MONITORING CUSP CLEFT TOPOLOGY USING PC5 ULF WAVES/

Induction magnetometer data recorded at three closely spaced sites (si milar to 120 Km) in Antarctica (mlat similar to-75 degrees) have been examined for ionospheric signatures of the cusp/cleft region of the ma gnetosphere. Crossphase analysis of the 1-10 mHz band, using pure-stat e filtering techniques reveal diurnally varying field line resonances embedded in the spectra, while interstation phase lag measurements ind icate azimuthal propagation of waves away from local magnetic noon. Us ing the T89 external field model crossphase measurements are put in th e 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, ind icating closed field lines thread these sites during periods of low to moderate geomagnetic activity (Kp<3).