Statistical properties of low-frequency waves and ion beams in the plasma sheet boundary layer: Geotail observations

摘要

Statistical properties of low-frequency (0.01–0.1 Hz) electromagnetic waves and their relations to ion distribution functions in “transition regions” from lobe to the plasma sheet, which include the plasma sheet boundary layer (PSBL) and boundary plasma sheet, are investigated based on 5-year data of the Geotail spacecraft observations in X GSM = [?31, ?15] and ∣Y GSM∣ < 5 R E . It is shown that the amplitude of the magnetic field fluctuations increases with increasing plasma β (the ratio of the plasma thermal pressure to the magnetic pressure), while the electric field amplitude decreases in high-β regions. These tendencies are consistent with a decrease of the local Alfvén velocity (V A ) in the transition region with increasing β. The statistical results also indicate that the low-frequency wave power has clear correlation with the energy flux of ion flows parallel to the magnetic field. If 10% of the beam energy is converted to the wave power, the ion beams could be the source of free energy of the large-amplitude electromagnetic waves. The estimated Poynting flux of the waves is distributed in the range from 1.0 × 10?6 to 5.6 × 10?2 mW/m2. The maximum Poynting flux is the same order of the pointing flux of Alfvén waves observed by the Polar spacecraft at altitudes of 4–7 R E , when mapped along converging magnetic field lines to the ionosphere at an altitude of 100 km. The good agreement of the Poynting fluxes is consistent with the idea that the low-frequency electromagnetic waves in the tail PSBL are the source of kinetic Alfvén waves in the high-latitude auroral regions. The results of partial moment calculations with data bins selected by careful eye inspection for each 12-s ion data during large-amplitude wave events show that in most of the events, the relative drift speed between cold-core and hot-beam ion components is below 2 V A , the density ratio of the cold-core to the hot-beam is typically a few tens of percent, and the beam component has a strong temperature anisotropy of T /T ⊥ ～ 0.44. The linear dispersion analysis using the observed distribution functions suggests the importance of the ion cyclotron anisotropy instability modified by the existence of cold-core ions for the generation of low-frequency large-amplitude electromagnetic waves in the PSBL.