Diurnal and seasonal variation of D-region electron density at low latitude

The electron density in D-region has been measured from Thumba (dip = 0), India during different times of the day and different seasons. The measurements were carried out over the last twentyfive years covering the period 1968 to 1993. The solar cycle effect was observed. The solar zenith angle dependence of the D-region electron density is in agreement with the theory. The occurrence of a ledge in the altitude region 70-90 ion is a unique feature of D-region over low latitudes. The ledge is a sudden reversal in electron density gradient with height in the region 70-90 km. (C) 1998 COSPAR. Published by Elsevier Science Ltd.