Westward electric fields as the cause of nighttime enhancements in electron concentrations in midlatitudeFregion

Abstract

Large enhancements in electron concentrations are frequently observed in the midlatitude nighttime F region. These enhancements are explained in terms of enhanced downward diffusion flux from the protonosphere induced by substorm-associated electric fields in the ionosphere. A mechanism is proposed by which a westward electric field enhances downward flux from the protonosphere while lowering the F layer. (An eastward electric field has opposite effects.) This mechanism plus height variations of recombination loss can explain the observed behavior of the nighttime F layer during substorms. Crude estimates of electric fields and protonospheric fluxes are made from ionospheric parameters observed during a substorm. Under favorable conditions ground-based ionosonde records may be used to infer spatial and temporal variations of electric fields in the ionosphere during substorms. The results of this study suggest that in middle latitudes the nighttime ionosphere is controlled by vertical drifts and fluxes from the protonosphere.