L. Svalgaard, Stanford University:

As the solar wind approaches the Earth, the wind will press up against the magnetic field of the Earth. Where the pressure of the solar wind balances the pressure of the magnetic field is the boundary between the solar wind and the Earth's magnetosphere. Electric currents flow along the boundary. Because the density and velocity of the solar wind varies continuously, the currents are constantly changing. Furthermore, whatever configurations of magnetic fields, plasma regimes, and electric currents that were established to maintain the pressure balance are constantly 'buffeted' and changing, often explosively. All of these processes involve electric currents having magnetic effects felt on the ground as geomagnetic 'activity'.

J. Dickey,  NASA, Jet Propulsion Laboratory, Pasadena:
One possibility is the movements of Earth's core (where Earth's magnetic field originates) might disturb Earth's magnetic shielding of charged-particle (i.e., cosmic ray) fluxes that have been hypothesized to affect the formation of clouds. This could affect how much of the sun's energy is reflected back to space and how much is absorbed by our planet. Other possibilities are that some other core process could be having a more indirect effect on climate, or that an external (e.g. solar) process affects the core and climate simultaneously.