Excitation of magnetospheric waveguide modes by magnetosheath flows

Standard models of the Earth's outer magnetospheric waveguide assume that a perfectly reflecting magnetopause can trap energy inside the waveguide. In contrast, we show that the near-noon magnetopause often acts as a leaky bo undary, wave trapping only being possible for large magnetosheath flow spee ds. Moreover, for sufficiently fast flow speeds, we show how waveguide mode s may be energized by magnetosheath flows via the overreflection mechanism. Unbounded simulations of the growth of surface waves via the development o f a Kelvin-Helmholtz instability (KHI) vortex sheet show growth rates which increase without limit proportional to wavenumber (k(y)), until the assump tion of a thin boundary is, no longer valid. For a bounded magnetosphere, h owever, overreflected body type waveguide modes can introduce wavenumber se lection, that is, generate modes with maximum linear growth rates at finite k(y) A necessary condition is that the wave is propagating in the magnetos phere, that is, the wave's turning point lies inside the magnetosphere. By developing a new description of both KHI and waveguide mode growth in terms of overreflection and the propagation of negative energy waves, we show ho w the maximum growth rate can be understood in terms of the reflection coef ficient of waves incident upon the magnetopause. Our model can also explain the observed local time dependence of Pc5 field line resonance wave power, and can explain the observed correlation between high solar wind speeds an d Pc5 wave power. Finally, we show how a waveguide with a free magnetopause boundary supports quarter-wavelength modes. These modes have lower frequen cies than the standard (magnetopause velocity node) half-wavelength modes, perhaps generating the millihertz waveguide mode eigenfrequencies which app ear to drive field line resonances in HF radar data.