磁暴恢复相后期超低频波观测分析

利用Radiation Belt Storm Probes（RBSP）和Magnetospheric Multiscale（MMS）的联合观测研究了2016年1月25日和2月22日的两个超低频（ULF）波动事件.这两个事件均发生在磁暴的恢复相后期，波动分布在晨侧与正午之间的区域.RBSP观测到的两次波动都发生在低L-shell（4.7～5.8），MMS观测到的两次波动都发生在高L-shell（8.0～14.2）.在高L-shell，超低频波偏振以环向波为主，沿磁力线方向为基频谐波.在低L-shell，超低频波偏振以径向波为主，沿磁力线方向为二次谐波.对内磁层（L<6）中的ULF波，通过比较?f/?w|μ,L和（dL）/（dW）（?f）/（?L）|μ,W的绝对值可知，bump-on-tail的离子分布和向地的离子相空间密度梯度通过N=1的弹跳共振，共同为波动的产生提供自由能.多重事例分析表明，在磁暴恢复相后期，约10keV的离子伴随着波动的存在而存在，并且为波动的产生提供自由能. 

Observations of ULF Waves at the Late Recovery Phase of Magnetic Storm

Two ULF wave events on 25 Jan. and 22 Feb. 2016 are studied by using the combined observations of Radiation Belt Storm Probes (RBSP) and Magnetospheric Multiscale (MMS). Both events occurred in the late recovery phase of the magnetic storm, and both fluctuations occurred in the area between morning and noon. The two events observed by RBSP occurred in the low L-shell (4.7~5.8), and the two events observed by MMS occurred in the high L-shell (8.0~14.2). In the high L-shell, the ULF wave is dominated by toroidal mode, and it is the first harmonic along the field line. In the low L-shell, the ULF wave is dominated by the poloidal wave, and it is the second harmonic along the field line. In addition, by comparing the absolute values of ?f/?w|μ,L and （dL）/（dW）（?f）/（?L）|μ,W, the bump-on-tail ion distribution and inward gradient of ion phase space density together provide free energy for the wave through N=1 bounce resonance. According to the analysis of these two cases, about 10keV ions exist with the waves and provide free energy for the generation of waves at the late recovery phase of the magnetic storm.