“Geography” of ion acceleration in the magnetotail: X‐line versus current sheet effects

Abstract

Field‐aligned ion beams are often observed in the Plasma Sheet Boundary Layer (PSBL) of Earth's magnetotail. We studied two types of ion beams observed in the magnetotail at different times. Both types of ion beams are pitch angle‐collimated along the magnetic field direction but they differ significantly with the width of ion velocity distribution functions in parallel energies. The first type represents energy‐collimated ion beams with observed durations of 3–23 min. The majority of such beams have energies ≤20 keV. Together with such ion beams, isotropic electron distributions are observed. We suggest that such ion beams are accelerated at spatially localized regions located rather far from the distant X‐line where magnetic field lines are already closed. Another type of PSBL ion distributions represents wide in parallel energy ion beams with typical energies ≥ tens of keV. The registered durations of such beams are 1–6 min. They are observed together with anisotropic electron velocity distributions formed by the cold and hot counter‐streaming components. This feature is peculiar for the magnetic separatrix and indicates that the spacecraft crosses still open or very recently closed magnetic field lines. We have analyzed 987 crossings of the high‐latitude edge of the PSBL by Geotail at −220 RE < X < −20 RE. The majority of Type‐I ion beams are moving earthward at X ≥ −110 RE and so have their sources located earthward from the distant X‐line (at X < −110 RE). These ion beams were observed during quiet geomagnetic periods. More than 50% of Type‐II ion beams move tailward at X ≤ −50 RE. Thus their acceleration sources are located closer to Earth. Type‐II ion beams were observed during both quiet and disturbed periods. The energy distribution of the first type of ion beams along the dawn‐dusk direction approximately conforms to the assumption about their non‐adiabatic acceleration by the quasi‐steady dawn‐dusk electric field. On the other hand, we suggest that ion beams of the second type are generated in close proximity to the X‐line. The energy distribution of such ion beams along the Y direction indicates ion energization by a strong (presumably inductive) electric field, especially in cases of ion acceleration tailward from the X‐line.

Acceleration and transport of ions in turbulent current sheets: formation of non-maxwelian energy distribution

“…The figures correspond to the case v ⊥ >0 (for v ⊥ <0, the distributions are mirror symmetric). In-situ measurements also reveal that, in the vicinity of active CS regions, the ion distributions may display larger energies as well as larger velocity dispersion (particle temperature) than the particles could get due to electrostatic effect even if all cross-tail potential drop would be converted to particle energy (Grigorenko et al, 2009). According to our model, the observed effects result from particle acceleration by inductive electric fields in the DM case (Fig.…”

“…Moreover, the presence of TEMF allows to explain additional particle accelerations (different from acceleration due to E y ) often observed in velocity distribution on open field lines (Grigorenko et al, 2009). …”

Formation of the high-energy ion population in the earth's magnetotail: spacecraft observations and theoretical models

“…For specific regions in the magnetotail, such acceleration can be effective for a large number of particles. In this case, accelerated particles form so-called beamlets: beams of almost monoenergetic particles observed in the magnetosphere boundary layer (e.g., Bosqued et al, 1993;Ashour-Abdalla et al, 2006;Zelenyi et al, 2007;Grigorenko et al, 2009). For the magnetotail configuration far from the X line, particles can gain 10-20 keV due to this mechanism.…”

Proton and heavy ion acceleration by stochastic fluctuations in the Earth's magnetotail

“…In addition, the observed proton temperatures, of the order of 5-10 keV, are also much larger than the energy corresponding to their original source, but the mechanism that can generate such heated particles is not fully understood (Runov et al, 2006;Artemyev et al, 2011). Grigorenko et al (2009) pointed out that 100 keV ions are accelerated near reconnecting regions, possibly by a process related to time-dependent, patchy reconnection. Spacecraft observations suggest that both electrons and ions are accelerated not only in the vicinity of the reconnection X line but also in a larger area around the reconnection region (e.g., Imada et al, 2007;Grigorenko et al, 2009;Ono et al, 2009).…”

“…Grigorenko et al (2009) pointed out that 100 keV ions are accelerated near reconnecting regions, possibly by a process related to time-dependent, patchy reconnection. Spacecraft observations suggest that both electrons and ions are accelerated not only in the vicinity of the reconnection X line but also in a larger area around the reconnection region (e.g., Imada et al, 2007;Grigorenko et al, 2009;Ono et al, 2009). Indeed, during disturbed periods, strong electric (Cattel and Mozer, 1982), velocity and magnetic fluctuations (e.g., Hoshino et al, 1994;Borovsky et al, 1997;Zimbardo et al, 2010), as well as energetic ions, are observed in the magnetotail.…”