Nongyrotropic distribution of pickup ions at comet P/Grigg‐Skjellerup: A possible source of wave activity

Abstract

The pickup of newborn cometary ions in the solar wind excites strong wave activity. At comet P/Grigg‐Skjellerup the interplanetary magnetic field is nearly perpendicular to the solar wind velocity; thus the pickup ions should form a ring distribution. In opposition to P/Halley, comet P/Grigg‐Skjellerup is rather small, and its inhomogeneity scale is even smaller than the pickup scale. Therefore the ring will be filled incompletely, and the distribution becomes nongyrotropic. The dispersion of a nongyrotropic cometary pickup distribution in a cold proton‐electron background plasma is studied. Nongyrotropy couples the gyrotropic eigenmodes and drives an instability. Left‐hand polarized waves close to the cometary ion cyclotron frequency are excited.