| Slowly growing low-frequency Farley-Buneman and gradient-drift waves in the E-region cannot be described accurately by a local linear instability. Instead, the decrease in neutral density with altitude introduces an inhomogeneity in the traditional dispersion relation, which causes the instability to switch from absolute to convective and requires a nonlocal description. With the magnetic field being nearly vertical at high latitudes, the parallel wavevector component has to increase in time as a result.; This increase in the parallel wavevector and the corresponding parallel electric fields stabilize the wave and ultimately limit its amplitude. From an equivalent point of view, the instability develops a large upward group velocity that limits the time spent in the unstable region, and places an upper limit on the time available for growth. In addition to the above, the wave often develops shocks, which cause it to switch to strongly damped ion-acoustic modes and further limit the growth.; For Farley-Buneman modes, the largest amplitude usually occurs near the ion-acoustic speed (though Doppler-shifted by the ion drift). For gradient-drift modes, conservation of wave action provides an important new growth mechanism from vertical motion through a plasma density gradient. This causes the wave amplitude to grow or decay so as to conserve wave energy. For nonlocal instabilities, the resulting change in amplitude is significantly larger than growth from the gradient-drift instability per se.; For very long wavelengths, the lack of significant diffusion allows the perturbed electric field to grow substantially from the new vertical gradient term. This can produce small scale secondary instabilities in directions near perpendicularity with the flow, via a two-step process. The symmetry of density enhancements and depletions in the primary wave means that when observed by radars, the spectrum from secondary instabilities might be fairly wide with a mean Doppler shift that is given by the line-of-sight component of the primary wave phase speed. This may explain recent HF radar observations of E-region plasma structures that show a mean Doppler shift apparently proportional to the cosine of the flow angle. |
