Stochastic and direct acceleration mechanisms in the Earth's magnetotail

Ion beams with energies of the order of several tens of keV are frequently observed in the Earth's magnetotail. Here we consider two possible acceleration mechanisms, the cross tail electric field E_y and the stochastic acceleration due to the electromagnetic fluctuations present in the magnetotail. Electromagnetic perturbations are generated by random oscillating ``clouds'' moving in the x - y plane. A test particle simulation has been performed in order to reproduce the interaction between protons and electromagnetic fluctuations and the constant dawn-dusk electric field, E_y, in the magnetotail current sheet. Protons are accelerated via a stochastic Fermi-like process and, by varying the features of the electromagnetic fluctuations, the combined effect of E_y and of the moving clouds can explain the typical energy range of ion observations from 20 to 100 keV, and the characteristic acceleration times of several minutes.