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 experimental drift mode with the aim of resolving the height variation of drift velocity in the polar cap ionosphere. We devised control programs for a Digisonde Portable Sounder 4 to collect data at separate frequency-range gates corresponding to the E and F regions to investigate the differences in their motions, During a 4-day campaign commencing March 11, 1996, the mode values of the drift perpendicular to the magnetic field (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 obtained within +/-3 hours of magnetic noon the average profile showed that in the lower E region V-perpendicular to increased approximately exponentially 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 mean 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 lower heights. The F region field-aligned component of drift (V-parallel 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 meridional components of the generally antisunward neutral wind (U-N) and perpendicular drift (V-perpendicular to S) moving plasma down the field lines during the day and up the field lines during the night, with U-N and V-perpendicular to S having net equatorward values when averaged over all day. While the E region drift direction tended to be aligned with the basic antisunward convection which dominates the F region above Casey, it also tended to show greater temporal variability in direction, suggesting a smaller-scale size and lifetime for the E region structures giving rise to the echoes. There were events lasting over 2 hours during which the drifts in the two regions were clearly resolved into different azimuths (by nearly 180 degrees for two events). These transient directional shears show the time variability in the phase transition between an F region collisionless, magnetized plasma driven by the E x B/B-2 convection to an E region collisional, unmagnetized plasma driven by E and irregular neutral winds.