近地磁尾方位角流能量转换过程

Energy conversion by azimuthal flow in the near-Earth magnetotail

高速流是磁尾等离子体片中质量、能量和磁通量最重要的输运形式.高速流在地向运动到近地磁尾后流速会降低,主导方向也经常转为晨昏方向.在等离子体流速及其主导分量、密度和温度等宏观参数变化的过程中,可能伴随有不同类型的能量转换.为此,我们分析近地磁尾方位角流事件期间的能量转换过程.基于整体流速的主导分量为晨昏向,及事件期间的平均温度比其前后增强的选取原则,从2008-2020年期间THEMIS中的三颗卫星(THA,THD,THE)运行在磁尾时的观测数据中,共筛选出821个平均温度比其前/后(事件之前或事件之后10 min)升高的方位角流事件.2011年5月16日的方位角流事例研究发现:能量超过1 keV尤其是超过10 keV的高能离子通量增加,可导致低速低温等离子体的密度、温度和流速增加;磁场减小及磁场功率谱密度的分析表明等离子体和磁能的转换发生在频率为0.01~0.334 Hz的低频波动,仅1%能量耗散.统计给出方位角流事件的平均温度约为3.7 keV,比其前后增加的幅度大部分集中在100~1000 eV.事件期间的平均数密度可能升高或者降低,二者概率大致相当.约96%的平均温度和密度同时大于其前后的方位角流事件,当地等离子体通量管熵也皆大于事件的前后值;电磁能流密度增大,34%的事件期间电磁能流密度主要沿着背景磁力线方向,可影响电离层内方位角方向上的极光膨胀过程.我们的研究结果表明温度和密度同时增加的方位角流事件等离子体宏观参数变化可能暗含磁层-电离层能量耦合过程,有助于理解极光椭圆带赤道向边界的局部增亮现象和磁尾等离子体片中的磁通量.

High-speed flows play the most important role in the transport of mass, energy and magnetic flux in the magnetotail plasma sheet. When earthward fast flows move closer to the Earth, the bulk flow velocity decreases, and the predominantly earthward flow often diverts into the dawn-dusk direction. Taking into account the significant variations of the flow velocity, its dominant component, density, temperature and other macroscopic parameters, the temporal-spatial evolution of the plasma flow could be accompanied by various energy conversions. To clarify this, we investigate the energy conversion process of the azimuthal flow event in the near Earth magnetotail. Based on the dominant component of the flow was dawn-dusk, we selected 821 azimuthal flow events measured by the three satellites(THA, THD and THE) of THEMIS in 2008-2020 for which the average temperature during these events was enhanced. One typical event occurred on the 16th May in 2011 indicated that the enhancement of energetic particle flux with energies above 1 keV(and especially 10 keV) led to an increase of bulk flow velocity, density, and temperature of the cold and slow flows. The magnetic field in this event also decreased and the corresponding magnetic power spectral density indicated a conversion of plasma to magnetic energy occurred at low frequencies from 0.01 to 0.334 Hz. Only 1% of the total energy was dissipated. Statistical results showed that the average temperature among all the selected events was 3.7 keV mainly accompanied by 100~1000 eV temperature increase compared with that seen 10 mins before and after each event(for similar intervals). The average number densities during the event can be larger or smaller than those before and after, and the probability was roughly the same. More than 50% of the events whose average thermal energy density is simultaneously larger than those before and after, which is consistent with the trends for the entropy of a local plasma flux tube relative to the previous and the latter intervals. About 96% of these events whose average temperature and density are simultaneously larger than those before and after the events, have a local plasma flux-tube entropy which is larger than both of those before and after accompanied by enhanced electromagnetic energy density. Energy conversion was dominant during these events with a very few percentages of energy dissipation, furthermore, the enhanced electromagnetic energy density during 34% of these cases were mainly field-aligned and might affect the auroral expansion process in the azimuthal direction. Our research suggests that changes of the plasma macroscopic parameters may imply a magnetospheric-ionospheric energy coupling process, which will be helpful for understanding phenomena such as the local aurora breakup at the equatorward boundary of the auroral oval and magnetic flux in the plasma sheet of the magnetotail.