Survey of energetic (E > 35 keV) ion anisotropies in the deep geomagnetic tail

Abstract

An analysis of the direction and magnitude of the anisotropy of energetic ions (E > 35 keV) in the plasma sheet of the deep geomagnetic tail has been undertaken using data from the energetic particle anisotropy spectrometer on board ISEE 3 during its geotail mission. An ensemble of samples, consisting of 10‐min averages of the data, has been chosen using selection criteria of magnetic field direction and particle intensity such that the samples are predominantly in the plasma sheet or energetic ion boundary layer. No distinction between quiet and disturbed times is made in this study. Tailward and earthward flows are seen with nearly equal probability out to a distance of 100 RE from the earth, beyond which tailward flow dominates. Beyond 150 RE, tailward flow occurs in 95% of the samples. The average derived ion flow velocity in the tailward direction increases steadily with distance to 650 km s−1 at 230 RE. The most common direction of ion anisotropy is 10° duskward of the GSM −X axis. The data are better ordered about this direction than about the direction of the magnetic field vector. This is explained by the presence of a transverse component of ion anisotropy in the duskward direction, the average value of which is independent of the strength of the tailward/earthward flow. It is argued that this transverse anisotropy is caused by a density gradient in the plasma sheet of scale length 2–3 RE.