|dc.description.abstract||Absorption of radio waves in the polar ionosphere near the magnetic noon was observed on October 8, 1991, by the 30 MHz imaging riometer at Ny-Alesund, Svalbard (invariant latitude 76.1°). These observations showed that the initially widespread absorption features became localized and enhanced in the high-latitude sector of the field of view, and followed a poleward motion. This behavior occurred quasi-periodically 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 106 el/cm3, drifted poleward from the polar cusp to the cap during the same period. This coincidence suggested that the ionospheric absorption was associated with F-region plasma patches in the polar cap. Theoretical absorption values of 0.14 dB, estimated using the electron densities and the electron-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 experimentally-determined collision frequencies, as indicated by Wang et al. (1994). These localized, enhanced, and poleward drifting absorption features over Ny-Alesund may be explained as F-region plasma patches produced by a magnetosheath-like particle precipitation into the cusp, and as small-scale irregularities caused by density gradients of the patches drifting into the polar cap.
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