Abstract
Abstract. We have studied the occurrence characteristics of bi-directional electron pitch angle anisotropy (enhanced flux in field-aligned directions, F^ /F|| > 1.5) at energies of 0.1–30 keV using plasma and magnetic field data from the AMPTE/IRM satellite in the near-Earth plasma sheet. The occurrence rate increases in the tailward direction from XGSM = - 9 RE to - 19 RE . The occurrence rate is also enhanced in the midnight sector, and furthermore, whenever the elevation angle of the magnetic field is large while the magnetic field intensity is small, B ~ 15 nT. From these facts, we conclude that the bi-directional electrons in the central plasma sheet are produced mainly in the vicinity of the neutral sheet and that the contribution from ionospheric electrons is minor. A high occurrence is also found after earthward high-speed ion flows, suggesting Fermi-type field-aligned electron acceleration in the neutral sheet. Occurrence characteristics of bi-directional electrons in the plasma sheet boundary layer are also discussed.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; magnetotail; plasma sheet)
Highlights
Bi-directional electron pitch angle anisotropy is commonly observed in the nightside magnetosphere (e.g. Klumpar, 1993 and references therein). Hada et al (1981) were the first to analyze the field-aligned bi-directional electrons systematically at radial distances of ∼8–30 RE using data from the IMP-6 satellite
We investigate the occurrence characteristics of the bi-directional electrons with a parallel and perpendicular flux ratio of more than 1.5 in the plasma sheet at |XGSM| = 9 − 19 RE using data obtained by the AMPTE/IRM satellite
We have studied the occurrence characteristics of fieldaligned bi-directional electrons with F /F⊥ > 1.5 observed by the AMPTE/IRM satellite in the near-Earth plasma sheet at |XGSM| = 9 − 19 RE
Summary
Bi-directional electron pitch angle anisotropy is commonly observed in the nightside magnetosphere (e.g. Klumpar, 1993 and references therein). Hada et al (1981) were the first to analyze the field-aligned bi-directional electrons systematically at radial distances of ∼8–30 RE using data from the IMP-6 satellite. We investigate the occurrence characteristics of the bi-directional electrons with a parallel and perpendicular flux ratio of more than 1.5 in the plasma sheet at |XGSM| = 9 − 19 RE using data obtained by the AMPTE/IRM satellite. CPS and PSBL are defined as the regions of ne < 0.94 · n0p.86 and ne > 0.94·n0p.86, where ne and np denote measured electron and ion densities including low-energy photoelectrons, respectively. Using data from the AMPTE/IRM satellite for February– May, 1985 and March–June, 1986, we examine the dependence of the bi-directional electron occurrence on different plasma sheet regions, magnetic activity, local magnetic field, and high-speed ion flow. The occurrence rate of bi-directional electrons was defined by normalizing the number of bi-directional electron distributions (events) by the total number of distribution samples in the relevant regions
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