Monitoring cusp/cleft topology using Pc5 ULF waves

Induction magnetometer data recorded at three closely spaced sites (~120 Km) in Antarctica (mlat ~−75°) have been examined for ionospheric signatures of the cusp/cleft region of the magnetosphere. Crossphase 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 T89 external field model crossphase measurements are put in the context of diurnally changing field line topology due to compression at the subsolar region and stretching along the dawn and dusk flanks. On six of the eight days of this study we have identify a consistent two dimensional phase pattern projected in the dayside ionosphere, indicating closed field lines thread these sites during periods of low to moderate geomagnetic activity (Kp<3).