CALCULATIONS OF RIOMETER ABSORPTION ASSOCIATED WITH F-REGION PLASMA STRUCTURES BASED ON SONDRE STROMFJORD INCOHERENT-SCATTER RADAR OBSERVATIONS

On several occasions significant riometer absorption has been measured at South Pole station in association with F region plasma patches for med in the cusp and propagating into the polar cap. The analysis of th ese events has used imaging riometer, photometer, and ionosonde data f rom South Pole as well as simultaneous observations of the patches by the Halley HF coherent scatter radar. However, detailed information ab out the F region structures such as plasma densities and temperatures, which is required to quantitatively study the response of riometer ab sorption, is unavailable from the instrumentation in Antarctica. Becau se both incoherent scatter radar and imaging riometer data are availab le from Sondre Stromfjord, it is possible to study such phenomena from that location. We have analyzed an event that occurred on September 1 4, 1991, when the radar and local ionosonde observed an F region plasm a patch moving poleward on the dayside. Calculated values of the zenit hal riometer absorption, arising from electron-ion collisions in the F region, were of the same order as the measured absorption when the pa tch was overhead. After the patch had passed, however, the measured ze nithal absorption significantly exceeded the calculated contribution f rom the F region. We attribute this to the later presence of absorptio n in lower altitude regions of the ionosphere, which is confirmed by t he ionosonde measurements of increased f(min).