Magnetospheric ULF waves with an increasing amplitude induced by solar wind dynamic pressure changes: THEMIS observations 

Xiaochen Shen1,2, Qiugang Zong3, Quanqi Shi1, Anmin Tian4, Weijie Sun3, Yongfu Wang3, Xuzhi Zhou3, Suiyan Fu3, Vassilis Angelopoulos5, Zuyin Pu3 and Michael Hartinger6, (1)Shandong University at Weihai, Weihai, China, (2)Shandong University, Shandong, China, (3)Peking University, School of Earth and Space Sciences, Beijing, China, (4)Shandong University at Weihai, Institute of Space Science, Weihai, China, (5)University of California Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, United States, (6)Space Science Institute, Boulder, United States
Abstract:
We report the in situ observation of the magnetospheric ultra-low frequency (ULF) waves with an increasing amplitude induced by solar wind dynamic pressure changes. We examine the magnetospheric responses to solar wind dynamic pressure enhancements from April 1, 2007 to December 31, 2012, and find six ULF wave events with slow but clear wave amplitude increase. The amplitudes of ion velocities and magnetic field of these waves continuously increase by 2.1 ∼ 4.4 times during three to six wave cycles. We choose two typical cases to further investigate the cause of this wave amplitude increase. We find that the wave amplitude growth is mainly contributed by the toroidal mode wave. Interestingly, toroidal mode waves are standing, while compressional and poloidal mode waves are not. Thus, we suspect that the wave amplitude increase may be caused by the superposition of two wave sources. One wave source is the standing wave excited by the solar wind dynamic impulse. Additionally, fast mode compressional wave continuously shakes the magnetic field lines. The azimuthal component of this magnetic perturbation is the second wave source. Furthermore, the simple model calculation of superposing two waves match the observations pretty well.
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