The effect of gravity and pressure in the electrodynamics of the low-latitude ionosphere

The source of the plasma drift structure in the low-latitude ionosphere during magnetically quiet times is generally understood. Nearly all the quiet time electric field structure has been attributed to divergences in the neutral wind current dynamo. However, there are other current drivers active in the low latitude ionosphere: a gravity-driven current (g x B drift of O+) and a gradient-pressure current (delP x B drift of e and O+). The gravity-driven current is important in the development of the Rayleigh-Taylor instability and equatorial spread F (ESF) but is ignored in global dynamo electric field models. The gradient pressure and gravity terms in the ionosphere momentum equation normally oppose each other. This paper examines the magnitude of the large-scale electric fields generated by divergences in these lesser currents in a coupled ionosphere-electrodynamics model. The low-latitude electric fields generated by the neutral wind dynamo are altered only slightly by these additional terms. The resulting vertical electric fields (zonal plasma drifts) do not change significantly but the vertical plasma drifts are affected by similar to10 to 15 m/s in the hours before sunrise and after sunset. These low-latitude plasma drift differences are due entirely to the inclusion of the gravity-driven current. The gravity current term produces a downward drift after sunset, which reduces the evening prereversal enhancement. Additionally, the positive vertical drift before sunrise is sometimes large enough to cause a predawn vertical enhancement. The gradient pressure terms can be ignored for large and medium scale ionosphere features.