Effect of the day night ionospheric conductivity gradient on polar cap convective flow

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Abstract
The electric field data in the polar cap do not display the expected mirror symmetry for positive and negative values of the solar magnetospheric y component of the interplanetary magnetic field, suggesting that an additional effect is squeezing the antisunward flow toward the dawnside of the polar cap. It is shown that a conductivity decrease toward the nightside will produce such an effect. Results are both for a model with a discontinuous decrease in density and for models with a smooth exponential decrease in conductivity with distance across the polar cap. Steeper conductivity gradients cause stronger squeezing of the flow toward the dawnside of the polar cap. Outside the polar cap the return flow towards the dayside spreads over a wider latitude range on the dawnside than on the duskside. This is a likely explanation of the observed asymmetry in the thickness of the auroral oval.