What are the Source of MF Signatures Recorded on DEMETER Satellite?

Abstract

We found new signatures on statistical maps of HF DEMETER data compiled for narrow frequency window and for seasonal variations. Beside the known signal enhancements between 1.8–2.9MHz above the powerful VLF transmitters that can be attributed to the effect of heating of the ionosphere by the VLF waves, we found signal decrease above the VLF transmitters. The low level of the seasonal variation on signal enhancement and decrease above the VLF transmitter may question the origin of the variation that could be attributed to high frequency part of terrestrial lightnings. We have found signatures of increased signal level over certain areas above 2.7MHz for local winter periods. They may be attributed to MF radio broadcast transmitters and can propagate in ordinary mode above the maximum local plasma frequency through the ionosphere as during the local winter. However some of the area found are not expected to exhibit radio broadcasting activity (Himalaya, Tibet, North sea), while there are areas spatially close the ones found, that do not exhibit such signatures (East US cf. West US or Central Europe cf. Western Europe). The absence of signal increase/decrease above the three Russian Alpha transmitters is not yet understood also.