Field-line resonance structures in Mercury’s multi-ion magnetosphere

Recently, MESSENGER spacecraft detected transverse waves at Mercury’s inner magnetosphere. The magnetic field fluctuations of theses waves are approximately perpendicular to the gradient of magnetic field magnitude which is similar to the field-line resonance characteristics predicted by numerical simulations in two-ion plasmas. In this paper, we perform a wave simulation in a three-ion plasma to consider the effect of multiple heavy ions at Mercury. Because recently observed wave frequencies are near the He+, He2+ and H+, we adopt multi-ion plasmas that contain H+, He2+, and He+. The simulation results show that several resonant waves between the ion gyrofrequencies can occur at the same location and also show the modulation of amplitude in time histories. Therefore, the simulation results suggest that it could be possible to observe two or three different resonant frequencies at certain locations in Mercury’s magnetosphere.