Monitoring of Ionospheric D-Region Behavior Utilizing the Dual VLF Experiment

Studying Earth's ionosphere applying very low radio frequency (VLF) ground-based networks or only single station instrumentation provides spatially and temporal limited information. The majority of the VLF experiments utilize strong naval communication transmitters as their signal sources. In most of those cases, the signal is relatively unknown, except perhaps for its frequency and field strength. Other than ionosondes (vertical ionospheric sounder), VLF radio transmitters are usually located at different locations than their receiver stations. This results in very different radio wave propagation paths and because of different ionospheric regimes (longitudinal, latitudinal, or seasonal variations).

Because VLF radio transmission can be influenced by regular natural sources, e.g. sunrise or sunset, or irregular natural sources, e.g. solar X-ray flare or earthquake, it is non-trivial to provide accurate ionospheric weather forecasts or detect precursors for possible hazards. For natural hazards, such as earthquakes, ionospheric disturbances are often found only after the occurrence of the hazard. Therefore, it is essential to experiment with modified approaches, other than the common ionospheric investigation methods.

The aim of this study is a case-based analysis of the two VLF transmissions from Iceland at 37.5 kHz and 57.4 kHz originating from the Naval Radio Transmitter Facility at Grindavik. Utilizing passive broadband VLF ground-based measurements, we compare observed delay times of the signal behaviors at sunrise and sunset, as well as sudden ionospheric disturbances (SID). Other than the zenith angle dependence, which causes changes of the photoionization at the ionospheric D-region, SID is caused by solar X-ray flare radiation. The solar X-ray flare flux data are provided by NOAA's GOES satellite. The temporal difference in the VLF signal level is of the order of minutes for dusk and dawn. The difference between the two VLF signal levels can not be caused by significant differences in their propagation paths. However, it is assumed that this temporal delay reflects vertical ionospheric composition changes.

Usually, VLF monitoring networks are used for comparing an observed SID event at various VLF frequencies and recorded from various VLF monitoring stations. The D-region enhancement during daytime is stronger during solar flare events. Especially, by comparing the VLF signal levels the spatial effects of solar flares are studied. However, the present study focuses mainly on temporal variation of the signal levels during dusk, dawn, and during SID events, which again could be caused by vertical ionospheric composition changes.