ATMOSPHERIC GREENHOUSE-EFFECT AND IONOSPHERIC TRENDS

Theoretical calculations predict that CO2 doubling would produce a 50 K decrease in the thermospheric temperature which can result in about 20 km decrease in the F2 peak height (hmF2) and a minor decrease in th e F2 layer critical frequency (foF2) [Rishbeth and Roble 1992]. In thi s paper we analyze ionosonde data for some 31 stations to study the lo ng term trends in hmF2 and foF2. Regression coefficients for hmF2 and foF2 as a function of solar activity, are obtained for each station an d departures (anomalies) from expected values derived for both these p arameters. An analysis of hmF2 and foF2 anomalies indicates negative t rends for some stations and positive trends for others. These varied b etween +29 to -20 km for hmF2 and +0.7 to -0.6 MHz for foF2 in the 34 year period since 1957, the International Geophysical Year (IGY). We c onclude that the present data do not provide a definitive evidence of any global long term trend in the ionosphere.