Electrodynamic coupling of high and low latitudes: Simulations of shielding/overshielding effects

The penetration of the electric field and associated magnetic perturbations from high latitudes to low latitudes is studied with the Magnetosphere-The rmosphere-Ionosphere-Electrodynamics General Circulation Model (MTIEGCM) of Peymirat et al. [1998] in response to variations of the polar cap potentia l drop, For a sudden decrease of the polar cap potential of similar to 40 k V, the model reproduces the well-known overshielding phenomenon correspondi ng to a sudden reversal of the low-latitude electric field. For quasi-perio dic oscillations of the polar cap potential drop of similar to 40 min perio d, the model predicts that the poleward electric field and the eastward Hal l current in the auroral zone lag slightly in phase (< 1 min), while the ea stward electric field and current at the magnetic equator are advanced slig htly in phase, with respect to the potential-drop oscillations. These phase differences are interpreted as the consequence of the succession of shield ing and overshielding episodes induced by the response of the region-a fiel d-aligned currents to the polar cap variations. The phase differences among the polar cap potential and the auroral and equatorial electric fields and currents increase with the plasma sheet pressure, The amplitude of the ass ociated magnetic perturbations is very dependent on the distribution of the potential along the polar cap boundary. The model predictions are tested a gainst the observations of Kobea et al. [this issue] for two events, one re presenting simple overshielding and the other associated with polar cap pot ential oscillations. The model underestimates the decay time of the magneti c perturbations by a factor of 2 during the overshielding event, and the mo del gives results compatible with the observations during the second event. The disagreements may be due to limitations of the model and uncertainties of the input parameters.