Excitation of O+ Band EMIC Waves Through H+ Ring Velocity Distributions: Van Allen Probe Observations

A typical case of electromagnetic ion cyclotron (EMIC) emissions with both He+ band and O+ band waves was observed by Van Allen Probe A on 14 July 2014. These emissions occurred in the morning sector on the equator inside the plasmasphere, in which region O+ band EMIC waves prefer to appear. Through property analysis of these emissions, it is found that the He+ band EMIC waves are linearly polarized and propagating quasi‐parallelly along the background magnetic field, while the O+ band ones are of linear and left‐hand polarization and propagating obliquely with respect to the background magnetic field. Using the in situ observations of plasma environment and particle data, excitation of these O+ band EMIC waves has been investigated with the linear growth theory. The calculated linear growth rate shows that these O+ band EMIC waves can be locally excited by ring current protons with ring velocity distributions. The comparison of the observed wave spectral intensity and the calculated growth rate suggests that the density of H+ rings providing the free energy for the instability has decreased after the wave grows. Therefore, this paper provides a direct observational evidence to the excitation mechanism of O+ band EMIC waves: ring current protons with ring distributions provide the free energy supporting the instability in the presence of rich O+ in the plasmasphere. Plain Language Summary Electromagnetic ion cyclotron (EMIC) waves can play an important role in the dynamics of the magnetosphere. Excitation of EMIC waves is usually considered through temperature‐anisotropy of ring current hydrogen ions. Considering that EMIC waves in O+ band have different properties in comparison with those in H+ and He+ bands, theoretical prediction has been proposed that O+ band EMIC waves might be excited by protons with ring velocity distributions, which is different to the excitations of H+ or He+ band EMIC waves through temperature‐anisotropy of ring current protons. However, direct observational evidence supporting this theoretical result has not been provided yet. In this letter, we report a typical case of excitation of O+ band EMIC waves. Combining of observations and theory analysis, we demonstrate that ring current protons with ring velocity distributions can drive O+ band EMIC waves, which is in accordance with previous theoretical predictions. Key Points In situ observations were used to investigate the excitation of O+ band EMIC waves Combination of observati