VLF waves: Conjugated ground-satellite relationships

The French mobile station for recording geophysical data has been put in operation at Husafell, Iceland (64°5N, 20°8W) between the 10th of July and the 22nd of September, 1977. This place was more or less conjugated with GEOS when this satellite was near its apogee. The equipments installed in the station for recording VLF and ULF phenomena have characteristics (band-pass, sampling rates) which are identical to the similar equipments installed onboard GEOS. Intercomparison between signals recorded at both points are therefore easy. We present here the results which were obtained in the VLF range.In many occasions, VLF emissions (mainly hiss) do present identical variations in amplitude, with a very abrupt (<1 mn) and very large (>20 dB) decrease in amplitude. Because of their simultaneity at both points, such abrupt variations cannot be interpreted in terms of a sudden ionospheric absorption (associated with an enhanced particle precipitation) nor in terms of a sudden crossing of detached plasma regions. In some cases, these abrupt changes in the VLF intensity are associated with the appearance and disappearance of strong ULF emissions, in the Pc-1 frequency range. Some examples of associated onboard measurements of high energy electron fluxes or cold plasma density (when available) are given, which may help understanding these VLF conjugated relationships.