Statistical properties of substorm auroral onset beads/rays
Statistical properties of substorm auroral onset beads/rays
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DOI：
10.1002/2016ja022801
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发表时间：
2016-09
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影响因子：
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通讯作者：
Y. Nishimura;J. Yang;P. Pritchett;F. Coroniti;Eric Donovan;L. Lyons;Richard A. Wolf;V. Angelopoulos;S. Mende
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文献类型：
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作者：
Y. Nishimura;J. Yang;P. Pritchett;F. Coroniti;Eric Donovan;L. Lyons;Richard A. Wolf;V. Angelopoulos;S. Mende
极光亚暴在初始增亮期间（亚暴极光起始波）通常与光线或珠状结构有关。利用在亚暴期间事件的时间历程和宏观相互作用（THEMIS）全天空成像仪数据中识别出的112次亚暴事件，对亚暴起始波的发生概率和特性进行了表征，并与赖斯对流模型 - 平衡态（RCM - E）以及动力学不稳定性特性进行了比较。发现所有亚暴起始都与光波有关，因此，光波是亚暴起始的一个常见特征。向东传播的波事件比向西传播的波事件更频繁，并且往往发生在低纬度亚暴期间（更强的太阳风驱动）。波的传播方向由初始增亮弧的方向所决定。我们还确定了向西和向东传播的波在波速、波长（波数）、周期和持续时间方面存在显著差异。相比之下，波的增长率不取决于传播方向或亚暴强度，而是与波的持续时间成反比。这表明波在达到一定强度阈值时会演变为向极区扩展，并且波的特性与亚暴强度没有直接关系。然而，波对于调节亚暴增长阶段和向极区扩展之间的转换仍然很重要。与弧方向的关系可以用RCM - E中的磁尾结构来解释，这表明相对于压力峰值的亚暴起始位置决定了波的传播方向。测量到的波的特性与动力学气球模交换不稳定性非常吻合，而交叉场电流不稳定性和电磁离子回旋不稳定性给出的传播速度要大得多，波周期要小得多。
Auroral substorms are often associated with optical ray or bead structures during initial brightening (substorm auroral onset waves). Occurrence probabilities and properties of substorm onset waves have been characterized using 112 substorm events identified in Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky imager data and compared to Rice Convection Model–Equilibrium (RCM‐E) and kinetic instability properties. All substorm onsets were found to be associated with optical waves, and thus, optical waves are a common feature of substorm onset. Eastward propagating wave events are more frequent than westward propagating wave events and tend to occur during lower‐latitude substorms (stronger solar wind driving). The wave propagation directions are organized by orientation of initial brightening arcs. We also identified notable differences in wave propagation speed, wavelength (wave number), period, and duration between westward and eastward propagating waves. In contrast, the wave growth rate does not depend on the propagation direction or substorm strength but is inversely proportional to the wave duration. This suggests that the waves evolve to poleward expansion at a certain intensity threshold and that the wave properties do not directly relate to substorm strengths. However, waves are still important for mediating the transition between the substorm growth phase and poleward expansion. The relation to arc orientation can be explained by magnetotail structures in the RCM‐E, indicating that substorm onset location relative to the pressure peak determines the wave propagation direction. The measured wave properties agree well with kinetic ballooning interchange instability, while cross‐field current instability and electromagnetic ion cyclotron instability give much larger propagation speed and smaller wave period.