Long-term changes in ionospheric parameters over Ahmedabad

Ionospheric data over Ahmedabad obtained from regular radio soundings made during past four decades are analyzed to study long term changes. Midday values of the critical frequencies of E, F-1 and F-2 layers show a small positive trend with changes of less than 0.1 MHz for E and F-2 layers and about 0.2 MHz for F-2 layer in of 40 years. Increase in the peak altitude of F-2 layer is also noticed. However the increases are within the limits of accuracies involved in estimation. (C) 1997 COSPAR. Published by Elsevier Science Ltd.