Decay of the cometary bow shock

Abstract

Cooling processes will weaken the bow shock that is expected to form when the solar wind plasma encounters a gassy comet. As the supersonic wind penetrates the comet's outer coma, accreting freshly ionized cometary atoms and molecules, it needs a shock to adjust to the inner subsonic conditions. The cometary ions, implanted in the plasma stream, are accelerated by the associated fields and take up much of the decrease in streaming energy. The subsonic flow in comets is distinguished by strong cooling, effected primarily through ion–molecule reactions between the energetic implanted ions and the neutral gas coma. We argue here that such cooling can cause complete decay of the shock's flanks, as probed by the ICE (International Cometary Explorer) spacecraft at comet Giacobini–Zinner.