Beam-driven ECH waves: A parametric study
Beam-driven ECH waves: A parametric study
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DOI：
10.1063/5.0053187
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发表时间：
2021-04
影响因子：
2.2
通讯作者：
Xu Zhang;V. Angelopoulos;A. Artemyev;Xiao‐jia Zhang
中科院分区：
文献类型：
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作者：
Xu Zhang;V. Angelopoulos;A. Artemyev;Xiao‐jia Zhang
电子回旋谐波（ECH）波在驱动漫射极光中起着重要作用，漫射极光占输入电离层的粒子能量的75%以上。磁层等离子体中的ECH波一直被认为主要是由行星偶极子场中自然产生的损耗锥各向异性（速度-空间梯度）激发的。然而，最近的THEMIS观测表明，电子束也可以激发地球磁尾中的这种波。环境和电子束等离子体条件下，电子束激发可以发生是未知的。了解这些条件将使我们能够进一步探索这种激发机制对地球和外行星磁层电子ECH波散射的相对贡献。使用热等离子体色散关系，我们解决了光束驱动的ECH波的性质，并进行全面的参数调查这种不稳定性。我们发现，增长是由束电子回旋共振的第一和更高的订单。我们还发现，这些波是不稳定的等离子体条件下的范围很广。生长速率随束流密度、束流速度和热电子温度的增加而增加;随束流温度和束流温度各向异性（T <$T <$）、热电子密度和冷电子密度和温度的增加而降低。这种情况在地球的磁尾中比比皆是，在那里，磁层电子被向地对流和磁场重联加热，与较冷的电离层电子共存。
Electron cyclotron harmonic (ECH) waves play a significant role in driving the diffuse aurora, which constitutes more than 75% of the particle energy input into the ionosphere. ECH waves in magnetospheric plasmas have long been thought to be excited predominantly by the loss cone anisotropy (velocity-space gradients) that arises naturally in a planetary dipole field. Recent THEMIS observations, however, indicate that an electron beam can also excite such waves in Earth’s magnetotail. The ambient and beam plasma conditions under which electron beam excitation can take place are unknown. Knowledge of such conditions would allow us to further explore the relative contribution of this excitation mechanism to ECH wave scattering of magnetospheric electrons at Earth and the outer planets. Using the hot plasma dispersion relation, we address the nature of beam-driven ECH waves and conduct a comprehensive parametric survey of this instability. We find that growth is provided by beam electron cyclotron resonances of both first and higher orders. We also find that these waves are unstable under a wide range of plasma conditions. The growth rate increases with beam density, beam velocity, and hot electron temperature; it decreases with increasing beam temperature and beam temperature anisotropy (T⊥ T∥ ⁄ ), hot electron density, and cold electron density and temperature. Such conditions abound in Earth’s magnetotail, where magnetospheric electrons heated by earthward convection and magnetic reconnection coexist with colder ionospheric electrons.