The average geomagnetic field has also varied. The average aa value was about 18 in 1850. It fell to a minimum of 14.4 in about 1900 and then rose to a high of about 23 over the last five solar cycles. Long term variation of geomagnetic activity as deduced from the aa index. Heavy line shows the 11-year solar cycle average aa index beginning two years after solar maximum.
Error bars are probable errors in the mean.
In conclusion it is clear that there have been long term changes in geomagnetic activity both in the average activity and in the amplitude of the 22-year cycle. These variations in turn imply that there are very long term variations in the sun. Since the 22-year cycle has been linked to the polar field of the sun, we infer that variations of its amplitude are due to variations in the magnitude of the polar magnetic field of the sun. An alternative way of visualizing this is to consider the control of the heliospheric current sheet by a tilted dipole in the sun. When the dipole axis is in the ecliptic plane, the amount of inward and outward IMF is equal at all latitudes and there is no heliographic latitude effect in the semi-annual variation of geomagnetic activity.