Energetic ion anisotropies in the geomagnetic tail: 1. A statistical survey

Abstract

During 1978 and 1979, the medium energy particles experiment on board the ISEE 2 spacecraft (apogee 22.7 RE) collected almost 122 days of data in the central region of the Earth's magnetotail. We have surveyed 16‐s averages of the energetic ion (E > 25 keV) and plasma measurements in order to establish the statistical properties of high ion anisotropy events (|A| > 1) which are frequently observed in the near‐Earth plasma sheet. Our major findings from the analysis of 1944 high‐anisotropy samples (HAS) are as follows: (1) Strong energetic ion anisotropies are not maintained over long periods of time but occur in bursts lasting for fractions of a minute up to a few minutes. (2) For almost 80% of the HAS, the anisotropy vector points either earthward or tailward rather than across the tail (3) The ratio of HAS toward the Earth and HAS toward the tail is approximately 4.8:1. (4) The probability of observing high‐anisotropy events is well enhanced beyond ≈16 RE downtail distance, on the duskside, and close (within 2–3 RE) to the neutral sheet. (5) Strong tailward anisotropies can be observed anywhere in the near‐Earth plasma sheet. However, their occurrence frequency is considerably above average beyond xSE ≈ −19 RE and within 2 RE of the neutral sheet. (6) Both plasma bulk flow and ion anisotropy at higher energy (>115 keV) are generally in the same direction as the energetic ion anisotropy at E > 25 keV. (7) Streaming energetic electron populations in the same direction as the ions are significantly more often observed when the ions are streaming tailward than simultaneously with earthward streaming ions when the electrons are usually isotropic. We conclude that statistically the observation of strong tailward ion streaming indicates the existence of a particle source in the near‐Earth magnetotail between the spacecraft and the Earth. Certain features suggest that this source might be identical with a near‐Earth magnetic neutral line.