Imaging riometer and HF radar measurements of drifting F region electron density structures in the polar cap

The authors report riometer, photometer, and ionosonde data from an unusual event observed in the south pole polar cap region. F region radio absorption is not generally a factor for riometer fluctuations because the F region critical frequency foF2 is usually 5 to 10 times lower than the riometer operating frequency. When this frequency is greater than 6 MHz and the D region is relatively small with a large F region, it is possible that F region absorption can be appreciable. In the event observed, OI 630 nm emission is enhanced but not N2+ 427 nm emission, which argues against keV energy electron precipitation increases in the D or E region explaining this observation. Riometer, ionosonde, and simultaneous radar imaging indicate it could be an electron density structure from the F regions drifting into the polar cap region