日侧外磁层ULF波调制EMIC波的MMS观测

磁层多尺度卫星（MMS-1）在日侧06:30 MLT（磁地方时，Magnetic Local Time）外磁层大于2R_e（L 为 7.5~10.1）的范围内观测到多达21个波包的准周期性电磁离子回旋波（EMIC）事件。超低频（ULF）波和能量质子温度各向异性准周期性增强也被同步观测到。频率分析显示，ULF波的周期、质子各向异性周期和EMIC波包的周期非常接近。MMS-4卫星在约1 h后经过附近空间区域，研究发现随着ULF波的幅度减弱，EMIC波包的准周期性也逐渐减弱。研究结果为ULF波在日侧外磁层调制质子各向异性从而产生周期性EMIC波包提供了完整的观测证据链。同时，观测结果进一步证明，这种ULF波调制的EMIC波包能够在大于2R_e的空间尺度内发生，且能够持续存在于几个小时以上的时间尺度。

Magnetospheric Multiscale Observation of Electromagnetic Ion Cyclotron Wave Modulated by ULF Wave in Outer Magnetosphere

A quasi-periodic Electromagnetic Ions Cyclotron (EMIC) wave event was reported. The event was observed by the Magnetospheric Multiscale (MMS-1) at dayside (06:30 MLT) in the outer magnetosphere. EMIC waves with more than 21 wave packets occurred within L value is from 7.5 to 10.1. Ultra-Low Frequency (ULF) waves and the quasi-periodic enhancement of energy protons temperature anisotropy are simultaneously observed. Frequency analysis shows that the period of the ULF wave, the proton anisotropy period and the period of the EMIC wave packet are very close. The MMS-4 satellite passed the very close region about an hour later and found that the quasi-periodicity of the EMIC wave packet decreased as the amplitude of the ULF wave decreased. The current results provide evidences that the ULF wave can modulate the energetic proton anisotropy in the outer magnetosphere at dayside and thus generate periodic EMIC wave packets. Moreover, correlated observations by MMS-1 and MMS-4 show that the ULF-modulation of EMIC waves can occur in a spatial scale that greater than 2 R_e, and last for several hours.