Intermediate electromagnetic turbulence at comets

Low-frequency electromagnetic turbulence observed in cometary environments mostly peaks at the gyrofrequency of the water-group ions of cometary origi n, because of cyclotron resonance fueled by relative drifts parallel to the interplanetary magnetic field. Nevertheless, some of the observations poin t to pickup proton cyclotron turbulence, to single-cycle magnetic pulses or solitary waves at the proton gyrofrequency, and also to intermediate frequ encies between the water-group and the proton gyrofrequencies. The generati on of turbulence at such frequencies has been attributed to cascade via wav e-wave interactions, but here an alternative and direct mechanism for the g eneration of these waves is suggested in an unstable wavelength band when t he solar wind is sufficiently mass-loaded by cometary material. It is shown , for average parameters near comet 1P/Halley, that unstable modes of the i ntermediate frequency kind can indeed occur up to several million kilometer s from the cometary nucleus and are easier to excite in higher-velocity sol ar wind flows.