在多流体描述中日冕的漂移不稳定性
Drift instabilities in the solar corona within the multi-fluid description
摘要：
最近的观测表明，太阳大气在密度、温度和磁场方面具有高度的结构性。这些梯度的存在可能会导致出现在等离子体中的电流，这在弱碰撞电晕可以构成驱动微观不稳定性的自由能源。这种不稳定性是非常重要的，因为它们代表了离子回旋波的可能来源，离子回旋波已被证实在日冕加热中起着重要作用，但其太阳起源仍不清楚。考虑横向于磁场的密度分层，研究了冕洞典型条件下梯度诱导等离子体微不稳定性的可能性。考虑到WKB（Wentzel-Kramers-Brillouin）近似，我们使用无碰撞多流体模型进行傅立叶平面波分析。通过忽略电子惯性，该模型允许我们考虑离子回旋波的影响，是从磁流体力学（MHD）的单流体模型缺席。现实的密度和温度模型，以及2-D分析磁场模型，被用来定义在极冕洞的开放场漏斗的背景等离子体。射线追踪理论被用来计算射线路径的不稳定波，以及它们的增长率在传播过程中的演变。我们表明，在典型的冕洞条件下，当假设典型的横向密度长度尺度从无线电观测，由相对的电子-离子漂移产生的电流提供足够的自由能驱动模式不稳定。这种不稳定性是由反向传播的慢模波之间的耦合引起的。
英文摘要：
Recent observations revealed that the solar atmosphere is highly structured in density, temperature and magnetic field. The presence of these gradients may lead to the appearance of currents in the plasma, which in the weakly collisional corona can constitute sources of free energy for driving micro-instabilities. Such instabilities are very important since they represent a possible source of ion-cyclotron waves which have been conjectured to play a prominent role in coronal heating, but whose solar origin remains unclear. Considering a density stratification transverse to the magnetic field, this paper aims at studying the possible occurrence of gradient-induced plasma micro-instabilities under typical conditions of coronal holes. Taking into account the WKB (Wentzel-Kramers-Brillouin) approximation, we perform a Fourier plane waves analysis using the collisionless multi-fluid model. By neglecting the electron inertia, this model allows us to take into account ion-cyclotron wave effects that are absent from the one-fluid model of magnetohydrodynamics (MHD). Realistic models of density and temperature, as well as a 2-D analytical magnetic-field model, are used to define the background plasma in the open-field funnel in a polar coronal hole. The ray-tracing theory is used to compute the ray path of the unstable waves, as well as the evolution of their growth rates during the propagation. We demonstrate that in typical coronal hole conditions, and when assuming typical transverse density length scales taken from radio observations, the current generated by a relative electron-ion drift provides enough free energy for driving the mode unstable. This instability results from a coupling between oppositely propagating slow-mode waves.
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