The comet/solar wind transition region at Giacobini‐Zinner

Between about 7 and 12 × 10⁴ km from the nucleus of Comet Giacobini‐Zinner, the International Cometary Explorer (ICE) found a region in which the solar wind flow speed decreased and the electron density and temperature increased by factors of ∼2. This transition region was characterized by large fluctuations in the plasma parameters and by highly variable electron velocity distributions. Electron temperature and density variations through the transition region reveal that ICE never crossed a short‐scalelength bow shock during the encounter, although the scalelength for the gross transition in plasma properties is not incompatible with a shock dominated by the length scales of cometary ions. However, many of the electron distributions in the transition region and sheath are similar to those seen behind weak collisionless shocks elsewhere in the heliosphere. A model of the comet/solar wind interaction is suggested in which a standing shock exists sunward of the ICE trajectory, but, due to large variations in the upstream conditions, it is highly variable and perhaps only intermittent along the flanks.