Long term ionospheric trends over Ahmedabad

Abstract

Ionospheric data over Ahmedabad (23°N, 73°E) during 1955–96 have been examined for the long term changes arising due to the predicted cooling of the mesosphere and thermosphere because of the increased concentration of trace gases resulting from anthropogenic causes. The long term changes in the critical frequencies of the ionospheric layers and the height of the maximum ionization as characterized by h P F 2 , the virtual height at 0.834 of f o F 2 , have been obtained after removing the seasonal and solar cycle variations. While the trend of a small decrease in f o E (0.15 MHz in 40 years) may not be considered significant, there is a clear indication of a decrease in f o F 2 (1.6 MHz for midday, 1.0 MHz for midnight). A small increase is noted in f o F 1 (0.3 MHz). There is a decrease in h P F 2 (16 km for midday and 10 km for midnight). The lowering of F 2 layer peak will affect radio propagation.