Trends in planetary wave activity in the upper middle atmosphere inferred from the nighttime LF radio wave absorption measurements

Summary The nighttime LF radio wave absorption in the lower ionosphere measured at two frequencies in central Europe over 1963–1985 is used to infer planetary wave activity and its long-term trend in the upper middle atmosphere (∼90–100 km). The observed positive trend is roughly consistent with results based on daytime absorption. Nighttime results are less pronounced and less statistically significant probably due to perturbing effects of geomagnetic activity. The observed trends, which are probably of anthropogenic origin, are together with the daytime results [3,4] the first evidence of long-term trends in planetary wave activity in the upper middle atmosphere.