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'. (L. Svalgaard, Stanford University).