Geomagnetic storms are major disturbances of the magnetosphere that occur when the interplanetary magnetic field turns southward and remains southward for an prolonged period of time. In 1931, Sydney Chapman and Vincenzo C. A. Ferraro wrote an article, A New Theory of Magnetic Storms, that sought to explain the phenomenon of geomagnetic storms. They argued that whenever the Sun emits a solar flare it will also emit a plasma cloud, now known that time as a coronal mass ejection. This plasma will travel at a velocity such that it reaches Earth within 113 days, though we now know this journey takes 1 to 5 days. The cloud will then compress the Earth’s magnetic field and thus increase this magnetic field at the Earth’s surface.
Geomagnetic storms, as observed by ground-based magnetometers, commonly begin with an increase in the strength of the geomagnetic field and the enhancements are associated with sudden increases in the dynamic pressure of the solar wind. A geomagnetic storm has three phases: an initial phase, a main phase, and a recovery phase. The initial phase is characterized by Dst (disturbance – storm time) index increasing by 20 to 50 nT in tens of minutes. The size of a geomagnetic storm is classified as moderate (-50 nT > minimum of Dst < -100 nT), intense (-100 nT > minimum Dst < -250 nT) or super-storm (minimum of Dst > -250 nT).
The initial phase is also referred to as a storm sudden commencement (SSC). However, not all geomagnetic storms have an initial phase and not all sudden increases in Dst are followed by a geomagnetic storm. The main phase of a geomagnetic storm is defined by Dst decreasing to less than -50 nT. The selection of -50 nT to define a storm is somewhat arbitrary. The minimum value during a storm will be between -50 and approximately -600 nT. The duration of the main phase is typically between 2 and 8 hours. The recovery phase is the period when Dst changes from its minimum value to its quiet time value. The period of the recovery phase may be as short as 8 hours or as long as 7 days.
During a geomagnetic storm’s main phase, which can last as long as two to two and a half days in the case of a severe storm, charged particles in the near-Earth plasma sheet are energized and injected deeper into the inner magnetosphere, producing the storm-time ring current. This phase is characterized by the occurrence of multiple intense substorms, with the attendant auroral and geomagnetic effects. The nature of the relationship between magnetic storms and substorms is a matter of some controversy. When the interplanetary field turns northward again, the rate of plasma energization and inward transport slows and the various loss processes that remove plasma from the ring current can begin to restore it to its pre-storm state. In the case of a great storm, such as the one of 6 February 1986, the ring current can take over a month to fully return to its quiet state. The drop in the surface magnetic field strength during the main phase of a geomagnetic storm is typically preceded by a brief rise in the field strength (see the entry for Dst index). This increase is caused by an intensification of the magnetopause current that occurs as increased solar wind dynamic pressure drives the magnetopause inward by as much as four Earth radii.
Recurrent vs. non-recurrent storms:
Geomagnetic storms are further classified as recurrent and non-recurrent. Recurrent storms occur every 27 days, corresponding to the Sun’s rotation period. They are triggered by the Earth’s encounters with the southward- oriented magnetic field of the high-pressure regions formed in the interplanetary medium by the interaction of low- and high-speed solar wind streams co-rotating with the Sun. Recurrent storms occur most frequently in the declining phase of the solar cycle. Non-recurrent geomagnetic storms, on the other hand, occur most frequently near solar maximum. They are caused by interplanetary disturbances driven by fast coronal mass ejections (CMEs) and typically involve an encounter with both the interplanetary shock wave and the CME that drives it.
The first observation of the effects of a geomagnetic storm occurred early in the 19th century: From May 1806 until June 1807 the German Alexander von Humboldt recorded the bearing of a magnetic compass in Berlin. On 21 December 1806 he noticed that his compass had become erratic during a bright auroral event. On September 1–2, 1859, the largest recorded geomagnetic storm occurred. From August 28 until September 2, 1859, numerous sunspots and solar flares were observed on the Sun, the largest flare occurring on September 1. This is referred to as the Solar storm of 1859 or the Carrington Event. The horizontal intensity of geomagnetic field was reduced by 1600 nT as recorded by the Colaba Observatory. It is estimated that Dst would have been approximately -1760 nT. Telegraph wires in both the United States and Europe experienced induced emf, in some cases even shocking telegraph operators and causing fires. Aurorae were seen as far south as Hawaii, Mexico, Cuba, and Italy — phenomena that are usually only seen near the poles. Ice cores show evidence that events of similar intensity recur at an average rate of approximately once per 500 years. Since 1859, less severe storms have occurred, notably the aurora of November 17, 1882 and the May 1921 geomagnetic storm, both with disruption of telegraph service and inititation of fires, and 1960, when widespread radio disruption was reported.
The March 1989 geomagnetic storm caused the collapse of the Hydro-Québec power grid in a matter of seconds as equipment protection relays tripped in a cascading sequence of events. Six million people were left without power for nine hours, with significant economic loss. The storm even caused aurorae as far south as Texas. The geomagnetic storm causing this event was itself the result of a coronal mass ejection, ejected from the Sun on March 9, 1989. The minimum of Dst was -589 nT .
On July 14, 2000, an X5 class flare erupted on the Sun (known as the Bastille Day event) and a coronal mass ejection was launched directly at the Earth. A geomagnetic super storm occurred on July 15–17; the minimum of the Dst index was – 301 nT. Despite the strength of the geomagnetic storm, no electrical power distribution failures were reported. The Bastille Day event was observed by Voyager 1 and Voyager 2, thus it is the farthest out in the solar system that a solar storm has been observed.
Seventeen major flares erupted on the Sun between 19 October and 5 November 2003, including a huge X28 flare, resulting in an extreme radio blackout, on 4 November. These flares were associated with CME events which impacted the Earth. The CMEs caused three geomagnetic storms between Oct 29 and November 2 during which the second and third storms were initiated before the previous storm period had fully recovered. The minimum Dst values were -151, -353 and -383 nT. Another storm in this event period occurred on November 4 – 5 with a minimum Dst of -69.nT. The whole sequence of events is known as the Halloween Solar Storm. The Wide Area Augmentation System (WAAS) operated by the Federal Aviation Administration (FAA) was offline for approximately 30 hours due to the storm. The Japanese ADEOS-2 satellite was severely damaged and the operation of many other satellites were interrupted due to the storm.