We investigate theoretically and experimentally the spectral characteristics of plasma density irregularities in the postsunset equatorial F region ionosphere during spread F conditions. We focus on irregularities occurring in the so-called inertial regime of the ionosphere, where polarization currents dominate Pedersen currents in the maintenance of charge neutrality on the boundaries of field-aligned plasma depletions. The equations of motion for the inertial regime flow are similar to the Navier Stokes equation for two-dimensional, neutral fluid turbulence. A model is presented for determining the location of the inertial regime, which may be found in a narrow window of local times and magnetic apex heights for a given longitude, season, and solar cycle. Large-amplitude depletions are shown to be necessary for inertial regime flow. We examined the most auspicious DE II plasma density fluctuation data for signatures of Navier Stokes turbulence or other processes that may be unique to the inertial regime. Approximately 50 orbital elements have been analyzed. Just 11 of these are predicted by our model to fall within the inertial regime. The density power spectra computed from data from all 11 exhibit k-5/3 power law scaling, suggesting that inertial-range turbulence operates at intermediate and large scales in the topside ionosphere. Meanwhile, spectra computed from plasma density data determined to lie outside the inertial regime exhibit a broad range of spectral indices. ¿ 1997 American Geophysical Union