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.
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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
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) and complex ejecta, which can be the result of the interaction of multiple CMEs.