“Phobos events”—Signatures of solar wind interaction with a gas torus?

Following recent simulations of the Phobos dust belt formation (Krivov and Hamilton, 1997), the effective dust-induced charge density as estimated is too small to account for the significant solar wind (sw) plasma and magnetic field perturbations observed by the Phobos-2 spacecraft in 1989 near the crossings of the Phobos orbit. In this paper the sw interaction with the Phobos neutral gas torus is re-investigated in a two-ion plasma model in which the newly created ions are treated as unmagnetized, forming a beam (not a ring beam) in the sw frame. A linear instability analysis based on both a cold fluid and a kinetic approach shows that electromagnetic ion beam waves in the whistler range of frequencies, driven most unstable at oblique propagation and appearing as almost purely growing waves in the beam frame, aquire high growth rates and provide a likely mechanism to cause the observed events.