NONGYROTROPY OF HEAVY NEWBORN IONS AT COMET GRIGG-SKJELLERUP AND CORRESPONDING INSTABILITY

The existence of the nongyrotropic velocity distribution of newborn io ns is a very important feature for comet Grigg-Skjellerup (P/G-S); thi s kind of distribution may be a reservoir of free energy and play a ra ther important role in the generation of plasma waves observed at P/G- S, especially close to the comet. In this paper we predict the distrib ution function of the newborn ion population, taking into account the source term for this population (implantation from neutrals) and the l oss term (pickup, i.e., velocity diffusion leading to assimilation in the solar wind). We integrate the equation backward along the solar wi nd plasma flow line modified by the ion gyration motion using the obse rvations of the Giotto spacecraft at P/G-S on July 10, 1992. We obtain the nongyrating zero-order nongyrotropic velocity distribution of the newborn ions which depends on the average pickup time of implanted io ns and on the distance from the comet. The stability of the above equi librium is then studied by adding linear perturbations and solving num erically the inferred dispersion equation. We show that this kind of d istribution function triggers a left-handed instability which can be c alled ''nongyrotropic ring instability'' and which arises from the cou pling between the two transverse electromagnetic modes and the longitu dinal electrostatic mode.