Sodankylä Geophysical Observatory, FIN-99600 Sodankylä, Finland
Increased concentrations of greenhouse gases in the air cause a cooling of the strato-, meso-, and thermosphere (Roble and Dickinson, 1989); the atmosphere is expected to shrink. Rishbeth (1990) predicted a lowering of the peak height of the ionospheric F2 layer by 15-20 km for doubling of the mixing ratios of carbon dioxide and methane. Lidar measurements in southern France show a cooling of about 4 K between 60 and 70 km altitude for a period of 10 years between 1980 and 1990 (Hauchecorne et al., 1991). These records, however, span less than a solar cycle and are therefore too short to reveal long-term changes. Bremer (1992) observed lowering of the F2 layer peak height at Juliusruh/Rügen, Germany, which he intepreted in agreement with Rishbeth's prediction to be due to greenhouse cooling. We compare long-term data from Sodankylä, Finland (Ulich and Turunen, 1997), with other ionosonde records. The evolution of the F2 peak height varies considerably among the stations studied in this work; while Sodankylä data shows generally lowering of the F2 peak height, this is not observed in all four seasons and not at all locations analyzed in this study.