Case study of propagation characteristics of EMIC wave using multipoint observation by Arase, Cluster, and Ground Station

Electromagnetic ion cyclotron (EMIC) waves are important for the loss of radiation belt electrons and ring current ions. After being excited by an instability driven by the temperature anisotropy followed by nonlinear wave-particle interactions occurring near the magnetic equator of the inner magnetosphere, EMIC waves propagate parallel along the magnetic field lines with left-handed polarization. Their wave normal angles with respect to the magnetic field increase as the waves propagate to higher latitudes. At latitudes where the wave frequency is the same as the crossover frequency, the polarization changes from left-handed to right-handed, called polarization reversal. Polarization reversal is one of the mechanisms that allow EMIC waves to propagate to the ground without being reflected in the magnetosphere, because based on the dispersion relation of the cold plasma, they could only exist in the frequency range above the cutoff frequency if they remained with the left-handed polarization. The crossover frequency at which the polarization reversal occurs depends highly on the surrounding plasma environment. To investigate the polarization reversal in the magnetosphere, conjugate observation, in which the same event is observed at different latitudes, is useful for discussing the propagation process of plasma waves and changes in the surrounding plasma environment.

In this study, we analyzed EMIC waves simultaneously observed by the Arase, Cluster and ground-based induction magnetometer at the Gakona station ( 62.39^° N and 214.78° E geographic coordinates). We used the electric and magnetic field waveform data observed by the PWE-EFD and MGF onboard the Arase satellite and the magnetic field waveform data observed by STAFF onboard the C1 satellite. Also, we used the induction magnetometer data from the Gakona station. The event of interest was observed from 21:20 to 21:40 UT on July 25,2020, with the same L-value-(L=6) and MLT-(12.9 MLT). In the spectra observed by Arase located in the equatorial region (MLAT= 5°), we identified the enhancement of electromagnetic waves in the frequency range from 0.65Hz to 1.1Hz, corresponding to the proton-band EMIC waves. The same EMIC wave was observed by C1 and Gakona. At this time C1 was located away from the equator-(MLAT= -22° ). While the frequency range of the EMIC wave observed at Arase was higher than the He⁺ cyclotron frequency, ƒ_He⁺ , the EMIC wave observed at C1 appeared in the spectra close to ƒ_He⁺. Considering the cold plasma dispersion relation, it was suggested that polarization reversal may have occurred during the wave propagation from the equatorial region at Arase to the higher latitude at C1. We have also performed the Singular Value Decomposition (SVD) method(Santolik et al. 2003) for each satellite data, which allows us to derive polarization properties. As a result, it was confirmed that the polarization in C1 changed from linear polarization to right-handed polarization below a specific frequency. With these results, it is observationally clear that the conditions for the EMIC wave propagation to the ground are satisfied. We also discussed the surrounding plasma environment and the generation process of the observed EMIC wave.