Resonant absorption of ULF waves at Mercury's magnetosphere

Abstract

[1] 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.

2D full-wave simulation of waves in space and tokamak plasmas

“…While these models can follow the flow of energy due to a particular excited wave, this approach will fail if the wave enters a region in space in which it can couple power to other propagating modes via tunneling and /or mode conversion. One the other hand, time-dependent fluid wave models have been used for MHD waves [9] and EMIC waves [10]. Although they successfully demonstrate mode conversion process for MHD and EMIC waves, since MHD wave models are only valid for waves with ω << Ω ion , where ω is angular frequency, and multi-ion fluid wave code adopted a slab coordinate so that the magnetic field curvature effect on waves could not be captured.…”

Comparison of ultra‐low‐frequency waves at Mercury under northward and southward IMF

“…Field line resonances, cavity modes, and local instabilities have all been proposed as explanations for these waves [Russell, 1989;Glassmeier et al, 2003;Kim and Lee, 2003;Kim et al, 2008;Boardsen et al, 2009]. The first panels display the frequency of the ULF wave events during each encounter; the solid lines denote f cH+ , f cHe++ , and f cHe+ .…”

Localization of Ultra-Low Frequency Waves in Multi-Ion Plasmas of the Planetary Magnetosphere

“…For Mercury, where the magnetic field is relatively weak, the wavelength of field-aligned modes can be comparable to the size of the magnetosphere. Therefore, IIH waves oscillate globally along the magnetic field lines for Mercury, similar to the field line resonance on Earth (Othmer et al 1999;Glassmeier et al 2003Glassmeier et al , 2004Klimushkin et al 2006;Kim et al 2008Kim et al , 2011aKim et al , 2013Kim et al , 2015a. On the other hand, on Earth, the magnetic field strength is larger and the wavelength is shorter, which typically localizes mode converted waves between the Buchsbaum cutoff locations, which occur at around 10 degrees latitude.…”