Magnetic activity effects on range type spread-F and vertical plasma drifts at Fortaleza and Huancayo as studied through ionosonde measurements and theoretical modelling

Ionosonde measurements are used to study the effects of magnetic activity on spread F and on the postsunset enhancement (Vzmax) of the F region vertical plasma drift at two longitudinally separated stations, Fortaleza and Huancayo, situated along the magnetic equator, in the American sector, during a period of high solar activity. The analysis is made for three different levels of magnetic activity. It is seen that, during the winter, Vzmax and the range spread F occurrence (RSFO) in the premidnight hours systematically increase with Kp at both locations. During the summer and equinoxes the RSFO decreases with magnetic activity in the premidnight hours and increases in the postmidnight hours, the increase being generally more pronounced during very disturbed periods. On the other hand Vzmax deduced from ionosonde measurements shows almost no variation with magnetic activity during equinoxes and summer although during individual months of those periods both increases and decreases are seen. With the help of theoretical model calculations for the electrodynamical coupling between the equatorial E and F regions it is shown that a disturbance F region dynamo associated neutral wind could account for the variations of Vzmax with magnetic activity, that are consistently observed during winter months. On the other hand, competing influences of penetration electric fields, and/or those of disturbance meridional winds, seem to mask the expected gross effect of zonal winds in the summer and equinoctial months