A magnetic storm is a disturbance of the Earth's upper atmosphere brought on by solar flares--i.e., bright eruptions from the visible portion of the Sun's chromosphere. The material associated with these flares consists primarily of protons and electrons with an energy of a few thousand electron volts. Called plasma, this material moves through the interplanetary medium at speeds ranging from 1,000 to 2,000 km (600 to 1,200 miles) per second, so that the ejected material reaches the Earth in approximately 21 hours. The pressure of the incoming plasma is transmitted to the outer edge of the Earth's magnetosphere; this causes an increase in the observed geomagnetic field at the ground, perhaps through hydromagnetic waves.
During these few minutes--the sudden-commencement phase--of the storm, the horizontal component of the geomagnetic field increases suddenly over the entire globe. The increase persists for two to six hours and is classified as the initial phase of the storm. One theory holds that, during the initial phase, added pressure on the Earth's magnetosphere causes the tail of the magnetosphere to be extended. In response to this unstable condition, the newly created magnetic lines in the interior of the tail contract rapidly, thereby sending plasma from the neutral sheet of the magnetosphere toward the night side of the Earth. This plasma injection results in intense auroral displays in the polar regions, while the contractions are observed on the Earth as a severe magnetic disturbance known as a polar substorm. This portion of the storm is followed by the storm's main phase, lasting 12 to 48 hours, during which the horizontal component of the field decreases, probably because of the injection or inflation of the magnetosphere by the incoming plasma. In the last stages, or recovery phase, the newly injected plasma drains slowly over several days into the interplanetary medium or the atmosphere, and the geomagnetic field approaches its pre-storm condition.
Excerpt from the Encyclopedia Britannica without permission.