Latitude-energy structure of multiple ion beamlets in Polar/TIMAS data in plasma sheet boundary layer and boundary plasma sheet below 6 R_E radial distance: basic properties and statistical analysis

摘要

Velocity dispersed ion signatures (VDIS) occurring at the plasma sheet boundary layer (PSBL) are a well reported feature. Theory has, however, predicted the existence of multiple ion beamlets, similar to VDIS, in the boundary plasma sheet (BPS), i.e. at latitudes below the PSBL. In this study we show evidence for the multiple ion beamlets in Polar/TIMAS ion data and basic properties of the ion beamlets will be presented. Statistics of the occurrence frequency of ion multiple beamlets show that they are most common in the midnight MLT sector and for altitudes above 4 R_E, while at low altitude (≤ 3 R_E), single beamlets at PSBL (VDIS) are more common. Distribution functions of ion beamlets in velocity space have recently been shown to correspond to 3-dimensional hollow spheres, containing a large amount of free energy. We also study correlation with ～100 Hz waves and electron anisotropies and consider the possibility that ion beamlets correspond to stable auroral arcs.