Electrodynamic coupling of high and low latitudes: Observations on May 27,1993

The penetration of disturbance electric fields from the polar region to the magnetic equator on the dayside of the Earth is examined with geomagnetic data on May 27, 1993. First, we examine a dayside equatorial disturbance th at followed the rapid recovery of magnetic activity from a storm and that h as the characteristics of overshielding caused by persistent region-2 field -aligned currents. It lasted similar to 3 hours. Second, we analyze a serie s of fluctuations with periods of 25-75 min, to determine the variations of amplitude and phase with magnetic latitude and magnetic local time. The fl uctuations were highly coherent at all latitudes between the magnetic equat or and the auroral zone, but the coherency decreased in the polar cap. A no rthward fluctuation at the equator during midday hours accompanied auroral zone fluctuations that were southward before noon, eastward around noon, an d northward after noon. The amplitudes decreased away from the auroral zone toward midlatitudes but were amplified under the equatorial electrojet. No detectable phase differences are found, indicating that any temporal lags which might be induced by persistence in the region-2 field-aligned current s are less than 1 min for fluctuations having periods like those examined h ere. A synoptic inversion analysis of the high-latitude magnetic data to es timate the time-varying high-latitude electric potential patterns shows tha t fluctuations of the high-latitude east-west potential gradient tended to be concentrated around midday, where they were in phase with fluctuations i n the midday east-west potential gradient at the magnetic equator.