LONG-TERM VARIATIONS IN THE GEOMAGNETIC-ACTIVITY LEVEL .2. ASCENDING PHASES OF SUNSPOT CYCLES

Monthly averages of the Helsinki Ak-values have been reduced to the eq uivalent aa-induces to extend the aa-data set back to 1844. A periodic ity of about five cycles was found for the correlation coefficient (r) between geomagnetic indices and sunspot numbers for the ascending pha ses of sunspot cycles 9 to 22, confirming previous findings based on a minor number of sunspot cycles. The result is useful to researchers i n topics related to solar-terrestrial physics, particularly for the in terpretation of long-term trends in geomagnetic activity during the pa st, and to forecast geomagnetic activity levels in the future.