Energy spectra and angular distributions of particles in the plasma sheet and their comparison with rocket measurements over the auroral zone

Abstract

This paper presents examples of electron and proton differential energy spectra between ∼100 ev and 18 kev measured with hemispherical plate electrostatic analyzers on the Vela satellites at a geocentric distance of ∼18 RE in the magnetotail plasma sheet. The spectra were obtained under several conditions of geomagnetic activity: (a) during very quiet conditions, (b) during the expansive phase of substorms, and (c) following a sudden impulse (si) at the earth. Most of the published auroral particle differential energy spectra measured by rockets are sketched in a single figure for easy comparison with the plasma sheet spectra. The plasma sheet particle fluxes generally, but not always, appear nearly isotropic (to within a factor of 2) to a relatively wide-angle detector such as the analyzer used here. Comments (a) through (e) below cite features of these sample spectra, referring to averages of measurements made in the antisolar direction and perpendicular to this, i.e., directionally ‘averaged’ results. This is not a statistical study and we do not claim that the features cited are strictly ‘typical’ for the corresponding geomagnetic conditions. But we know, from observation of hundreds of spectra, that these are fairly representative samples and that the conclusions of this study would remain essentially unchanged upon selection of another group of spectra obtained under the same range of geomagnetic conditions. (a) Plasma sheet electron spectra are usually smooth single-peaked distributions often resembling Maxwellian distributions from several hundred ev to a few kev and frequently (especially during magnetically active periods) becoming power-law spectra above several kev. (b) In quiet times peak electron intensities were ∼108 el/cm2 sec ster kev and occurred from one hundred to several hundred ev. Following polar magnetic substorms the peak electron intensities were ∼107 el/cm2 sec ster kev and occurred at ∼1 to a few kev. The electron number density varied between these two conditions about inversely as the electrons' average energy so that the electron energy density changed relatively little, remaining at ∼30 ev/cm3 ster. (c) Proton peak fluxes during quiettimes were ∼ 106 p/cm2 sec ster kev and appeared near or below 1 kev. Following substorms the peak flux was ∼105 p/cm2 sec ster kev and occurred near 10 kev. (d) After a sudden impulse the electron and proton energy densities rose to unusually high values (3 to 4 times the values seen during quiettimes and following substorms). This was achieved by the fluxes reaching their quiettime values (∼108 and 106 particles/cm2 sec ster kev, respectively) but with peak energies several times higher than the quiettime values, (e) The electron and proton fluxes measured at rocket altitudes typically exceed fluxes in the plasma sheet by factors of 10 to 100 over most of the energy range (∼100 ev to 18 kev) of the Vela measurements. At rocket altitudes the electron spectra often have secondary intensity peaks in the range of a few kev in sharp contrast to the usual smooth single-peaked distributions in the plasma sheet. (f) A careful analysis of the data shows that the angular distribution of the plasma sheet electrons can be sharply peaked along the presumed direction of the magnetic field lines; certain unique characteristics of the measurements with the Vela electrostatic analyzers permitted the detection of this feature and a rough measure of the degree of the anisotropy. In one example studied, the peak in the angular distribution was ∼15° wide (full width, half maximum), and the intensity parallel to the magnetic field was ≳27 times the intensity perpendicular to it. We infer that at these times the protons and also perhaps higher energy electrons (Ee>45 kev) have similarly peaked angular distributions. The much higher fluxes of all of these particles measured by rockets simply reflect the fact that the rockets measure only the flux in the very narrow (∼1°) loss cone of the plasma sheet angular distribution, while the detectors (generally wide-angle) in the plasma sheet average over a large fraction of the total angular distribution.