Accelerated cometary ions observed downstream of the comet Halley bow shock

Abstract

Fluxes of energetic ions with energies exceeding 100 keV were observed upstream of the bow shock of comet Halley by the Tünde instrument which was on board the VEGA 1 spacecraft. Downstream of the shock, ion fluxes in the energy range 100 to 800 keV were observed. Cometary ions, such as O+, newly picked up by the solar wind have energies of about 15 keV in the solar wind frame of reference; hence the measured ion fluxes indicate that acceleration processes must have been operating near comet Halley. The measured ion fluxes were transformed into distribution functions in the solar wind frame using a variety of assumptions concerning the energy dependence of the distribution function and the identity of the ion species. The derived distribution function upstream of the shock falls off steeply with energy between 100 and 150 keV, with an effective temperature of about 7 keV or spectral index about −15. The distribution function increases with decreasing cometocentric distance, on average, reaching a maximum at the bow shock. Downstream of the shock, the spectra can be represented in two ways: (1) one population with a single spectral index based on a power law energy dependence or (2) two exponential function populations with different effective temperatures. In the latter case a soft component exists with an effective temperature of about 30 keV for energies less than about 250 keV and a harder component exists for higher energies with an effective temperature of about 100 keV. Downstream of the shock, the ion fluxes decrease with decreasing cometocentric distance. The measured distribution functions are compared with those obtained by similar instruments on Giotto and the International Cometary Explorer as well as with the predictions of several theoretical models that employ different acceleration mechanisms.