Ion and electron heating at collisionless shocks near the critical Mach number

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Abstract
The evolution of the ion and electron distribution functions across a set of 10 low‐Mach number, nominally subcritical, quasi‐perpendicular shocks is examined in high time resolution (full distribution every 3 s) using data from the ISEE 1 and 2 spacecraft. Both ions and electrons sometimes show slight preheating upstream of the shock, but otherwise the ion and electron temperatures rise together in the magnetic ramp and show no further increase downstream. Contrary to the usual assumption based on early laboratory and theoretical work that at subcritical shocks the bulk of the energy dissipation occurs as resistive heating of the electrons, it is found that the ion temperature increase exceeds that of the electrons. This difference is attributed to the distinction between dispersive shocks, such as those studied here, and resistive shocks, such as those observed in most laboratory studies. The increase in ion temperature is predominantly in the perpendicular direction and is due to heating of the entire distribution rather than to the formation of a high‐energy tail. The perpendicular temperature increase is typically a factor of 10–20, much greater than the usual assumption of adiabatic heating. The downstream to upstream ratio of perpendicular electron temperature is equal to the magnetic field ratio (∼2–2.5). The electrons also show significant heating in the parallel direction, with the downstream T∥/T⊥ ∼1–1.2. The downstream electron distribution exhibits the characteristic flattop seen downstream of supercritical shocks, and there is evidence for the field‐aligned electron beam identified previously within those shocks. As previously reported, the downstream ion and electron total temperatures are nearly equal. These observations are interpreted as evidence for the simultaneous operation of several plasma instabilities, including the modified two‐stream instability, driven by the cross‐field current within the shock, and the ion acoustic instability, driven by the field‐aligned electron beam.