Electron velocity distributions near the Earth's bow shock

Abstract

A survey of two-dimensional electron velocity distributions, f(¾), measured near the earth's bow shock using Los Alamos/Garching plasma instrumentation aboard ISEE 2 is presented. This survey provides clues to the mechanisms of electron thermalization within the shock and the relaxation of both the upstream and downstream velocity distributions. First, near the foreshock boundary, fluxes of electrons having a power law shape at high energies backstream from the shock. Although most often they appear as a monotonically decreasing extension of solar wind distributions in the backward hemisphere along the magnetic field direction, /•, they occasionally appear as a resolved peak in energy. Within the interior of the foreshock, in addition to the hot, isotropic electrons at higher energies, field-aligned depressions in oe (V) are observed at the lowest energies (E • 15 eV) and twin angular peaks centered on/• are observed at intermediate energies (15 eV • E • 45 eV). Such distributions are associated closely with 1-Hz whistler waves. Second, within the shock, cuts through oe(V) along/•, oe(V•), often show single maxima offset toward the magnetosheath by speeds comparable to, but larger than, the upstream thermal speed. When sequences of such distributions ar• observed in a single shock transition, offset speeds increase and peak heights of f(V•) decrease with increasing penetration toward the downstream (magnetosheath) side. Third, magnetosheath distributions generally have flat tops out to an energy, Eo, with maxima substantially lower than that in the solar wind. Occasionally, cuts through f(¾) along/• show one and sometimes two small peaks at the edge of the flat tops making them appear concave upward. The magnetosheath distributions often have strong angular anisotropies which depend on energy. For energies less than Eo, f(V•) > f(V•_) at constant E, whereas for E > Eo, f(V•) < f(V•_). The electron distributions characteristic of these three regions are interpreted as arising from the effects of macroscopic (scale size comparable to or larger than the shock width) electric and magnetic fields and the subsequent effects of microscopic (scale size small in comparison with the shock width) fields. In particular, our results suggest that field-aligned instabilities are likely to be present in the earth's bow shock. 1. INTRODUCTION Much work has been done over the past two decades to understand the physics of collisionless shocks (see e.g., Sagdeer and Galeev [1969], Tidman and Krall [1971], Biskamp [1973], and Galeev [1976]). For any shock above a critical Mach number, Mc, some type of dissipation is necessary to provide a transition between upstream and downstream parameters and satisfy the Rankine-Hugoniot relations [Tidman and Krall , 1971]. Since a collisionless shock has, by definition, a width much less than a mean free path, the dissipation cannot be due to classical collisions. The usual mechanism which is invoked for heating electrons is that of an anomalous, or more precisely, wave-particle dissipation due t...

Electron Temperature Gradient Scale at Collisionless Shocks

“…Interestingly, Fig. 4 shows that features previously reported with the ramp, e.g., the beam vestige of the solar wind peak [28], are present only in the more gradual initial rise that precedes the steep ramp. That beam has been totally eroded by the time this electron scale ramp is encountered.…”

“…3 shows that both the parallel and perpendicular electron temperatures closely track the steep rise in magnetic field, with half the electron heating taking place on a scale of 17.3 km, corresponding to 6.4 electron inertial lengths and a small fraction (0.15) of an ion inertial length. Although much of the electron dynamics is linked to the DC electric and magnetic fields within the ramp [8,24,28,29] and is therefore reversible, the fact that both T e and T e⊥ rise together suggests an infla- tion of the particle phase space distribution that is not reversible, due primarily to the filling in and/or entrapment of electrons in regions of phase space that would otherwise be inaccessible. This infilling can be seen in the cuts of the distributions shown in Fig.…”

Plasma waves in the Earth's electron foreshock: 1. Time‐of‐flight electron distributions in a generalized Lorentzian plasma and dispersion solutions

“…The resulting reduced distributions close to the leading edge of the foreshock display a pronounced peak with a sharp cutoff near the cutoff velocity. While individual observational examples of discernible "bumps" in distributions that may be unstable have been reported [Fitzenreiter et al, 1984[Fitzenreiter et al, , 1990, only stable distributions with "plateaus" are commonly observed [Feldman et al, 1982[Feldman et al, , 1983, which questions the temporal persistence of the bump-on-tail distributions.…”

Dust-acoustic solitary structures in plasmas with nonthermal electrons and positive dust

“…On the other hand, space plasma observations indicate the presence of ion and electron populations which are not in thermodynamic equilibrium (Asbridge et al, 1968;Feldman et al, 1983; Correspondence to: F. Verheest (frank.verheest@ugent.be) Lundin et al, 1989;Futaana et al, 2003). Often, these nonthermal velocity distributions include a ring structure, and the simplest analytical way to model such effects is by the Cairns distribution (Cairns et al, 1995).…”