Modulation of Whistler Mode Waves by Ion‐Scale Waves Observed in the Distant Magnetotail

Wave activities in tailward flows have been explored in the distant magnetotail at ~54 RE, where the coupling between whistler mode waves and ion‐scale waves was observed. The whistler mode waves periodically appeared at each cycle of the ion‐scale waves, and the electron distribution functions associated with the whistler mode waves showed enhancement in the direction parallel to background magnetic field. Wave analyses show that the field‐aligned electron components act as the energy source of the whistler mode waves. The ion‐scale waves are generated by the interaction of the hot ion beam with background ions. A likely candidate of the ion‐scale wave is the kinetic Alfven wave, which can generate the enhanced field‐aligned electron populations by the parallel electric field. Plain Language Summary The ionized particles constrained by the Earth's magnetic field involve dynamics of different spatial and temporal scales. Because of large mass difference, the scales characterizing the ion and electron dynamics are very different, and the related processes are often considered separately. In this study, we have found that the electron scale waves can be modulated by ion‐scale waves, indicating that the dynamics of electrons and ions are coupled. The ion‐scale waves are induced by the streaming ion beams superposed on the background ions. In the process, electrons are accelerated parallel to the magnetic field direction by the ion‐scale wave, resulting in the deviation of electron populations from the equilibrium state. The unstable electrons then release the excess energy by generating electron scale waves, called whistler mode waves. Key Points Modulation of whistler mode waves by ion‐scale waves are observed in the distant magnetotail The whistler mode waves are excited by field‐aligned electron populations generated by the ion‐scale waves The ion‐scale waves, possibly kinetic Alfven waves, are driven by tailward streaming ion beams