Diurnal asymmetry in the lower ionosphere — seasonal variability

Summary On the basis of long period measurements of ionospheric absorption in five A3 circuits in Central Europe, it is shown that the considerable seasonal variation of the diurnal asymmetry of absorption, found in[1], exhibits practically no year-to-year variability and is well-developed at equivalent frequencies f_eq1 MHz, while it vanishes at F_eq 2 MHz. The limited data on the diurnal asymmetry of the D-region electron concentration are consistent with the seasonal pattern of the diurnal asymmetry in absorption. A tentative hypothesis of nitric oxide variability as the cause of the seasonal variation of the asymmetry is proposed.