Summary
A Lowell DPS-4 digital ionosonde has operated at the Australian Antarctic polar cap station of Casey (-80.6o CGM latitude) since early 1993. During that time the DPS-4 has been operated in a routine mode to obtain an extended database of background ionospheric drift measurements [1]. Special campaigns have been conducted in which the instrument has been operated in special new modes to study, for example, the variation in drift with height [2], polar patches and formation of sporadic E [3]. New display modes for ionosonde data have been exploited to elucidate features of the wide range of dynamic phenomena that occur in the polar cap ionosphere. The background ionospheric drift data have been used to develop a model of drift at Casey dependent on IMF and local time. This paper will review the experimental and data analysis techniques developed, including a Dopplionogram technique and Doppler-sorted interferometry. The general behaviour of ionospheric convection at Casey will be discussed, including the seasonal variation, one feature of which is the development during summer of a lobe cell circulating on open field lines, embedded within one of the merging cells. Examples of polar patches, TIDs and variations between E and F region drifts will also be presented. Overall the results clearly show that flexibility of operation in modern digital ionosondes make them powerful instruments for studying the wide range of phenomena that occur in the polar cap ionosphere.