Imprints of non-adiabatic ion acceleration in the earth's magnetotail: Interball observations and statistical analysis

In this paper we discuss observations of short (duration 1-2 min) high-velocity field-aligned ion beams (beamlets) in two regions of the Earth's magnetosphere: in the magnetotail and in the high-latitude auroral region made at Interball-1, -2 satellites (1995-1998). We statistically defined the latitudinal sizes of the regions of beamlet localization in the magnetotail and in auroral region and show that beamlets are concentrated inside a rather thin (∼0.8°) interval of latitudes, in the region adjacent to the boundary of the Plasma Sheet. Statistical study of beamlet occurrence frequency and their spatial distribution over the Neutral Sheet performed for different interplanetary conditions reveals that these distributions strongly depend on interplanetary magnetic field clock-angle. Moreover, these distributions look rather different for midnight sector and magnetotail flanks. While at the midnight sector beamlets are observed almost under various IMF directions, at flanks they are registered mainly during periods of strong By component in IMF direction. In general, our results confirm the model of burst ion acceleration in the magnetotail that takes place even during absolutely quiet magnetosphere periods.