IMAGING RIOMETER AND HF RADAR MEASUREMENTS OF DRIFTING F-REGION ELECTRON-DENSITY STRUCTURES IN THE POLAR-CAP

A study of riometer, photometer, and ionosonde data from South Pole St ation has revealed a class of unusual dayside absorption events which occur near local magnetic noon when South Pole is located poleward of the dayside cusp. These events, which can reach values in excess of 1 dB in narrow-beam 38.2-MHz riometer measurements, are accompanied by s ignificant enhancement of the O(1D2) 630.0-nm emission. However, becau se there is no corresponding change in the N2+ 427.8-nm emission, it i s unlikely that these events are caused by the conventional D or lower E region ionization increases usually attributed to the precipitation of auroral electrons of keV energy. Rather, the comparison of South P ole imaging riometer and ionosonde data with simultaneous Halley PACE HF radar data suggests that these unusual absorption events are relate d to F region electron density structures drifting from the dayside ov al into the polar cap. If further work sustains this interpretation, t hen new prospects will be opened up for using imaging riometers to exa mine aspects of polar cap convection.