EMPIRICAL-MODELS OF STORM TIME EQUATORIAL ZONAL ELECTRIC-FIELDS

Ionospheric plasma drifts often show highly complex and variable signa tures during geomagnetically active periods due to the effects of diff erent disturbance processes. We describe initially a methodology for t he study of storm time dependent ionospheric electric fields. We prese nt empirical models of equatorial disturbance zonal electric fields ob tained using extensive F region vertical plasma drift measurements fro m the Jicamarca Observatory and auroral electrojet indices. These mode ls determine the plasma drift perturbations due to the combined effect s of short-lived prompt penetration and longer lasting disturbance dyn amo electric fields. We show that the prompt penetration drifts obtain ed from a high time resolution empirical model are in excellent agreem ent with results from the Rice Convection Model for comparable changes in the polar cap potential drop. We also present several case studies comparing observations with results obtained by adding model disturba nce drifts and season and solar cycle dependent average quiet time dri ft patterns. When the disturbance drifts are largely due to changes in magnetospheric convection and to disturbance dynamo effects, the meas ured and modeled drift velocities are generally in good agreement. How ever, our results indicate that the equatorial disturbance electric fi eld pattern can be strongly affected by variations in the shielding ef ficiency, and in the high-latitude potential and energy deposition pat terns which are not accounted for in the model. These case studies and earlier results also suggest the possible importance of additional so urces of plasmaspheric disturbance electric fields.