Increased magnetic storm activity from 1868 to 1995
The aa index provides the longest continuous data set which can be used in the analysis of magnetospheric and ionospheric phenomenology. All phases of the solar cycle show increases in activity since cycle 14. The activity increase does not appear to be associated with any instrumental, ionospheric or magnetospheric effects. Barely significant effects (in terms of the results presented in this paper) have been identified in the long-term change in magnetic latitude of the observatory sites, the positions of high-latitude ionospheric features such as the cusp, and ionospheric Pedersen and Hall conductivities due to changing magnetic field orientation and strength. The prime cause of the change in geomagnetic activity is an increase in solar activity. The number of storms at solar minimum has typically increased by 40% more than the other phases. This is principally due to increased recurrent storm activity to such an extent that conditions at minimum in recent cycles could be thought of as being more representative of the declining phase.
Authors: Clilverd, M.A., Clark, T.D.G., Clarke, E., Rishbeth, H.