Long-term trends in the lower ionosphere

The radio wave absorption in the lower ionosphere over Europe is analyzed and/or previous results are updated in three areas: (1) Trends in absolute values of absorption. (2) Trends in amplitudes of annual and semiannual variations. (3) Trends in inferred planetary wave activity. Trends in absolute values of absorption do not provide a consistent pattern, maybe due to their high sensitivity to instrumental problems. Similarly, there is no consistent pattern in trends in relative amplitudes of annual variations. Some data provide a positive trend, other a negative trend. On the other hand, there appears to be a systematic tendency to positive trend in amplitudes of semiannual variations. Some increase of the absorption-inferred planetary wave activity in the upper middle atmosphere over the recent four decades was found in the 1970s and 1980s, whereas no change was found in the 1960s. In the late 1980s and 1990s, the increase stopped and the planetary wave activity seems to stagnate. (C) 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.