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Plasmas

Main articles: Radiation astronomy/Plasmas and Plasma astronomy

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 . 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. 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

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

An explosive limb flare occurred above 30,000 km in the corona of the Sun. "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." "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." "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." "[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."

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

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. 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.

"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."