Instabilities of Alfven and magnetosonic waves in dusty cometary plasmas with an ion ring beam

The effects of charged dust grains on the instabilities of electromagnetic waves in the solar wind plasma due to ring-beam cometary ion velocity distr ibutions are considered. The waves are assumed to propagate parallel to the solar wind magnetic field. Collisionless cold plasma theory is used to der ive the dispersion relations of linear waves in the solar wind, for a delta -function ring-beam velocity distribution of cometary water-group ions. The resonant and nonresonant instabilities are considered, and it is shown tha t the dust has its greatest effect on the nonresonant firehose instability at very long wavelength. The dust can cause an unstable dust-free firehose mode to be stable, and vice versa. It is also shown that as the frequency a pproaches the cometary ion cyclotron frequency, there is an upper limit to the wave number for the unstable firehose mode. For the left-hand polarized wave, the presence of dust increases this upper wave number limit, until t he firehose mode merges with the resonant mode. The right-hand polarized fi rehose mode also has an upper wave number limit, which is almost independen t of the presence of dust. It is shown that, although there is a high frequ ency resonant instability of the normal whistler mode induced by the ring b eam, there is no corresponding instability of the dust-induced whistler-lik e mode at low frequencies. (C) 1999 American Institute of Physics. [S1070-6 64X(99)03101-8].