Long-term variation in the north-south asymmetry of geomagnetic activity with respect to the solar equatorial plane

North-south asymmetry of geomagnetic activity with respect to the solar equatorial plane has been studied for the period 1878-1988, utilizing the seasonal change in the heliographic latitude of the Earth. It was found that the north-south asymmetry of the aa index showed a variation with a period of 11 years in the solar cycles 12-15 (1878-1923), followed by a variation with a 22 year period in the solar cycles 17-21 (1933-1988). This peculiar behavior is discussed in comparison with the variation in the north-south asymmetry of the sunspot number on the solar disc. (author)