IMPLICATIONS OF THE SMALL ASPECT ANGLES OF EQUATORIAL SPREAD F

Small-scale equatorial spread F irregularities are almost perfectly al igned with the geomagnetic field. We develop here an analytic plasma k inetic theory of small-scale, quasi-field-aligned irregularities that includes ion viscosity and finite Larmor radius effects. We conclude, for one thing, that the measured aspect angles are too small to be con sistent with a dissipative drift wave source of 3-m irregularities. No nlinearly driven flute modes appear to be the only available mechanism . We compare the relative influence of parallel and perpendicular diss ipation and conclude that the aspect width depends only weakly on any single geophysical parameter, such as collision frequency, gradient le ngth, temperature, etc. This finding is consistent with our observatio n that the measured aspect angles vary little with altitude and only w eakly with instability level.