Electron velocity distributions near interplantary shocks

Abstract

Solar wind electron velocity distributions measured across interplanetary shocks using the Los Alamos plasma analyzer on ISEE 3 have been studied to understand electron heating mechanisms for weak and intermediate strength collisionless shocks. This study thus complements earlier studies of electron heating at the earth's bow shock, an example of a highly supercritical collisionless shock. At the weakest interplanetary shocks (downstream to upstream density ratio N(d/u) ≲ 2 and velocity difference V(d‐u) ≲ 70 km/s), heating perpendicular to the magnetic field B predominates over parallel heating. Such heating may be a consequence of conservation of magnetic moment across the shock. At the stronger interplanetary shocks (N(d/u) ≳ 2 and V(d‐u) ≳ 70 km/s), heating parallel to B is dominant and the downstream velocity distributions are flat‐topped, similar to what is observed downstream of the earth's bow shock. This similarity suggests that electron heating in all collisionless shocks with N(d/u) ≳ 2 and V(d‐u) ≳ 70 km/s results in part from an acceleration parallel to B produced by the macroscopic shock electric field, followed by beam driven plasma instabilities.