资源环境科技发展态势分析平台

We present a Magnetospheric Multiscale mission observation of quasiperiodic electromagnetic ion cyclotron (EMIC) variations on 24 December 2015 corresponding to magnetic field depressions as well as proton pitch angle anisotropy enhancements. Proton dynamic pressure obtained by WIND indicates that the solar wind was periodically compressing the magnetosphere with a period similar to 7.5 min, which is consistent with the EMIC modulation period observed by the Magnetospheric Multiscale. Numerical calculations demonstrate that the EMIC modulation period is comparable with the magnetic field line resonance period, suggesting that quasiperiodic solar wind enhancement compressing the magnetosphere and propagating as the fast mode can lead to the magnetic field line oscillation and hence the presence of quasiperiodic magnetic compressions and discrete proton anisotropy elements, which provides optimal conditions for modulating the EMIC instability. Current results provide a complete chain of self-consistent observational evidences to illustrate the process connecting solar wind variations to EMIC wave amplitude modulation in the magnetosphere.