STUDY OF IONOSPHERIC VARIABILITY AT FIXED HEIGHTS USING DATA FROM SOUTH-AMERICA

A series of about 800 electron density profiles calculated by inversion of ionograms obtained at four stations in Argentina(low to middle geomagnetic latitudes) have been used to study the variability of electron density at fixed heights. The variability index used is the percentage ratio of the standard deviation over the mean. A set of groups of days has been selected representing different seasons and two years with different levels of solar activity. Also used were ''composite'' noon-profiles of San Juan ( in three years 219 profiles each representative for the given hour in a group of five consecutive days). Around noon minimum variability is consistently found at 170-190 km. Variability increases with height in the F2 region both by day and night, reaching a maximum somewhere below 300 km. At greater heights the variability show a tendency to decrease. Night-time variability is apparently larger. By day the variability decreases with increasing solar activity. This behaviour is reversed by night above 220 km. A certain increase of the variability with latitude appears to exist.