UPSTREAM WAVE ACTIVITY AT COMET P GRIGG-SKJELLERUP/

Comet P/Grigg-Skjellerup is a comet with a moderate gas production rat e. Quasi-linear theory, which has successfully described waves upstrea m of the bow shock of comet P/Halley (a comet with a much higher gas p roduction rate), is not appropriate in this case. In this paper, we de velop the nonlinear theory of laminar MHD wave activity upstream of th e bow shock of P/Grigg-Skjellerup using a combination of analytical an d numerical methods. The nonlinear saturation mechanism is the trappin g of cometary ions, resulting in their bounce oscillations over pitch angle. For the specific case of the Giotto encounter with P/Grigg-Skje llerup, during which the solar wind propagation was almost transverse to the magnetic field, the excitation of left-hand circularly polarize d waves propagating toward the comet is initially dominant. Later on, as a result of pitch angle scattering of resonant particles, we find t hat waves propagating toward the Sun are also excited. The numerical s olutions show that under the action of waves propagating in both direc tions the cometary ions acquire a spread in energy. The results of thi s theoretical analysis are compared with measurements made during the Giotto encounter with P/Grigg-Skjellerup and are found to be in good q ualitative agreement.