Low‐frequency instabilities and the resulting velocity distributions of pickup ions at comet Halley

Abstract

The interaction between the solar wind and newborn cometary ions is studied using a new analytical theory as well as one‐ and two‐dimensional hybrid simulations. Using the observed parameters upstream of the comet Halley, a detailed study of wave excitation and the resulting particle 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 Alfvén ion cyclotron waves can be excited. However, parallel propagating waves are found to be dominant in the wave spectrum and to control the scattering of the pickup ions. Several features of the observed distributions of pickup protons are explained. In particular, it is shown that the observed asymmetric pitch angle distribution for the pickup protons is due to the small saturation amplitude of the waves for the given parameters. Water group associated waves can lead to energy diffusion and further pitch angle scattering of protons. This effect is most likely to be important 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 shown that the observed increase in the radius of the proton velocity shell just outside the bow shock can be due to water group waves. The nearly 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 the magnetic field data. Just outside the bow shock, simulations show that parallel propagating water group waves can steepen with attached whistler wave packets. The steepening process at parallel propagation is a transient effect, in an important contrast to the case of steepening at oblique angles. The smaller beam densities at comet Halley appears to be the main reason not only why waves at comet Halley have smaller amplitudes but also why oblique, steepening magnetosonic waves have not been detected at comet Halley, whereas they have been seen at comet Giacobini‐Zinner.