Signatures of the ionospheric cusp in digital ionosonde measurements of plasma drift above Casey, Antarctica

Signatures of the ionospheric cusp in HF digital ionosonde measurements of plasma drift made at the polar cap station Casey, Antarctica (-80.8 degrees geomagnetic latitude), are investigated. Measurements recorded during the campaign interval February 13-17, 1996, are considered in this case study b ecause the summer dipole tilt effect, and an interplanetary magnetic field (IMF) northward condition on February 16, were favorable for the detection of the cusp at a higher than usual latitude. On February 14 and 15 the magn itude of the IMF-was about 4-6 nT, and the station probably passed just pol eward of the cusp. The most general signatures of the cusp were enhanced el ectric field and electric field turbulence shown by increased drift velocit y and velocity scatter in the convection throat, respectively. Broadband, u nstructured fluctuations in the geomagnetic field measured by magnetometers near to noon are well known to be a signature of cusp currents and were as sociated with the intervals of enhanced convection turbulence. A major cusp event occurred above Casey on February 16, when the magnitude of the IMF i ncreased from about 5 to >10 nT. Cusp signatures during this event included the drift velocity surging to large values just before and after an interv al during which the F region echoes were lost because of an absence of I; r egion ionization and the formation of electron density patches. The loss of echoes was only partly explained by increased absorption and scatter of th e transmitted radio waves; Although the spectral width of Doppler peaks inc reased, this, by itself, was not a unique signature of the cusp because the obliquity of echoes also controlled the spectral width in our near-vertica l interferometry. However, signatures of the cusp were easily recognized in digisonde data, and the cusp's location and dynamics can be monitored usin g digisondes.