Statistical properties of beamlets in the Earthʹs magnetotail Original Research Article

Plasma sheet boundary layer represents the complicated dynamical region filled by the transient plasma structures. We used 4-year observations of CORALL plasma analyzer onboard Interball-1 spacecraft to infer the statistical distribution of these structures and its relation to interplanetary conditions. Short lived ion beams with the bursty appearance (beamlets) observed at this transitional region are moving mainly earthward and their velocity dependence on the distance from the neutral sheet (ΔZ) is strongly controlled by the IMF direction. During quiet periods and when IMF has southward direction normal velocity dependence on ΔZ is observed statistically. When IMF is northward or during active periods statistical velocity dependence of beamlets on ΔZ is inverse. Occurrence frequency of beamlet-like structures during quiet periods and southward IMF maximizes at ΔZ = 5 – 6 Re over the nominal position of the neutral sheet, which corresponds to the boundary between open and closed field lines, and could be mapped to the position of the acceleration region in the distant magnetotail. For these periods the asymmetry in the beamlet energy distribution over Y GSM direction is also observed. Beamlets with highest energies are registered mostly in the dawn flank while beamlets with lower energies are observed in the dusk flank. During disturbed periods statistical ΔZ-Profile of beamlet velocities indicates the existence of at least 2 acceleration sites (e.g. at the near-Earth and distant X-lines). When the IMF turns northward beamlets could be observed at very high altitudes above the neutral sheet. We discuss possible mechanisms explaining these statistical properties of beamlet distributions.