Ionogram-recorded equatorial spread-F and height changes at Huancayo during sunspot-maximum years

The analyses presented here consider equatorial nighttime disturbance conditions as indicated by spread-F traces on ionograms, which record the presence of medium-scale structures (MSSs) created by the passage of atmospheric gravity waves. Relationships between these MSSs and geomagnetic activity (GA) have been examined statistically, particularly considering the ionospheric height (h'F) changes (increases and decreases) associated with the presence or absence of these MSSs. It is shown that for the postsunset height increases with spread-F present, the GA a few hours earlier has well-defined low levels. In contrast, if these height increases are not accompanied by spread-F, the GA a few hours earlier is high. When there are no height rises, similar inverse associations with GA are recorded for both (1) spread-F present and (2) spread-F absent. For the postsunset height decreases, when no spread-F occurs, high GA levels are well defined at times about 7 h earlier. Thus all these results indicate an inverse relationship between the MSSs and GA. The experimental evidence suggests that the presunrise height rises are produced by large-scale traveling ionospheric disturbances generated in auroral-zone regions by substorm onsets.