Statistical properties of low-frequency waves and ion beams in the plasma sheet boundary layer: Geotail observations
Statistical properties of low-frequency waves and ion beams in the plasma sheet boundary layer: Geotail observations
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DOI：
10.1029/2004ja010395
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发表时间：
2005-02
影响因子：
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通讯作者：
T. Takada;K. Seki;M. Hirahara;M. Fujimoto;Yoshifumi Saito;H. Hayakawa;T. Mukai
中科院分区：
文献类型：
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作者：
T. Takada;K. Seki;M. Hirahara;M. Fujimoto;Yoshifumi Saito;H. Hayakawa;T. Mukai
[1]根据GeoTail航天器在XGSM=[−31，−15]和|YGSM|<；5 RE中的5年观测数据，研究了从波瓣到等离子体片“过渡区”(包括等离子体片边界层和边界等离子体片)低频(0.01-0.1赫兹)电磁波的统计特性及其与离子分布函数的关系。结果表明，随着等离子体β(等离子体热压与磁压之比)的增大，磁场起伏的幅度增大，而高β区的电场起伏幅度减小。这些趋势与过渡区局部阿尔芬速度(VA)随β的增大而减小相一致。统计结果还表明，低频波功率与平行于磁场的离子流能流有明显的相关性。如果将10%的束流能量转换为波能，则离子束可以成为大振幅电磁波的自由能源。估算的坡印亭通量分布在1.0×10−6~5.6×10−2 mW/m2之间。当沿会聚磁力线映射到100公里高度的电离层时，最大Pointing通量与极地航天器在4-7RE高度观测到的Alfven波的指向通量相同数量级。Pointing通量的良好一致性与尾部PSBL中的低频电磁波是高纬度极光区动力Alfven波源的观点一致。对大幅度波事件期间的每个12-S离子数据进行的局部矩计算结果表明，在大多数事件中，冷核和热束离子的相对漂移速度小于2VA，冷核和热束的密度比通常为几十%，束流分量具有很强的温度各向异性，T∥/T⟂∼为0.44。利用观测到的分布函数进行的线性色散分析表明，冷核离子的存在修正的离子回旋各向异性不稳定性对于在PSBL中产生低频大振幅电磁波具有重要意义。
[1] 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 XGSM = [−31, −15] and ∣YGSM∣ < 5 RE. 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 Alfven velocity (VA) 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 Alfven waves observed by the Polar spacecraft at altitudes of 4–7 RE, 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 Alfven 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 VA, 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.