Field Line Resonance at Mercury

When an incident compressional wave propagates across an Alfven velocity gradient in multi-fluid plasmas, the compressional wave can couple with the Alfven resonance for lower frequency and/or the ion-ion hybrid (IIH) resonance for higher frequency. Recently, a wave simulation in electron-hydrogen-sodium plasmas suggested that the field line resonance at Mercury is expected to occur when the IIH and/or Alfven resonance conditions are satisfied. However, the relative efficiency of wave energy absorption at these resonances has not been studied in the context of Mercury's magnetosphere. To understand the efficiency of wave absorption, we evaluate absorption coefficients (A) at the IIH and Alfven resonances for variable concentrations of sodium, ηNa = NNa / Ne, where Nj is number density for particle species j, and azimuthal (p) and field-aligned (q) wave numbers. Our results show that (1) The maximum values of absorption are up to 100 %; (2). The values of ηNa, where A has a maximum, increase as q increases; (3) The value of p where A has a maximum is almost same for different q values, but the width p of the absorption window become wider as q increases; and (4) When q increases, A oscillates in ηNa. These results suggest that the mode conversion efficiency is sensitive to the azimuthal and field aligned wave numbers and heavy ion concentration rates. Therefore when MESSENGER enters orbit around Mercury and detects field-line resonances, it is expected that discontinuities in the resonance frequencies could be observed in the radial direction.