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Theoretical calculations predict that CO 2 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 the F2 layer critical frequency (foF2) Rishbeth and Roble 1992. In this paper we analyze ionosonde data for some 31 stations to study the long term trends in hmF2 and foF2. Regression coefficients for hmF2 and foF2 as a function of solar activity, are obtained for each station and departures (anomalies) from expected values derived for both these parameters. An analysis of hmF2 and foF2 anomalies indicates negative trends for some stations and positive trends for others. These varied between +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 conclude that the present data do not provide a definitive evidence of any global long term trend in the ionosphere.
Upadhyay et al. (Tue,) studied this question.