Anomalous Trapping of Low Pitch Angle Electrons by Coherent Whistler Mode Waves
Anomalous Trapping of Low Pitch Angle Electrons by Coherent Whistler Mode Waves
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DOI：
10.1029/2019ja026493
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发表时间：
2018-12
期刊：
影响因子：
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通讯作者：
M. Kitahara;Y. Katoh
中科院分区：
文献类型：
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作者：
M. Kitahara;Y. Katoh
由相干哨声模波组成的合唱发射是高能电子的俯仰角散射的原因。这种散射与高能电子沉降到大气中密切相关，从而形成脉动极光。传统上，高能电子被认为满足回旋共振条件，能量范围在几到几十千电子伏特之间，并以波的形式向损耗锥散射。然而，以往的仿真研究表明，低俯仰角的电子容易被相干哨声模式波从损耗锥散射出去。我们研究了在低俯仰角下反常捕获的机制，推导了一个具有低俯仰角假设的粒子方程。另一个通常被忽略的项表示由波动磁场和平行粒子速度引起的洛伦兹力。因此，由于v‖×Bw较大的洛伦兹力，满足回旋加速器谐振条件的低俯仰角电子被散射出损耗锥，有效地被波捕获。我们在一维偶极磁场中使用哨声模式波模型进行了测试粒子模拟，并重现了电子的异常捕获。仿真结果表明，高、中俯仰角的电子大部分向低俯仰角区域散射，而低俯仰角的电子则向高俯仰角区域强烈散射。因此，相干合唱元素在电子俯仰角分布中产生一个凸起。
Chorus emissions composed of coherent whistler mode waves are responsible for pitch angle scattering of energetic electrons. This scattering is closely related to energetic electron precipitation into the atmosphere, contributing to pulsating auroras. Conventionally, energetic electrons are considered to satisfy the cyclotron resonance condition over the energy range of a few to tens of kiloelectron volts and are scattered toward the loss cone by waves. However, previous simulation studies indicate that low pitch angle electrons tend to be scattered away from the loss cone by coherent whistler mode waves. We examine the mechanism of anomalous trapping at low pitch angles, deriving a particle equation with low pitch angle assumptions. An additional term that is conventionally neglected represents the Lorentz force caused by the wave magnetic field and the parallel particle velocity. Therefore, due to the large v‖×Bw Lorentz force, low pitch angle electrons satisfying the cyclotron resonant condition are scattered away from the loss cone and effectively trapped by waves. We perform test particle simulations in a one‐dimensional dipole magnetic field with a whistler mode wave model and reproduce the anomalous trapping of electrons. The simulation results show that the majority of electrons at high and moderate pitch angles are scattered toward low pitch angle regions while low pitch angle electrons are strongly scattered toward high pitch angle regions. Consequently, a coherent chorus element produces a bump in the electron pitch angle distribution.