Electronic Collision Frequency in the F Region at Sunrise & Sunset

193-194The variation of the effective collision frequency of electrons veff with solar zenith angle has been studied around sunrise and sunset by group retardation method [Appleton, E. V., Nature, Lond., 135(1935), 618] at different radio frequencies. The variation is found to be steeper at sunrise than at sunset. Taking a linear relationship between 'veff' and f2, the square of radio frequency, a rough estimate of the electron temperature Te has been made. The resulting values of Te at sunrise and sunset are found to be 1113 and 987o K., respectively