Atmospheric Impact on VLF Radio Waves

Abstract

Radar waves with Very Low Frequency (VLF) are reflected in the D-region, the lower edge of the ionosphere. The D-region (60 - 90km) is influenced by the solar zenith angle and space weather from above and the mesosphere's dynamic and chemical processes. For example, during October there is a sharp decrease in the daytime VLF amplitude between transmitter and receiver combinations whose great circle paths lie in polar latitudes. Until now we do not know what causes the October effect. Space weather phenomena can be ruled out as a cause since their time scales are too short or too long and a similar behaviour cannot be observed in spring. We assume that dynamical and chemical processes in the mesosphere play a major role. A strong warming coinciding in the lower mesosphere alone cannot explain the sharp decrease in the VLF daytime amplitude. We suspect and confirm that the water vapour in the lower mesosphere, having similar characteristics as the warming and the VLF amplitude decrease, plays a crucial role in the formation mechanism of the October effect. Another example of the atmosphere's impact on VLF radio waves is sudden stratospheric warmings (SSWs). SSWs change the temperature and wind dramatically in the middle atmosphere. Thus, the circulation changes and hence the vertical transport of chemical constituents. These dramatic changes influence the VLF radio reflection during day and night up to several weeks after the SSW occurred depending on the type of the SSW. These two examples show the importance of understanding the impact of the atmosphere on VLF radio wave propagation.