Studies on the sunrise effect in regions E and F

Abstract

The characteristics of an ionospheric layer are known to undergo variations during sunrise in the layer (sunrise-effect). The sunrise-effect begins not when the solar rays first strike the layer, but later. This delay phenomenon in the “sunrise-effect” has been studied for the E- and F-regions of the ionosphere at Calcutta during the period January 1947 to June 1952. It is shown that the observed delay times can be explained as due to the difference between the night-time and daytime heights of an ionospheric layer. The early-morning rays which strike the night side of the ionospheric layer and increase its ionization, must pass above the daytime height of the layer, in order that the active wavelengths do not get absorbed by the thickness of the air through which the rays pass. The measured daytime height of the layer (after the occurrence of the sunrise-effect) agrees reasonably well with the height grazing which the solar rays strike the nighttime height of the layer at the time of commencement of the sunrise-effect.