Digital ionosonde measurements of the height variation of drift velocity in the southern polar cap ionosphere: Initial results

Abstract

During the late austral summer of 1995–1996 we operated an HF digital ionosonde located at Casey, Antarctica (66.3°S, 110.5°E, −80.8° 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⊥) 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⊥ 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⊥ 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‖) showed a strong diurnal variation, with mean values of −30 m s−1 near noon and +60 m 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 (UN) and perpendicular drift (V⊥s) moving plasma down the field lines during the day and up the field lines during the night, with UN and V⊥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° 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 × B/B2 convection to an E region collisional, unmagnetized plasma driven by E and irregular neutral winds.