Whistler Waves Associated With Electron Beams in Magnetopause Reconnection Diffusion Regions
Whistler Waves Associated With Electron Beams in Magnetopause Reconnection Diffusion Regions
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DOI：
10.1029/2022ja030882
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发表时间：
2022-09
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通讯作者：
Shan Wang;N. Bessho;D. Graham;O. Le Contel;F. Wilder;Y. Khotyaintsev;K. Genestreti;B. Lavraud
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作者：
Shan Wang;N. Bessho;D. Graham;O. Le Contel;F. Wilder;Y. Khotyaintsev;K. Genestreti;B. Lavraud
在与电子束有关的磁层顶重联中，经常可以观察到呼啸波。我们分析了电子扩散区(EDR)周围的七个MMS交叉点，以研究电子束在哨声激发中的作用。波有两种主要类型：(A)椭圆度高的窄带波和(B)椭圆度和波法向角显著变化的静电性更强的宽带波。虽然这两种类型的波都与电子束有关，但关键的区别是背景粒子的各向异性，分别具有垂直和平行的各向异性。线性不稳定性分析表明，第一类波主要是由背景各向异性引起的，束流贡献了额外的回旋共振来促进波的增长。第二种类型的宽带波通过朗道共振被激发，正如在一个事件中所看到的那样，束流的各向异性诱导了一个额外的回旋模式。结果得到了细胞内粒子模拟的支持。我们推测，第一种类型发生在中心EDR的下游，在那里背景电子经历强子加速形成垂直各向异性；第二种类型发生在导场重联的中心EDR中。用线性失稳分析方法进行了参数研究。∼3以上的束流各向异性本身就可能激发回旋模波。较大的光束漂移会引起多普勒频移，并可能导致离子框架中的左旋偏振。未来的研究需要确定观测是否涵盖更广泛的参数制度，并理解哨声和其他不稳定性之间的竞争。
Whistler waves are often observed in magnetopause reconnection associated with electron beams. We analyze seven MMS crossings surrounding the electron diffusion region (EDR) to study the role of electron beams in whistler excitation. Waves have two major types: (a) Narrow‐band waves with high ellipticities and (b) broad‐band waves that are more electrostatic with significant variations in ellipticities and wave normal angles. While both types of waves are associated with electron beams, the key difference is the anisotropy of the background population, with perpendicular and parallel anisotropies, respectively. The linear instability analysis suggests that the first type of wave is mainly due to the background anisotropy, with the beam contributing additional cyclotron resonance to enhance the wave growth. The second type of broadband waves are excited via Landau resonance, and as seen in one event, the beam anisotropy induces an additional cyclotron mode. The results are supported by particle‐in‐cell simulations. We infer that the first type occurs downstream of the central EDR, where background electrons experience Betatron acceleration to form the perpendicular anisotropy; the second type occurs in the central EDR of guide field reconnection. A parametric study is conducted with linear instability analysis. A beam anisotropy alone of above ∼3 likely excites the cyclotron mode waves. Large beam drifts cause Doppler shifts and may lead to left‐hand polarizations in the ion frame. Future studies are needed to determine whether the observation covers a broader parameter regime and to understand the competition between whistler and other instabilities.