Relationship between ULF wave mode mix, equatorial electric fields, and ground magnetometer data

The magnetized plasma of near‐Earth space supports the shear and fast Alfvén, ultralow‐frequency (ULF; 1–100 mHz), magnetohydrodynamic wave modes. The fast mode may propagate across the magnetic field, spreading ULF wave energy throughout the magnetosphere, and couple with the shear Alfvén mode to form field line resonances (FLRs). The FLR electric field in the magnetosphere may have sufficiently large amplitudes to energize electrons and enhance radiation belt particle diffusion rates. Ozeke et al. (2009) recently described a technique that related the observed north‐south ULF magnetic component at the ground, b_v^g, with the radial electric field component in the equatorial plane of the magnetosphere, e_v^eq, via the fields in the ionosphere. In this paper we use a fully coupled ULF wave model to determine the ratio e_v^eq/b_v^g for a 5 mHz FLR formed at high latitudes. We find that Ozeke et al. (2009) underestimated the ULF wave magnetic field on the ground which varies with ionosphere Hall conductance. This difference is found to be caused by assuming a decoupled wave mode model for the ionosphere fields. Any relationship that involves ULF wavefields in the ionosphere must include the effects of ULF wave mode mixing.