THE WHISTLER-MODE BOW WAVE OF AN ASTEROID

The Galileo spacecraft has flown by two asteroids, Gaspra in 1990 and Ida in 1993. In both cases the magnetometer detected magnetic field perturbations that are believed to be produced by an interaction of the asteroid with the solar wind. Kivelson et al. have proposed that these perturbations are caused by whistler-mode waves excited by the solar wind flow around the asteroid. This paper presents an analysis of whistler-mode waves generated by the interaction of a small object with the solar wind. Three cases are considered, with the solar wind magnetic field (1) parallel, (2) perpendicular, and (3) at an arbitrary angle to the velocity vector. Using the Cerenkov condition and the quasi-longitudinal approximation for the whistler mode, the angular limits of the wave pattern are determined. For the parallel case the waves are confined to the upstream region, and for the perpendicular case the waves are confined to two wedge-shaped regions extending downstream from the magnetic field line through the object. For intermediate magnetic field orientations the accessibility region maintains the wedge-shaped configuration, with the leading edge of the wedge roughly following the direction of the magnetic field. The outer boundary of the accessibility region is a caustic surface, along which large field amplitudes are expected, similar to the bow wave of a ship. In all cases the interaction involves a characteristic wavelength, lambda(c) = f(c)c(2)/(f(p)(2)V), that is comparable to the size of an asteroid.