Relationships between the AE , ap and Dst indices near solar minimum ( 1974 ) and at solar maximum ( 1979 )

Three-hourly average values of the Dst, AE and ap geomagnetic activity indices have been studied for 1 year's duration near the solar minimum (1974) and also at the solar maximum (1979). In 1979 seven intense geomagnetic storms (Dst <A100 nT) occurred, whereas in 1974 only three were reported. This study reveals: (1) the yearly average of AE is greater in 1974 than in 1979, whereas the inverse seems to be true for the yearly average of Dst, when a higher number of intense storms is present. These averages indicate the kind of activity occurring on the sun as shown in earlier work. (2) The seasonal variation of Dst is higher than that of ap and is almost negligible in AE. (3) The correlation coecient of ap´AE is in general the highest, as the magneto-meters that monitor both indices are close, and is surpassed only by the ap´Dst correlation during geomagnetic storms, when the in¯uence of the ring current is dominant. The correlation of ap´Dst also shows a seasonal variability. (4) For the ®rst time a study of correlation between ap and a linear combination of AE and Dst has also been made. We found higher correlation coecients in this case as compared to those between ap´Dst and ap´AE.