On the possibility of inferring the inter-planetary sector structure from daily variations of geomagnetic ap indices

Abstract

The linear Bartels ap indices, which by definition should have no average U.T. variations, show in fact two different average U.T. variations if the data is divided into two groups according to the two Interplanetary Magnetic Field (IMF) polarities. These differences are found to be similar for all seasons and activity ranges. Correlating the ap variations of individual days to the average ap variations for days with interplanetary away and toward polarities, a simple but objective precept of calculations is given to infer the IMF sector structure with a success rate of 73% of the days for the years 1963–1973. The same method is employed to infer the IMF sector structure since 1932, and the results are compared to the sector structure inferred from polar cap magnetograms. Some known features of solar sector fields, e.g. the heliographic latitude dependence of the dominant polarity, are also found in the polarity classification based on ap variations, whereas the significant higher geomagnetic activity during intervals of toward polarity before 1963, which was found in the sector structure inferred from polar cap magnetograms, is not observed.