Quasi‐steady, marginally unstable electron cyclotron harmonic wave amplitudes
Quasi‐steady, marginally unstable electron cyclotron harmonic wave amplitudes
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DOI：
10.1002/jgra.50319
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发表时间：
2013-06
期刊：
影响因子：
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通讯作者：
Xiao‐jia Zhang;V. Angelopoulos;B. Ni;R. Thorne;R. Horne
中科院分区：
文献类型：
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作者：
Xiao‐jia Zhang;V. Angelopoulos;B. Ni;R. Thorne;R. Horne
电子回旋谐波（ECH）一直被认为是地球磁尾扩散极光的潜在驱动力。然而，在外磁尾强烈的ECH排放的稀缺性表明，我们的理解的放大和这些波的电子散射的相对重要性是缺乏的。我们对波增长和准线性扩散进行了全面的研究，以估计一旦扩散和增长平衡但在对流或损失改变背景热等离子体片种群之前损失锥驱动的ECH波的振幅。我们预计这将是最常见的状态之间的快速对流的情节等离子体片。对于任何给定的波振幅，我们使用二维反弹平均Fokker-Planck方程模拟与ECH波相互作用引起的电子扩散。在将得到的电子分布拟合为多分量减去双麦克斯韦的叠加之后，我们使用HOTRAY射线追踪代码估计最大路径积分增益。我们认为，在准稳态期间的波振幅是增益振幅曲线上的拐点。在准稳态期间，ECH波振幅在L ~ 8处可能很大（~1 mV/m），但在外磁尾（L ~ 16）中下降到非常低的值（<~0.1 mV/m），并且可能低于相对接近地球的典型仪器的灵敏度，主要是因为损失锥很小。我们的研究结果加强了潜在的重要作用，ECH波在驱动扩散极光，并建议，仔细比较理论波振幅估计和观测需要解决赤道散射机制的扩散极光降水。
Electron cyclotron harmonic (ECH) waves have long been considered a potential driver of diffuse aurora in Earth's magnetotail. However, the scarcity of intense ECH emissions in the outer magnetotail suggests that our understanding of the amplification and the relative importance of these waves for electron scattering is lacking. We conduct a comprehensive study of wave growth and quasi‐linear diffusion to estimate the amplitude of loss‐cone‐driven ECH waves once diffusion and growth balance but before convection or losses alter the background hot plasma sheet population. We expect this to be the most common state of the plasma sheet between episodes of fast convection. For any given wave amplitude, we model electron diffusion caused by interaction with ECH waves using a 2‐D bounce‐averaged Fokker‐Planck equation. After fitting the resultant electron distributions as a superposition of multicomponent subtracted bi‐Maxwellians, we estimate the maximum path‐integrated gain using the HOTRAY ray‐tracing code. We argue that the wave amplitude during quasi‐steady state is the inflection point on a gain‐amplitude curve. During quasi‐steady state, ECH wave amplitudes can be significant (~1 mV/m) at L ~ 8 but drop to very low values (<~0.1 mV/m) in the outer magnetotail (L ~ 16) and likely fall below the sensitivity of typical instrumentation relatively close to Earth mainly because of the smallness of the loss cone. Our result reinforces the potentially important role of ECH waves in driving diffuse aurora and suggests that careful comparison of theoretical wave amplitude estimates and observations is required for resolving the equatorial scattering mechanism of diffuse auroral precipitation.