Remote sensing of a flux transfer event with energetic particles

Abstract

We have used the medium energy particle spectrometer on board ISEE 2 to perform an analysis of the azimuthal asymmetry at the onset and end of a flux transfer event (FTE) on the dayside magnetopause. The event chosen was on November 8, 1977, at 0212 UT, during a period of high bitrate. The resulting angular resolution (32 sectors) permitted a very detailed analysis. We demonstrate that it is possible to describe the ion intensity as a function only of pitch angle and distance of the gyro‐center from a moving planar surface separating the particle‐rich and empty regions of the FTE. The intensity changes sharply within a fraction of a gyro‐radius. In order to match the location of the step at different pitch angles, it is necessary to define the particle boundary with respect to the gyro‐center itself, i.e., the particle intensity changes sharply when the gyro‐center crosses the boundary, and this boundary is the same for all pitch angles. The FTE boundary velocities that are derived from the analysis are in the range 10–50 km/s. When the results are transformed into the magnetopause normal coordinate system (LMN), the FTE is seen to sweep over the spacecraft from the magnetopause side, to reverse and to return in the same direction. At both the beginning and end, the FTE boundary motion has a northward component. The event appears as a flux tube filled with streaming particles (primarily ions) moving northward in the plane of the magnetopause with a speed of 50–100 km/s.