Ionospheric electric fields play fundamental roles in the dynamics of the Earth's ionosphere and thermosphere. Enhanced geomagnetic activity has long been known to cause large ionospheric electric field and current perturbations from high to equatorial latitudes. These electrodynamic disturbances can effect strongly the altitudinal and latitudinal distribution of ionization and the generation of plasma irregularities over a large area of the Earth. Over the last decade, experimental and global convection modeling studies have significantly improved our understanding of the electrodynamic response of the middle and low latitude ionosphere to enhanced geomagnetic activity. The dominant electric field disturbance processes have been identified as due to the prompt penetration of high latitude electric fields to lower latitudes, and to ionospheric disturbance dynamo electric fields driven by enhanced energy input into the high latitude ionosphere. The typical latitudinal, local time, and storm time dependence of these electric field disturbances don not depend on the details of magnetospheric physics, and are now reasonably well explained by global convection and ionospheric dynamo models. On the other hand, the mechanisms responsible for the large spatial and temporal variability of these perturbation electric fields, particularly during and shortly after strong geomagnetic activity, are not well understood. In this work, we first review the recent progress in the study of the middle and low latitude disturbance electric fields, focusing on prompt penetration electric fields. Then we discuss some of the solar wind-magnetosphere-ionosphere processes responsible of their large variability. These include magnetic field line stretching, dipolarization, and ionospheric conductivity effects.
