Why does the October effect not occur at night?

Radar waves with very low frequency (VLF) are reflected at the lower edge of the ionosphere, in the D-region. The D-region (60 - 90km) is influenced by the solar zenith angle and space weather from above as well as by dynamical and chemical processes in the mesosphere. During October there is a well-known sharp decrease of the daytime VLF amplitude between transmitter and receiver combinations whose great circle paths lie mainly 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 either too short or too long. The solar zenith angle, strongly influencing the seasonal variation of the VLF amplitude can also be ruled out as a similar behavior is not observed in spring. Thus, there is a strong assumption that neutral dynamical processes in the mesosphere play a major role. We assume and confirm that a regional warming in the lower mesosphere, occurring simultaneously and with similar characteristics as the October effect, plays a major role in the formation process of the October effect. The VLF reflection height is about 15km higher during nighttime than during daytime. This difference in combination with the location of the regional warming explains, why the October effect can not be observed during nighttime.