Field line resonant frequencies and ionospheric conductance: results from a 2-D MHD model

The magnetised plasma of the near-Earth space environment supports ultra-low frequency (ULF; 1 - 100 mHz), magnetohydrodynamic (MHD) oscillations. For sufficiently large ionospheric conductances, field line resonances (FLRs) form between the northern and southern ionospheres. These conditions are usually met for daytime ionosphere conductance values. The FLRs are normal modes of the system and may be used to remote sense plasma mass densities in the magnetosphere. The oscillations lose energy in the ionosphere whose properties determine the boundary conditions, particularly resonance damping effects. Using a two dimensional (2D) MHD model of the magnetosphere and realistic ionosphere boundary conditions, the variation in resonant frequency with ionosphere conductivity is reported. For typical mid to low latitude summer and winter ionosphere parameters, the FLRs change by less than 5%. This translates to an uncertainty of 7% in plasma mass density.