Strong electron heating at the Earth's bow shock

Abstract

The electron heating at collisionless shocks in near-Earth space is normally found to be relatively small. We report here on two sets of bow shock crossings observed by the ISEE 1 and ISEE 2 spacecraft in which very large electron temperature increases were found. The two sets of shocks are part of a larger set of 52 bow shock crossings compiled from the ISEE data set. When the shocks with the large electron heating are compared to the rest of the shocks in the compiled set, it is found that the only upstream parameter which is out of the ordinary is the upstream solar wind flow speed V u. Indeed, for the entire shock set the only upstream parameters which correlate well with the electron heating are the solar wind speed and quantities derived from it. The best correlation among those tested is between the temperature increase (Tea-Teu ) and the difference in the square of the bulk flow speed (V• 2 -Va 2) across the shock, which is proportional to the total amount of bulk flow energy dissipated by the shock. A subsequent search for bow shock crossings which occur during intervals of high solar wind speed confirms that the electron heating is very large under high-speed conditions. The first-order dependence of the electron temperature increase on the available bulk flow energy is consistent with a heating process which is dominated by the macroscopic cross-shock electrostatic potential jump. This study suggests that the temperature difference is the appropriate measure of electron heating at shocks (rather than the ratio T•a/T•,,) and that the first-order dependence on (V•2 _ Va2) should be normalized out in future studies of electron heating at space shocks.

Role of overshoots in the formation of the downstream distribution of adiabatic electrons

“…The prevailing view on the electron heating in shocks is that it is due to the quasi-static electric field in the shock front [Feldman, 1985;Thomsen et al, 1987;Schwartz et al, 1988]. The model suggested for not very thin shocks is based on the conservation of the magnetic moment in the weakly inhomogeneous (at the scale much larger than the electron gyroradius) magnetic field and energy conservation in the de Hoffman-Teller frame, where the cross-shock potential depends only on the coordinate along the shock normal [Feldman et al, 1982;Goodrich and Scudder, 1984;Feldman, 1985;Scudder et al, 1986c;Schwartz et al, 1988;Scudder, 1995].…”

Ion and electron heating at the Martian bow shock. Common for bow shocks or not?

“…However, it could be explained by rather high solar wind with bulk velocities of 790 km/sec and 550 km/sec on 1 February 1989 and 4 February 1989, respectively. Indeed, a few observations made in the near-Earth under the high-speed conditions in the solar wind (V sw > 550 km/sec) reveal also very large increase of the electron temperature (T d /T u > 10) at the bow shock which exceeds the jump of the magnetic field (Thomsen et al, 1987). In spite of poor statistics of electron observations onboard the Phobos-2 spacecraft, general similarities between the electron behavior at Earth and Mars can be stated.…”

Analytic model for the electrostatic potential jump across collisionless shocks, with application to Earth's bow shock

“…It is widely known that the reflection, heating and acceleration of charged particles at collisionless magnetosonic shock waves are strongly influenced by the cross–shock electrostatic potential jump Δϕ [ Feldman et al , 1982; Scudder et al , 1986a, 1986b, 1986c; Thomsen et al , 1987; Schwartz et al , 1988; Hull et al , 1998, 2000; Hull and Scudder , 2000]. In particular, the cross–shock potential is fundamental to understanding particle energetics and dynamics; this is true not only for thermal and mildly superthermal particles, but also for highly superthermal particles undergoing significant shock acceleration [ Zank et al , 1996; Lee et al , 1996].…”