Seasonal and magnetic activity variations of ionospheric electric fields above the southern mid-latitude station, Bundoora, Australia

We investigate the seasonal, local solar time, and geomagnetic activity var iations of the average Doppler velocity measured by an HF digital ionosonde deployed at Bundoora, Australia(145.1 degrees E, 37.7 degrees S, geographi c; 49 degrees S magnetic). The Doppler velocities were heavily averaged to suppress the short-term effects (<3 hours) of atmospheric gravity waves, an d thereby obtain the diurnal variations attributed to the tidally-driven io nospheric dynamo and electric fields generated by magnetic disturbances. Th e observed seasonal variations in Doppler velocity were probably controlled by variations in the lower thermospheric winds and ionospheric conductivit y above Bundoora and in the magnetically conjugate location. The diurnal va riations of the meridional (field perpendicular) drifts and their perturbat ions exhibited a complex structure, and were generally smaller than the var iations in the zonal drifts. The latter were basically strongly westward du ring the evening to early morning, and weakly eastward during the late morn ing to just past noon. The zonal perturbations were strongly enhanced by in creasing geomagnetic activity, and closely resembled the perturbation drift s measured by the incoherent scatter radar (ISR) at Millstone Hill (71.5 de grees W,42.6 degrees N, 57 degrees N). There was also some resemblance betw een the diurnal variations in the meridional drifts. Overall. the compariso ns suggest that with sufficient averaging, Doppler velocities measured with digital ionosondes at mid-latitudes correspond to true ion motions driven by ionospheric electric fields. This is a useful result because apart from the ISRs located in the American-European sector, there are no ground-based instruments capable of measuring electric fields in the mid-latitude ionos phere.