On several occasions significant riometer absorption has been measured at South Pole station in association with F region plasma patches formed in the cusp and propagating into the polar cap. The analysis of these events has used imaging riometer, photometer, and ionosonde data from South Pole as well as simultaneous observations of the patches by the Halley HF coherent scatter radar. However, detailed information about the F region structures such as plasma densities and temperatures, which is required to quantitatively study the response of riometer absorption, is unavailable from the instrumentation in Antarctica. Because both incoherent scatter radar and imaging riometer data are available from Sondre Stromfjord, it is possible to study such phenomena from that location. We have analyzed an event that occurred on September 14, 1991, when the radar and local ionosonde observed an F region plasma patch moving poleward on the dayside. Calculated values of the zenithal riometer absorption, arising from electron-ion collisions in the F region, were of the same order as the measured absorption when the patch was overhead. After the patch had passed, however, the measured zenithal absorption significantly exceeded the calculated contribution from the F region. We attribute this to the later presence of absorption in lower altitude regions of the ionosphere, which is confirmed by the ionosonde measurements of increased ƒmin.
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