LARGE-M WAVES GENERATED BY SMALL-M FIELD LINE RESONANCES VIA THE NONLINEAR KELVIN-HELMHOLTZ INSTABILITY

Recently, ultralow-frequency waves with large azimuthal wavenumber (la rge m) have been observed on similar L shells and with the same (or si milar) frequencies as small-m field line resonances (FLRs). The large- m waves appeared to the west of the small-m FLRs and had westward phas e propagation while the small-m FLRs had tailward phase propagation. W e propose an extension to an earlier waveguide model to explain these observations. We suggest that small-m tailward propagating waveguide m odes 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 io nospheric 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 io n distributions amplify the seed K-H waves as the ions drift westward. This leads to observable large-m drift waves at or beyond the westwar d limits of the small-m FLR regions.