Proton acceleration at two-dimensional dipolarization fronts in the magnetotail

Transient reconnection events in the planetary magnetotail give rise to fast plasma jets, whose leading edges are called dipolarization fronts. We perform a test particle simulation of ion acceleration in a 2-D model of dipolarization fronts. We study the dependence of acceleration on several parameters of the model. We obtain that the interaction between the dipolarization front and particles depends on their initial energy h(0), resulting in a moderate increase of the gained energy with h(0). The front amplitude B-max has been varied from 10 nT to 40 nT in order to study its influence on the particle gyro-radii and, in turn, on interaction with the front. As expected, the energy gained by particles increases with B-max. We find that the acceleration strongly depends on the front velocity v, which we chose to vary from 200 km/s to 750 km/s. The simulations pointed out that higher energies have been reached with fast fronts, ranging from 50 to 100 keV. We also study how the average maximum energy attained by ions, h(max), depends on the y extent of the front: at variance with expectations, the energy gain decreases with the increase of the y size of the front for small values of that size; increasing further, the scaling of h(max) with the y size seems to indicate a slow growth. We also study the energy and velocity distributions functions of ions accelerated by dipolarization fronts and compare these distributions with spacecraft observations.