Ground‐satellite correlations during presubstorm magnetic field configuration changes and plasma sheet thinning in the near‐Earth magnetotail

Abstract

Observations of electron precipitation were made with balloon‐borne X ray detectors and with riometers in Scandinavia on August 9 and 17, 1968, while Ogo 5 was inbound from the magnetotail making observations of the plasma sheet. Clear substorm growth phase effects were observed in terms of changes in field configuration, particle intensity, and pitch angle distributions at Ogo 5 and in the enhanced electron precipitation observed on the ground. For each of the five substorm studies a clear southward turning of the interplanetary magnetic field preceded enhanced plasma sheet thinning, contrasted with the natural thinning during substorm recovery at distances 10–15 RE from the earth near midnight. Pi 2 magnetic pulsations along with mid‐latitude and auroral zone magnetograms established substorm expansion onset. During three of the substorms, conditions were ideal for observing precipitation. The observed precipitation is readily interpreted in terms of eastward azimuthally drifting electrons encountering an increasingly taillike field configuration during the substorm growth phase, a situation resulting in scattering of the electrons near the neutral sheet owing to the taillike magnetic field configuration. As the thinning proceeds, the precipitation region near the earth moves to the lower latitudes, later to move to higher latitudes during expansion. The results are quite in agreement with current growth phase models, at least in the case of isolated substorms.