Truncated VLF Saucers Observed by the Low Altitude Satellite DEMETER

We report DEMETER observations of a truncated VLF (Very Low Frequency) saucer in the auroral ionosphere. The very unusual shape of the frequency-time spectrogram arises from the sharp disappearance of the emissions below a frequency which displays a small and regular variation along the orbit. This low frequency cutoff results from the altitude variation of the LHF (Lower Hybrid Frequency) which is higher below the satellite than at the satellite altitude.