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High-energy radiation originating from thunderclouds can be registered by detectors on satellites and on the ground surface. Intense bursts of photons with energy 10 keV-100 MeV lasting 0.1-5 ms are called terrestrial gamma ray flashes (TGFs) and are usually observed from satellites (Fishman et al., 1994;Mailyan et al., 2016;Østgaard et al., 2019). Thunderstorm ground enhancements (TGEs) and gamma ray glows can be observed under thunderclouds and have a duration of up to several hours (Chilingarian, 2011;A. Gurevich et al., 2016;Torii et al., 2009). The gamma-radiation of thunderclouds is caused by bremsstrahlung of runaway electrons, which accelerate and multiply in the electric field, forming relativistic runaway electron avalanches (RREAs) (

Many physical phenomena in space involve energy dissipation which generally leads to charged particle acceleration, often up to very high energies. In the Earth magnetosphere energy accumulation and release occur in the magnetotail, namely in its Current Sheet (CS). The kinetic analysis of non-adiabatic ion trajectories in the CS region with finite but positive normal component of the magnetic field demonstrated that this region is essentially non-uniform in terms of scattering characteristics of ion orbits and contains spatially localized, well-separated sites of enhanced and reduced chaotization. The latter represent sources from which accelerated and energy-collimated ions are ejected into Plasma Sheet 134 E.E. Grigorenko et al.Boundary Layer (PSBL) and stream towards the Earth. Numerical simulations performed as part of a Large-Scale Kinetic Model have shown the multiplet ion structure of the PSBL is formed by a set of ion beams (beamlets) localized both in physical and velocity space. This structure of the PSBL is quite different from the one produced by CS acceleration near a magnetic reconnection region in which more energetic ion beams are generated with a broad range of parallel velocities. Multi-point Cluster observations in the magnetotail PSBL not only showed that non-adiabatic ion acceleration occurs on closed magnetic field lines with at least two CS sources operating simultaneously, but also allowed an estimation of their spatial and temporal characteristics. In this paper we discuss and compare the PSBL manifestations of both mechanisms of CS particle acceleration: one based on the peculiar properties of non-adiabatic ion trajectories which operates on closed magnetic field lines and the other representing the well-explored mechanism of particle acceleration during the course of magnetic reconnection. We show that these two mechanisms supplement each other and the first operates mostly during quiescent magnetotail periods.

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[1] Several studies, both experimental, analytical, and numerical, show that a substantial electrostatic field E z 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 E z 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 E y is also present to accelerate particles. We find that for electric field E z 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 E z is an overlapping of beamlet energies to form a wide energetic beam. Implications for beamlet observations are discussed.Citation: Dolgonosov, M. S., G. Zimbardo, and A. Greco (2010), Influence of the electric field perpendicular to the current sheet on ion beamlets in the magnetotail,