High-latitude ionospheric response to a geomagnetic sudden commencement

A geomagnetic sudden commencement on March 25, 1987 was simultaneous with the onset of a large-scale depletion of F-region electron density observed by the EISCAT incoherent scatter radar in the dusk sector of the auroral zone (69.6 N, 19.2 E). Ionospheric signatures of the impulse were a transient increase in the northward component of the electric field and in the ion temperature. The maximum density depletion occurred about 1 h after the magnetic impulse and corresponded to maxima in the electric field and ion temperature. Ionosonde measurements from stations south of the radar showed that the density depletion progressed steadily equatorwards by more than 10 deg of latitude during the 2 h following the impulse. A brief recovery of F-region densities after the passage of the depletion was observed by the ionosondes, but the ionosphere above the radar remained depleted. It is concluded that the main cause of the observed density depletion was sunward transport of lower density nightside plasma following an equatorward expansion of the high-latitude plasma convection pattern associated with the magnetic impulse.