Nonlinear equatorial spread F: The effect of neutral winds and background Pedersen conductivity

Abstract

A simple model of the interaction of the equatorial ionosphere with the eastward F region neutral wind in the presence of evolving spread F bubbles is given. A consequence of the model is that the upper portions of bubbles will take on a westward tilt, while the lower portions will tilt eastward, giving rise to the ‘fishtails’ and ‘C’s’ observed by coherent backscatter radar measurements of field‐aligned small‐scale irregularities. The essence of the model is that the plasma away from the equatorial plane (e.g., a background nighttime E region at higher latitudes) makes a finite contribution to the magnetic field line‐integrated Pedersen conductivity, causing an incomplete coupling of the plasma motion to the neutral wind. The degree of coupling is then a function of the Pedersen conductivities both near the equator in the F region and in the higher‐latitude E region, giving rise to vertical shears of east‐west plasma motion having opposing signs on either side of the equatorial Pedersen conductivity maximum. Evolving spread F bubbles are caught up in this shear as they rise vertically, resulting in the characteristic ‘C’ shape seen by backscatter radar and in the westward motion of plasma bubbles observed by satellite in situ measurements. Numerical simulations, incorporating an eastward neutral wind in the equatorial F region and E region Pedersen conductivity effects, are presented to further support the model and analysis. The simulations also show the result that it may be the eastward as well as the westward wall of a bubble which is subject to secondary instabilities in the presence of an eastward neutral wind. In addition, even without the neutral wind the numerical simulations show that E region Pedersen conductivity effects can result in a slowing down of equatorial spread F and attendant bubble evolution.