Tailward Propagation of Magnetic Energy Density Variations With Respect to Substorm Onset Times
Tailward Propagation of Magnetic Energy Density Variations With Respect to Substorm Onset Times
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DOI：
10.1029/2017ja025147
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发表时间：
2018-06
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影响因子：
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通讯作者：
J. Coxon;M. Freeman;C. Jackman;C. Forsyth;I. J. Rae;R. Fear
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作者：
J. Coxon;M. Freeman;C. Jackman;C. Forsyth;I. J. Rae;R. Fear
在地磁亚暴期间，大约1015 J的能量从太阳风中提取并由地球的磁层处理。在亚暴膨胀阶段开始之前，这种能量被认为主要是作为磁尾叶磁场的增加而储存的。然而，如何，何时以及在磁尾内存储和释放这些能量还不清楚。使用数据从集群航天器和亚暴发作从亚暴发作和阶段指数的电喷流（SOPHIE），我们研究的变化叶磁能密度相对于亚暴发作541孤立发作。基于互相关分析和一个简单的模型，我们推导出以下结论：平均而言，磁能密度在发病前一小时内近似线性增加，发病后以类似的速度下降。这些变化的时间和幅度随尾下距离的变化而变化，从中尾（X <$-9 RE）的观测显示，在近尾（X <$-9 RE）变化后约20分钟，磁能密度发生了较大的变化。在SOPHIE确定的地面爆发之前，观测到近尾区域的能量密度下降，这意味着亚暴是从磁尾驱动并传播到电离层的。这些结果的含义是，在亚暴膨胀阶段，近尾区域的能量首先释放，能量转换随着时间的推移远离地球传播。
During geomagnetic substorms, around 1015 J of energy is extracted from the solar wind and processed by the Earth's magnetosphere. Prior to the onset of substorm expansion phases, this energy is thought to be largely stored as an increase in the magnetic field in the magnetotail lobes. However, how, when, and where this energy is stored and released within the magnetotail is unclear. Using data from the Cluster spacecraft and substorm onsets from Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE), we examine the variation in the lobe magnetic energy density with respect to substorm onset for 541 isolated onsets. Based on a cross‐correlation analysis and a simple model, we deduce the following: On average, the magnetic energy density increases approximately linearly in the hour preceding onset and decreases at a similar rate after onset. The timing and magnitude of these changes varies with downtail distance, with observations from the mid‐tail ( X≾−9RE ) showing larger changes in the magnetic energy density that occur ∼20 min after changes in the near‐tail ( X≿−9RE ). The decrease in energy density in the near‐tail region is observed before the ground onset identified by SOPHIE, implying that the substorm is driven from the magnetotail and propagates into the ionosphere. The implication of these results is that energy in the near‐tail region is released first during the substorm expansion phase, with energy conversion propagating away from the Earth with time.