Plasma Sheet Pressure Variations in the Near-Earth Magnetotail During Substorm Growth Phase: THEMIS Observations
Plasma Sheet Pressure Variations in the Near-Earth Magnetotail During Substorm Growth Phase: THEMIS Observations
复制标题
亚暴增长阶段近地磁尾的等离子体片压力变化：THEMIS 观测
DOI：
10.1002/2017ja024603
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发表时间：
2017
影响因子：
2.8
通讯作者：
Shen X C
中科院分区：
文献类型：
--
作者：
Sun W J;Fu S Y;Wei Y;Yao Z H;Rong Z J;Zhou X Z;Slavin J A;Wan W X;Zong Q G;Pu Z Y;Shi Q Q;Shen X C
我们利用亚暴期间事件和大尺度相互作用的时间历史(THEMIS)观测了亚暴生长阶段近地磁尾(半径距离R，从7.5RE到12RE，磁当地时间，MLT，从18：00到06：00)中等离子体片压力的变化。结果发现，在亚暴发展阶段，约39.4%(76/193)的入选事件出现赤道等离子体压(Peq)下降现象。晨间(04：00~06：00)和黄昏(18：00~20：00)出现率明显高于午夜地区(20：00~04：00，40%)。亚暴发育期最大降雨量百分比的平均值在黎明和黄昏侧(约为−20%)大于午夜区域(约为−16%)。亚暴发展末期峰值增高率的平均值在午夜前(22：00~00：00)最高(~40%)，而在黎明和黄昏两侧基本不变。进一步研究表明，在亚暴增长期结束时，有13.0%的事件与增长期前相比有10%以上的峰值下降，有28.0%的事件有较小的变化(<10%)，有59.0%的事件有10%以上的上升。本研究还揭示了电子压强(Pe)在亚暴增长期Peq值变化中的重要作用。这些变化往往占Peq变化的50%以上，而且Peto离子压比往往表现出很大的变化(~150%)。在所研究的事件中，在增长阶段观测到了赤道等离子体对流的增强，它在午夜尾部发散，并以~20 Km/S的速度向日侧磁层方位向传播，提出亚暴增长阶段近地等离子体层的Peq减少可能是由于日侧磁层顶重联驱动的闭合磁通量向日侧磁层输送所致。太阳风和电离层电导率效应都可能影响研究区峰值减少事件的发生率和峰值增加百分比的分布。
We investigate the plasma sheet pressure variations in the near‐Earth magnetotail (radius distance,R, from 7.5REto 12REand magnetic local time, MLT, from 18:00 to 06:00) during substorm growth phase with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. It is found that, during the substorm growth phase, about 39.4% (76/193) of the selected events display a phenomenon of equatorial plasma pressure (Peq) decrease. The occurrence rates ofPeqdecrease cases are higher in the dawn (04:00 to 06:00) and dusk (18:00 to 20:00) flanks (> 50%) than in the midnight region (20:00 to 04:00, < 40%). The mean values of the maximum percentages ofPeqdecrease during the substorm growth phases are larger in the dawn and dusk flanks (~ −20%) than in the midnight region (~ > −16%). The mean value ofPeqincrease percentages at the end of substorm growth phase is the highest (~ 40%) in the premidnight MLT bin (22:00 to 00:00) and is almost unchanged in the dawn and dusk flanks. Further investigations show that 13.0% of the events have more than 10% ofPeqdecrease at the end of substorm growth phase comparing to the value before the growth phase, and ~ 28.0% of the events have small changes (< 10%), and ~ 59.0% events have a more than 10% increase. This study also reveals the importance of electron pressure (Pe) in the variation ofPeqin the substorm growth phase. ThePevariations often account for more than 50% of thePeqchanges, and the ratios ofPeto ion pressure often display large variations (~ 50%). Among the investigated events, during the growth phase, an enhanced equatorial plasma convection flow is observed, which diverges in the midnight tail region and propagates azimuthally toward the dayside magnetosphere with velocity of ~ 20 km/s. It is proposed that thePeqdecreases in the near‐Earth plasma sheet during the substorm growth phase may be due to the transport of closed magnetic flux toward the dayside magnetosphere driven by dayside magnetopause reconnection. Both solar wind and ionospheric conductivity effects may influence the distributions of occurrence rates forPeqdecrease events and thePeqincrease percentages in the investigated region.