The following describes a magnetic cloud observed by the ACE spacecraft early in February 1998. Both MAG and SWEPAM data are presented here in a very preliminary examination of the disturbance.
The event was used in an ACE News dated March 13, 1998. The initial text is taken from the ACE News account.
Click here to open a new window showing MAG and SWEPAM data for this period:
500 KByte pdf image mag_swepam_cloud_1998-034.pdf.
1.2 MByte postscript image mag_swepam_cloud_1998-034.ps.
On Feb 4 and 5 (days 35 and 36) an unusually large magnetic cloud moved past the ACE spacecraft. At this time, ACE was already at the L1 orbit.
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. Typically these structures last about a day. The above figure shows all of the characteristics of a magnetic cloud where theta is the latitude of the magnetic field vector and phi is its longitude. The front boundary was associated with a magnetic hole early on day 35 in which the density was elevated.
The magnetic cloud was moving unusually slowly, on average about 320 km/s. The slow speed, long duration and asymmetric magnetic field strength profile indicate that it was relatively 'old', i.e. more evolved than typical magnetic clouds at 1 AU. The 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. An analysis of the composition for clouds may ultimately provide new insights into the source mechanisms that create them.
Return to the ACE List of Disturbances and Transients.