The 22-year variation of geomagnetic activity: Implications for the polar magnetic field of the Sun

Authors


Abstract

We examine the average behavior of the aa index and its recent extension using proxy data over the period 1844 to the present. Over this period the daily average aa index fell from an average of about 18 in 1850 to a low of about 14.4 in 1900 to a high of 23 over the most recent solar cycles. Independently the amplitude of the 22-year variation of geomagnetic activity has also changed. It was its largest in the years 1862 to 1908 when it reached 30% of the mean activity but has been close to 0 in other years except from 1949 to 1970 when it was about 15% of the mean level of activity. Because the 22-year variation of geomagnetic activity is thought to be associated with the reversals of the polar magnetic field of the sun, these latter variations suggest that the strength of the polar magnetic field of the sun varies considerably over the long term.

Ancillary