  We develop a basic model of the time variation of geomagnetic activity and
show that the model predicts, with decreasing levels of exactitude, the time
variation of the ~27 day period components of geomagnetic aa index, cloud cover
and surface temperature during several years near solar cycle minima. We
interpret this as indicating that there is a connection between the ~27 day
variation of geomagnetic activity and the ~27 day variations of cloud cover and
surface temperature with the decreasing correlations between model variation
and aa index, cloud cover and surface temperature variation due to delays and
phase shifts between the three variables some of which are obvious, such as the
180 degree seasonal phase shift between cloud cover and surface temperature,
and others less certain. We find that, while the components of cloud cover and
surface temperature influenced by geomagnetic activity amount to, on average,
about 20% of the overall variations, the influence may be several times higher
during the semi-annual maxima in geomagnetic activity that occur around the
equinoxes.

Earth and Planetary Astrophysics;Atmospheric and Oceanic Physics