An extension of the geomagnetic activity index series <i><b>aa</b></i> for two solar cycles (1844–1868)

Abstract

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.