Triggering process of whistler mode chorus emissions in the magnetosphere
Triggering process of whistler mode chorus emissions in the magnetosphere
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DOI：
10.1029/2010ja016280
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发表时间：
2011-05-11
影响因子：
2.8
通讯作者：
Nunn, David
中科院分区：
文献类型：
--
作者：
Omura, Yoshiharu;Nunn, David
合唱团的发射是由磁层中高能电子（10-100 keV）的温度各向异性驱动的线性回旋不稳定性触发的。在磁赤道附近，由于速度空间中电磁电子空穴的存在，合唱辐射以绝对非线性不稳定性的形式增长。从恒定频率的线性波增长到音调频率上升的非线性波增长的过渡过程是由于形成了与波磁场反平行的谐振电流-J（B）。上升频率在赤道处的电子空穴引入相移，并导致与波电场-J（E）反平行的谐振电流分量，这导致非线性波增长。为了证实这种触发机制，我们进行了Vlasov混合模拟与J（B）和没有J（B）。没有J（B）的运行不再现合唱发射，而具有J（B）的运行成功地再现合唱发射。由J（B）引起的非线性频移ω（1）在触发过程中起关键作用。发现频率偏移的非线性过渡时间T-N与非线性捕获周期具有相同的数量级，这一点通过模拟和观测得到了证实。所建立的频率扫描速率为ω（1）/T-N，这给出了合唱发射的最佳波幅。
Chorus emissions are triggered from the linear cyclotron instability driven by the temperature anisotropy of energetic electrons (10-100 keV) in the magnetosphere. Chorus emissions grow as an absolute nonlinear instability near the magnetic equator because of the presence of an electromagnetic electron hole in velocity space. The transition process from the linear wave growth at a constant frequency to the nonlinear wave growth with a rising tone frequency is due to formation of a resonant current -J(B) antiparallel to the wave magnetic field. The rising-tone frequency introduces a phase shift to the electron hole at the equator and results in a resonant current component antiparallel to the wave electric field -J(E), which causes the nonlinear wave growth. To confirm this triggering mechanism, we perform Vlasov hybrid simulations with J(B) and without J(B). The run without J(B) does not reproduce chorus emissions, while the run with J(B) does successfully reproduce chorus emissions. The nonlinear frequency shift omega(1) due to J(B) plays a critical role in the triggering process. The nonlinear transition time T-N for the frequency shift is found to be of the same order as the nonlinear trapping period, which is confirmed by simulations and observation. The established frequency sweep rate is omega(1)/T-N, which gives an optimum wave amplitude of chorus emissions.