Decaying equatorial F region plasma depletions

We analyze plasma density data from the AE‐E satellite to determine how F region irregularities decay. This satellite had a low inclination orbit that was ultimately circular at an altitude of 434 km. It frequently observed plasma depletions in the postsunset regime during spread F conditions, sometimes along a trajectory affording a “horizontal cut” perspective through the field‐aligned irregularities. On one occasion, the satellite passed through nearly the same region of depleted flux tubes on sequential orbits. Data from these orbits imply that the irregularities decay at a constant rate over a broad range of horizontal scale sizes. We introduce a statistical model of the intermediate‐scale (10 km – 100 m) plasma irregularities and use it to infer a value for the perpendicular ambipolar diffusion coefficient from the measured decay rate. The value we estimate (2 m 2 /s) is close to the expected classical value. The uniform decay rate and the quasi‐universal nature of the intermediate‐scale spectra hint that the irregularities themselves maintain a characteristic shape as they decay. High‐resolution satellite density measurements show that irregularities have the form of kilometer‐scale quasi‐periodic depletions. We propose a one‐dimensional, nonlinear model of the collisional interchange instability which admits a closed‐form, steady state solution predicting the shape of the depletions. Computer simulations of the model produce kilometer‐scale waves that resemble the in situ observations.