DAYTIME IONOSPHERIC ABSORPTION FEATURES IN THE POLAR-CAP ASSOCIATED WITH POLEWARD DRIFTING F-REGION PLASMA PATCHES

Absorption of radio waves in the polar ionosphere near the magnetic no on was observed on October 8, 1991, by the 30 MHz imaging riometer at My-Alesund, Svalbard(invariant latitude 76.1 degrees). These observati ons showed that the Initially widespread absorption features became lo calized and enhanced in the high-latitude sector of the field of view, and followed a poleward motion. This behavior occurred quasi-periodic ally and repeated every 10-20 min. Simultaneous observations by EISCAT ''Polar'' experiments showed that nine discrete plasma patches, with F-region electron density enhanced by an order of 10(6) el/cm(3), drif ted poleward from the polar cusp to the cap during the same period. Th is coincidence suggested that the ionospheric absorption was associate d with F-region plasma patches in the polar cap. Theoretical absorptio n values of 0.14 dB, estimated using the electron densities and the el ectron-ion collision frequencies from the EISCAT F-region plasma data, are smaller than the observed values (<0.8 dB). This discrepancy may be related to the difference between the theoretically-and experimenta lly-determined collision frequencies, as indicated by Wang et al. (199 4). These localized, enhanced, and poleward drifting absorption featur es over Ny-Alesund may be explained as F-region plasma patches produce d by a magnetosheathlike particle precipitation into the cusp, and as small-scale irregularities caused by density gradients of the patches drifting into the polar cap.