Statistical characterization of the growth and spatial scales of the substorm onset arc.
Statistical characterization of the growth and spatial scales of the substorm onset arc.
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DOI：
10.1002/2015ja021470
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发表时间：
2015-10
影响因子：
2.8
通讯作者：
Owen, C. J.
中科院分区：
文献类型：
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作者：
Kalmoni, N. M. E.;Rae, I. J.;Watt, C. E. J.;Murphy, K. R.;Forsyth, C.;Owen, C. J.
我们提出了第一个关于极光时空结构的多事件研究，以提供导致亚暴爆发弧的近地等离子体不稳定的统计证据。利用地面极光成像仪的数据，我们研究了沿弧极光珠的可重复特征，这些特征被认为代表了近地等离子体片中磁层不稳定性的电离层投影。我们发现，这些波状波动的增长和空间尺度在多个事件中是相似的，这表明每次极光突然变亮都有一个共同的解释。我们从统计上发现，极光珠的生长速率在低波数处达到峰值，最不稳定的空间尺度映射到赤道磁层中大约 9-12 R E 的方位角波长 λ≈ 1700-2500 km。我们将生长速率和空间尺度与一系列磁尾不稳定性的理论预测进行比较，包括交叉场电流不稳定性和剪切流气球不稳定性。我们的结论是，尽管交叉场流不稳定性可以产生类似幅度的增长率，但不稳定波数的范围表明剪切流气球不稳定性是我们观测结果最可能的解释。极光珠是沿着亚暴爆发弧的可重复特征 极光珠的亮度在广泛的空间尺度上呈指数增长 在剪切流气球预测的那些空间尺度上增长率达到峰值
We present the first multievent study of the spatial and temporal structuring of the aurora to provide statistical evidence of the near‐Earth plasma instability which causes the substorm onset arc. Using data from ground‐based auroral imagers, we study repeatable signatures of along‐arc auroral beads, which are thought to represent the ionospheric projection of magnetospheric instability in the near‐Earth plasma sheet. We show that the growth and spatial scales of these wave‐like fluctuations are similar across multiple events, indicating that each sudden auroral brightening has a common explanation. We find statistically that growth rates for auroral beads peak at low wave number with the most unstable spatial scales mapping to an azimuthal wavelength λ≈ 1700–2500 km in the equatorial magnetosphere at around 9–12 R E. We compare growth rates and spatial scales with a range of theoretical predictions of magnetotail instabilities, including the Cross‐Field Current Instability and the Shear Flow Ballooning Instability. We conclude that, although the Cross‐Field Current instability can generate similar magnitude of growth rates, the range of unstable wave numbers indicates that the Shear Flow Ballooning Instability is the most likely explanation for our observations. Auroral beads are a repeatable signature along the substorm onset arc Bead brightness grows exponentially across a wide range of spatial scales Growth rates peak at those spatial scales predicted by shear flow ballooning