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 Vu. 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 (Ted ‐ Teu) and the difference in the square of the bulk flow speed (Vu² ‐ Vd²) 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 Ted/Teu) and that the first‐order dependence on (Vu² ‐ Vd²) should be normalized out in future studies of electron heating at space shocks.