Nonlinear wave growth theory of coherent hiss emissions in the plasmasphere
Nonlinear wave growth theory of coherent hiss emissions in the plasmasphere
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DOI：
10.1002/2015ja021520
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发表时间：
2015-05
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影响因子：
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通讯作者：
Y. Omura;Satoko Nakamura;C. Kletzing;D. Summers;M. Hikishima
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文献类型：
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作者：
Y. Omura;Satoko Nakamura;C. Kletzing;D. Summers;M. Hikishima
最近由Van Allen探测器对等离子体层HISS发射的观测表明，等离子体层中的宽带HISS发射由具有上升和下降音频的短时相干元素组成。基于哨声模式合唱发射的非线性波增长理论，考察了非线性理论对相干嘶嘶发射的适用性。我们将触发升调合唱发射的最佳波幅的推导推广到升调和降调嘶鸣元素的情况。发出嘶嘶声的幅度分布与触发上升或下降音调的最佳波幅很好地接近。通过形成上升音调的电子空穴和下降音调的电子丘陵，相干波的演化达到最佳幅度。电磁粒子模拟证实了非线性波增长机制是嘶嘶声产生过程的初始阶段。我们发现，作为瞬时频率的函数，理论上的最佳振幅与观测到的振幅之间有很好的一致性。我们计算了赤道的非线性增长率，发现上升色调发射的非线性增长率远远大于线性增长率。观测到的HISS辐射的时间尺度也与非线性理论所预测的时间尺度一致。根据这一理论，我们可以推断出等离子体层赤道区产生嘶嘶发射的高能电子的性质。
Recent observations of plasmaspheric hiss emissions by the Van Allen Probes show that broadband hiss emissions in the plasmasphere comprise short‐time coherent elements with rising and falling tone frequencies. Based on nonlinear wave growth theory of whistler mode chorus emissions, we have examined the applicability of the nonlinear theory to the coherent hiss emissions. We have generalized the derivation of the optimum wave amplitude for triggering rising tone chorus emissions to the cases of both rising and falling tone hiss elements. The amplitude profiles of the hiss emissions are well approximated by the optimum wave amplitudes for triggering rising or falling tones. Through the formation of electron holes for rising tones and electron hills for falling tones, the coherent waves evolve to attain the optimum amplitudes. An electromagnetic particle simulation confirms the nonlinear wave growth mechanism as the initial phase of the hiss generation process. We find very good agreement between the theoretical optimum amplitudes and the observed amplitudes as a function of instantaneous frequency. We calculate nonlinear growth rates at the equator and find that nonlinear growth rates for rising tone emissions are much larger than the linear growth rates. The time scales of observed hiss emissions also agree with those predicted by the nonlinear theory. Based on the theory, we can infer properties of energetic electrons generating hiss emissions in the equatorial region of the plasmasphere.