A computational and theoretical analysis of falling frequency VLF emissions
A computational and theoretical analysis of falling frequency VLF emissions
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DOI：
10.1029/2012ja017557
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发表时间：
2012-08
影响因子：
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通讯作者：
D. Nunn;Y. Omura
中科院分区：
文献类型：
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作者：
D. Nunn;Y. Omura
[1]近年来，在升频触发发射和升调合唱的模拟和理论研究方面取得了很大进展。PIC和Vlasov VHS码都在VLF波传播的赤道下游区域产生了振荡。VHS代码只产生瀑布或向下钩困难，由于波粒子相互作用的相干性质跨越赤道。使用VHS代码，我们现在将相互作用区域限制在赤道上游的区域，其中不均匀因子S为正。这就抑制了赤道上的相关波粒相互作用效应和代码触发降音的倾向，并允许形成适当的降调生成区域。VHS编码现在可以轻松地、可重复地触发降调。共振粒子流JE在空间和时间上的演化显示了在−5224 km处的生成点，并且波场在穿过非线性生成区域时经历了大约25 dB的放大。平行于波磁场（JB）的电流分量为正，而平行于波磁场（JB）的电流分量为负。共振粒子阱显示出增强的分布函数或“山”，而共振粒子阱有一个“洞”。根据最近的理论（Omura等人，2008年，2009年）扫描频率主要是由于平流项。非线性频移项现在是负的（10 - 12 Hz），并且−800 Hz/s的扫描速率近似为非线性频移除以TN，TN是俘获时间量级的过渡时间。
[1] Recently much progress has been made in the simulation and theoretical understanding of rising frequency triggered emissions and rising chorus. Both PIC and Vlasov VHS codes produce risers in the region downstream from the equator toward which the VLF waves are traveling. The VHS code only produces fallers or downward hooks with difficulty due to the coherent nature of wave particle interaction across the equator. With the VHS code we now confine the interaction region to be the region upstream from the equator, where inhomogeneity factor S is positive. This suppresses correlated wave particle interaction effects across the equator and the tendency of the code to trigger risers, and permits the formation of a proper falling tone generation region. The VHS code now easily and reproducibly triggers falling tones. The evolution of resonant particle current JE in space and time shows a generation point at −5224 km and the wavefield undergoes amplification of some 25 dB in traversing the nonlinear generation region. The current component parallel to wave magnetic field (JB) is positive, whereas it is negative for risers. The resonant particle trap shows an enhanced distribution function or ‘hill’, whereas risers have a ‘hole’. According to recent theory (Omura et al., 2008, 2009) sweeping frequency is due primarily to the advective term. The nonlinear frequency shift term is now negative (∼−12 Hz) and the sweep rate of −800 Hz/s is approximately nonlinear frequency shift divided by TN, the transition time, of the order of a trapping time.