Stochastic Fermi acceleration in the magnetotail current sheet:A numerical study

摘要

A numerical study has been carried out in order to investigate the effects of modeltime-dependent electromagnetic fields along with stationary, constant electric andmagnetic field components on the ion dynamics. In the Earth's magnetotail, the trajectoriesof thermal protons are integrated in a 2-D model where they can experience a Fermi-likeacceleration process by interacting with synthetic oscillating electromagnetic clouds,randomly positioned within the xy plane. Besides the time-dependent fluctuations, adawn-dusk electric field component E_(0y)is present, as well as a constant out-of-planemagnetic field component B_n. When the size of the electromagnetic clouds is smallcompared to the simulation box sides, the proton acceleration is mostly due to the dawn-duskelectric field; increasing B_ncauses a reduction of the obtained energies, reduction whichdecreases when B_nis further increased. For larger sizes of the electromagnetic blobs,Fermi acceleration becomes more efficient, and proton energies up to 100 keV areobtained. In such a case, the influence of the constant fields E_(0y)andB_nbecomes weak,and the final energy distribution does not depend on B_n. This means that the stochasticFermi acceleration can be efficient even in the presence of a background magnetic field.Since the typical energies of protons are in good agreement with those observed forproton beams in the Earth's magnetotail, the joint effects of the random fields and thesteady ones can explain particle acceleration processes in the distant and the near-Earthmagnetotail regions.