is the only remote sensing instrument on SOHO that does not look at the Sun. It watches the rest of the sky, measuring hydrogen that is ‘blowing’ into the Solar System from interstellar space. By studying the interaction between the solar wind and this hydrogen gas, SWAN determines how the solar wind is distributed. As such, it can be qualified as SOHO’s solar wind ’mapper’."[53]

"The sensors on the IBEX spacecraft are able to detect energetic neutral atoms (ENAs) at a variety of energy levels."[54]

The satellite's payload consists of two energetic neutral atom (ENA) imagers, IBEX-Hi and IBEX-Lo. Each of these sensors consists of a collimator that limits their fields-of-view, a conversion surface to convert neutral hydrogen and oxygen into ions, an electrostatic analyzer (ESA) to suppress ultraviolet light and to select ions of a specific energy range, and a detector to count particles and identify the type of each ion.

"IBEX–Lo can detect particles with energies ranging from 10 electron–volts to 2,000 electron–volts (0.01 keV to 2 keV) in 8 separate energy bands. IBEX–Hi can detect particles with energies ranging from 300 electron–volts to 6,000 electron–volts (.3 keV to 6 keV) in 6 separate energy bands. ... Looking across the entire sky, interactions occurring at the edge of our Solar System produce ENAs at different energy levels and in different amounts, depending on the process."[54]

Proton–hydrogen charge-exchange collisions [such as those shown at right] are often the most important process in space plasma because [h]ydrogen is the most abundant constituent of both plasmas and background gases and hydrogen charge-exchange occurs at very high velocities involving little exchange of momentum.

"Energetic neutral atoms (ENA), emitted from the magnetosphere with energies of ∼50 keV, have been measured with solid-state detectors on the IMP 7/8 and ISEE 1 spacecraft. The ENA are produced when singly charged trapped ions collide with the exospheric neutral hydrogen geocorona and the energetic ions are neutralized by charge exchange."[55]

"The IMAGE mission ... High Energy Neutral Atom imager (HENA) ... images [ENAs] at energies between 10 and 60 keV/nucleon [to] reveal the distribution and the evolution of energetic [ions, including protons] as they are injected into the ring current during geomagnetic storms, drift about the Earth on both open and closed drift paths, and decay through charge exchange to pre‐storm levels."[56]

"In 2009, NASA's Interstellar Boundary Explorer (IBEX) mission science team constructed the first-ever all-sky map [at right] of the interactions occurring at the edge of the solar system, where the sun's influence diminishes and interacts with the interstellar medium. A 2013 paper provides a new explanation for a giant ribbon of energetic neutral atoms – shown here in light green and blue -- streaming in from that boundary."[57]

"[T]he boundary at the edge of our heliosphere where material streaming out from the sun interacts with the galactic material ... emits no light and no conventional telescope can see it. However, particles from inside the solar system bounce off this boundary and neutral atoms from that collision stream inward. Those particles can be observed by instruments on NASA’s Interstellar Boundary Explorer (IBEX). Since those atoms act as fingerprints for the boundary from which they came, IBEX can map that boundary in a way never before done. In 2009, IBEX saw something in that map that no one could explain: a vast ribbon dancing across this boundary that produced many more energetic neutral atoms than the surrounding areas."[57]

""What we are learning with IBEX is that the interaction between the sun's magnetic fields and the galactic magnetic field is much more complicated than we previously thought," says Eric Christian, the mission scientist for IBEX at NASA's Goddard Space Flight Center in Greenbelt, Md. "By modifying an earlier model, this paper provides the best explanation so far for the ribbon IBEX is seeing.""[57]

## Neutrons

This image shows a Bonner Ball Neutron Detector which is housed inside the small plastic ball when the top is put back on. Credit: NASA.

Because free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions).

"The neutrons are produced by the energetic protons interacting with a number of different nuclei."[58]

"Observations made with the gamma-ray spectrometer (GRS) on the Solar Maximum Mission (SMM) satellite and with the Jungfraujoch neutron monitor are used to determine the directional solar neutron emissivity spectrum from ~100 MeV to ~2 GeV during the solar flare on 1982 June 3. The experimental data require a time-extended emission of the neutrons at the Sun with the majority of the neutrons produced after the impulsive phase."[31]

"The first detection of ~400 MeV solar neutrons near the Earth [occurred] following an impulsive solar flare on 1980 June 21 [...] For three events, solar neutron decay protons have been observed near the Earth".[31]

The "existence of neutrons at the Sun, producing the n-p capture γ-ray line at 2.223 MeV, have been reported for several events".[31]

The "average energy of the solar nucleons causing the flare enhancement must be less than for the cosmic-ray primaries above ~3.5 GeV. This means that the atmospheric cascade, producing the excess count rate, was initiated by solar neutrons in the energy range 300 MeV-3.5 GeV."[31]

Around EeV (1018 eV) energies there may be associated ultra high energy neutrons “observed in anisotropic clustering ... because of the relativistic neutrons boosted lifetime.”[59] “[A]t En = 1020 eV, [these neutrons] are flying a Mpc, with their directional arrival (or late decayed proton arrival) ... more on-line toward the source.”[59] Although “neutron (and anti-neutron) life-lengths (while being marginal or meaningless at tens of Mpcs, the growth of their half-lives with energy may naturally explain an associated, showering neutrino halo.”[59]

The Bonner Ball Neutron Detector "BBND ... determined that galactic cosmic rays were the major cause of secondary neutrons measured inside ISS. The neutron energy spectrum was measured from March 23, 2001 through November 14, 2001 in the U.S. Laboratory Module of the ISS. The time frame enabled neutron measurements to be made during a time of increased solar activity (solar maximum) as well as observe the results of a solar flare on November 4, 2001."[60]

"BBND results show the overall neutron environment at the ISS orbital altitude is influenced by highly energetic galactic cosmic rays, except in the South Atlantic Anomaly (SAA) region where protons trapped in the Earth's magnetic field cause a more severe neutron environment. However, the number of particles measured per second per square cm per MeV obtained by BBND is consistently lower than that of the precursor investigations. The average dose-equivalent rate observed through the investigation was 3.9 micro Sv/hour or about 10 times the rate of radiological exposure to the average US citizen. In general, radiation damage to the human body is indicated by the amount of energy deposited in living tissue, modified by the type of radiation causing the damage; this is measured in units of Sieverts (Sv). The background radiation dose received by an average person in the United States is approximately 3.5 milliSv/year. Conversely, an exposure of 1 Sv can result in radiation poisoning and a dose of five Sv will result in death in 50 percent of exposed individuals. The average dose-equivalent rate observed through the BBND investigation is 3.9 micro Sv/hour, or about ten times the average US surface rate. The highest rate, 96 microSv/hour was observed in the SAA region."[60]

## Neutron colors

The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is then adopted to the Maxwellian distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy is of the free neutron. Kinetic energy, speed and wavelength of the neutron are related through the De Broglie relation.

Moderated and other, non-thermal neutron energy distributions or ranges are

• Fast neutrons: kinetic energies greater than 1 eV, 0.1 MeV or approximately 1 MeV, depending on the definition.
• Slow neutrons: a kinetic energy less than or equal to 0.4 eV.
• Epithermal neutrons: an energy from 1 eV to 10 keV.
• Hot neutrons: an energy of about 0.2 eV.
• Thermal neutrons: an energy of about 0.025 eV.[61] This is the most probable energy, while the average energy is 0.038 eV.
• Cold neutrons: an energy from 5 × 10−5 eV to 0.025 eV.
• Very cold neutrons: an energy from 3 × 10−7 eV to 5 × 10−5 eV.
• Ultra cold neutrons: an energy less than 3 × 10−7 eV.
• Continuum region neutrons: an energy from 0.01 MeV to 25 MeV.
• Resonance region neutrons: an energy from 1 eV to 0.01 MeV.
• Low energy region neutrons: an energy less than 1 eV.

The principal component of radiation through great thicknesses of shielding (such as concrete or regolith) consists of neutrons in the very high energy range (above 50 MeV) associated with a 20 GeV synchrotron.[62]

## Fast neutrons

The 14
C
produced by fast neutrons in the reaction 16O(n,3He)14C in the stratosphere depends on oxygen composing 28 % of the atmosphere; i.e. 25-28 % more 14
C
is likely to be produced because fast neutrons are much more plentiful in the stratosphere. This 14
C
may be produced even before neutrons thermalize for the expected reaction.

"THE first suggestion that appreciable 14
C
might be produced in situ in polar ice was made by Fireman and Norris1, who studied 14
C
in CO2 extracted from both accumulation and ablation samples. In some ablation samples they observed 14
C
activities between four and six times higher than those expected due to trapped atmospheric CO2."[63]

"The 14C is produced mainly by nuclear spallations of oxygen in ice. The observed concentration of 14C in ablation ice samples is 1–3 x 103 atom per g ice, three orders of magnitude higher than expected from the amount of trapped atmospheric CO2 in this ice."[63]

"The in situ 14C has a unique signature: about 60% exists as 14CO and the remainder as 14CO2."[63]

"Significant in situ production of radiocarbon by fast neutrons is restricted to the first ~15 m of firn, while the pore closure at this site occurs at 71 m depth [7]. So it is to be expected that most of the in situ produced 14
CO
2
and 14
CO
will diffuse out of the firn matrix and subsequently escape via the pores before these are closed, although a small fraction may stay behind."[64]

"The 14
C
activities of the CO fractions are close to the background value for CO blanks. A mean concentration of 9 ± 6 molecules 14
CO
/g ice was deduced for the three ice samples. The relatively large error is primarily caused by the observed fluctuations in the background level. When this result is combined with the in situ 14
CO
2
/14
CO
ratio of 3.8 [4], this leads to approximately 40 in situ produced 14
C
atoms per gram of ice. For ice still containing all in situ produced 14
C
atoms (no escape before pore closure), the 14
C
concentration can be calculated using the model by Lal et al. [1]. For the altitude, latitude and meteorological data [7,9] of the present location we find approximately 2400 at./g. So we observe that ~98% of the in situ produced 14
C
escaped from the firn before pore closure. This result compares well with the ~3% retained in situ 14
C
, obtained by Wilson and Donahue [5] on two ice samples of the GISP ice core."[64]

Disregarding "in situ production of radiocarbon would make the correlation worse: the radiocarbon ages would become younger. The large uncertainty in radiocarbon AD age for the youngest samples is mainly caused by the radio- carbon calibration curve."[64]

"To compare our radiocarbon ages with ages derived from volcanic horizon identification with di-electrical profiling (DEP)/electrical DC conductivity (ECM) measurements, the age difference between trapped air and the ice matrix must be known. The age of the ice matrix at pore closure, at this site, can be calculated from the accumulation rate (62 mm water equivalent/yr), the -10 m temperature (-38.5°C) and the initial density of the snow pack (325 kg/m3) [7,9,11], which leads to 740 yr. According to Schwander and Stauffer [12], the average age difference between the air captured in the ice and the ice matrix is equal to the age of the ice matrix at a density of 815 kg/m3. For this site [Dronning Maud Land, Antarctica], this leads to 670 yr (estimated error ± 100 yr), [...]. (At 815 kg/m3 ca. 50% of the air which will be eventually in the ice has been trapped.)"[64]

The "results obtained at this site by radiocarbon dating of ice at shallow depth cannot compete in accuracy with those obtained by the DEP/ECM methods. However, for drill sites with very low accumulation rates, sites where hiati exist, or at greater depth where stratigraphical methods become more uncertain due to layer thinning, 14
C
measurements can provide absolute age estimates of the captured air from which ages of the ice matrix can be approximated."[64]

"The Earth's magnetic field deflects incoming charged particles so that the equatorial cosmic-ray flux is four times less than the polar flux [...]."[65]

"Spallation of atmospheric oxygen nuclei might contribute up to 20% to production of 14
C
produced in the atmosphere (Lal and Peters 1967)."[20]

"The fraction of cosmogenic 14
C
produced below the atmosphere at the earth’s surface is estimated to be less than 0.1% of the total (Lal 1988a, 1992b)."[20]

## Slow neutrons

"The sharp dependence on energy of the cadmium cross section for neutrons of energies near 0.35 eV [slow neutrons] has been used to investigate the energy distribution of 0.35-ev neutrons scattered through 90° by lead, aluminum, diamond, and graphite."[66]

## Thermal neutrons

A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV (about 4.0×10−21 J or 2.4 MJ/kg, hence a speed of 2.2 km/s), which is the most probable energy at a temperature of 290 K (17 °C or 62 °F), the mode of the Maxwell–Boltzmann distribution for this temperature.

After a number of collisions with nuclei (scattering) in a medium (neutron moderator) at this temperature, neutrons arrive at about this energy level, provided that they are not absorbed.

Thermal neutrons have a different and sometimes much larger effective neutron absorption cross-section for a given nuclide than fast neutrons, and can therefore often be absorbed more easily by an atomic nucleus, creating a heavier, often unstable isotope of the chemical element as a result (neutron activation).

## Neutron sources

Neutron emitters to left of lower dashed line
Z → 0 1 2
n ↓ n H He 3 4
0 1H Li Be 5 6
1 1n 2H 3He 4Li 5Be B C 7
2 2n 3H 4He 5Li 6Be 7B 8C N 8
3 4H 5He 6Li 7Be 8B 9C 10N O 9
4 4n 5H 6He 7Li 8Be 9B 10C 11N 12O F 10
5 6H 7He 8Li 9Be 10B 11C 12N 13O 14F Ne 11
6 7H 8He 9Li 10Be 11B 12C 13N 14O 15F 16Ne Na 12
7 9He 10Li 11Be 12B 13C 14N 15O 16F 17Ne 18Na Mg 13
8 10He 11Li 12Be 13B 14C 15N 16O 17F 18Ne 19Na 20Mg Al 14
9 12Li 13Be 14B 15C 16N 17O 18F 19Ne 20Na 21Mg 22Al Si
10 14Be 15B 16C 17N 18O 19F 20Ne 21Na 22Mg 23Al 24Si
11 16B 17C 18N 19O 20F 21Ne 22Na 23Mg 24Al 25Si
12 18C 19N 20O 21F 22Ne 23Na 24Mg 25Al 26Si
13 20N 21O 22F 23Ne 24Na 25Mg
26Al
27Si
14 22O 23F 24Ne 25Na 26Mg 27Al 28Si

Neutrons are produced when alpha particles impinge upon any of several low atomic weight isotopes including isotopes of lithium, beryllium, carbon and oxygen.

Gamma radiation with an energy exceeding the neutron binding energy of a nucleus can eject a neutron. Two examples and their decay products:

9Be + >1.7 Mev photon → 1 neutron + 2 4He
2H (deuterium) + >2.26 MeV photon → 1 neutron + 1H

Traditional particle accelerators with hydrogen (H), deuterium (D), or tritium (T) ion sources may be used to produce neutrons using targets of deuterium, tritium, lithium, beryllium, and other low-Z materials. Typically these accelerators operate with voltages in the > 1 MeV range.

Neutrons (so-called photoneutrons) are produced when photons above the nuclear binding energy of a substance are incident on that substance, causing it to undergo giant dipole resonance after which it either emits a neutron (photodisintegration) or undergoes fission (photofission). The number of neutrons released by each fission event is dependent on the substance. Typically photons begin to produce neutrons on interaction with normal matter at energies of about 7 to 40 MeV, which means that megavoltage photon radiotherapy facilities may produce neutron radiation as well, and require special shielding for it. In addition, electrons of energy over about 50 MeV may induce giant dipole resonance in nuclides by a mechanism which is the inverse of internal conversion, and thus produce neutrons by a mechanism similar to that of photoneutrons.[67]

A spallation source is a high-flux source in which protons that have been accelerated to high energies hit a target material, prompting the emission of neutrons.

Nuclear fusion, the combining of the heavy isotopes of hydrogen, also has the potential to produce large quantities of neutrons.

Neutron emission is a type of radioactive decay of atoms containing excess neutrons, in which a neutron is simply ejected from the nucleus. Two examples of isotopes which emit neutrons are beryllium-13 (mean life 2.7x10-21 sec) and helium-5 (7x10-22 sec).

Neutron emission usually happens from nuclei that are in an excited state, such as the excited O-17* produced from the beta decay of N-17. The neutron emission process itself is controlled by the nuclear force and therefore is extremely fast, sometimes referred to as "nearly instantaneous." The ejection of the neutron may be as a product of the movement of many nucleons, but it is ultimately mediated by the repulsive action of the nuclear force that exists at extremely short-range distances between nucleons. The life time of an ejected neutron inside the nucleus before it is emitted is usually comparable to the flight time of a typical neutron before it leaves the small nuclear "potential well," or about 10-23 seconds.[68] A synonym for such neutron emission is "prompt neutron" production, of the type that is best known to occur simultaneously with induced nuclear fission. Many heavy isotopes, most notably californium-252, also emit prompt neutrons among the products of a similar spontaneous radioactive decay process, spontaneous fission.

Most neutron emission outside prompt neutron production associated with fission (either induced or spontaneous), is from neutron-heavy isotopes produced as fission products. These neutrons are sometimes emitted with a delay, giving them the term delayed neutrons, but the actual delay in their production is a delay waiting for the beta decay of fission products to produce the excited-state nuclear precursors that immediately undergo prompt neutron emission. Thus, the delay in neutron emission is not from the neutron-production process, but rather its precursor beta decay which is controlled by the weak force, and thus requires a far longer time. The beta decay half lives for the precursors to delayed neutron-emitter radioisotopes, are typically fractions of a second to tens of seconds.

## Protons

This figure shows a detected 94 % correlation between scaled sunspot numbers and neutrino detections. Credit: John N. Bahcall.
The diagram shows one of the Van Allen Probes with various components and subsystems labeled. Credit: JHU/APL.
This graph displays the flux of high energy protons measured by GOES 11 over four days from November 2, 2003, to November 5, 2003. Credit: NOAA.

"Proton astronomy should be possible; it may also provide indirect information on inter-galactic magnetic fields."[69]

"The Relativistic Proton Spectrometer (RPS) [measures] inner radiation belt protons with energies from 50 MeV-2 GeV. Such protons are known to pose a number of hazards to humans and spacecraft, including total ionizing dose, displacement damage, single event effects, and nuclear activation. The objectives of the investigation are to: (1) support the development of a new AP9/AE9 standard radiation model for spacecraft design; (2) to develop and test the model for RBSP data in general and RPS specifically; and, (3) to provide standardized worst-case specifications for dose rate, internal and deep dielectric chargins, and surface charging."[70]

"Neutrinos can be produced by energetic protons accelerated in solar magnetic fields. Such protons produce pions, and therefore muons, hence also neutrinos as a decay product, in the solar atmosphere."[71]

"Energetic protons in the solar corona could explain Figure 2 [at right] only if (1) they tap a substantial fraction of the entire energy generated in the corona, (2) the energy generated in the corona is at least 3 times what has been deduced from the observations, (3) the vast majority of energetic protons do not escape the Sun, (4) the proton energy spectrum is unusually hard (p0 = 300 MeV c-1, and (5) the sign of the variation is opposite to what one would predict. As the likelihood of all of these conditions being fulfilled seems extremely small, we do not believe that neutrinos produced by energetic protons in the solar atmosphere contribute significantly to the neutrino capture in the 37Cl experiment."[71]

"Proton astronomy should be possible; it may also provide indirect information on inter-galactic magnetic fields."[72]

Proton astronomy per se often consists of directly or indirectly detecting the protons and deconvoluting a spatial, temporal, and spectral distribution.

“[A]t the high end of the proton energy spectrum (above ≈ 1018 eV) [the Larmor radius] deflection becomes small enough that proton astronomy becomes possible.”[73]

"The third largest solar proton event in the past thirty years took place during July 14-16, 2000, and had a significant impact on the earth's atmosphere."[74]

## Mesons

Mesons are hadronic subatomic particles, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometre, which is about ​23 the size of a proton or neutron.

Charged mesons decay (sometimes through intermediate particles) to form electrons and neutrinos. Uncharged mesons may decay to photons.

Mesons are not produced by radioactive decay, but appear in nature only as short-lived products of very high-energy interactions in matter. In cosmic ray interactions, for example, such particles are ordinary protons and neutrons. Mesons are also frequently produced artificially in high-energy particle accelerators that collide protons, anti-protons, or other particles.

Mesons are subject to both the weak and strong interactions. Mesons with net electric charge also participate in the electromagnetic interaction.

While no meson is stable, those of lower mass are nonetheless more stable than the most massive mesons, and are easier to observe and study in particle accelerators or in cosmic ray experiments. They are also typically less massive than baryons, meaning that they are more easily produced in experiments, and thus exhibit certain higher energy phenomena more readily than baryons composed of the same quarks would.

Potential mesons to be detected astronomically include: π, ρ, η, η′, φ, ω, J/ψ, ϒ, θ, K, B, D, and T.

## B mesons

"The K0-K0 bar, D0-D0 bar, and B0-B0 bar oscillations are extremely sensitive to the K0 and K0 bar energy at rest. The energy is determined by the values mc2 with the related mass as well as the energy of the gravitational interaction. Assuming the CPT theorem for the inertial masses and estimating the gravitational potential through the dominant contribution of the gravitational potential of our Galaxy center, we obtain from the experimental data on the K0-K0 bar oscillations the following constraint: |(mg/mi)K0 - (mg/mi)K0 bar| ≤ 8·10-13, CL=90%. This estimation is model dependent and in particular it depends on a way we estimate the gravitational potential. Examining the K0-K0 bar, B0-B0 bar, and D0-D0 bar oscillations provides us also with weaker, but model independent constraints, which in particular rule out the very possibility of antigravity for antimatter."[75]

## Upsilon mesons

A plot of the invariant mass of muon pairs, the peak at about 9.5 GeV is due to the contribution of the Upsilon meson. Credit: Leon Lederman and the E288 collaboration, Fermilab.

The plot on the right shows a peak at about 9.5 GeV due to the Upsilon meson.

## Psions

J/Ψ production is graphed. Credit: Fermilab.

On the right is a graph of the production of psions at Fermilab.

## Omega mesons

Omega meson production:[76]

1. ${\displaystyle p+d\rightarrow He^{3}+\omega ,}$
2. ${\displaystyle {\bar {p}}+p\rightarrow \omega +\eta +\pi _{0},}$
3. ${\displaystyle \pi ^{-}+p\rightarrow \omega +n,}$
4. ${\displaystyle p+{\bar {p}}\rightarrow \mathrm {K} ^{+}+\mathrm {K} ^{-}+\omega ,}$
5. ${\displaystyle p+{\bar {p}}\rightarrow \mathrm {K} 1+\mathrm {K} 1+\omega ,}$

## Phi mesons

The phi meson ${\displaystyle \Phi ^{0}}$(1020) has a mass of 1019.445 MeV. It decays per[77]

1. ${\displaystyle \Phi ^{0}\rightarrow \mathrm {K} ^{+}+\mathrm {K} ^{-}or}$
2. ${\displaystyle \Phi ^{0}\rightarrow \mathrm {K} _{S}^{0}+\mathrm {K} _{L}^{0}.}$

## Rho mesons

Rho mesons occur in three states: ρ+, ρ-, and ρ0.[77] The rest masses are apparently the same at 775.4±0.4 and 775.49±0.34.[77] Decay products are π± + π0 or π+ + π-, respectively.[77]

## Eta mesons

Eta mesons (547.863 ± 0.018 MeV) have the decay schemes:[76]

1. η : ${\displaystyle \eta \rightarrow \gamma +\gamma ,}$
2. η : ${\displaystyle \eta \rightarrow \pi ^{0}+\pi ^{0}+\pi ^{0},or}$
3. η : ${\displaystyle \eta \rightarrow \pi ^{+}+\pi ^{0}+\pi ^{-},}$

Eta prime mesons (957.78 ± 0.06 MeV) have the decay schemes:[76]

1. η' : ${\displaystyle \eta ^{'}\rightarrow \pi ^{+}+\pi ^{-}+\eta or}$
2. η' : ${\displaystyle \eta ^{'}\rightarrow \pi ^{0}+\pi ^{0}+\gamma ,}$

The charmed eta meson ηC(1S) has a rest mass of 2983.6 ± 0.7 MeV.[76]

## D mesons

${\displaystyle D_{S}\rightarrow \tau +{\bar {\nu }}_{\tau }\rightarrow \nu _{\tau }+{\bar {\nu }}_{\tau }.}$[78]

## Kaons

"The muons created through decays of secondary pions and kaons are fully polarized, which results in electron/positron decay asymmetry, which in turn causes a difference in their production spectra."[79]

The "highest energy neutrinos from GRBs mainly come from kaons."[80]

## Pions

"The Gamma-Ray Spectrometer (GRS) on [Solar Maximum Mission] SMM has detected [...] at least two of the flares have spectral properties >40 MeV that require gamma rays from the decay of neutral pions. [Pion] production can occur early in the impulsive phase as defined by hard X-rays near 100 keV."[81]

## Tauons

"For ultrahigh energies the neutrino spectrum at the detector is influenced by neutrino-nucleon interactions and tauon decays during the passage through the interior of the earth."[82]

## Muons

The Moon's cosmic ray shadow, as seen in secondary muons generated by cosmic rays in the atmosphere, and detected 700 meters below ground, at the Soudan II detector. Credit: J. H. Cobb et al. (The Soudan 2 Collaboration).{{fairuse}}

"TeV muons from γ ray primaries ... are rare because they are only produced by higher energy γ rays whose flux is suppressed by the decreasing flux at the source and by absorption on interstellar light."[83]

"[T]here is a window of opportunity for muon astronomy with the AMANDA, Lake Baikal, and MILAGRO detectors."[83]

## Neutrinos

The diagram contains the reactions in the proton-proton chain including neutrino production. Credit: Dorottya Szam.

The highest flux of solar neutrinos come directly from the proton-proton interaction, and have a low energy, up to 400 keV. There are also several other significant production mechanisms, with energies up to 18 MeV. [84]

A neutrino created with a specific lepton flavor (electron, muon or tau) can later be measured to have a different flavor.

A great deal of evidence for neutrino oscillation has been collected from many sources, over a wide range of neutrino energies and with many different detector technologies.[85]

Solar neutrinos have energies below 20 MeV and travel an astronomical unit between the source in the Sun and detector on the Earth. At energies above 5 MeV, solar neutrino oscillation actually takes place in the Sun through a resonance known as the MSW effect, a different process from the vacuum oscillation.

The presence of electrons in matter changes the energy levels of the propagation eigenstates (mass eigenstates) of neutrinos due to charged current coherent forward scattering of the electron neutrinos (i.e., weak interactions). The coherent forward scattering is analogous to the electromagnetic process leading to the refractive index of light in a medium. This means that neutrinos in matter have a different effective mass than neutrinos in vacuum, and since neutrino oscillations depend upon the squared mass difference of the neutrinos, neutrino oscillations may be different in matter than they are in vacuum. With antineutrinos, the conceptual point is the same but the effective charge that the weak interaction couples to (called weak isospin) has an opposite sign.

The effect is important at the very large electron densities of the Sun where electron neutrinos are produced. The high-energy neutrinos seen, for example, in SNO (Sudbury Neutrino Observatory) and in Super-Kamiokande, are produced mainly as the higher mass eigenstate in matter ν2m, and remain as such as the density of solar material changes. (When neutrinos go through the MSW resonance the neutrinos have the maximal probability to change their nature, but it happens that this probability is negligibly small—this is sometimes called propagation in the adiabatic regime). Thus, the neutrinos of high energy leaving the sun are in a vacuum propagation eigenstate, ν2, that has a reduced overlap with the electron neutrino νe = ν1 cosθ + ν2 sinθ seen by charged current reactions in the detectors.

"For high-energy solar neutrinos the MSW effect is important, and leads to the expectation that Pee = sin²θ, where θ = 34° is the solar mixing angle. This was dramatically confirmed in the Sudbury Neutrino Observatory (SNO), which has resolved the solar neutrino problem. SNO measured the flux of Solar electron neutrinos to be ~34% of the total neutrino flux (the electron neutrino flux measured via the charged current reaction, and the total flux via the neutral current reaction). The SNO results agree well with the expectations.

For the low-energy solar neutrinos, on the other hand, the matter effect is negligible, and the formalism of oscillations in vacuum is valid. The size of the source (i.e. the Solar core) is significantly larger than the oscillation length, therefore, averaging over the oscillation factor, one obtains Pee = 1 − sin²2θ / 2. For the same value of the solar mixing angle (θ = 34°) this corresponds to a survival probability of Pee ≈ 60%. This is consistent with the experimental observations of low energy Solar neutrinos by the Homestake experiment (the first experiment to reveal the solar neutrino problem), followed by GALLEX, GNO, and SAGE (collectively, gallium radiochemical experiments), and, more recently, the Borexino experiment. These experiments provided further evidence of the MSW effect.

The transition between the low energy regime (the MSW effect is negligible) and the high energy regime (the oscillation probability is determind by matter effects) lies in the region of about 2 MeV for the Solar neutrinos.

Here on the Earth's surface the νe flux is about 1011 νe cm-2 s-1 in the direction of the Sun.[86]

"The total number of neutrinos of all types agrees with the number predicted by the computer model of the Sun. Electron neutrinos constitute about a third of the total number of neutrinos. [...] The missing neutrinos were actually present, but in the form of the more difficult to detect muon and tau neutrinos."[86]

For antiproton-proton annihilation at rest, a meson result is, for example,

${\displaystyle p^{+}+{\bar {p}}^{-}\rightarrow \pi ^{+}+\pi ^{-},}$[87]
${\displaystyle {\pi }^{+}\rightarrow {\mu }^{+}+{\nu }_{\mu }\rightarrow e^{+}+{\nu }_{e}+{\bar {\nu }}_{\mu }+{\nu }_{\mu },}$[88] and
${\displaystyle D_{S}\rightarrow \tau +{\bar {\nu }}_{\tau }\rightarrow \nu _{\tau }+{\bar {\nu }}_{\tau }.}$[78]

"All other sources of ντ are estimated to have contributed an additional 15%."[78]

${\displaystyle \tau \rightarrow e+\nu _{\tau }+\nu _{e},}$[78]

for two neutrinos.[78]

${\displaystyle \tau \rightarrow h+\nu _{\tau }+X,}$[78]

where ${\displaystyle h}$ is a hadron, for two neutrinos.[78]

## Electrons

Solitary electrons constitute much of the remaining 1% of cosmic rays.

"The conventional procedure of delta-ray counting to measure charge (Powell, Fowler, and Perkins 1959), which was limited to resolution sigmaz = 1-2 because of uncertainties of the criterion of delta-ray ranges, has been significantly improved by the application of delta-ray range distribution measurements for 16O and 32S data of 200 GeV per nucleon (Takahashi 1988; Parnell et al. 1989)."[89] Here, the delta-ray tracks in emulsion chambers have been used for "[d]irect measurements of cosmic-ray nuclei above 1 TeV/nucleon ... in a series of balloon-borne experiments".[89]

## Positrons

Observation of positrons from a terrestrial gamma ray flash is performed by the Fermi gamma ray telescope. Credit: NASA Goddard Space Flight Center.

A few antiprotons and positrons are in primary cosmic rays.

"In the first 18 months of operations, AMS-02 [image under Cherenkov detectors] recorded 6.8 million positron (an antimatter particle with the mass of an electron but a positive charge) and electron events produced from cosmic ray collisions with the interstellar medium in the energy range between 0.5 giga-electron volt (GeV) and 350 GeV. These events were used to determine the positron fraction, the ratio of positrons to the total number of electrons and positrons. Below 10 GeV, the positron fraction decreased with increasing energy, as expected. However, the positron fraction increased steadily from 10 GeV to 250 GeV. This increase, seen previously though less precisely by instruments such as the Payload for Matter/antimatter Exploration and Light-nuclei Astrophysics (PAMELA) and the Fermi Gamma-ray Space Telescope, conflicts with the predicted decrease of the positron fraction and indicates the existence of a currently unidentified source of positrons, such as pulsars or the annihilation of dark matter particles. Furthermore, researchers observed an unexpected decrease in slope from 20 GeV to 250 GeV. The measured positron to electron ratio is isotropic, the same in all directions."[90]

A High-Energy Antimatter Telescope (HEAT) has been developed and tested in the mid 1990s to measure the positron fraction in cosmic rays.[91]

There is an "unexpected rise of the positron fraction, observed by HEAT and PAMELA experiments, for energies larger than a few GeVs."[92]

"[T]he HEAT balloon experiment [30] ... has mildly indicated a possible positron excess at energies larger than 10 GeV ... In October 2008, the latest results of PAMELA experiment [36] have confirmed and extended this feature [37]."[92]

Earlier measurements indicate that "the positron fraction, [f = ] e+/(e- + e+), increases with energy at energies above 10 GeV. Such an increase would require either the appearance of a new source of positrons or a depletion of primary electrons."[91] All results taken together suggest a slight decrease with increasing energy from about 1 GeV to 10 GeV, but overall the fraction may be constant, per Figure 2.[91]

## Gamma rays

"The 2.2 MeV line is formed in the reaction which synthesizes deuterium: 1H(n,γ)2H ... The line has been observed in a number of solar flares by the SMM, Hinotori and Prognoz satellites".[93]

"The 2.2-MeV line fluence throughout the [May 24, 1990] flare was 345 ± 6 photons/cm2, which corresponds to the observed synthesis of over 3 tons [some ~3.3 metric tons] of deuterium on the solar surface."[93]

"Surface fusion is no longer bizarre since the 2.2 MeV gamma ray line of the P(n,γ)D reaction was observed[93] during the solar flare of May 24 1990."[94]

"[M]ost of the sun’s fusion must occur near the surface rather than the core."[94]

## Ultrasoft X-rays

"[T]he ultrasoft X-ray emission (peak energy 30-50 eV) observed in the three strong (≥ 4 1037-1038 erg s-1) LMC X-ray sources CAL83, CAL87 and RXJ0527.8-6954 can be explained by steady nuclear burning of hydrogen accreted onto white dwarfs with masses in the range of 0.7 to 1.2 Mʘ."[95]

## Coronal clouds

The image is a schematic view of the Mount Norikura solar neutron telescope. Credit: Y. Muraki, K. Murakami, M. Miyazaki, K. Mitsui. S. Shibata, S. Sakakibara, T. Sakai, T. Takahashi, T. Yamada, and K. Yamaguchi.
RHESSI observes high-energy phenomena from a solar flare. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.
"This graph shows the neutrons detected by a neutron detector at the University of Oulu in Finland from May 16 through May 18, 2012. The peak on May 17 represents an increase in the number of neutrons detected, a phenomenon dubbed a ground level enhancement or GLE. This was the first GLE since December of 2006. Credit: University of Oulu/NASA's Integrated Space Weather Analysis System"[57].

"[C]oronal magnetic bottles, produced by flares, [may] serve as temporary traps for solar cosmic rays ... It is the expansion of these bottles at velocities of 300–500 km/s which allows fast azimuthal propagation of solar cosmic rays independent of energy. A coronagraph on Os 7 observed a coronal cloud which was associated with bifurcation of the underlying coronal structure."[96]

"A persistent problem of solar cosmic-ray research has been the lack of observations bearing on the timing and conditions in which protons that escape to the interplanetary medium are first accelerated in the corona."[36]

Fairly large fluxes of neutrons have been observed during solar flares such as that of November 12, 1960, with a flux of 30-70 neutrons per cm-2 s-1.[97]

"The neutrons are produced by the energetic protons interacting with a number of different nuclei."[58]

A "new detector to observe solar neutrons [has been in operation] since 1990 October 17 [...] at the Mount Norikura Cosmic Ray Laboratory (CRL) of [the] Institute for cosmic Ray Research, the University of Tokyo."[98]

"On 1991 June 1, an active sunspot appeared at N25 E90 on the Sun (NOAA region 6659). The commencement of an enormous bright flare was observed at 03:37 UT on 1991 June 4 [...] The flare was classified as 3 B and the location was at N31 E70 of the solar surface."[98]

"The solar neutron telescope [image at right] consists of 10 blocks of scintillator [...] and several lead plates which are used to place kinetic energies Tn of incoming particles into three bands (50-360 MeV, 280-500 MeV, and ≥ 390 MeV)."[98] The telescope is inclined to the direction of the Sun by 15°.[98] The plane area of the detector is 1.0 m2 and protected by lead plates (Pb) to eliminate gamma-ray and muon background from the side of the detector.[98] The anti-coincident counter (A) is used to reject the muons and gamma rays, coming from the side of the detector and the top scintillators.[98] (P) and (G) are used to identify the proton events and gamma rays.[98] The central scintillator blocks are optically separated into 10 units.[98]

"The horizontal scintillator just above the 10 vertical scintillators distinguishes neutral particles (neutrons) from the charged particles (mainly muons, protons and electrons)."[98]

"Mount Norikura Cosmic-Ray Laboratory has an elevation of 2770 m above sea level. The geographical latitude is 36.10° N and the longitude is 137.55° E. The zenith angle of the Sun at 03:37 UT on June 4 is 18.9° and the solar neutron telescope was set at a zenith angle of 15° on this day."[98]

The solar flare at Active Region 10039 on July 23, 2002, exhibits many exceptional high-energy phenomena including the 2.223 MeV neutron capture line and the 511 keV electron-positron (antimatter) annihilation line. In the image at right, the RHESSI low-energy channels (12-25 keV) are represented in red and appear predominantly in coronal loops. The high-energy flux appears as blue at the footpoints of the coronal loops. Violet is used to indicate the location and relative intensity of the 2.2 MeV emission.

During solar flares “[s]everal radioactive nuclei that emit positrons are also produced; [which] slow down and annihilate in flight with the emission of two 511 keV photons or form positronium with the emission of either a three gamma continuum (each photon < 511 keV) or two 511 keV photons. Higher energy protons and ®-particles produce charged and neutral pions that decay to produce high-energy electrons/positrons and photons, respectively; these were detected in the 1991 June 11 flare by EGRET (Kanbach et al. 1993)."[99]

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) made the first high-resolution observation of the solar positron-electron annihilation line during the July 23, 2003 solar flare.[99]

There "is only a narrow range of temperatures around 6 × 103 K, and only in a quiet solar atmosphere, where the line shape is dominated by the formation of positronium in flight (the positron replaces the proton in the hydrogen atom). The positronium can be formed in either the singlet or triplet state (Crannell et al. 1976). When it annihilates from the singlet state, it emits two 511 keV γ rays (2γ) in the center-of-mass frame; the lines are broadened by the velocity of the positronium."[99]

The observations are somewhat consistent with electron-positron annihilation in a quiet solar atmosphere via positronium as well as during flares.[99] Line-broadening is due to "the velocity of the positronium."[99]

"The width of the annihilation line is also consistent ... with thermal broadening (Gaussian width of 8.1 ± 1.1 keV) in a plasma at 4-7 x 105 K. In a quiet solar atmosphere, these temperatures are only reached in the transition region at densities ≤ 1012 H cm−3. ... The RHESSI and all but two of the SMM measurements are consistent with densities ≤ 1012 H cm-3 [but] <10% of the p and α interactions producing positrons occur at these low densities. ... positrons produced by 3He interactions form higher in the solar atmosphere ... all observations are consistent with densities > 1012 H cm-3. But such densities require formation of a substantial mass of atmosphere at transition region temperatures."[99]

The "positrons annihilate at such low densities [...] positrons produced by 3He interactions form higher in the solar atmosphere; however, in order to explain the line width, it would require a much higher 3He/4He ratio than the upper limit set for this flare by RHESSI. Alternatively, all the observations are consistent with densities > 1012 H cm−3. But such densities require formation of a substantial mass of atmosphere at transition region temperatures."[99]

"On May 17, 2012 an M-class flare exploded from the sun. The eruption also shot out a burst of solar particles traveling at nearly the speed of light that reached Earth about 20 minutes after the light from the flare. An M-class flare is considered a "moderate" flare, at least ten times less powerful than the largest X-class flares, but the particles sent out on May 17 were so fast and energetic that when they collided with atoms in Earth's atmosphere, they caused a shower of particles to cascade down toward Earth's surface. The shower created what's called a ground level enhancement (GLE)."[100]

"[O]n Saturday, May 5, ... a large sunspot rotated into view on the left side of the sun. ... [J]ust before [Active Region 1476] disappeared over the right side of the sun, it ... erupted with an M-class flare."[100]

### Zodiacal lights

The Zodiacal Light is over the Faulkes Telescope, Haleakala, Maui. Credit: 808caver.

The Zodiacal light is a faint, roughly triangular, diffuse white glow seen in the night sky that appears to extend up from the vicinity of the Sun along the ecliptic or zodiac.[101] It is best seen just after sunset and before sunrise in spring and autumn when the zodiac is at a steep angle to the horizon. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders it invisible. The zodiacal light decreases in intensity with distance from the Sun, but on very dark nights it has been observed in a band completely around the ecliptic. In fact, the zodiacal light covers the entire sky, being responsible for major part[102] of the total skylight on a moonless night. There is also a very faint, but still slightly increased, oval glow directly opposite the Sun which is known as the gegenschein. The dust forms a thick pancake-shaped cloud in the Solar System collectively known as the zodiacal cloud, which occupies the same plane as the ecliptic. The dust particles are between 10 and 300 micrometres in diameter, with most mass around 150 micrometres.[103]

### Gegenscheins

The Gegenschein is seen directly opposite to the sun's position in the sky. It is much fainter than the Zodiacal light, and is caused by sunlight reflecting off dust particles outside the Earth's orbit.

The Van Allen radiation belt is split into two distinct belts, with energetic electrons forming the outer belt and a combination of protons and electrons forming the inner belts. In addition, the radiation belts contain lesser amounts of other nuclei, such as alpha particles.

The large outer radiation belt extends from an altitude of about three to ten Earth radii (RE) or 13,000 to 60,000 kilometres above the Earth's surface. Its greatest intensity is usually around 4–5 RE. The outer electron radiation belt is mostly produced by the inward radial diffusion[104][105] and local acceleration[106] due to transfer of energy from whistler mode plasma waves to radiation belt electrons. Radiation belt electrons are also constantly removed by collisions with atmospheric neutrals,[106] losses to magnetopause, and the outward radial diffusion. The outer belt consists mainly of high energy (0.1–10 MeV) electrons trapped by the Earth's magnetosphere. The gyroradii for energetic protons would be large enough to bring them into contact with the Earth's atmosphere. The electrons here have a high flux and at the outer edge (close to the magnetopause), where geomagnetic field lines open into the geomagnetic "tail", fluxes of energetic electrons can drop to the low interplanetary levels within about 100 km (a decrease by a factor of 1,000).

## Jovian electrons

"Jovian electrons, both at Jupiter and in the interplanetary medium near Earth, have a very hard spectrum that varies as a power law with energy (see, e.g., Mewaldt et al. 1976). This spectral character is sufficiently distinct from the much softer solar and magnetospheric electron spectra that it has been used as a spectral filter to separate Jovian electrons from other sources ... A second Jovian electron characteristic is that such electrons in the interplanetary medium tend to consist of flux increases of several days duration which recur with 27 day periodicities ... A third feature of Jovian electrons at 1 AU is that the flux increases exhibit a long-term modulation of 13 months which is the synodic period of Jupiter as viewed from Earth".[107]

Jovian electrons propagate "along the spiral magnetic field of the interplanetary medium [from Jupiter and its magnetosphere to the Sun]".[107]

## Electromagnetics

"Carbon materials for electromagnetic interference (EMI) shielding [...] include composite materials, colloidal graphite and flexible graphite. Carbon filaments of submicron diameter are effective for use in composite materials, especially after electroplating with nickel."[108]

## Mars

This graph shows the preliminary results from Curiosity's first radiation measurements on Mars, specifically the flux of radiation detected by Curiosity's Radiation Assessment Detector (RAD) on Mars over three and a half hours on Aug. 6 PDT (Aug. 7 UTC). Credit: NASA/JPL-Caltech/SWRI.
This image contains polar maps of thermal and epithermal neutrons as detected by the Mars Odyssey spacecraft in orbit around Mars. The images are from July 22, 2009. Credit: NASA/JPL-Caltech.

"NASA's Curiosity rover ... Radiation Assessment Detector instrument, or RAD, collected data for about 3 1/2 hours on Wednesday (Aug. 8)"[109]. As the Sun was relatively quiet in the direction of Mars, most of the spikes in the collected, unprocessed temporal spectrum are considered to be from galactic cosmic-radiation.[110]

"The data show that the radiation levels measured on Mars during this period of quiet solar activity are reduced from the average radiation detected in space during Curiosity's cruise to Mars. This is explained by the rover being on the planet versus out in space, where it would have more exposure to radiation from all directions. Red arrows point to spikes in the radiation dose rate from heavy ion particles, which would be the most dangerous to astronauts. ... RAD measures 26 kinds of charged particles as well as neutrons and gamma rays."[111] Several neutron detectors and spectrometers have been and are currently being used to measure surface properties associated with neutron emission. The Dynamic Albedo of Neutrons (DAN) spectrometer is aboard the Curiosity rover.

"The Dynamic Albedo of Neutrons (DAN) is an active/passive neutron spectrometer that measures the abundance and depth distribution of H- and OH-bearing materials (e.g., adsorbed water, hydrated minerals) in a shallow layer (~1 m) of Mars' subsurface along the path of the MSL rover. In active mode, DAN measures the time decay curve (the "dynamic albedo") of the neutron flux from the subsurface induced by its pulsing 14 MeV neutron source."[112] "The science objectives of the DAN instrument are as follows: 1) Detect and provide a quantitative estimation of the hydrogen in the subsurface throughout the surface mission; 2) Investigate the upper <0.5 m of the subsurface and determine the possible layering structure of hydrogen-bearing materials in the subsurface; 3) Track the variability of hydrogen content in the upper soil layer (~1 m) during the mission by periodic analysis; and 4) Track the variability of neutron radiation background (neutrons with energy < 100 keV) during the mission by periodic analysis."[112]

Both the neutron spectrometer, from Los Alamos National Laboratories in New Mexico, and the High Energy Neutron Detector (HEND), from the the Russian Aviation and Space Agency, are operating aboard the Odyssey spacecraft in orbit around Mars since 2001.

## Interplanetary medium

The interplanetary medium includes interplanetary dust, cosmic rays and hot plasma from the solar wind. The temperature of the interplanetary medium varies. For dust particles within the asteroid belt, typical temperatures range from 200 K (−73 °C) at 2.2 AU down to 165 K (−108 °C) at 3.2 AU[113] The density of the interplanetary medium is very low, about 5 particles per cubic centimeter in the vicinity of the Earth; it decreases with increasing distance from the sun, in inverse proportion to the square of the distance. It is variable, and may be affected by magnetic fields and events such as coronal mass ejections. It may rise to as high as 100 particles/cm³.

"[I]nterplanetary space ... is a stormy and sometimes very violent environment permeated by energetic particles and radation constantly emanating from the Sun."[39]

## Heliospheres

"The sun emits a plasma wind with an embedded magnetic field that tends to exclude low energy galactic cosmic rays from the heliosphere."[27]

The "observed cosmic ray flux at Earth is inversely correlated with solar activity. [...] At a period of high solar activity (for example in 1983), the flux below a GeV can be suppressed by as much as an order of magnitude."[27]

The "flux of cosmic rays in the heliosphere varies with the eleven year solar cycle".[27]

## Interstellar medium

Def. cosmic rays that are created when primary cosmic rays interact with interstellar matter are called secondary cosmic rays.

Carbon and oxygen nuclei collide with interstellar matter to form lithium, beryllium and boron in a process termed cosmic ray spallation. Spallation is also responsible for the abundances of scandium, titanium, vanadium, and manganese ions in cosmic rays produced by collisions of iron and nickel nuclei with interstellar matter.

Observations of the lunar shadowing of galactic cosmic rays (GCRs) has demonstrated that there does not appear to be an antiproton component of the galactic cosmic rays, but the antiprotons detected are instead produced by the GCR interaction with interstellar hydrogen gas.[114]

For an interstellar medium "composed of 90% H and 10% He, [with a density of 0.3 atoms cm-3] and using the most recently measured cross sections (Webber, 1989; Ferrando et al., 1988b), the escape length has been found equal to 34βR-0.6 g cm-2 for rigidities R above 4.4 GV, and 14β g cm-2 below. ... where R and β are the interstellar values of the rigidity and the ratio of the velocity of the particle to the velocity of light."[115]

## Probability for absorption

When a gamma ray passes through matter, the probability for absorption is proportional to the thickness of the layer, the density of the material, and the absorption cross section of the material. The total absorption shows an exponential decrease of intensity with distance from the incident surface:

${\displaystyle I(x)=I_{0}\cdot e^{-\mu x}}$

where μ = nσ is the absorption coefficient, measured in cm−1, n the number of atoms per cm3 in the material, σ the absorption cross section in cm2 and x the thickness of material in cm.

The time evolution of the number of emitted scintillation photons N in a single scintillation event can often be described by the linear superposition of one or two exponential decays. For two decays, we have the form:[116]

${\displaystyle N=A\exp \left(-{\frac {t}{{\tau }_{f}}}\right)+B\exp \left(-{\frac {t}{{\tau }_{s}}}\right)}$

where τf and τs are the fast (or prompt) and the slow (or delayed) decay constants. Many scintillators are characterized by 2 time components: one fast (or prompt), the other slow (or delayed). While the fast component usually dominates, the relative amplitude A and B of the two components depend on the scintillating material. Both of these components can also be a function the energy loss dE/dx.

In cases where this energy loss dependence is strong, the overall decay time constant varies with the type of incident particle. Such scintillators enable pulse shape discrimination, i.e., particle identification based on the decay characteristics of the PMT electric pulse. For instance, when BaF2 is used, gamma rays typically excite the fast component, while alpha particles excite the slow component: it is thus possible to identify them based on the decay time of the PMT signal.

The monochromatic flux density radiated by a greybody at frequency ${\displaystyle \nu }$ through solid angle ${\displaystyle \Omega }$ is given by ${\displaystyle F_{\nu }=B_{\nu }(T)Q_{\nu }\Omega }$ where ${\displaystyle B_{\nu }(T)}$ is the Planck function for a blackbody at temperature T and emissivity ${\displaystyle Q_{\nu }}$.

For a uniform medium of optical depth ${\displaystyle \tau _{\nu }}$ radiative transfer means that the radiation will be reduced by a factor ${\displaystyle e^{-\tau _{\nu }}}$. The optical depth is often approximated by the ratio of the emitting frequency to the frequency where ${\displaystyle \tau =1}$ all raised to an exponent β.

For cold dust clouds in the interstellar medium β is approximately two. Therefore Q becomes,

${\displaystyle Q_{\nu }=1-e^{-\tau _{\nu }}=1-e^{-\tau _{0}(\nu /\nu _{0})^{\beta }}}$. (${\displaystyle \tau _{0}=1}$, ${\displaystyle \nu _{0}}$ is the frequency where ${\displaystyle \tau _{0}=1}$).

## Dose equivalents

Def. the dose received in one hour at a distance of 1 cm from a point source of 1 mg of radium in a 0.5 mm thick platinum enclosure is called a sievert.

Equivalent dose to a tissue is found by multiplying the absorbed dose, in gray, by a weighting factor (WR). The relation between absorbed dose D and equivalent dose H is thus:

${\displaystyle H=W_{R}\cdot D}$.

The weighting factor (sometimes referred to as a quality factor) is determined by the radiation type and energy range.[117]

${\displaystyle H_{T}=\sum _{R}W_{R}\cdot D_{T,R}\ ,}$

where

HT is the equivalent dose absorbed by tissue T
DT,R is the absorbed dose in tissue T by radiation type R
WR is the weighting factor defined by the following table
electrons, muons, photons (all energies) 1
protons and charged pions 2
alpha particles, fission fragments, heavy ions 20
neutrons
(function of linear energy transfer L in keV/μm)
L < 10 1
10 ≤ L ≤ 100 0.32·L − 2.2
L > 100 300 / sqrt(L)

Thus for example, an absorbed dose of 1 Gy by alpha particles will lead to an equivalent dose of 20 Sv. The maximum weight of 30 is obtained for neutrons with L = 100 keV/μm.

## Effective doses

The effective dose of radiation (E), absorbed by a person is obtained by averaging over all irradiated tissues with weighting factors adding up to 1:[117][118]

${\displaystyle E=\sum _{T}W_{T}\cdot H_{T}=\sum _{T}W_{T}\sum _{R}W_{R}\cdot D_{T,R}}$.

## Little Ice Age

Changes in the 14C record, which are primarily (but not exclusively) caused by changes in solar activity, are graphed over time. Credit: Leland McInnes.{{free media}}
The periods of highest 14C production as measured from tree rings coincide with the periods of highest cooling during the past 1200 years. Credit: Max-Planck-Institut für Aeronomie Katlenburg-Lindau.{{fairuse}}

The Little Ice Age (LIA) appears to have lasted from about 1218 (782 b2k) to about 1878 (122 b2k).

The second image down shows the 14C data obtained from tree rings. The lower the solar activity, the higher the cosmic radiation, which determines the isotope content. The periods of highest 14C production as measured from tree rings coincide with the periods of highest cooling during the past 1200 years.

## Late Middle Ages

The Shroud of Turin: modern photo of the face, is shown positive left, digitally processed image right. Credit: Dianelos Georgoudis.

The Late Middle Ages extends from about 700 b2k to 500 b2k.

Italian humanism began in the first century of the late Middle Ages (c.1350-1450).[119]

The processed image at the right in the images on the right is the product of the application of digital filters. Digital filters are mathematical functions that do not add any information to the image, but transform it in such a way that information already present in it becomes more visible or easier to appreciate by the naked eye. The processed image was produced by inverting the brightness of the pixels in the positive image but without inverting their hue, and then by increasing both the brightness contrast and the hue saturation. Finally noise and so-called “salt and pepper” filters automatically removed the noisy information from the original image which hinders the appreciation of the actual face. To my knowledge the resulting image is the best available and indeed the only one that reveals the color information hidden in the original.

Radiocarbon dating of a corner piece of the shroud placed it between the years 1260 and 1390,[120] in the High to Late Middle Ages, which is consistent with "its first recorded exhibition in France in 1357."[121]

## Medieval Warm Period

"A proof-of-concept self-calibrating chronology [based upon the Irish Oak chronology] clearly demonstrates that third order polynomials provide a series of statistical calibration curves that highlight lacunae in the samples."[122]

## Early Middle Ages

Third order polynomials provide a series of statistical calibration curves that highlight lacunae in the carbon-14 samples. Credit: Gunnar Heinsohn.
The Δ14C values in a chronology can clearly be used to identify apparent catastrophic gaps and catastrophic rises in carbon-14. Credit: Gunnar Heinsohn.
The time profile of the carbon-14 spike around 774 AD. The colored dots represent the measurements in Japanese (M12) and German (Oak) trees, while the black lines represent the modeled profile corresponding to the instant production of carbon-14. Credit: Isosik.
The AD 774/5 event shows in 10Be, 14C and 36Cl. Credit: Florian Mekhaldi, Raimund Muscheler, Florian Adolphi, Ala Aldahan, Jürg Beer, Joseph R. McConnell, Göran Possnert, Michael Sigl, Anders Svensson, Hans-Arno Synal, Kees C. Welten & Thomas E. Woodruff.

The Early Middle Ages date from around 1,700 to 1,000 b2k.

At left is an attempt to correlate the change in 14C with time before 1950. The different data sets are shown with different colored third order polynomial fits to each data set.

"The Δ14C values in a chronology can clearly be used to identify catastrophic gaps and catastrophic rises in carbon-14."[123]

The first four gaps have a jump up in 14C with a fairly quick return to the calibration curve shown in the figure on the second left. However, from about 2000 b2k there is a steady rise in the Δ14C values.

The 993–994 carbon-14 spike was a rapid increase in carbon-14 content from tree rings, and followed the 774–775 carbon-14 spike.[124] This event is also confirmed by a sharp increase of beryllium-10 and hence considered as solar-origin.[125] It may have come from a massive solar storm as a series of auroral observations are known to be observed in late 992.[126]

The 774–775 Carbon-14 Spike is an observed increase of 1.2% in the concentration of carbon-14 isotope in tree rings dated to the years 774 or 775 AD (1226 or 1225 b2k), which is about 20 times as high as the normal background rate of variation. It was discovered during a study of Japanese cedar trees, with the year of occurrence determined through dendrochronology.[127] A surge in beryllium isotope 10Be, detected in Antarctic ice cores, has also been associated with the 774–775 event.[128]

The event appears to have been global, with the same carbon-14 signal found in tree rings from Germany, Russia, the United States, and New Zealand.[128][129][130]

The signal exhibits a sharp increase of ~1.2% followed by a slow decline (see Figure 1), which is typical for an instant production of carbon-14 in the atmosphere,[128] indicating that the event was short in duration. The globally averaged production of carbon-14 for this event is calculated as Q = (1.1–1.5)×108 atoms/cm2.[128][131][132]

Several possible causes of the event have been considered.

"This year the Northumbrians banished their king, Alred, from York at Easter-tide; and chose Ethelred, the son of Mull, for their lord, who reigned four winters. This year also appeared in the heavens a red crucifix, after sunset; the Mercians and the men of Kent fought at Otford; and wonderful serpents were seen in the land of the South-Saxons."[133]

A "red crucifix" was recorded by the Anglo-Saxon Chronicle as appearing in the skies of Britain for the year 774; since no supernova remnant has been found for this year, it is interpreted as red Sprite lightning.

In China, there are no clear references to an aurora in the mid 770s, as happened on 762; and "comet"-sightings of the 770s do not match the expected atmospheric phenomena.[134] Instead an anomalous "thunderstorm" was recorded for 775.[135]

The common paradigm is that the event was caused by a solar-particle event (SPE), or a consequences of events as often happen, from a very strong solar flare(s), perhaps the strongest ever known, but still within the Sun's abilities.[128][131][136][137][125]

Another discussed scenario of the event origin, involving a gamma-ray burst,[132][138] appears unlikely, since the event was also observed in isotopes 10Be and 36Cl.[139]

The event of 774 is the strongest spike over the last 11,000 years in the record of cosmogenic isotopes,[136] but it is not unique. A similar event occurred in 993 or 994 (1007 or 1006 b2k), but it was only 0.6 times as strong.[124] Several other events of the same kind are also suspected to have occurred during the Holocene epoch.[136]

From these statistics, one may expect that such strong events occur once per tens of millennia, while weaker events may occur once per millennium or even century. The event of 774 did not cause catastrophic consequences for life on Earth,[140][137] but had it happened in modern times, it may have produced catastrophic damage to modern technology, particularly to communication and space-borne navigation systems. In addition, a solar flare capable of producing the observed isotopic effect would pose considerable risk to astronauts.[141]

As of 2017, there is "little understanding"[142] of 14C past variations because annual-resolution measurements are only available for a few periods (e.g., the AD 774-775). In 2017, another "extraordinarily large" 14C increase (20‰) has been associated with the 5480 BC event, but it is not associated with a solar event because of its long duration, but rather to an unusually fast grand minimum of solar activity.[142]

"The Roman Climate Optimum [RCO] (sometimes called the "Roman Warm Period") [dates] ca. 200 BC--AD 150, […]. Buoyed by high levels of insolation and weak volcanic activity, the RCO was a period of warm, wet, and stable climate across much of the vast Roman Empire.46 [The] RCO was a phase of high and stable solar activity. Between a grand solar minimum centered at 360 BC and another at 690 AD, solar radiation fluctuated within a modest band, reaching one peak at a grand maximum around AD 305.48"[143]

## Iron Age

The iron age history period began between 3,200 and 2,100 b2k.

"After a typological analysis and a cross-dating of bronze artifacts recovered north and south of the Alps, the Roman school of Peroni set the 1020 [3020 b2k] as the beginning of the Iron Age (De Marinis 2005, p. 21; Pacciarelli 2005). The date is in agreement with the chronology supported by Lothar Sperber (Sperber 1987). The recent works of Nijboer based on the analysis of radiocarbon dates from Latial contexts agree with this high chronology (Nijboer et al. 1999-2000; Nijboer & Van der Plicht 2008; Van der Plicht et al. 2009)."[144]

## Subatlantic period

The "calibration of radiocarbon dates at approximately 2500-2450 BP [2500-2450 b2k] is problematic due to a "plateau" (known as the "Hallstatt-plateau") in the calibration curve [...] A decrease in solar activity caused an increase in production of 14C, and thus a sharp rise in Δ 14C, beginning at approximately 850 cal (calendar years) BC [...] Between approximately 760 and 420 cal BC (corresponding to 2500-2425 BP [2500-2425 b2k]), the concentration of 14C returned to "normal" values."[145]

"The main discontinuity in the climatic condition during the Bronze Age and Iron Age transition can be identified in the boundary from Subatlantic to Subboreal (2800-2500 BP; 996/914-766/551 2σ cal. BC). Such period “has globally been identified as a time of marked climatic change. Stratigraphical, paleobotanical and archaeological evidence point to a change from a dry and warm to a more humid and cool climate in central and northwestern Europe” (Tinner et al. 2003). The climatic deterioration which characterizes this chronological range is directly responsible of the plateau in the calibration curve between 760 and 420 BC (2500-2425 BP) (see chapter 4.3.2.1). The climatic oscillation around 2700 BP (896/813 2σ cal. BC) has been detected worldwide. Van Geel et al. (1996, 1998) and Speranza et al. (2002) found an abrupt shift around 850 BC in changing species composition of peat-forming mosses in European Holocene raised bog deposits. The change was from mosses preferring warm conditions to those preferring colder and wetter environments. Archaeological evidence supports such a change. Bronze Age settlements located in the Netherlands were suddenly abandoned after a long period of occupation which last around one millennium (Dergachev et al. 2004). Other studies confirmed the climatic discontinuity; Schilman et al. (2001) studied δ18O and δ13C in deposits from the southeastern Mediterranean, off Israel, and recognized the presence of two humid events in the time ranges of 3500-3000 BP (1884/1772-1263/1215 2σ cal. BC) and 1700-1000 BP (332/389-1016/1030 2σ cal. AD) and a period of arid conditions between 3000 and 1700 BP (1263/1215 2σ cal. BC- 332/389 2σ cal. AD). Barber and Langdon (2001) identified three main long climatic deteriorations 2900-2830 BP (1119/1037-1012/934 2σ cal. BC), 2630-2590 BP (810/797-801/788 2σ cal. BC) and 1550-1400 BP (430/549-637/658 2σ cal. AD) through the analysis of plant macrofossils in a peat deposit of Walton Moss located in Northern England and comparing such data with a temperature reconstruction based on chironomids in the sediment of a nearby lake."[144]

## "Hallstatt disaster"

"Hallstatt disaster" refers to the plateau located in the calibration curve between 760 and 420 cal BC (2500-2425 BP). Credit: Giacomo Capuzzo.{{fairuse}}

"With the term “Hallstatt disaster” the scientific community refers to the plateau located in the calibration curve between 760 and 420 cal BC (2500-2425 BP) [the graph on the right]. The term is due to the chronological analogy to the Hallstatt society which developed in the late Bronze Age and the beginning of Iron Age in the northern part of the Alps (Austria). The flat shape of the calibration curve in this time-span is the result of the decrease, and hence the return to normal values, of the percentage of 14C after a period characterized by an increase in the concentration of radiocarbon in the atmosphere, which is mirrored in the calibration curve as a sharp descent between 850 and 760 BC (2700-2450 BP) (Speranza et al. 2000). As asserted by many authors (Van Geel et al. 1996; Van Geel et al. 1998; Tinner et al. 2003; Dergachev et al. 2004; Van der Plicht et al. 2004; Swindles et al. 2007) the chronological range 850-760 BC is characterized by an abrupt increase of the amount of 14C in the atmosphere and it corresponds chronologically to the boundary from Subatlantic to Subboreal (2800-2500 BP), which “has globally been identified as a time of marked climatic change. Stratigraphical, paleobotanical and archaeological evidence point to a change from a dry and warm to a more humid and cool climate in central and northwestern Europe” (Tinner et al. 2003)."[144]

## Subboreal period

The "period around 850-760 BC [2850-2760 b2k], characterised by a decrease in solar activity and a sharp increase of Δ 14C [...] the local vegetation succession, in relation to the changes in atmospheric radiocarbon content, shows additional evidence for solar forcing of climate change at the Subboreal - Subatlantic transition."[145]

The "apparent reality of social equality testified by LBA urnfield burials can be definitely discarded at the Iron Age transition by the archaeological excavation at the Hexenbergle site, near Wehringen in Bayern (Germany). The monumental radiocarbon dated mound with a cremation burial of an adult male accompanied by a great amount of objects, including a sword, elements decorating a wagon and an extensive set of painted pottery (Hennig 1995). The dendrochronological date obtained on the wagon (778±5BC) provides a precise temporal location for an upper-class deceased with sepulchral paraphernalia in the Hallstatt period (Friedrich & Henning 1995, 1996)."[144]

## Late Bronze Ages

The Late Bronze Ages begin about 3550 b2k and end about 2900 b2k.

The "abandonment of lakeshore Swiss pile-dwellings has been dated to around 1520 BC [3520 b2k] (Menotti 2001). [Slightly] "later in time episodes of flood events and lake-level highstand at 3100 BP (1415/1311 2σ cal. BC) and 2800 BP (996/914 2σ cal. BC) have been recently detected in the Southern Alps, in the sediment cores extracted from the Lake Ledro, located in the province of Trento (Joannin et al. 2014)."[144]

Radiocarbon "data indicate that the New Kingdom of Egypt started between 1570 and 1544 B.C.E [3270 - 3544 b2k]."[146]

## Middle Bronze Ages

Fresco of The Fisherman is from Akrotiri, Santorini, Greece at a height of 1.10 m. Credit: Yann Forget.{{free media}}

The Middle Bronze Ages begin about 4100 b2k and end about 3550 b2k.

The Fisherman is a Minoan Bronze Age fresco from Akrotiri, on the Aegean island of Santorini (classically Thera), dated to the Neo-Palatial period (c. 1640–1600 BC). The settlement of Akrotiri was buried in volcanic ash (dated by radiocarbon dating to c. 1627 BC [c. 3626 b2k]) by the Minoan eruption on the island, which preserved many Minoan frescoes like this.

High precision radiocarbon dating of 18 samples from Jericho, including six samples of carbonized cereal from the burnt stratum, gave the age of the strata as 1562 BC, with a margin of error of 38 years [3562 ± 38 b2k].[147]

## Atlantic period

The "Atlantic period [is] 4.6–6 ka [4,600-6,000 b2k]."[148]

## Boreal transition

These are measured results [Δ14
C
: defined in Stuiver and Polach (29)] of four series that were measured at different AMS laboratories. Credit: Fusa Miyakea, A. J. Timothy Jull, Irina P. Panyushkina, Lukas Wacker, Matthew Salzer, Christopher H. Baisan, Todd Lange, Richard Cruz, Kimiaki Masuda, and Toshio Nakamura.{{fairuse}}

"In recent years, the German oak chronology has been extended to 7938 BC [9938 b2k]. For earlier intervals, tree-ring chronologies must be based on pine, because oak re-emigrated to central Europe at the Preboreal/Boreal transition, at about 8000 BC [10,000 b2k]."[149]

"The age range, 7145-7875 BC [9145-9875 b2k], is represented by the oak chronology, 'Main9'."[149]

"The age range, 7833-9439 BC [9833-11439 b2k], is covered by the 1784-yr pine chronology."[149]

In the image on the right: the upper graph shows measured results of Δ14
C
: defined in Stuiver and Polach (1977) from three series at different AMS laboratories (Arizona, Nagoya, and ETH [Swiss Federal Institute of Technology]). The lower shows a comparison of bristlecone pine (diamonds), with the original datasets of the IntCal and the IntCal13 curve.

"Grand solar minima are defined as the periods when the solar activity is at a very low level, i.e., it is defined as the group sunspot number becoming less than 15 during at least two consecutive decades, according to Usoskin et al. (18). In a grand solar minimum, the 14
C
content increases largely due to a reduction of the solar modulation parameter Φ. It is estimated that Φ was ∼160 MV during the Maunder Minimum, while the present-day value varies between 400 MV and 1,000 MV (19)."[150]

"On average, the increase rate of the [Maunder, the Spörer, the Oort, the AD seventh century, and the fourth century BC] grand solar minima, where [there is] annual resolution 14
C
data (5, 20–22) [of] about 0.3‰/y. Against the normal grand solar minima, the increase rate of the 5480 BC event is 2‰/y. Although the total 14
C
increment of the 5480 BC event is almost equal to the other minima (∼20‰), the 5480 BC event increases much faster than the others."[150]

"To explain a rapid and large 14
C
increase, a dramatic decrease of the solar magnetism, or extreme [solar proton events] SPEs, is necessary."[150]

"Over ∼3,000 y from ca. 6000 BC to 3000 BC, the geomagnetic dipole field was ∼0.9 times smaller than today’s field, and had an almost constant value (23)."[150]

Hypotheses:

1. special state of the grand solar minimum,
2. successive extreme SPEs over ∼20 y, or
3. a combination of some extreme SPEs and a normal grand solar minimum (or solar magnetic activities).[150]

Regarding hypothesis 1: "The structure of the 5480 BC event indicates a rapid increase in 14
C
after 5470 BC followed by a gradual plateau-like increase for the next 15 y and then a gradual decay. Although the initial increase in 14
C
for this event is different from the behavior in a normal grand solar minima, the time scale of the plateau and the following decay is consistent with the Maunder Minimum [assuming the Maunder Minimum is a standard variation of a grand solar minimum]."[150]

Regarding hypothesis 2: It "is less likely that the plateau and the following decay can be explained by only [successive] SPEs."[150]

It "is difficult to divide 14
C
variations into the contribution by solar magnetic activities and that by SPEs".[150]

Either "(i) several SPEs occurred during the early normal grand solar minimum, or (ii) several SPEs occurred, and then the solar magnetic activity became gradually higher."[150]

Aassume the "SPEs only occurred in the increasing period 5481–5468 BC. Knowing that a 14
C
production rate during the Maunder Minimum varies between 2.1 atoms per square centimeter per second and 2.6 atoms per square centimeter per second (19), we can accept that a production rate for the low solar magnetic activity during the increasing 14
C
period is an average of upper values, 2.35 atoms per square centimeter per second. In contrast, [...] the average production rate of 14
C
during normal solar magnetic activity is 1.8 atoms per square centimeter per second. Based on these hypotheses, a total 14
C
production by SPEs above 2.35 and 1.8 atoms per square centimeter per second during the increase can be shown to be 6.0 ± 2.4 and 10.5 ± 3.0 atoms per square centimeter per second, respectively. The total production rate of the AD 775 event has been estimated to be 3.9 atoms per square centimeter per second to 6.9 atoms per square centimeter per second (8, 10, 11, 14). Then, it is possible that the total production by SPEs of the event is comparable to or larger than the AD 775 event."[150]

"From direct measurements of the sun, [...] solar flares tend to occur during a solar maximum, e.g., the number of SPEs increases in solar maximum periods (27). Also, the two annual 14
C
events (AD 774–775 and AD 993–994) occurred in higher solar activity periods, or at least did not occur during grand solar minimum periods (7). Thus, [hypothesis 1] (several SPEs occurred in the grand solar minimum) seems less plausible; however, the viability of one scenario over another is currently limited by our poor understanding of the mechanism for occurrence of extreme SPEs."[150]

"In the observation of solar-type stars by the Kepler telescope, stars where several superflares occurred over several years were detected (28). Such observations of solar-type stars may support an extreme SPE origin of the 5480 BC event. Further investigations of the 14
C
record, or of other radionuclides such as annual 10
Be
data in ice cores, may turn up similar events, which would help to further discussion of the cause of this event. In any case, the 14
C
variation of the 5480 BC event indicates an unprecedented anomaly in solar activity compared to other periods."[150]

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