Particle acceleration at planetary bow shock waves

Abstract

We can extend our understanding of collisionless shocks by comparing their behaviour in a variety of plasma conditions at several different planets. One property of such shocks is the occurrence of upstream magnetohydrodynamic waves associated with particle beams accelerated at these shocks, and flowing back towards the Sun¹. We report here observations of one of these classes of wave at Mercury, Venus, Earth and Jupiter. First, we show that the empirical relationship between the interplanetary field strength and the frequency of the waves in the observer's rest frame, derived originally from ground-based pulsation studies, and later, in situ, is approximately true at all the planets. Second, the observed frequencies are consistent with resonance with beams of ions of essentially the same energy at each of the planets. This observation is consistent with Sonnerup's² geometrical model of ion reflection at collisionless shocks. The universality of this ion acceleration mechanism in the Solar System suggests that like acceleration occurs in similar astrophysical systems outside our Solar System and thus may provide a source of acceleration for cosmic rays.