Observation and Numerical Simulation of Cold Ions Energized by EMIC Waves

This is the first report of significant energization (up to 7,000 eV) of low-energy He⁺ ions, which occurred simultaneously with H-band electromagnetic ion cyclotron (EMIC) wave activity, in a direction mostly perpendicular to the ambient magnetic field. The event was detected by the Arase satellite in the dayside plasmatrough region off the magnetic equator on 15 May 2019. The peak energy of the He⁺ flux enhancements is mostly above 1,000 eV. At some interval, the He⁺ ions are energized up to ∼7,000 eV. The H-band waves are excited in a frequency band between the local crossover and helium gyrofrequencies and are close to a linear polarization state with weakly left-handed or right-handed polarization. The normal angle of the waves exhibits significant variation between 0° and 80°, indicating a non-parallel propagation. We run a hybrid code with parameters estimated from the Arase observations to examine the He⁺ energization. The simulations show that cold He⁺ ions are energized up to more than 1,000 eV, similar to the spacecraft observations. From the analysis of the simulated wave fields and cold plasma motions, we found that the ratio of the wave frequency to He⁺ gyrofrequency is a primary factor for transverse energization of cold He⁺ ions. As a consequence of the numerical analysis, we suggest that the significant transverse energization of He⁺ ions observed by Arase is attributed to H-band EMIC waves excited near the local helium gyrofrequency.