Influence of the electric field perpendicular to the current sheet on ion beamlets in the magnetotail

摘要

Several studies, both experimental, analytical, and numerical, show that a substantialelectrostatic field E_zperpendicular to the magnetotail current sheet can be found. Thiselectric field has a typical bipolar structure. We propose a theoretical analysis describingthe ion behavior in the vicinity of the current sheet under the influence of E_zin atwo-dimensional taillike magnetic field reversal. It is shown that for some initialparameters resonant acceleration is possible, forming beamlets (small-scale and almostmonoenergetic field-aligned ion beams). To check the analytical results, a test particlesimulation has been performed, where the dawn-dusk electric field Ey is also presentto accelerate particles. We find that for electric field E_zpointing away (or toward) thecurrent sheet, the beamlet resonant regions are shifted toward the Earth (or awayfrom the Earth) resulting in decreasing (or increasing) typical energy of ions leading tothe modification of the universal scaling predicted by Zelenyi et al. (2007). Anothereffect due to the presence of E_zis an overlapping of beamlet energies to form a wideenergetic beam. Implications for beamlet observations are discussed.