Chorus intensity modulation driven by time‐varying field‐aligned low‐energy plasma
Chorus intensity modulation driven by time‐varying field‐aligned low‐energy plasma
复制标题
由时变场对准低能等离子体驱动的合唱强度调制
DOI：
10.1002/2015ja021330
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发表时间：
2015
期刊：
影响因子：
--
通讯作者：
J. Bonnell
中科院分区：
文献类型：
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作者：
Y. Nishimura;J. Bortnik;W. Li;J. Liang;R. Thorne;V. Angelopoulos;O. Le Contel;U. Auster;J. Bonnell
最近的研究表明，合唱波负责散射和沉淀驱动脉动极光的高能电子。虽然一些合唱强度调制事件与<~100 eV的电子密度调制相关，但午夜后扇区中的大多数合唱强度调制事件发生时没有明显的密度变化。虽然由于航天器电势和静电分析仪（ESA）能量范围限制的限制，通常很难测量低能（<20 eV）电子通量的演变，但我们使用事件时间历史和亚暴期间的大尺度相互作用（THEMIS）卫星数据确定，约100 eV的低能离子显示出与合唱强度调制相关的密度调制。这些低能离子和电子与0°（对于北方冬季事件）和180°（对于北方夏季事件）俯仰角中的主峰场对准，表明来自阳光照射的半球的流出等离子体是赤道附近低能等离子体密度调制的来源。等离子体片等离子体密度，和周围的电场和磁场不显示调制与合唱强度调制。假设电荷中性，低能量离子可以用来表示波增长率计算中的冷等离子体密度，并且发现低能量等离子体密度的增强对合唱线性增长率的贡献最有效。这些结果表明，合唱强度调制是由一个反馈过程，其中流出的等离子体由于高能电子沉淀增加赤道密度，驱动进一步的电子沉淀。
Recent studies have shown that chorus waves are responsible for scattering and precipitating the energetic electrons that drive the pulsating aurora. While some of the chorus intensity modulation events are correlated with <~100 eV electron density modulation, most of the chorus intensity modulation events in the postmidnight sector occur without apparent density changes. Although it is generally difficult to measure evolution of low‐energy (<~20 eV) electron fluxes due to constraints imposed by the spacecraft potential and electrostatic analyzer (ESA) energy range limit, we identified using Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite data that low‐energy ions of ~100 eV show density modulation that is correlated with chorus intensity modulation. Those low‐energy ions and electrons are field‐aligned with major peaks in 0° (for northern hemisphere winter event) and 180° (for northern hemisphere summer event) pitch angle, indicating that outflowing plasma from the sunlit hemisphere is the source of the low‐energy plasma density modulation near the equator. Plasma sheet plasma density, and ambient electric and magnetic fields do not show modulations that are correlated with the chorus intensity modulation. Assuming charge neutrality, the low‐energy ions can be used to represent cold plasma density in wave growth rate calculations, and the enhancements of the low‐energy plasma density are found to contribute most effectively to chorus linear growth rates. These results suggest that chorus intensity modulation is driven by a feedback process where outflowing plasma due to energetic electron precipitation increases the equatorial density that drives further electron precipitation.