Long-term changes in the ionosphere

The ionosphere is subject to many variations, notably the seasonal and solar activity cycles and the influences of geomagnetic activity, which are well documented even though imperfectly understood. These well-known variations make it difficult to detect long-term or secular changes, whether natural and man-made. Nevertheless, the ionospheric record now extends over 65 years and it is becoming feasible to investigate possible long-term changes. Possible causes of change include variations of solar activity, the secular variation of the geomagnetic field, global cooling in the upper atmosphere which would lower the ionospheric F2-layer, and changes in minor constituents. To detect any such changes, it is essential to maintain a basic world-wide programme of upper atmospheric and geomagnetic monitoring, and to ensure that the data are readily accessible to researchers. (C) 1997 COSPAR. Published by Elsevier Science Ltd.