Production of small‐scale Alfvén waves by ionospheric depletion, nonlinear magnetosphere‐ionosphere coupling and phase mixing
Production of small‐scale Alfvén waves by ionospheric depletion, nonlinear magnetosphere‐ionosphere coupling and phase mixing
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通过电离层损耗、非线性磁层-电离层耦合和相位混合产生小尺度阿尔文波
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发表时间：
2013
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通讯作者：
A. Streltsov
中科院分区：
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作者：
A. Russell;A. Wright;A. Streltsov
火箭和卫星以前曾在大规模向下场向电流中观察到小规模阿尔文波，数值模拟将其形成与自洽磁层-电离层耦合联系起来。这些波的起源以前归因于电离层反馈不稳定性，但是，我们表明，它们出现在数值实验中的不稳定性被排除在外。提出了一种新的解释，其中强烈的电离层耗尽和相关的电流加宽（非线性陡化/波破碎过程）在向下的电流区域内形成磁层电离层波，这些振荡驱动上覆等离子体中的上行惯性阿尔文波。由此产生的波是由特征周期，这是一个很好的匹配，以合理的假设条件下，以前观察到的周期。与此同时，垂直于磁场的波长最初映射到与低空磁层的电子惯性长度相当的电离层尺度，但由于边界波的基于频率的相位混合（相位混合的新表现），随着时间的推移变得更短。在适当的条件下，这些可以作为电离层反馈不稳定性的种子。
Rockets and satellites have previously observed small‐scale Alfvén waves inside large‐scale downward field‐aligned currents, and numerical simulations have associated their formation with self‐consistent magnetosphere‐ionosphere coupling. The origin of these waves was previously attributed to ionospheric feedback instability; however, we show that they arise in numerical experiments in which the instability is excluded. A new interpretation is proposed in which strong ionospheric depletion and associated current broadening (a nonlinear steepening/wave‐breaking process) form magnetosphereionosphere waves inside a downward current region and these oscillations drive upgoing inertial Alfvén waves in the overlying plasma. The resulting waves are governed by characteristic periods, which are a good match to previously observed periods for reasonable assumed conditions. Meanwhile, wavelengths perpendicular to the magnetic field initially map to an ionospheric scale comparable to the electron inertial length for the low‐altitude magnetosphere, but become shorter with time due to frequency‐based phase mixing of boundary waves (a new manifestation of phase mixing). Under suitable conditions, these could act as seeds for the ionospheric feedback instability.