DIGITAL IONOSONDE MEASUREMENTS OF THE HEIGHT VARIATION OF DRIFT VELOCITY IN THE SOUTHERN POLAR-CAP IONOSPHERE - INITIAL RESULTS

During the late austral summer of 1995-1996 we operated an HF digital ionosonde located at Casey, Antarctica (66.3 degrees S, 110.5 degrees E, -80.8 degrees corrected geomagnetic (CGM) latitude), in an experime ntal drift mode with the aim of resolving the height variation of drif t velocity in the polar cap ionosphere. We devised control programs fo r a Digisonde Portable Sounder 4 to collect data at separate frequency -range gates corresponding to the E and F regions to investigate the d ifferences in their motions, During a 4-day campaign commencing March 11, 1996, the mode values of the drift perpendicular to the magnetic f ield (V-perpendicular to) were 85 m s(-1) in the E region and 485 m s( -1) in the F region (using 10 m s(-1) bins and echoes from all heights in each region). Vertical profiles of drift velocity were obtained by sorting echoes into 10-km group-height bins. For measurements obtaine d within +/-3 hours of magnetic noon the average profile showed that i n the lower E region V-perpendicular to increased approximately expone ntially with true height. The corresponding velocity scale height was <9.0 km at 105 km, where the gradient was >46.7 m s(-1) km(-1). The me an value of V-perpendicular to leveled off to about 700 m s(-1) above 120 km, where it remained up to the F region peak height. The vertical gradient was caused by the increase in collision frequencies at the l ower heights. The F region field-aligned component of drift (V-paralle l to) showed a strong diurnal variation, with mean values of -30 m s(- 1) near noon and +60 in s(-1) during the night at a height of 180 km. The average over the whole day reveals a net upward drift of 30 m s(-1 ). This behavior is attributed to the interaction between the meridion al components of the generally antisunward neutral wind (U-N) and perp endicular drift (V-perpendicular to S) moving plasma down the field li nes during the day and up the field lines during the night, with U-N a nd V-perpendicular to S having net equatorward values when averaged ov er all day. While the E region drift direction tended to be aligned wi th the basic antisunward convection which dominates the F region above Casey, it also tended to show greater temporal variability in directi on, suggesting a smaller-scale size and lifetime for the E region stru ctures giving rise to the echoes. There were events lasting over 2 hou rs during which the drifts in the two regions were clearly resolved in to different azimuths (by nearly 180 degrees for two events). These tr ansient directional shears show the time variability in the phase tran sition between an F region collisionless, magnetized plasma driven by the E x B/B-2 convection to an E region collisional, unmagnetized plas ma driven by E and irregular neutral winds.