Identification of the Nature of Electromagnetic Waves near the Proton-cyclotron Frequency in Solar-terrestrial Plasmas

Low-frequency (quasi-)monochromatic electromagnetic waves near the ion-cyclotron frequency always exhibit both left-hand (LH) and right-hand (RH) polarization in solar-terrestrial spacecraft observations. However, due to the Doppler frequency shift resulting from the bulk flow of charged particles, the nature of these waves in the plasma frame is still unclear. This paper proposes a useful method to directly identify the nature of the observed waves. Using three wave parameters including polarization, direction of the parallel Poynting flux, and correlation between perpendicular magnetic field and perpendicular ion/electron velocity, we could discriminate the wave mode (Alfvén/ion-cyclotron wave or fast-magnetosonic/whistler wave) and its propagation direction (along or against the magnetic field) in the plasma frame. Using Magnetospheric Multiscale spacecraft measurements, we analyze two wave events containing both LH- and RH-polarized low-frequency electromagnetic waves in the Earth's magnetosheath, and find that these waves correspond to counter-propagating Alfvén/ion-cyclotron waves in the plasma frame. Our method is helpful for studying low-frequency electromagnetic waves detected by satellites that have particle measurements with an adequate temporal resolution.