The October Effect in the Neutral Atmosphere Simulated by Different Whole Atmosphere Models

Abstract

The October effect is long known as a sharp decrease in the amplitude of radio waves with Very Low Frequency (VLF) reflected in the D-region (60-90km). However, the mechanism of the October effect is unclear until today. Recent studies show that neutral atmosphere dynamics might cause the October effect. Simultaneously with the decrease in the VLF amplitude, there is a warming and drying in the lower mesosphere which cannot be observed in spring resulting in a spring-fall asymmetry similar to the VLF amplitude. This spring-fall asymmetry is reproduced by MERRA-2 only in years after 2005 when satellite observations are assimilated in the mesosphere. Other models like WACCM-X also have difficulties reproducing this asymmetry while CMAM30 can reproduce the spring-fall asymmetry. A modelling study and various analysis techniques are used to investigate the mechanism of the October effect in the neutral and ionized atmosphere. Based on our results we assume that the onset of the planetary wave activity and westward gravity wave drag after the quiet summer season induces a poleward and downward motion resulting in the observed warming and drying in the lower mesosphere.