LOW-FREQUENCY INSTABILITIES AND THE RESULTING VELOCITY DISTRIBUTIONS OF PICKUP IONS AT COMET HALLEY

The interaction between the solar wind and newborn cometary ions is st udied using a new analytical theory as well as one- and two-dimensiona l hybrid simulations. Using the observed parameters upstream of the co met Halley, a det ailed study of wave excitation and the resulting par ticle distributions is presented. Linear theory as well as simulations show that a variety of modes such as the fast magnetosonic mode, high frequency whistlers and obliquely propagating Alfven ion cyclotron wa ves can be excited. However, parallel propagating waves are found to b e dominant in the wave spectrum and to control the scattering of the p ickup ions. Several features of the observed distributions of pickup p rotons are explained. In particular, it is shown that the observed asy mmetric pitch angle distribution for the pickup protons is due to the small saturation amplitude of the waves for the given parameters. Wate r group associated waves can lead to energy diffusion and further pitc h angle scattering of protons. This effect is most likely to be import ant in the vicinity of the bow shock of comet Halley where the density of water group ions becomes comparable to that of protons. It is show n that the observed increase in the radius of the proton velocity shel l just outside the bow shock can be due to water group waves. The near ly isotropic proton pitch angle distribution observed by Neugebauer et al. [1989] just outside the bow shock may, however, be related to the presence of a rotational discontinuity which has been identified in t he magnetic field data. Just outside the bow shock, simulations show t hat parallel propagating water group waves can steepen with attached w histler wave packets. The steepening process at parallel propagation i s a transient effect, in an important contrast to the case of steepeni ng at oblique angles. The smaller beam densities at comet Halley appea rs to be the main reason not only why waves at comet Halley have small er amplitudes but also why oblique, steepening magnetosonic waves have not been detected at comet Halley, whereas they have been seen at com et Giacobini-Zinner.