A magnetic cloud is defined as a structure with strong magnetic fields, low proton temperatures and a smooth rotation of the magnetic field direction through a large angle as it moves past a spacecraft in a period on the order of a day. The above figure shows all of these features defining a magnetic cloud where theta is the latitude of the magnetic field vector and phi is its longitude.
The front boundary of the February magnetic cloud was associated with a magnetic hole in which the density was high. The magnetic cloud was moving slowly, at about 320 km/s. The slow speed, large duration and asymmetric magnetic field strength profile indicate that the magnetic cloud was relatively 'old', i.e. more evolved than typical magnetic clouds at 1 AU. The magnetic cloud was still expanding as it moved past ACE as is seen by the continual drop in wind speed across the cloud.
The relatively high density at the rear of the magnetic cloud might be related to a high-density feature in the source, e.g. a solar prominence at the rear of a CME.
Magnetic clouds are thought to be associated with discrete solar ejecta. As such, the composition of their thermal ion component is frequently distinct from other periods of solar wind activity. Comparison of those differences may ultimately provide new insights into the source mechanisms that creates a magnetic cloud.
....contributed by Charles Smith, Bartol Research Inst./Univ. of Delaware
Last modified 13 March 1998, mrt
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