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

Abstract

Several studies, both experimental, analytical, and numerical, show that a substantial electrostatic field Ez perpendicular to the magnetotail current sheet can be found. This electric field has a typical bipolar structure. We propose a theoretical analysis describing the ion behavior in the vicinity of the current sheet under the influence of Ez in a two‐dimensional taillike magnetic field reversal. It is shown that for some initial parameters resonant acceleration is possible, forming beamlets (small‐scale and almost monoenergetic field‐aligned ion beams). To check the analytical results, a test particle simulation has been performed, where the dawn‐dusk electric field Ey is also present to accelerate particles. We find that for electric field Ez pointing away (or toward) the current sheet, the beamlet resonant regions are shifted toward the Earth (or away from the Earth) resulting in decreasing (or increasing) typical energy of ions leading to the modification of the universal scaling predicted by Zelenyi et al. (2007). Another effect due to the presence of Ez is an overlapping of beamlet energies to form a wide energetic beam. Implications for beamlet observations are discussed.