Poloidal ULF wave observed in the plasmasphere boundary layer
Poloidal ULF wave observed in the plasmasphere boundary layer
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DOI：
10.1002/jgra.50427
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发表时间：
2012-12
期刊：
影响因子：
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通讯作者：
W. Liu;J. Cao;X. Li;T. Sarris;Q. Zong;M. Hartinger;K. Takahashi;H. Zhang;Q. Shi;V. An
中科院分区：
文献类型：
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作者：
W. Liu;J. Cao;X. Li;T. Sarris;Q. Zong;M. Hartinger;K. Takahashi;H. Zhang;Q. Shi;V. An
我们报告了一个与等离子体层边界层(PBL)的形成有关的罕见的超低频(ULF)波产生事件，THEMIS的一颗卫星TH-D在随后的出站过程中很好地观测到了这一事件。2011年9月13日，TH-D在L=3.4时观测到一次尖锐的等离子体顶层顶。等离子体圈开始扩大，并于9月14日继续补充。9月15日，在L=4.4点和6.5点分别形成了两个密度梯度的PBL。在这两个密度梯度内，观察到强烈的径向磁场和方位向电场振荡，表明存在极向超低频波。根据磁场和电场信号之间的相位延迟，以及将观测到的波频率与预测的谐本征频率进行比较，我们发现观测到的振荡是二次谐波极向波。进一步的研究表明，观测到的波很可能是在~10keV的“尾部碰撞”等离子体分布下的漂移反弹共振产生的。我们证明了在本征频率接近这些热质子的反弹频率的PBL内激发波，而在本征频率偏离反弹频率的PBL之外激发波。最后，通过改变局域场线的本征频率，我们认为冷等离子体密度似乎也是超低频波产生的一个控制因素，除了尾部碰撞的能量源。
We report on a rare ultra‐low‐frequency (ULF) wave generation event associated with the formation of a plasmasphere boundary layer (PBL), which was well observed by one of the THEMIS satellites, TH‐D, during subsequent outbound passes. On 13 September 2011, TH‐D observed a sharp plasmapause at L = 3.4. The plasmasphere started to expand and continued to be refilled on 14 September. On 15 September, a PBL was formed with two density gradients at L = 4.4 and 6.5, respectively. Within the two density gradients, strong radial magnetic field and azimuthal electric field oscillations were observed, suggesting poloidal ULF waves. Based on the phase delay between magnetic and electric field signals, as well as the comparison between the observed wave frequency and predicted harmonic eigenfrequency, we find that the observed oscillations are second harmonic poloidal waves. Further investigation shows that the observed waves are likely generated by drift‐bounce resonance with “bump‐on‐tail” plasma distributions at ~10 keV. We demonstrate that the waves are excited within the PBL where the eigenfrequency is close to the bounce frequency of these hot protons, but not outside the PBL where the eigenfrequency deviates from the bounce frequency. Finally, we suggest that cold plasma density seems to be a controlling factor for ULF wave generation as well, in addition to the bump‐on‐tail energy source, by altering eigenfrequency of the local field lines.