View source for Astronomy:Magnetic cloud

{{short description|Region of enhanced magnetic field strength in the solar wind}}
A '''magnetic cloud''' is a transient event observed in the [[Astronomy:Solar wind|solar wind]]. It was defined in 1981 by Burlaga et al. 1981 as a region of enhanced [[Physics:Magnetic field|magnetic field]] strength, smooth rotation of the magnetic field vector, and low [[Physics:Proton|proton]] temperature.<ref>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</ref> Magnetic clouds are a possible manifestation of a [[Astronomy:Coronal mass ejection|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 [[Astronomy:Solar Maximum Mission|SMM]].<ref>Burlaga, L. F. et al., "A magnetic cloud and a coronal mass ejection" in "Geophysical Research Letter"s, 9, 1317-1320, 1982</ref> 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 [[Astronomy:Advanced Composition Explorer|ACE]] is a fast-mode [[Physics:Shock wave|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 [[Physics:Electron|electron]]s, unusual charge state or abundance of [[Chemistry:Iron|iron]], [[Chemistry:Helium|helium]], [[Chemistry:Carbon|carbon]], and/or [[Chemistry:Oxygen|oxygen]].

The typical time for a magnetic cloud to move past a satellite at the L1 point is 1 day corresponding to a [[Astronomy:Earth radius|radius]] of 0.15 AU with a typical speed of {{convert|450|km/s|mi/s|abbr=on}} and magnetic field strength of 20 [[Tesla (unit)|nT]].<ref>Lepping, R. P. et al. "Magnetic field structure of interplanetary magnetic clouds at 1 AU" in "Journal of Geophysical Research", 95, 11957-11965, 1990</ref>

==Other types of ejecta observed at Earth==
Magnetic clouds represent about one third of ejecta observed by satellites at Earth. Other types of ejecta are multiple-magnetic cloud events (a single structure with multiple subclouds distinguishable)<ref>Wang, Y. M., et al., Multiple magnetic clouds in interplanetary space, Solar Physics, 211, 333-344, 2002.</ref><ref>Wang, Y. M., et al., Multiple magnetic clouds: Several examples during March - April, 2001, J. Geophys. Res., 108(A10), 1370, 2003.</ref> and complex ejecta, which can be the result of the interaction of multiple CMEs.

==See also==
* [[Astronomy:List of solar storms|List of solar storms]]
* [[Astronomy:Coronal cloud|Coronal cloud]]
* [[Physics:Interplanetary magnetic field|Interplanetary magnetic field]] (IMF) and [[Earth:Earth's magnetic field|Earth's magnetic field]] ([[Astronomy:Magnetosphere|magnetosphere]])
* [[Astronomy:Space weather|Space weather]] and [[Physics:Heliophysics|heliophysics]]

==References==
{{reflist|2}}

==External links==

{{The Sun}}
{{Doomsday}}
{{Magnetospherics}}
{{Solar storms}}

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[[Category:Stellar phenomena]]
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[[Category:Plasma physics]]

{{Sourceattribution|Magnetic cloud|1}}