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Magnetic cloud
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.〔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〕 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.〔Burlaga, L. F. et al., "A magnetic cloud and a coronal mass ejection" in "Geophysical Research Letter"s, 9, 1317-1320, 1982〕 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 characteristics ==
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 〔Lepping, R. P. et al. "Magnetic field structure of interplanetary magnetic clouds at 1 AU" in "Journal of Geophysical Research", 95, 11957-11965, 1990.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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