Geomagnetic storm (magnetic storm) theory A geomagnetic storm - TopicsExpress

Geomagnetic storm (magnetic storm) theory A geomagnetic storm is a temporary disturbance of the Earths magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field which interacts with the Earths magnetic field. The increase in the solar wind pressure initially compresses the magnetosphere and the solar winds magnetic field interacts with the Earth’s magnetic field and transfers an increased energy into the magnetosphere. Both interactions cause an increase in movement of plasma through the magnetosphere (driven by increased electric fields inside the magnetosphere) and an increase in electric current in the magnetosphere and ionosphere. During the main phase of a geomagnetic storm, electric current in the magnetosphere creates a magnetic force which pushes out the boundary between the magnetosphere and the solar wind. The disturbance in the interplanetary medium which drives the geomagnetic storm may be due to a solar coronal mass ejection (CME) or a high speed stream (co-rotating interaction region or CIR)[1] of the solar wind originating from a region of weak magnetic field on the Sun’s surface. The frequency of geomagnetic storms increases and decreases with the sunspot cycle. CME driven storms are more common during the maximum of the solar cycle and CIR driven storms are more common during the minimum of the solar cycle. There are several space weather phenomena which tend to be associated with or are caused by a geomagnetic storm. These include: Solar Energetic Particle (SEP) events, geomagnetically induced currents (GIC), ionospheric disturbances which cause radio and radar scintillation, disruption of navigation by magnetic compass and auroral displays at much lower latitudes than normal. In 1989, a geomagnetic storm energized ground induced currents which disrupted electric power distribution throughout most of the province of Quebec[2] and caused aurorae as far south as *Definition of a geomagnetic storm A geomagnetic storm is defined[6] by changes in the DST[7] (disturbance – storm time) index. The Dst index estimates the globally averaged change of the horizontal component of the Earth’s magnetic field at the magnetic equator based on measurements from a few magnetometer stations. Dst is computed once per hour and reported in near-real-time.[8] During quiet times, Dst is between +20 and -20 nano-Tesla (nT). A geomagnetic storm has three phases:[6] an initial phase, a main phase and a recovery phase. The initial phase is characterized by Dst (or its one-minute component SYM-H) increasing by 20 to 50 nT in tens of minutes. 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 or SYM-H 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. 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 *Historical occurrences Early in the 19th century the first geomagnetic storm was observed, or to be more precise the effects of it were observed: 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.[9] 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. It can be assumed that a massive coronal mass ejection (CME), associated with the flare, was launched from the Sun and reached the Earth within eighteen hours — a trip that normally takes three to four days. 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.[10] 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.[11] GOES-7 monitors the space weather conditions during the Great Geomagnetic storm of March 1989, the Moscow neutron monitor recorded the passage of a CME as a drop in levels known as a Forbush decrease.[12] 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.[2][13] Six million people were left without power for nine hours, with significant economic loss. The storm even caused aurorae as far south as Texas.[3] The geomagnetic storm causing this event was itself the result of a coronal mass ejection, ejected from the Sun on March 9, 1989.[14] 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.[15] The Bastille Day event was observed by Voyager 1 and Voyager 2,[16] 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 perhaps the most intense flare ever measured on the GOES XRS sensor – a huge X28 flare,[17] 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 last geomagnetic storm was weaker than the preceding storms because the active region on the Sun had rotated beyond the meridian where the central portion CME created during the flare event passed to the side of the Earth. The whole sequence of events is known as the Halloween Solar Storm.[18] The Wide Area Augmentation System (WAAS) operated by the Federal Aviation Administration (FAA) was offline for approximately 30 hours due to the storm.[19] The Japanese ADEOS-2 satellite was severely damaged and the operation of many other satellites were interrupted due to the storm.[20]