A worldwide disturbance of the earth's magnetic field. Magnetic storms are frequently characterized by a sudden onset, in which the magnetic field undergoes marked changes in the course of an hour or less, followed by a very gradual return to normalcy, which may take several days. If extreme enough, they may interfere with the operation of electrical power lines and the operation of artificial satellites. Magnetic storms are caused by solar disturbances, though the exact nature of the link between the solar and terrestrial disturbances is not totally understood. They are more frequent during years of high sunspot number. Sometimes a magnetic storm can be linked to a particular coronal mass ejection. In these cases, the time between the ejection and onset of the magnetic storm is about one or two days. When these disturbances are observable only in the auroral zones, they may be termed polar magnetic storms. See also M-region.