From May 1806 until June 1807, Baron Alexander von Humboldt and a colleague observed local Berlin magnetic declination every half hour from midnight to morning. They used a microscope to identify which direction the magnetic needle was pointing. On December 21, 1806, strong magnetic disturbances were recorded. Humboldt noted that this magnetic disturbance was accompanied by strong auroral lights. In the morning, the aurora was gone, the magnetic disturbances were gone. Humboldt was left though with his discovery of the geomagnetic storm.
A geomagnetic storm is just what Humboldt recorded, a marked temporary disturbance of the Earth's magnetic field. It was initially thought that geomagnetic storms were produced by the influx of a greater than normal amount of solar particles released from the Sun during a flare or CME (coronal mass ejection). Solar flares and CMEs are related to geomagnetic storms, but not because of the increase influx of particles into the Earth's magnetosphere.
The solar wind carries with it the magnetic field of the Sun. This magnetic field or the IMF (interplanetary magnetic field) has a particular orientation - southward or northward. If the IMF of the solar wind is southward and the solar wind crosses the Earth for long durations of time or in shorter more energetic bursts (flares/CMEs), geomagnetic storms can be expected. Geomagnetic storms are complex multi-faceted phenomena that originate at the Sun and occur in the solar wind, the magnetosphere, the ionosphere and the thermosphere. Basically, the southward IMF causes magnetic reconnection of the dayside magnetopause, rapidly injecting magnetic and particle energy into the Earth's magnetosphere and modifying the large-scale ring current systems.
Just in the last 30 years have scientists truly begun to understand the coupled Sun-Earth system. Much of the new development and many of the improved theories are due to space-based observatories such as Yohkoh and Ulysses. Considerable uncertainties still exist in regards to geomagnetic storms. It is extremely important to understand such storms because of the effects they have on life on Earth. Geomagnetic storms can affect radio communication, satellite drag, auroral activity and even the safety of astronauts in Earth orbit.