Implications of the small aspect angles of equatorial spread F

Small‐scale equatorial spread F irregularities are almost perfectly aligned with the geomagnetic field. We develop here an analytic plasma kinetic 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 consistent with a dissipative drift wave source of 3‐m irregularities. Nonlinearly driven flute modes appear to be the only available mechanism. We compare the relative influence of parallel and perpendicular dissipation and conclude that the aspect width depends only weakly on any single geophysical parameter, such as collision frequency, gradient length, temperature, etc. This finding is consistent with our observation that the measured aspect angles vary little with altitude and only weakly with instability level.