AN EXTENSION OF THE GEOMAGNETIC-ACTIVITY INDEX SERIES AA FOR 2 SOLAR-CYCLES (1844-1868)

This paper deals with an extension of Mayaud's aa-index series (from 1868 to the present) for two solar cycles 1844-1868. The magnetic data sources are hourly declination readings at the Helsinki magnetic-meteorological observatory carried out in 1844-1880. The new data set provides a reliable and homogeneous series of declination for computations of equivalent aa-indices. For the time period 1868-1880 the linear correlation coefficient between the two indices was 0.96 on the monthly basis. Behaviour of the geomagnetic activity during the course of the sunspot cycles 9-12 revealed similar periodicities and linear dependencies (Ohl's and Feynman's relationships) as later activity data analysis has shown.