VLF step frequency sounding of the polar lower ionosphere

The lower ionosphere above Byrd Station, Antarctica, is probed using radio waves over the frequency range 3.0 kHz to 30 kHz. A new phase radar technique is used to determine an accurate phase height of reflection for a propagation path near vertical incidence. The results of the first seven months of synoptic sounding of the polar D region are presented. The analysis of these data has yielded the average phase height behavior during periods of total darkness, continuous sunlight, and day-night, night-day transition, under both quiet and disturbed magnetic conditions. The dominant ionization production mechanism is related to energetic particle precipitation and is only slightly affected by solar ultraviolet radiation. Phase height measurements obtained during a large polar cap absorption event are also shown.