Oblique ring instability driven by nongyrotropic ions: Application to observations at comet Grigg-Skjellerup

We investigate the oblique behavior of low-frequency left-handed electromagnetic waves driven by a ring of nongyrotropic ions. We use the unperturbed orbit integral technique to obtain for the first time the dispersion equation of oblique modes for a nongyrotropic particle distribution in a magnetoplasma. When the oblique propagation angle 8 with respect to the magnetic field increases, the polarization of the unstable wave mode rapidly loses the left hand circular nature of the parallel mode and tends to be linear. Electron density compression and magnetic field compression appear and both maximize at θ∼22°. The difference between gyrotropic ring instability and nongyrotropic ring instability diminishes with the increase of obliquity. The nongyrotropy of the ring ions decreases the growth rate for large wave numbers and to a lesser extent at its maximum. In addition, the nongyrotropy increases slightly the electron density and magnetic field compressions. The growth rate of the mode with θ>0 (k• V_p<0, V_p being the perpendicular plasma flow velocity) is larger than that of the mode with θ<0 (k• V_p>0); this effect does not exist in the gyrotropic case. The plasma parameters used in these theoretical calculations are those observed during the Giotto encounter with comet Grigg-Skjellerup, when nongyrotropic newborn heavy ion ring distributions as well as associated low frequency waves have been reported. The results on the electron density compression are in good quantitative agreement with the observations upstream of the cometary shock.