Unusual declining phase of solar cycle 23: Weak semi-annual variations of auroral hemispheric power and geomagnetic activity

The well known semi-annual variations of geomagnetic activity, with peaks occurring during equinoxes, were virtually absent during SC23-D. This makes SC23-D markedly different from previous solar cycles which had clear semi-annual variations.

In SC23-D, both Kp and HP had unusual peaks around the December solstice (in years 2003, 2004 and 2006) and August (in years 2004 and 2005), instead of at the equinoxes. These peaks appeared to be associated with solar wind/IMF and the consequent merging electric field peaks in the same periods. Furthermore, the absolute values and relative changes of the Kp index were much smaller in SC23-D than in other solar cycles.