ULF foreshock under radial IMF: THEMIS observations and global kinetic simulation Vlasiator results compared
ULF foreshock under radial IMF: THEMIS observations and global kinetic simulation Vlasiator results compared
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DOI：
10.1002/2015ja021526
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发表时间：
2015-04
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通讯作者：
M. Palmroth;M. Archer;R. Vainio;H. Hietala;Y. Pfau‐Kempf;S. Hoilijoki;O. Hannuksela;U. Ganse;A. Sandroos;S. Alfthan;J. Eastwood
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文献类型：
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作者：
M. Palmroth;M. Archer;R. Vainio;H. Hietala;Y. Pfau‐Kempf;S. Hoilijoki;O. Hannuksela;U. Ganse;A. Sandroos;S. Alfthan;J. Eastwood
几十年来，在准平行弓形激波的上游观测到了周期约为30 s的单色大尺度超低频（ULF）波。这些波通常相对于行星际磁场（IMF）倾斜地传播，而引起波的不稳定性的增长率在平行于磁场的方向上最大化。有人认为，斜传播的机制涉及波折射由于超热离子的空间变异性，起源于E × B漂移分量。我们使用Vlasiator研究了准径向IMF下的ULF前震，Vlasiator是一种新开发的全局混合Vlasov模拟，用于求解质子的Vlasov方程，而电子被视为电荷中和流体。我们观察到的30年代的超低频波的产生，并比较他们的属性，以前的文献和多点时间历史的事件和大尺度相互作用在亚暴（THEMIS）航天器观测。我们发现，Vrasiator再现前震ULF波在所有报告的观测方面。我们的结论是，反射回流离子的密度和速度的变化决定了前震的大尺度结构，这会影响波的频率，波长和斜传播。我们的结论是，波折射也可能在径向IMF条件下工作，这是早期被认为是一个例外的折射机制，由于小的E × B漂移分量。我们认为，额外的折射可能是由回流离子的密度和速度的大尺度空间变化引起的。
For decades, monochromatic large‐scale ultralow frequency (ULF) waves with a period of about 30 s have been observed upstream of the quasi‐parallel bow shock. These waves typically propagate obliquely with respect to the interplanetary magnetic field (IMF), while the growth rate for the instability causing the waves is maximized parallel to the magnetic field. It has been suggested that the mechanism for the oblique propagation concerns wave refraction due to the spatial variability of the suprathermal ions, originating from the E × B drift component. We investigate the ULF foreshock under a quasi‐radial IMF with Vlasiator, which is a newly developed global hybrid‐Vlasov simulation solving the Vlasov equation for protons, while electrons are treated as a charge‐neutralizing fluid. We observe the generation of the 30 s ULF waves and compare their properties to previous literature and multipoint Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft observations. We find that Vlasiator reproduces the foreshock ULF waves in all reported observational aspects. We conclude that the variability of the density and velocity of the reflected back streaming ions determines the large‐scale structure of the foreshock, which affects the wave frequency, wavelength, and oblique propagation. We conclude that the wave refraction may also be at work for radial IMF conditions, which has earlier been thought of as an exception to the refraction mechanism due to the small E × B drift component. We suggest that additional refraction may be caused by the large‐scale spatial variability of the density and velocity of the back streaming ions.