Trends in the characteristics of the annual and semiannual variations observed in the radio wave absorption in the lower ionosphere

[摘要] The continuous increase in concentration ofgreenhouse gases in the atmosphere is expected to cool higher levels of theatmosphere. There is some direct and indirect experimental evidence of long-termtrends in temperature and other parameters in the mesosphere and lowerthermosphere (MLT). Here we look for long-term trends in the annual andsemiannual variations of the radio wave absorption in the lower ionosphere,which corresponds to the MLT region heights. Data from central and southeasternEurope are used. A consistent tendency to a positive trend in the amplitude ofthe semiannual wave appears to be observed. The reality of a similar tendency inthe amplitude of the annual wave is questionable in the sense that the trend inthe amplitude of the annual wave is probably induced by the trend in the yearlyaverage values of absorption. The phases of both the annual and semiannual wavesdisplay a forward tendency, i.e. shift to an earlier time in the year. Atentative interpretation of these results in terms of changes of the seasonalvariation of temperature and wind at MLT heights does not contradict the trendsobserved in those parameters.

Key words. Ionosphere (ionosphere – atmosphereinteractions; mid-latitude ionosphere) · Meteorology and atmospheric dynamics(middle atmosphere dynamics)

[学科分类] 地球科学(综合)