Aurorae are mostly caused by energetic electrons precipitating into the atmosphere.[1] Credit: Samuel Blanc[1].

Although electron astronomy is usually not recognized as a formal branch of astronomy, the measurement of electron fluxes helps to understand a variety of natural phenomena.

""[E]lectron astronomy" has an interesting future".[2]

## Astronomy

Main source: Astronomy
The composite shows upper atmospheric lightning and electrical discharge phenomena. Credit: Abestrobi.

With respect to the rocky-object Earth, between the surface and various altitudes there is an electric field induced by the ionosphere. It changes with altitude from about 150 volts per meter at the suface to lower values at higher altitude. In fair weather, it is relatively constant, in turbulent weather it is accompanied by ions. At greater altitude these chemical species continue to increase in concentration. To dissipate the accumulation of greater charge differential between the surface and the ionosphere, the gases between suffer breakdown (ionization) that permits lightning to be either a draw of negative charge, usually electrons, upward from the surface or a transfer of positive charge to the ground.

Def. "the non-linear scattering of radiation off electrons" is called induced Compton scattering.[3]

"The effect of scattering is to move photons to lower frequencies."[3] "[T]he fact that the radio pulses [from a pulsar] are not suppressed by induced scattering suggests that the wind's Lorentz factor exceeds ~104.[3]

The Lorentz factor is defined as:[4]

${\displaystyle \gamma ={\frac {1}{\sqrt {1-v^{2}/c^{2}}}}={\frac {1}{\sqrt {1-\beta ^{2}}}}={\frac {\mathrm {d} t}{\mathrm {d} \tau }}}$

where:

• v is the relative velocity between inertial reference frames,
• β is the ratio of v to the speed of light c.
• τ is the proper time for an observer (measuring time intervals in the observer's own frame),
• c is the speed of light.

As an example, "[t]he power into the Crab Nebula is apparently supplied by an outflow [wind] of ~1038 erg/s from the pulsar"[3] where there are "electrons (and positrons) in such a wind"[3]. These beta particles coming out of the pulsar are moving very close to light speed.

## Electrons

Main sources: Charges/Electrons and Electrons

The electron is a subatomic particle with a negative charge, equal to -1.60217646x10-19 C. Current, or the rate of flow of charge, is defined such that one coulomb, so 1/-1.60217646x10-19, or 6.24150974x1018 electrons flowing past a point per second give a current of one ampere. The charge on an electron is often given as -e. note that charge is always considered positive, so the charge of an electron is always negative.

The electron has a mass of 9.10938188x10-31 kg, or about 1/1840 that of a proton. The mass of an electron is often written as me.

When working, these values can usually be safely approximated to:

-e = -1.60x10-19 C
me = 9.11x10-31kg

It has no known components or substructure; in other words, it is generally thought to be an elementary particle.[5][6] The intrinsic angular momentum (spin) of the electron is a half-integer value in units of ħ, which means that it is a fermion.

## Delta rays

A delta ray is characterized by very fast electrons produced in quantity by alpha particles or other fast energetic charged particles knocking orbiting electrons out of atoms. Collectively, these electrons are defined as delta radiation when they have sufficient energy to ionize further atoms through subsequent interactions on their own.

"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)."[7] 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".[7]

## Epsilon rays

Epsilon radiation is tertiary radiation caused by secondary radiation (e.g., delta radiation). Epsilon rays are a form of particle radiation and are composed of electrons. The term is very rarely used today.

## Planetary sciences

This is a schematic of Jupiter's magnetosphere and the components influenced by Io (near the center of the image). Credit: John Spencer.

The image at right represents "[t]he Jovian magnetosphere [magnetic field lines in blue], including the Io flux tube [in green], Jovian aurorae, the sodium cloud [in yellow], and sulfur torus [in red]."[8]

"Io may be considered to be a unipolar generator which develops an emf [electromotive force] of 7 x 105 volts across its radial diameter (as seen from a coordinate frame fixed to Jupiter)."[9]

"This voltage difference is transmitted along the magnetic flux tube which passes through Io. ... The current [in the flux tube] must be carried by keV electrons which are electrostatically accelerated at Io and at the top of Jupiter's ionosphere."[9]

"Io's high density (4.1 g cm-3) suggests a silicate composition. A reasonable guess for its electrical conductivity might be the conductivity of the Earth's upper mantle, 5 x 10-5 ohm-1 cm-1 (Bullard 1967)."[9]

As "a conducting body [transverses] a magnetic field [it] produces an induced electric field. ... The Jupiter-Io system ... operates as a unipolar inductor" ... Such unipolar inductors may be driven by electrical power, develop hotspots, and the "source of heating [may be] sufficient to account for the observed X-ray luminosity".[10]

"The electrical surroundings of Io provide another energy source which has been estimated to be comparable with that of the [gravitational] tides (7). A current of 5 x 106 A is ... shunted across flux tubes of the Jovian field by the presence of Io (7-9)."[11]

"[W]hen the currents [through Io] are large enough to cause ohmic heating ... currents ... contract down to narrow paths which can be kept hot, and along which the conductivity is high. Tidal heating [ensures] that the interior of Io has a very low eletrical resistance, causing a negligible extra amount of heat to be deposited by this current. ... [T]he outermost layers, kept cool by radiation into space [present] a large resistance and [result in] a concentration of the current into hotspots ... rock resistivity [and] contact resistance ... contribute to generate high temperatures on the surface. [These are the] conditions of electric arcs [that can produce] temperatures up to ionization levels ... several thousand kelvins".[11]

"[T]he outbursts ... seen [on the surface may also be] the result of the large current ... flowing in and out of the domain of Io ... Most current spots are likely to be volcanic calderas, either provided by tectonic events within Io or generated by the current heating itself. ... [A]s in any electric arc, very high temperatures are generated, and the locally evaporated materials ... are ... turned into gas hot enough to expand at a speed of 1 km/s."[11]

## Colors

Notation: WN5 is a component of V444 Cygni, with its Wolf-Rayet (W) spectrum dominated by NitrogenIII-V and HeliumI-II lines and WN2 to WN5 considered hotter or "early".

"The color temperature of the central part of the WN5 disk for λ < 7512 Å, where the main source of opacity is electron scattering, is Tc = 80,000-100,000 K. This high temperature represents the electron temperature slightly below the surface of the WN5 core--the level at which the star becomes optically thick in electron scattering."[12]

## Minerals

Main source: Minerals
Fluorescing fluorite is from Boltsburn Mine Weardale, North Pennines, County Durham, England, UK. Credit: .

"Many samples of fluorite exhibit fluorescence under ultraviolet light, a property that takes its name from fluorite.[13] Many minerals, as well as other substances, fluoresce. Fluorescence involves the elevation of electron energy levels by quanta of ultraviolet light, followed by the progressive falling back of the electrons into their previous energy state, releasing quanta of visible light in the process. In fluorite, the visible light emitted is most commonly blue, but red, purple, yellow, green and white also occur. The fluorescence of fluorite may be due to mineral impurities such as yttrium, ytterbium, or organic matter in the crystal lattice. In particular, the blue fluorescence seen in fluorites from certain parts of Great Britain responsible for the naming of the phenomenon of fluorescence itself, has been attributed to the presence of inclusions of divalent europium in the crystal.[14]

## Theoretical electron astronomy

We now assume that the γ-rays are produced [from 3C 279] by relativistic electrons via Compton scattering of synchrotron photons (SSC). In any such model, the fact that the γ-rays luminosity, produced via Compton scattering, is higher than that emitted at lower frequencies (1014 - 1016 Hz), supposedly via the synchrotron process, implies a radiation energy density, Ur, higher than the magnetic energy density, UB. From the observed power ratio we derive that Ur must be one order of magnitude greater than UB, which may be a lower limit if Klein-Nishina effects reduce the efficiency of the self-Compton emission. This result is independent of the degree of beaming, which, for a homogeneous source, affects both the synchrotron and the self-Compton fluxes in the same way. This source is therefore the first observed case of the result of a Compton catastrophe (Hoyle, Burbidge, & Sargent 1966)."[15]

Here's a theoretical definition:

Def. an observational astronomy that primarily detects electron fluxes to study their production and sources is called an electron astronomy.

## Sources

In this diagram, the higher harmonics of the first frequency at the top are shown. Credit: Y Landman, derivative work by W axell.

The process of ionization removes one or more electrons from a neutral atom to yield a variety of ions depending on the chemical element species and incidence of sufficient energy to remove the electrons.

The preflare solar material is observed "to be an elevated cloud of prominence-like material which is suddenly lit up by the onslaught of hard electrons accelerated in the flare; the acceleration may be inside or outside the cloud, and brightening is seen in other areas of the solar surface on the same magnetic field lines."[16]

A coronal mass ejection (CME) is an ejected plasma consisting primarily of electrons and protons.

"The suprathermal electrons in the solar wind and in solar particle events have excellent properties for this application: they move rapidly, they remain tightly bound to their field lines, and they may arrive "scatter-free" even at low energies, and from deep in the solar atmosphere (Lin 1985)."[2]

As of December 5, 2011, "Voyager 1 is about ... 18 billion kilometers ... from the [S]un [but] the direction of the magnetic field lines has not changed, indicating Voyager is still within the heliosphere ... the outward speed of the solar wind had diminished to zero in April 2010 ... inward pressure from interstellar space is compacting [the magnetic field] ... Voyager has detected a 100-fold increase in the intensity of high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside ... [while] the [solar] wind even blows back at us."[17]

"[W]hen the medium [behaves] like an amplifier to the incident radiation" it is "possible for negative absorption to arise at radio wavelengths".[18]

The necessary and sufficient conditions for negative absorption to occur at radio wavelengths are

1. "the kinetic energy distribution F(η) of the radiating electrons [is] markedly non-thermal with an appreciable excess of high energy electrons such that ∂F/∂η is positive over a finite range of the kinetic energy η" and
2. "the stimulated transition probability [has] a maximum at some finite value of the kinetic energy, the most favorable case occurring when this maximum is a sharp one at the value of η at which ∂F/∂η has a positive maximum."[18]

"These conditions can both be met in principle for the cases in which the dominant radiation process is due [to]

1. [the] Cerenkov effect,
2. gyro radiation by non-relativistic electrons, [and]
3. synchrotron-type radiation by highly relativistic electrons".[18]

Def. "radiation at the fundamental or at the first few harmonics of the gyro frequency by weakly relativistic electrons rotating in a magnetic field" is called gyro radiation.[18]

Def. "radiation by strongly relativistic electrons at high harmonics of the gyro frequency" is called synchrotron radiation.[18]

## Strong forces

"The idea behind baryon matter is that a macroscopic state may exist in which a smaller effective baryon mass inside some region makes the state energetically favored over free particles. [...] This state will appear in the limit of large baryon number as an electrically neutral coherent bound state of neutrons, protons, and electrons in β-decay equilibrium."[19]

## Continua

The X-ray continuum can arise from bremsstrahlung, black-body radiation, synchrotron radiation, or what is called inverse Compton scattering of lower-energy photons by relativistic electrons, knock-on collisions of fast protons with atomic electrons, and atomic recombination, with or without additional electron transitions.[20]

## Cosmic rays

Aluminium-26, 26Al, is a radioactive isotope of the chemical element aluminium, decaying by either of the modes beta-plus or electron capture, both resulting in the stable nuclide magnesium-26. The half-life of 26Al is 7.17×105 years. This is far too short for the isotope to survive to the present, but a small amount of the nuclide is produced by collisions of argon atoms with cosmic ray protons.

## Beta particles

This graph is a chart of the nuclides for carbon to fluorine. Decay modes:

Credit: original: National Nuclear Data Center, stitched: Neokortex, cropped: Limulus.

Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay. They are designated by the Greek letter beta (β).

At right is a graph or block diagram that shows the boundaries for nuclear particle stability. The boundaries are conceptualized as drip lines. The nuclear landscape is understood by plotting boxes, each of which represents a unique nuclear species, on a graph with the number of neutrons increasing on the abscissa and number of protons increasing along the ordinate, which is commonly referred to as the table of nuclides, being to nuclear physics what the more commonly known periodic table of the elements is to chemistry. However, an arbitrary combination of protons and neutrons does not necessarily yield a stable nucleus, and ultimately when continuing to add more of the same type of nucleons to a given nucleus, the newly formed nucleus will essentially undergo immediate decay where a nucleon of the same isospin quantum number (proton or neutron) is emitted; colloquially the nucleon has 'leaked' or 'dripped' out of the target nucleus, hence giving rise to the term "drip line". The nucleons drip out of such unstable nuclei for the same reason that water drips from a leaking faucet: the droplet, or nucleon in this case, sees a lower potential which is great enough to overcome surface tension in the case of water droplets, and the strong nuclear force in the case of proton emission or alpha decay. As nucleons are quantized, then only integer values are plotted on the table of isotopes, indicating that the drip line is not linear but instead looks like a step function up close.

Beta particles (electrons) are more penetrating [than alpha particles], but still can be absorbed by a few millimeters of aluminum. However, in cases where high energy beta particles are emitted shielding must be accomplished with low density materials, e.g. plastic, wood, water or acrylic glass (Plexiglas, Lucite). This is to reduce generation of Bremsstrahlung X-rays. In the case of beta+ radiation (positrons), the gamma radiation from the electron-positron annihilation reaction poses additional concern.

## Muons

"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."[21]

Muon decay produces three particles, an electron plus two neutrinos of different types.

## Gamma rays

Most astronomical gamma-rays may be produced from the same type of accelerations of electrons, and electron-photon interactions, that produce X-rays in astronomy (but occurring at a higher energy in the production of gamma-rays).

A number of different processes occurring in the universe may result in gamma-ray emission. These processes include the interactions of energetic electrons with magnetic fields.

The correlations of the high energy electrons energized during a solar flare and the gamma rays produced are mostly caused by nuclear combinations of high energy protons and other heavier ions.

## X-rays

X-rays remove electrons from atoms and ions, and those photoelectrons can provoke secondary ionizations. As the intensity is often low, this X-ray heating is only efficient in warm, less dense atomic medium (as the column density is small). For example in molecular clouds only hard x-rays can penetrate and x-ray heating can be ignored. This is assuming the region is not near an x-ray source such as a supernova remnant.

In an X-ray tube, electrons are accelerated in a vacuum by an electric field and shot into a piece of metal called the "target". X-rays are emitted as the electrons slow down (decelerate) in the metal. The output spectrum consists of a continuous spectrum of X-rays, with additional sharp peaks at certain energies characteristic of the elements of the target.

## Ultraviolets

Carbon has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 229.687 nm from C III, 227.089, 227.727, and 227.792 nm from C V, 207.025, 208.216, 313.864, and 343.366 nm from C VI.[22]

Argon has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 333.613, 334.472, 335.211, 335.849, and 336.128 nm from Ar III.[22]

## Visuals

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation. The emitted radiation may also be of the same wavelength as the absorbed radiation, termed "resonance fluorescence".[23]

## Violets

The spectrum shows the lines in the visible due to emission from elemental hydrogen. Credit:Teravolt.
The spectrum shows the lines in the visible due to emission from elemental oxygen. Credit:Teravolt.

Hydrogen has two emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas at 397.007 nm of the Balmer series (Hε) and 434.05 nm Hγ.[22]

Oxygen has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 406.963, 406.99, 407.22, 407.59, 407.89, 408.51, 435.12, 441.489, and 441.697 nm from O II, and 434.045 nm from O VIII.[22]

"Electron temperatures are generally derived from the ratio of auroral to nebular lines in [O III] or [N II]."[24] "[B]ecause of the proximity of strong night-sky lines at λ4358 and λλ5770, 5791, the auroral lines of [O III] λ4363 and [N II] λ5755 are often contaminated."[24]

Argon has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 426.653, 428.29, 433.12, 434.8064, 437.075, 437.967, 442.60, and 443.019 nm from Ar II.[22]

## Yellows

"The temperature of yellow coronal regions is ... about 2.5 [x] 106 [K]. ... although some ions Ca XV will exist at lower, as well as higher temperatures."[25]

"The AS prominences [AS in Menzel-Evans' classification [4];] move with velocities exceeding by far the velocities of other types of prominences [7], [8]. As short-living phenomena, they are condensed quickly and the temperature of the coronal gases should rise in the early stages of their condensation. Indeed, the AS prominences use to be allied with yellow line emission (λ 5694)."[25]

"The yellow line is namely due to the ion Ca XV, according to Edlen's and Waldmeier's identification. ... the line λ 5694 is emitted by 3P1 - 3P0 transition of Ca XV."[25]

"The solar corona is not in thermodynamical equilibrium. In particular, the photo-recombination is compensated with electron impact ionization, while the reverse processes viz. the photoionization and recombination by impact with two electrons are there negligible."[25]

## Infrareds

In infrared astronomy, the cosmic infrared background (CIB) causes a significant attenuation for very high energy electrons through inverse Compton scattering, photopion and electron-positron pair production.

## Submillimeters

Radio observations at 210 GHz taken by the Bernese Multibeam Radiometer for KOSMA (BEMRAK) of high-energy particle acceleration during the energetic solar flare of 2003 October 28 at submillimeter wavelengths reveal a gradual, long-lasting (>30 minutes) component with large apparent source sizes (~60"). Its spectrum below ~200 GHz is consistent with synchrotron emission from flare-accelerated electrons producing hard X-ray and γ-ray bremsstrahlung assuming a magnetic field strength of ≥200 G in the radio source and a confinement time of the radio-emitting electrons in the source of less than 30 s. There is a close correlation in time and space of radio emission with the production of pions".[26]

## Superluminals

There is a cut-off frequency above which the equation ${\displaystyle \cos \theta =1/(n\beta )}$ cannot be satisfied. Since the refractive index is a function of frequency (and hence wavelength), the intensity does not continue increasing at ever shorter wavelengths even for ultra-relativistic particles (where v/c approaches 1). At X-ray frequencies, the refractive index becomes less than unity (note that in media the phase velocity may exceed c without violating relativity) and hence no X-ray emission (or shorter wavelength emissions such as gamma rays) would be observed. However, X-rays can be generated at special frequencies just below those corresponding to core electronic transitions in a material, as the index of refraction is often greater than 1 just below a resonance frequency (see Kramers-Kronig relation and anomalous dispersion).

"Throughout the shower development, the electrons and positrons which travel faster than the speed of light in the air emit Cherenkov radiation."[27]

"High energy processes such as Compton, Bhabha, and Moller scattering, along with positron annihilation rapidly lead to a ~20% negative charge asymmetry in the electron-photon part of a cascade ... initiated by a ... 100 PeV neutrino"[28].

"The tachyonic spectral densities generated by ultra-relativistic electrons in uniform motion are fitted to the high-energy spectra of Galactic supernova remnants, such as RX J0852.0−4622 and the pulsar wind nebulae in G0.9+0.1 and MSH 15-52. ... Tachyonic cascade spectra are quite capable of generating the spectral curvature seen ... Estimates on the electron/proton populations generating the tachyon flux are obtained from the spectral fits"[29]

## Plasma objects

"Plasma is the fourth state of matter, consisting of electrons, ions and neutral atoms, usually at temperatures above 104 degrees Kelvin."[30]

Plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms (reduce or increase the number of electrons in them), thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions.[31]

For plasma to exist, ionization is necessary. The term "plasma density" by itself usually refers to the "electron density", that is, the number of free electrons per unit volume. The degree of ionization of a plasma is the proportion of atoms that have lost or gained electrons, and is controlled mostly by the temperature. Even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e., response to magnetic fields and high electrical conductivity). The degree of ionization, α is defined as α = ni/(ni + na) where ni is the number density of ions and na is the number density of neutral atoms. The electron density is related to this by the average charge state <Z> of the ions through ne = <Z> ni where ne is the number density of electrons.

## Gaseous objects

Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is due to the decreasing amount of H ions, which absorb visible light easily.[32] Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H ions.[33][34] The photosphere has a particle density of ~1023 m−3 (this is about 0.37% of the particle number per volume of Earth's atmosphere at sea level; however, photosphere particles are electrons and protons, so the average particle in air is 58 times as heavy).

## Materials

Main sources: Chemicals/Materials and Materials

Def. an object, usually made of glass, that focuses or defocuses the light or an electron beam that passes through it is called a lens.

## Coronal clouds

An abundance of helmet streamers is shown at solar maximum. Credit: NASA.
Helmet streamers are shown at solar minimum restricted to mid latitudes. Credit: NASA.

Helmet streamers are bright loop-like structures which develop over active regions on the sun. They are closed magnetic loops which connect regions of opposite magnetic polarity. Electrons are captured in these loops, and cause them to glow very brightly. The solar wind elongates these loops to pointy tips. They far extend above most prominences into the corona, and can be readily observed during a solar eclipse. Helmet streamers are usually confined to the "streamer belt" in the mid latitudes, and their distribution follows the movement of active regions during the solar cycle. Small blobs of plasma, or "plasmoids" are sometimes released from the tips of helmet streamers, and this is one source of the slow component of the solar wind. In contrast, formations with open magnetic field lines are called coronal holes, and these are darker and are a source of the fast solar wind. Helmet streamers can also create coronal mass ejections if a large volume of plasma becomes disconnected near the tip of the streamer.

In the corona thermal conduction occurs from the external hotter atmosphere towards the inner cooler layers. Responsible for the diffusion process of the heat are the electrons, which are much lighter than ions and move faster.

## Solar winds

Particles such as electrons are used as tracers of cosmic magnetic fields.[2]

"From a plasma-physics point of view, the particles represent the correct way to identify magnetic field lines."[2] "The suprathermal electrons in the solar wind and in solar particle events have excellent properties for this application: they move rapidly, they remain tightly bound to their field lines, and they may arrive "scatter-free" even at low energies, and from deep in the solar atmosphere (Lin 1985)."[2]

"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."[35]

## Mercury

Main sources: Wanderers/Mercury and Mercury

Mariner 10 has aboard "one backward facing electron spectrometer (BESA). ... An electron spectrum [is] obtained every 6 s, ... within the energy range 13.4-690 eV. ... [B]y taking into account [the angular] distortion [caused by the solar wind passing the spacecraft] and the spacecraft sheath characteristics ... some of the solar wind plasma parameters such as ion bulk speed, electron temperature, and electron density [are derived]."[36] Mariner 10 had three encounters with Mercury on March 29, 1974, September 21, 1974, and on March 16, 1975.[37] The BESA measurements "show that the planet interacts with the solar wind to form a bow shock and a permanent magnetosphere. ... The magnetosphere of Mercury appears to be similar in shape to that of the earth but much smaller in relation to the size of the planet. The average distance from the center of Mercury to the subsolar point of the magnetopause is ∼ 1.4 planetary radii. Electron populations similar to those found in the earth’s magnetotail, within the plasma sheet and adjacent regions, were observed at Mercury; both their spatial location and the electron energy spectra within them bear qualitative and quantitative resemblance to corresponding observations at the earth."[38]

"[T]he Mercury encounter (M I) by Mariner 10 on 29 March 1974 occurred during the height of a Jovian electron increase in the interplanetary medium."[39]

## Venus

Main sources: Wanderers/Venus and Venus
This Chandra X-ray Observatory image is the first X-ray image ever made of Venus. Credit: NASA/MPE/K.Dennerl et al.

The first ever X-ray image of Venus is shown at right. The "half crescent is due to the relative orientation of the Sun, Earth and Venus. The X-rays from Venus are produced by fluorescent radiation from oxygen and other atoms in the atmosphere between 120 and 140 kilometers above the surface of the planet. In contrast, the optical light from Venus is caused by the reflection from clouds 50 to 70 kilometers above the surface. Solar X-rays bombard the atmosphere of Venus, knock electrons out of the inner parts of atoms, and excite the atoms to a higher energy level. The atoms almost immediately return to their lower energy state with the emission of a fluorescent X-ray. A similar process involving ultraviolet light produces the visible light from fluorescent lamps."[40]

## Earth

Main source: Earth
This graph shows the relationship of the atmosphere and ionosphere to electron density. Credit: .
Bright X-ray arcs of low energy (0.1 - 10 keV) are generated during auroral activity. Observation dates: 10 pointings between December 16, 2003 and April 13, 2004. Instrument: HRC. Credit: NASA/MSFC/CXC/A.Bhardwaj & R.Elsner, et al.; Earth model: NASA/GSFC/L.Perkins & G.Shirah.
This image is a composite of the first picture of the Earth in X-rays over a diagram of the Earth below. Credit: NASA, Ruth Netting.

"[L]ow-altitude regions of downward electric current on auroral magnetic field lines are sites of dramatic upward magnetic field-aligned electron acceleration that generates intense magnetic field-aligned electron beams within Earth’s equatorial middle magnetosphere."[41]

The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km to more than 1000 km. It owes its existence primarily to ultraviolet radiation from the Sun.

The images [lower right] are superimposed on a simulated image of the Earth. The color code represents brightness, maximum in red. Distance from the North pole to the black circle is 3,340 km (2,080 mi).

"Auroras are produced by solar storms that eject clouds of energetic charged particles. These particles are deflected when they encounter the Earth’s magnetic field, but in the process large electric voltages are created. Electrons trapped in the Earth’s magnetic field are accelerated by these voltages and spiral along the magnetic field into the polar regions. There they collide with atoms high in the atmosphere and emit X-rays".[42]

At right is a composite image which contains the first picture of the Earth in X-rays, taken in March, 1996, with the orbiting Polar satellite. The area of brightest X-ray emission is red.

Energetic charged particles from the Sun energize electrons in the Earth's magnetosphere. These electrons move along the Earth's magnetic field and eventually strike the ionosphere, causing the X-ray emission. Lightning strikes or bolts across the sky also emit X-rays.[43]

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[44][45] and local acceleration[46] 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,[46] 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).

## Moon

Main sources: Wanderers/Moon and Moon
These two hemispheric Lambert azimuthal equal area projections show the total magnetic field strength at the surface of the Moon, derived from the Lunar Prospector electron reflectometer (ER) experiment. Credit: Mark A. Wieczorek.
The Moon where a prediction of a lunar double layer[47] was confirmed in 2003.[48] In the shadows, the Moon charges negatively in the interplanetary medium.[49] Credit: Mdf.

The electron reflectometer (ER) [aboard the Lunar Prospector determines] the location and strength of magnetic fields from the energy spectrum and direction of electrons. The instrument [measures] the pitch angles of solar wind electrons reflected from the Moon by lunar magnetic fields. Stronger local magnetic fields can reflect electrons with larger pitch angles. Field strengths as small as 0.01 [nanotesla] nT could be measured with a spatial accuracy of about 3 km (1.9 mi) at the lunar surface.

"[T]he shadowed lunar surface charges negative."[50]

## Jupiter

Main sources: Wanderers/Jupiter and Jupiter

"Field-aligned equatorial electron beams [have been] observed within Jupiter’s middle magnetosphere. ... the Jupiter equatorial electron beams are spatially and/or temporally structured (down to <20 km at auroral altitudes, or less than several minutes), with regions of intense beams intermixed with regions absent of such beams."[41]

"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".[39]

Jovian electrons propagate "along the spiral magnetic field of the interplanetary medium [from Jupiter and its magnetosphere to the Sun]".[39]

## Callisto

Main source: Callisto
This image of Callisto from NASA's Galileo spacecraft, taken in May 2001, is the only complete global color image of Callisto obtained by Galileo. Credit: NASA/JPL/DLR(German Aerospace Center).

At right is a complete global color image of Callisto. Bright scars on a darker surface testify to a long history of impacts on Jupiter's moon Callisto. The picture, taken in May 2001, is the only complete global color image of Callisto obtained by Galileo, which has been orbiting Jupiter since December 1995. Of Jupiter's four largest moons, Callisto orbits farthest from the giant planet. Callisto's surface is uniformly cratered but is not uniform in color or brightness. Scientists believe the brighter areas are mainly ice and the darker areas are highly eroded, ice-poor material.

Callisto's ionosphere was first detected during Galileo flybys;[51] its high electron density of 7–17 x 104 cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone.

## Saturn

Main sources: Wanderers/Saturn and Saturn

"[M]agnetospheric electron (bi-directional) beams connect to the expected locations of Saturn’s aurora".[52]

Powered by the Saturnian equivalent of (filamentary) Birkeland currents, streams of charged particles from the interplanetary medium interact with the planet's magnetic field and funnel down to the poles.[53] Double layers are associated with (filamentary) currents,[54][55] and their electric fields accelerate ions and electrons.[56]

## Heliospheres

These electrons "provide remote-sensing observations of distant targets in the heliosphere - the Sun, the Moon, Jupiter, and various heliospheric structures."[2]

## Electron winds

"The power into the Crab Nebula is apparently supplied by an outflow [wind] of ~1038 erg/s from the pulsar"[3] where there are "electrons (and positrons) in such a wind"[3]. These beta particles coming out of the pulsar are moving very close to light speed.

"An electron beam furnace (EB furnace) is a type of vacuum furnace employing a high-energy electron beam in vacuum as the mean for delivery of heat to the material being melted.

As of December 5, 2011, "Voyager 1 is about ... 18 billion kilometers ... from the [S]un [but] the direction of the magnetic field lines has not changed, indicating Voyager is still within the heliosphere ... the outward speed of the solar wind had diminished to zero in April 2010 ... inward pressure from interstellar space is compacting [the magnetic field] ... Voyager has detected a 100-fold increase in the intensity of high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside ... [while] the [solar] wind even blows back at us."[17]

The source of heat that brings the coronal cloud near the Sun hot enough to emit X-rays may be an electron beam heating effect due to "high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside"[17].

## Interstellar medium

The "many other types of radio sources in our galaxy [...] include so-called radio stars, emission nebula, flare stars and pulsars. [...] Pulsars were first discovered in 1967 by Cambridge post-graduate student Jocelyn Bell as she processed charts associated with an unrelated project to study twinkling radio sources. She noticed recurrent signals when the antenna was pointed in a certain direction. Further study revealed a precise timing interval of about 1 second. It also was found that the pulses were dispersed such that the lower frequencies arrived later than the higher frequencies. This dispersion could be attributed to scattering of the radiation by interstellar electrons and, if so, could provide an indication of the pulsar distance."[57]

"A particular subject of interest is the cluster ion series (NH3)nNH4+, since it is the dominant group of ions over the whole investigated temperature range."[58] For astrochemisty, "[t]hese studies are expected to throw light on the sputtering from planetary and interstellar ices and the possible formation of new organic molecules in CO--NH3–H2O ice by megaelectronvolt ion bombardment."[58]

"lnterstellar scintillation (ISS), fluctuations in the amplitude and phase of radio waves caused by scattering in the interstellar medium, is important as a diagnostic of interstellar plasma turbulence. ISS is also of interest because it is noise for other radio astronomical observations. [As a remote, sensing tool, ISS is used to diagnose the plasma turbulence in the interstellar medium (lSM). However, where ISS acts as a noise source in other observations, the plasma physics of the medium is only of secondary interest.] The unifying concern is the power spectrum of the interstellar electron density."[59]

"From measurements of angular broadening of pulsars and extragalactic sources, decorrelation bandwidth of pulsars, refractive steering of features in pulsar dynamic spectra, dispersion measure fluctuations of pulsars, and refractive scintillation index measurements, [...] a composite structure function that is approximately power law over 2 x 106 m < scale < 1013 m [is constructed]. The data are consistent with the structure function having a logarithmic slope versus baseline Iess than 2; thus there is a meaningful connection between scales in the radiowave fluctuation field and the scales in the electron density field causing the scattering."[59]

A "composite electron density spectrum [is] approximately power law over at least the ≈ 5 decade wavenumber range 10-13 m-1 < wavenumber < 10-8 m-1 and that may extend to higher wavenumbers."[59]

## Physics

Main source: Physics

Def. a quasiparticle produced as a result of electron spin-charge separation is called a chargon, or holon.

Def. a quasiparticle that electrons in solids are able to split into during the process of spin-charge separation, when extremely tightly confined at temperatures close to absolute zero is called a spinon.

## Electron volts

Energy of photons is shown in the visible spectrum. Credit: Vojtěch Hála and Gringer.
This is a graph of eV versus nm. Credit: User:Mrdupont.

Def. a unit for measuring the energy of subatomic particles; the energy equal to that attained by an electron moving through a potential of one volt is called an electron volt.

The electron volt (eV) is the energy gained (or lost) by an electron in passing through a potential difference of one volt. Since the charge on an electron is 1.60218 x 10-19 Coulombs, an eV is 1.60218 x 10-19 J. A keV is 1000 eV and a MeV is 1000 keV.

A photon with an energy of 1 eV has a frequency of 1 eV/h = 2.41799 x 1014 Hz or about 242 THz and a wavelength of c*h/1 eV = 1.23984 x 10-6 m or about 1,240 nm or 12,400 Å. That would put the photon in the infrared range. In practice, photon energies are seldom stated for such long wavelengths.

Generally, the energy E, frequency ν, and wavelength λ of a photon are related by

${\displaystyle E=h\nu ={\frac {hc}{\lambda }}={\frac {(4.13566733\times 10^{-15}\,{\mbox{eV}}\,{\mbox{s}})(299\,792\,458\,{\mbox{m/s}})}{\lambda }},}$

where h is the Planck constant, c is the speed of light. This reduces to

${\displaystyle E{\mbox{(eV)}}={\frac {1239.84187\,{\mbox{eV}}\,{\mbox{nm}}}{\lambda \ {\mbox{(nm)}}}}.}$

## Electron density

Electron density calculated for aniline, high density values indicate atom positions, intermediate density values emphasize bonding, low values provide information on a molecule's shape and size. Credit: .

Def. a measure of the number of electrons per unit volume of space is called an electron density.

Electron density is the measure of the probability of an electron being present at a specific location. In molecules, regions of electron density are usually found around the atom, and its bonds.

In quantum chemical calculations, the electron density, ρ(r), is a function of the coordinates r, defined so ρ(r)dr is the number of electrons in a small volume dr. For closed-shell molecules, ${\displaystyle \rho (\mathbf {r} )}$ can be written in terms of a sum of products of basis functions, φ:

${\displaystyle \rho (\mathbf {r} )=\sum _{\mu }\sum _{\nu }P_{\mu \nu }\phi _{\mu }(\mathbf {r} )\phi _{\nu }(\mathbf {r} )}$

where P is the density matrix. Electron densities are often rendered in terms of an isosurface (an isodensity surface) with the size and shape of the surface determined by the value of the density chosen, or in terms of a percentage of total electrons enclosed.

## Spin density

Spin density is electron density applied to free radicals. It is defined as the total electron density of electrons of one spin minus the total electron density of the electrons of the other spin. One of the ways to measure it experimentally is by electron spin resonance,[60] neutron diffraction allows direct mapping of the spin density in 3D-space.

## Electron temperatures

If the velocities of a group of electrons, e.g., in a plasma, follow a Maxwell-Boltzmann distribution, then the electron temperature is well-defined as the temperature of that distribution. For other distributions, two-thirds of the average energy is often referred to as the temperature, since for a Maxwell-Boltzmann distribution with three degrees of freedom, ${\displaystyle \langle E\rangle =(3/2)\langle k_{B}T\rangle }$. The SI unit of temperature is the kelvin (K), but using the above relation the electron temperature is often expressed in terms of the energy unit electronvolt (eV). Each kelvin (1 K) corresponds to 8.617343(15) x 10-5 eV; this factor is the ratio of the Boltzmann constant to the elementary charge. The electron temperature of a plasma can be several orders of magnitude higher than the temperature of the neutral species or of the ions. This is a result of two facts. Firstly, many plasma sources heat the electrons more strongly than the ions. Secondly, atoms and ions are much heavier than electrons, and energy transfer in a two-body collision is much more efficient if the masses are similar.

## Technology

Main source: Technology
Beam of electrons are moving in a circle in a magnetic field (cyclotron motion). Lighting is caused by excitation of atoms of gas in a bulb. Credit: Marcin Białek.

A cyclotron is a compact type of particle accelerator in which charged particles in a static magnetic field are travelling outwards from the center along a spiral path and get accelerated by radio frequency electromagnetic fields. Cyclotrons accelerate charged particle beams using a high frequency alternating voltage which is applied between two "D"-shaped electrodes (also called "dees"). An additional static magnetic field ${\displaystyle B}$ is applied in perpendicular direction to the electrode plane, enabling particles to re-encounter the accelerating voltage many times at the same phase. To achieve this, the voltage frequency must match the particle's cyclotron resonance frequency

${\displaystyle f={\frac {qB}{2\pi m}}}$,

with the relativistic mass m and its charge q. This frequency is given by equality of centripetal force and magnetic Lorentz force. The particles, injected near the center of the magnetic field, increase their kinetic energy only when recirculating through the gap between the electrodes; thus they travel outwards along a spiral path.

Cyclotron radiation is electromagnetic radiation emitted by moving charged particles deflected by a magnetic field. The Lorentz force on the particles acts perpendicular to both the magnetic field lines and the particles' motion through them, creating an acceleration of charged particles that causes them to emit radiation (and to spiral around the magnetic field lines). ... Cyclotron radiation is emitted by all charged particles travelling through magnetic fields, however, not just those in cyclotrons. Cyclotron radiation from plasma in the interstellar medium or around black holes and other astronomical phenomena is an important source of information about distant magnetic fields. The power (energy per unit time) of the emission of each electron can be calculated using:

${\displaystyle {-dE \over dt}={\sigma _{t}B^{2}V^{2} \over c\mu _{o}}}$

where E is energy, t is time, ${\displaystyle \sigma _{t}}$ is the Thomson cross section (total, not differential), B is the magnetic field strength, V is the velocity perpendicular to the magnetic field, c is the speed of light and ${\displaystyle \mu _{o}}$ is the permeability of free space.

"Electron beams can be generated by thermionic emission, field emission or the anodic arc method. The generated electron beam is accelerated to a high kinetic energy and focused towards the [target]. When the accelerating voltage is between 20 kV – 25 kV and the beam current is a few amperes, 85% of the kinetic energy of the electrons is converted into thermal energy as the beam bombards the surface of the [target]. The surface temperature of the [target] increases resulting in the formation of a liquid melt. Although some of incident electron energy is lost in the excitation of X-rays and secondary emission, the [target] material evaporates under vacuum."[61]

## Scintillation detectors

A scintillator is a material, which exhibits scintillation—the property of luminescence[62] when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light. Here, "particle" refers to "ionizing radiation" and can refer either to charged particulate radiation, such as electrons and heavy charged particles, or to uncharged radiation, such as photons and neutrons, provided that they have enough energy to induce ionization.

A scintillation detector or scintillation counter is obtained when a scintillator is coupled to an electronic light sensor such as a photomultiplier tube (PMT) or a photodiode. PMTs absorb the light emitted by the scintillator and reemit it in the form of electrons via the photoelectric effect. The subsequent multiplication of those electrons (sometimes called photo-electrons) results in an electrical pulse which can then be analyzed and yield meaningful information about the particle that originally struck the scintillator.

## Galileo Orbiter

This is an image of the Energetic Particles Detector (EPD) aboard the Galileo Orbiter. Credit: NASA.

The Energetic Particles Detector (EPD) aboard the Galileo Orbiter is designed to measure the numbers and energies of electrons whose energies exceed about 20 keV. The EPD can also measure the direction of travel of electrons. The EPD uses silicon solid state detectors and a time-of-flight detector system to measure changes in the energetic electron population at Jupiter as a function of position and time.

"[The] two bi-directional, solid-state detector telescopes [are] mounted on a platform which [is] rotated by a stepper motor into one of eight positions. This rotation of the platform, combined with the spinning of the orbiter in a plane perpendicular to the platform rotation, [permits] a 4-pi [or 4π] steradian coverage of incoming [electrons]. The forward (0 degree) ends of the two telescopes [have] an unobstructed view over the [4π] sphere or [can] be positioned behind a shield which not only [prevents] the entrance of incoming radiation, but [contains] a source, thus allowing background corrections and in-flight calibrations to be made. ... The 0 degree end of the [Low-Energy Magnetospheric Measurements System] LEMMS [uses] magnetic deflection to separate incoming electrons and ions. The 180 degree end [uses] absorbers in combination with the detectors to provide measurements of higher-energy electrons ... The LEMMS [provides] measurements of electrons from 15 keV to greater than 11 MeV ... in 32 rate channels."[63]

## Lunar Prospector

The electron reflectometer (ER) [aboard the Lunar Prospector determines] the location and strength of magnetic fields from the energy spectrum and direction of electrons. The instrument measures the pitch angles of solar wind electrons reflected from the Moon by lunar magnetic fields. Stronger local magnetic fields can reflect electrons with larger pitch angles. Field strengths as small as 0.01 nanotesla nT could be measured with a spatial accuracy of about {3 km (1.9 mi) at the lunar surface. The ER is located at the end of one of the three radial science booms on the Lunar Prospector.

## Imaging Compton Telescope

This is a schematic of the various experiments aboard the Gamma-ray Observatory. Credit: NASA/JPL.
The Imaging Compton Telescope (COMPTEL) utilizes the Compton Effect and two layers of gamma-ray detectors. Credit: NASA.

For cosmic gamma-ray events, the experiment required two nearly simultaneous interactions, in a set of front and rear scintillators. Gamma rays would Compton scatter in a forward detector module, where the interaction energy E1, given to the recoil electron was measured, while the Compton scattered photon would then be caught in one of a second layer of scintillators to the rear, where its total energy, E2, would be measured. From these two energies, E1 and E2, the Compton scattering angle, angle θ, can be determined, along with the total energy, E1 + E2, of the incident photon. The positions of the interactions, in both the front and rear scintillators, was also measured. The vector, V, connecting the two interaction points determined a direction to the sky, and the angle θ about this direction, defined a cone about V on which the source of the photon must lie, and a corresponding "event circle" on the sky.

"COMPTEL's upper layer of detectors are filled with a liquid scintillator which scatters an incoming gamma-ray photon according to the Compton Effect. This photon is then absorbed by NaI crystals in the lower detectors. The instrument records the time, location, and energy of the events in each layer of detectors which makes it possible to determine the direction and energy of the original gamma-ray photon and reconstruct an image and energy spectrum of the source."[64]

## Hypotheses

Main source: Hypotheses
1. Superluminal electrons exist.

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