Resonant absorption of ULF waves at Mercury's magnetosphere

Abstract

Ultra low frequency (ULF) waves, which are assumed to be standing waves on the field, are observed by the Mariner 10 spacecraft at Mercury. These waves are oscillating at 38% of the proton gyrofrequency. It is well known that heavy ions, such as Na+, are abundant in Mercury's magnetosphere. Because the presence of different ion species has an influence on plasma dispersion characteristics near the ion gyrofrequencies, magnetospheric eigenoscillations observed at Mercury with frequency in the gyrofrequency range require a multi‐fluid treatment for the plasma. Thus ULF waves at Mercury may have a distinct difference from typical ULF oscillations at Earth, which are often described in terms of magnetohydrodynamics. By adopting a multi‐fluid numerical wave model, we examine how magnetic eigenoscillations occur in Mercury's magnetosphere. Because protons and sodium ions are the main constituents at Mercury, we assume an electron‐proton‐sodium plasma in our model. Our results show: (1) the observed ULF waves are likely compressional waves rather than standing oscillations such as field line resonances (FLRs), (2) FLRs at Mercury are expected to occur when the ion‐ion hybrid and/or Alfvén resonance conditions are satisfied, (3) the magnetic field of FLRs at Mercury's magnetosphere oscillates linearly in the east‐west (azimuthal) meridian when the frequency is located between two ion gyrofrequencies, and (4) the resonance frequency enables us to estimate the local heavy ion concentration ratio.