Consequences of strong ion acceleration in current sheets and due to reconnection

Current sheet acceleration as well as reconnection are two major ion acceleration processes in space plasma and, therefore, their general characteristics have been well investigated in the past. Now, however, with the new opportunities of modern satellite missions, a much better energy, angular and time resolution of particle distribution functions is available rather than it was in the past. This causes a demand of improved theoretical understanding of the formation of particle distribution functions in space and velocity space. We currently have theoretically derived some appropriate fine features of beam distribution functions of ions, accelerated by reconnection (Buchner, 1996) and in two-dimensional current sheets (Buchner and Kuska, 1996). Our theory describes strong acceleration processes, the common regime of magnetotail acceleration. It corresponds to the acceleration of initially cold ions to much higher energies in a one-step process. The quantitative criterion for strong acceleration is that the characteristic deHofmann-Teller frame velocity has to exceed the initial thermal velocity of the particles. In strong acceleration phase bunching takes place. It controls the distribution of accelerated particles in space and velocity space. As a result the observed ion distribution functions carry important information about the acceleration process itself. Here in this paper we demonstrate the predicted consequences of strong acceleration by means of test particle calculations. In particular we present the distinguishing features of current sheet acceleration and reconnection: Cup-like distributions versus multiple ring structures. The distinctive features of ion beam distributions, caused by strong acceleration and phase,bunching, provide new tools for diagnosing acceleration processes and electric fields in space plasma. (C) 1998 COSPAR. Published by Elsevier Science Ltd.