This picture shows material from the Sun coming towards the Earth. It is the Sun's magnetic field carried in this material that causes magnetic storms.
NASA
Geomagnetic Storms
From May 1806 until June 1807, from his home in Berlin, Baron Alexander von Humboldt observed which way the magnetic needle was pointing. On December 21, 1806, he recorded strong magnetic disturbances. The same night he saw 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 disturbance of the Earth's magnetic field.
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 periods of time, geomagnetic storms can be expected. The southward IMF causes magnetic and particle energy to be injected into the Earth's magnetosphere creating storms.
Just in the last 30 years have scientists truly begun to understand the coupled Sun-Earth system. Many of the improved theories are due to satellites such as Yohkoh and Ulysses. 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.
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