Summary
Induction magnetometer data recorded at three closely spaced sites (110km) in Antarctica (mlat -75°) have been examined for ionospheric signatures of the outer magnetosphere and cusp/cleft regions. Cross-phase analysis of the 1–10 mHz band, using pure-state filtering techniques reveal diurnally varying field line resonances embedded in the spectra, while interstation phase lag measurements indicate azimuthal propagation of waves away from local magnetic noon. Using the T96 (Tsyganenko, 1995) external field model, cross-phase measurements are put in the context of diurnally changing field line topology, due to compression at the sub-solar region and stretching along the dawn and dusk flanks. This suggests field line stretching as an alternative or additional explanation to the Kelvin-Helmholtz instability for azimuthal wave propagation. A ‘wave cusp meridian’ identified in the propagation data shows a variable location about local magnetic noon which is under the control of IMF By and shows the same well-known behaviour as the cusp to these variations.