Radiation astronomy/Plasmas

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On July 19, 2012, an eruption occurred on the sun that produced a moderately powerful solar flare and a dazzling magnetic display known as coronal rain. Credit: NASA Goddard Space Flight Center, Music: 'Thunderbolt' by Lars Leonhard, courtesy of artist.

A coronal cloud is a cloud, or cloud-like, natural astronomical entity, composed of plasma and usually associated with a star or other astronomical object where the temperature is such that X-rays are emitted. While small coronal clouds are above the photosphere of many different visual spectral type stars, others occupy parts of the interstellar medium (ISM), extending sometimes millions of kilometers into space, or thousands of light-years, depending on the size of the associated object such as a galaxy.

Coronal clouds[edit]

"Coronal clouds, type IIIg, form in space above a spot area and rain streamers upon it."[1]

"[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."[2]

"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."[3]

Flares[edit]

"[A] medium-strength flare erupted from the sun on July 19, 2012. The blast also generated the enormous, shimmering plasma loops, which are an example of a phenomenon known as "coronal rain," agency officials said."[4]

"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."[5]

Plasma rains[edit]

"Hot plasma in the corona cooled and condensed along strong magnetic fields in the region" slowly falling back to the solar surface as plasma "rain".[4]

Coronal mass ejections[edit]

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.

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 [6]. 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[7]. 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.

Other signatures of magnetic clouds are now used in addition to the one described above: among other, bidirectional superthermal electrons, unusual charge state or abundance of iron, helium, carbon and/or oxygen. The typical time for a magnetic cloud to move past a satellite at the L1 point is 1 day corresponding to a radius of 0.15 AU with a typical speed of 450 km s−1 and magnetic field strength of 20 nT [8]

Def. a "massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space"[9] is called a coronal mass ejection (CME).

An explosive limb flare occurred above 30,000 km in the corona of the Sun.[10] "So the aftermath of the flare explosion, usually visible in disk pictures as extensive Hα brightening, but hidden from us in this case, was seen by the ionosphere as an intense flux of ionizing radiation from the coronal cloud created by the explosion."[10] "The November 20, 1960, event is very similar to that of February 10, 1956, which was observed at Sacramento Peak. A bright ball appears above the surface, grows in size and Hα brightness, and explodes upward and outward."[10] "The great breadth and intensity of the Hα emission from the suspended ball at 2013 U.T. testify to the large amount of energy stored there, as no corresponding macroscopic motion was observed until the explosion at 2023 U.T."[10] "[T]he great energy of the preflare cloud was released into the corona by the explosion of 2023 U.T., and Hα radiation disappeared by 2035 U.T."[10]

"On 16 June 1972, the Naval Research Laboratory's coronagraph aboard OSO-7 tracked a huge coronal cloud moving outward from the Sun."[11]

A coronal mass ejection (CME) is an ejected plasma consisting primarily of electrons and protons (in addition to small quantities of heavier elements such as helium, oxygen, and iron), plus the entraining coronal closed magnetic field regions. Evolution of these closed magnetic structures in response to various photospheric motions over different time scales (convection, differential rotation, meridional circulation) somehow leads to the CME.[12] Small-scale energetic signatures such as plasma heating (observed as compact soft X-ray brightening) may be indicative of impending CMEs.

The soft X-ray sigmoid (an S-shaped intensity of soft X-rays) is an observational manifestation of the connection between coronal structure and CME production.[12]

"Relating the sigmoids at X-ray (and other) wavelengths to magnetic structures and current systems in the solar atmosphere is the key to understanding their relationship to CMEs."[12]

Stellar winds[edit]

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.

"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."[3]

"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."[3]

"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."[3] 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."[3]

The solar wind originates through the polar coronal holes.

"The solar wind is a plasma, composed primarily of electrons and lone protons, and the variations in the index of refraction are caused by variations in the density of the plasma.[13] Different indices of refraction result in phase changes between waves traveling through different locations, which results in interference. As the waves interfere, both the frequency of the wave and its angular size are broadened, and the intensity varies.[14]"[15]

Auroras[edit]

This dramatic panorama shows a colourful, shimmering auroral curtain reflected in a placid Icelandic lake. Credit: Carlos Gauna. {{fairuse}}

Auroras can be caused by electrons being absorbed into an atmosphere.

The "dramatic panorama [on the right shows a colorful], shimmering auroral curtain reflected in a placid Icelandic lake. The image was taken on 18 March 2015 by Carlos Gauna, near Jökulsárlón Glacier Lagoon in southern Iceland."[16]

"The celestial display was generated by a coronal mass ejection, or CME, on 15 March. Sweeping across the inner Solar System at some 3 million km per hour, the eruption reached Earth, 150 million kilometres away, in only two days. The gaseous cloud collided with Earth’s magnetic field at around 04:30 GMT on 17 March."[16]

"When the charged particles from the Sun penetrate Earth's magnetic shield, they are channelled downwards along the magnetic field lines until they strike atoms of gas high in the atmosphere. Like a giant fluorescent neon lamp, the interaction with excited oxygen atoms generates a green or, more rarely, red glow in the night sky, while excited nitrogen atoms yield blue and purple colours."[16]

"Auroral displays are not just decorative distractions. They are most frequent when the Sun's activity nears its peak roughly every 11 years. At such times, the inflow of high-energy particles and the buffeting of Earth’s magnetic field may sometimes cause power blackouts, disruption of radio communications, damage to satellites and even threaten astronaut safety."[16]

Magnetohydrodynamics[edit]

"When magnetic fields "reconnect" in a turbulent magnetohydrodynamic (MHD) plasma, electric fields are generated in which particles can be accelerated (Matthaeus et al., 1984; Sorrell, 1984)."[17]

X-rays[edit]

The image shows the Pleiades in X-rays, taken by ROSAT, where the brightest optical stars are inside the green squares. Credit: Worldtraveller.

"The Pleiades star cluster is one of the jewels of the northern sky. To the unaided eye it appears as an alluring group of stars in the constellation Taurus, while telescopic views reveal cluster stars surrounded by delicate blue wisps of dust-reflected starlight. To the X-ray telescopes on board the orbiting ROSAT observatory, the cluster also presents an impressive, but slightly altered, appearance. This false color image [at right] was produced from ROSAT observations by translating different X-ray energy bands to visual colors - the lowest energies are shown in red, medium in green, and highest energies in blue. (The green boxes mark the position of the seven brightest visual stars.) The Pleiades stars seen in X-rays have extremely hot, tenuous outer atmospheres called coronas and the range of colors corresponds to different coronal temperatures."[18]

Ultraviolets[edit]

A coronal mass ejection is shown in the ultraviolet. Credit: NASA/SDO.

"One of the fastest CMEs in years was captured by the STEREO COR1 telescopes on August 1, 2010. ... This CME is seen to be heading towards Earth at speeds well over 1000 kilometers per second."[19]

"On August 1st, almost the entire Earth-facing side of the sun erupted in a tumult of activity. There was a C3-class solar flare, a solar tsunami, multiple filaments of magnetism lifting off the stellar surface, large-scale shaking of the solar corona, radio bursts, a coronal mass ejection and more. This extreme ultraviolet snapshot [at right] from the Solar Dynamics Observatory (SDO) shows the sun's northern hemisphere in mid-eruption. Different colors in the image represent different gas temperatures ranging from ~1 to 2 million degrees K."[19]

References[edit]

  1. Edison Pettit (July 1943). "The Properties of Solar Prominences as Related to Type". Astrophysical Journal 98 (7): 6-19. doi:10.1086/144539. 
  2. K. H. Schatten, D. J. Mullan (December 1, 1977). "Fast azimuthal transport of solar cosmic rays via a coronal magnetic bottle". Journal of Geophysical Research 82 (35): 5609-20. doi:10.1029/JA082i035p05609. http://www.agu.org/pubs/crossref/1977/JA082i035p05609.shtml. Retrieved 2013-07-07. 
  3. 3.0 3.1 3.2 3.3 3.4 E. W. Cliver, S. W. Kahler, M. A. Shea, and D. F. Smart (September 1 1982). "Injection onsets of ~2 GeV protons, ~1 MeV electrons, and ~100 keV electrons in solar cosmic ray flares". The Astrophysical Journal 260 (9): 362-70. 
  4. 4.0 4.1 Lua error in Module:Citation/CS1 at line 3723: bad argument #1 to 'pairs' (table expected, got nil).
  5. Lua error in Module:Citation/CS1 at line 3723: bad argument #1 to 'pairs' (table expected, got nil).
  6. Burlaga, L. F., E. Sittler, F. Mariani, and R. Schwenn, "Magnetic loop behind an interplanetary shock: Voyager, Helios and IMP-8 observations" in "Journal of Geophysical Research", 86, 6673, 1981
  7. Burlaga, L. F. et al., "A magnetic cloud and a coronal mass ejection" in "Geophysical Research Letter"s, 9, 1317-1320, 1982
  8. Lepping, R. P. et al. "Magnetic field structure of interplanetary magnetic clouds at 1 AU" in "Journal of Geophysical Research", 95, 11957-11965, 1990.
  9. coronal mass ejection. San Francisco, California: Wikimedia Foundation, Inc. June 21, 2013. Retrieved 2013-07-07.
  10. 10.0 10.1 10.2 10.3 10.4 Harold Zirin (October 1964). "The Limb Flare of November 20, 1960: a Coronal Phenomenon". Astrophysical Journal 140 (10): 1216-35. doi:10.1086/148019. 
  11. Martin Koomen and Russell Howard, Richard Hansen and Shirley Hansen (February 1974). "The coronal transient of 16 June 1972". Solar Physics 34 (2): 447-52. doi:10.1007/BF00153680. http://link.springer.com/article/10.1007/BF00153680. Retrieved 2013-07-10. 
  12. 12.0 12.1 12.2 Gopalswamy N, Mikic Z, Maia D, Alexander D, Cremades H, Kaufmann P, Tripathi D, Wang YM (2006). "The pre-CME Sun". Space Sci Rev 123 (1–3): 303. doi:10.1007/s11214-006-9020-2. 
  13. Jokipii (1973), pp. 11–12.
  14. Alurkar (1997), p. 11.
  15. Lua error in Module:Citation/CS1 at line 3723: bad argument #1 to 'pairs' (table expected, got nil).
  16. 16.0 16.1 16.2 16.3 Lua error in Module:Citation/CS1 at line 3723: bad argument #1 to 'pairs' (table expected, got nil).
  17. Thomas K. Gaisser (1990). Cosmic Rays and Particle Physics. Cambridge University Press. p. 279. ISBN 0521339316. Retrieved 2014-01-11.
  18. Robert Nemiroff & Jerry Bonnell (August 28, 1999). X-Ray Pleiades. Greenbelt, Maryland USA: NASA/GSFC. Retrieved 2013-07-07.
  19. 19.0 19.1 Holly Zell (August 4, 2010). Spacecraft Observes Coronal Mass Ejection. Washington, DC USA: NASA. Retrieved 2013-07-07.