The connection between the October effect in VLF observations and neutral atmosphere dynamics

Abstract

The October effect is long known as a strong decrease in VLF amplitudes occurring every October. However, neither it’s formation mechanism nor the characteristic of the October effect are known. Simultaneously with the October effect we observe a regional temperature increase between 55 and 75 km, i.e. in the region where VLF radio waves are reflected. In theory an increase in temperature can lead to an increased collision frequency and thus an increased VLF signal absorption resulting in a decreased VLF amplitude. Thus, there seems to be a connection between the October effect and the neutral atmosphere dynamics. Here we want to investigate the characteristics of the October effect in VLF signals in connection to neutral atmosphere temperature using different transmitter and receiver combiations as well as MLS satellite observations. We found that the October effect is strongest in polar latitudes and vanishes in low latitudes for temperature and VLF observations. Finally a possible formation mechanism for the October effect will be discussed.