USGS Frequently Asked Questions

Answer:

A magnetic storm is a period of rapid magnetic field variation. The causes of magnetic storms are explained, in general terms, in the Introduction to Geomagnetism page given on this website. Briefly, then, magnetic storms have two basic causes. First of all, let us be reminded that the Sun is always emitting a wind of charged particles that flows outward into space away from the Sun itself. Occasionally the Sun emits a strong surge of solar wind, something called a coronal mass ejection. When this gust of solar wind impacts upon the outer part of the Earth’s magnetic field, the magnetosphere, the field is disturbed and it undergoes a complex oscillation. This causes the generation of associated electric currents in the near-Earth space environment, which, in turn, generate additional magnetic-field variations -- all of which constitute a 'magnetic storm'. The second cause of magnetic storms is the occasional direct linkage of the Sun’s magnetic field with that of the Earth’s. This direct magnetic connection is not the normal state of affairs, but when it occurs, charged particles, traveling along magnetic-field lines, can easily enter the magnetosphere, generate currents, and cause the magnetic field to undergo time-dependent variation. On occasion, the Sun emits a coronal mass ejection at a time when the magnetic-field lines of the Earth and Sun are directly connected. Then we can experience a truly large magnetic storm.