A possible cause for the October effect in the D-region

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 by the mesosphere's dynamic and chemical processes. During October there is a sharp decrease of 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. The solar zenith angle can also be ruled out as a similar behavior is not observed in spring. Thus, there is an assumption that dynamical processes in the mesosphere play a major role. Previous studies showed that a strong warming occurs in the lower mesosphere shortly before the October effect is observed. While the characteristics of this warming help us to explain why the October effect occurs during daytime only, the warming alone cannot explain the sharp decrease in the VLF daytime amplitude. We suspect and confirm that the water vapor 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.

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