Long-term trends of the critical frequency of the F2 layer at northern and southern high latitude regions

The long-term behaviour of the plasma frequency of the F2 layer, foF2, is investigated to contribute to the discussion about the natural or anthropogenic origin of the ionospheric long-term changes. In this paper, long time series of hourly foF2 data from two high latitude ionospheric observatories of both the hemispheres are analysed. A decreasing trend of foF2 is found confirming a previous result obtained by using the foF2 monthly medians from Antarctic stations. This negative trend does not show a clear dependence on the long-term behaviour of the geomagnetic activity impact on the ionosphere. (C) 2002 Elsevier Science Ltd. All rights reserved.