Overview of Magnetic Field and Plasma Observations.

A plot of the magnetic field and solar wind parameters from Oct. 18 through Oct. 20 is shown in Figure 1. From top to bottom the figure shows the magnetic field strength (B), the elevation () and azimuth () of the magnetic field direction, the proton density (N), the most probable proton thermal speed assuming an isotropic Maxwellian velocity distribution (V_T = (2kT/m)), and the magnitude of the bulk velocity (V). The magnetic cloud is the region in which the direction of the strong magnetic field rotated smoothly from south to north. The speed of the cloud was 400 km/s, and one does not see the decrease in speed from the front to the rear that is typical of magnetic clouds at 1 AU. This speed profile indicates that the cloud was "old" [Farrugia et al., 1992; Oscherovich et al., 1993, 1995] and no longer rapidly expanding; further studies of this observation are needed. A shock wave was observed ahead of the magnetic cloud on calendar day 291.445 (Oct. 18 at hour 10.7, 1040 UT), probably produced by the supermagnetoacoustic motion of the magnetic cloud relative to the solar wind closer to the sun.

The magnetic cloud was being overtaken by a corotating stream. A corotating shock driven by the stream entered the rear of the magnetic cloud and was observed by WIND at 292.744. The stream compressed the plasma and field at the rear of the magnetic cloud. A stream interface was seen at 292.954, characterized by an abrupt decrease in density, a similar increase in temperature and a peak in the magnetic field intensity. A corotating stream, indicated by the low density and high temperature, followed the stream interface. WIND was clearly in the stream after 293.3, when the speed and temperature were high, the density was low, and the magnetic field strength was average. The region between the first stream interface at 292.954 and 293.3 was complicated, owing to a second density peak and several large changes in magnetic field direction in that interval. This too is a subject that merits further study.