ULF waves at Mercury

Ion cyclotron frequency range waves (or electromagnetic ion cyclotron wave, EMIC) have been often observed at Mercury's magnetospheres. The previous statistical study showed the magnetic compressional component is dominant near the magnetic equator and the transition from compressional to transverse dominance occurs roughly at magnetic latitudes of ±20˚. Because the observed waves also often show linearly polarization, the field-line resonance in the single or multiple ion plasmas have been suggested to discuss such waves. On the other hand, electromagnetic ion Bernstein wave (IBW) is also suggested because of strong power of compressional component. In this talk, we will address both field-line resonance and electromagnetic IBWs in order to discuss the ULF waves detected from MESSENGER. We adopted 2D full-wave code that recently developed at Princeton Plasma Physics Laboratory. When compressional fast waves are launched in the outer magnetosphere, the waves propagate to inner magnetosphere and strong field-aligned waves are mode-converted from the incoming compressional waves. Such mode-converted waves globally oscillate and have strong transverse components. Near the magnetic equator, due to mixture of incoming compressional waves and mode-converted field-line resonance, magnetic compressional component is dominant while transverse component is dominant off the equator, which is consistent with statistical study. We also used warm plasma ray-tracing to explore the propagation of the IBW mode in a dipole magnetic field and found that the electromagnetic IBWs are highly unstable to the proton loss cone distribution function and the wave's group velocity is highly field aligned. The wavelength of this mode is on the order of 100 km. We also discovered that as the waves propagate they can become highly compressional even in a moderate proton beta ~0.05 to 0.54 plasma, which is also consistent with observations.