The ionosonde data are studied for equatorial station, Thumba, to delineate various features of the evening height rise of F layer. Sharp increase of h'F and hpF2 is observed in the postsunset period for high solar activity. Seasonal variation is observed in this increase of h'F and it is maximum for equinox months. For summer months, there is a delay of about an hour in the time of occurence of h'F (peak) as compared to winter and equinox months. This delay is shown to be associated with the delay in sunset times in the conjugate E regions. As for magnetic activity dependence, it is found that this height increase is less pronounced for disturbed days for winter and equinox whereas for summer it is marginally higher over the quiet day values. Further, it is observed that the value of h'F (peak) during disturbed periods is almost at the same value of 350 km for all the three seasons. Thus the seasonal variation of magnetic activity effects appears to be mainly governed by the average seasonal variation for quiet times.

The increase in F layer height is due to zonal eastward electric fields developed after sunset which is believed to be due to F region dynamo fields. While the main driving force for these fields is the zonal neutral winds, the development of these fields depend on the ratio of the F region to E region conductivity and the longitudinal gradient in the E region conductivity. Experimental observations of both the neutral winds and ionospheric conductivities are examined for their variation with solar activity, season and magnetic activity as both these factors will contribute for the various observed features of the height rise. Through model calculations it is shown that the E region density (conductivity) gradient in the postsunset period is higher by a factor of 2 for high solar activity compared to low solar activity and hence it is partly responsible for the observed solar activity variation of the postsunset height rise of the F layer. The importance of the studies using available ionosonde data for understanding the F region dynamo electric fields is emphasized. ¿ Copyright 1990 by the American Geophysical Union