Large‐m waves generated by small‐m field line resonances via the nonlinear Kelvin‐Helmholtz instability

Abstract

Recently, ultralow‐frequency waves with large azimuthal wavenumber (large m) have been observed on similar L shells and with the same (or similar) frequencies as small‐m field line resonances (FLRs). The large‐m waves appeared to the west of the small‐m FLRs and had westward phase propagation while the small‐m FLRs had tailward phase propagation. We propose an extension to an earlier waveguide model to explain these observations. We suggest that small‐m tailward propagating waveguide modes drive the small‐m FLRs. Phase mixing within these FLRs allows the development of the nonlinear Kelvin‐Helmholtz (K‐H) instability near the resonant field lines. Phase‐mixing scale lengths are limited by ionospheric dissipation, and we show that realistic ionospheric Pedersen conductivities result in the dominance of a single zero‐frequency K‐H wave in each small‐m FLR region having m consistent with observation of the large‐m disturbances. K‐H growth rates are significant, but not large enough to disrupt the small‐m FLRs. We propose that unstable ion distributions amplify the seed K‐H waves as the ions drift westward. This leads to observable large‐m drift waves at or beyond the westward limits of the small‐m FLR regions.