Editing Geomagnetic storm (section)

==Definition==
A geomagnetic storm is defined<ref name="gonzalez">Gonzalez, W. D., J. A. Joselyn, Y. Kamide, H. W. Kroehl, G. Rostoker, B. T. Tsurutani, and V. M. Vasyliunas (1994), What is a Geomagnetic Storm?, J. Geophys. Res., 99(A4), 5771–5792.</ref> by changes in the [[Disturbance storm time index|Dst]]<ref>{{cite journal |url=http://wdc.kugi.kyoto-u.ac.jp/dstdir/dst2/onDstindex.html |last1=Sugiura |first1=M. |first2=T. |last2=Kamei |title=Equatorial Dst index 1957–1986 |journal=IAGA Bulletin |issue=40 |editor1=A. Berthelier |editor2=M. Menville |publisher=ISGI Publ. Off. |location=Saint. Maur-des-Fosses, France |date=1991}}</ref> (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.<ref>{{cite web |url=http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html|title=World Data Center for Geomagnetism, Kyoto}}</ref> During quiet times, Dst is between +20 and −20 nano-[[Tesla (unit)|Tesla]] (nT).{{citation needed|date = April 2022}}

A geomagnetic storm has three phases: initial, main and recovery. The initial phase is characterized by Dst (or its one-minute component SYM-H) increasing by 20 to 50&nbsp;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&nbsp;nT. The selection of −50&nbsp;nT to define a storm is somewhat arbitrary. The minimum value during a storm will be between −50 and approximately −600&nbsp;nT. The duration of the main phase is typically 2–8 hours. The recovery phase is when Dst changes from its minimum value to its quiet time value. The recovery phase may last as short as 8 hours or as long as 7 days.<ref name="gonzalez" />

[[File:Aurora borealis2, Churchill, MB.JPG|thumb|Aurora borealis]]

The size of a geomagnetic storm is classified as moderate (−50&nbsp;nT > minimum of Dst > −100&nbsp;nT), intense (−100&nbsp;nT > minimum Dst > −250&nbsp;nT) or super-storm (minimum of Dst < −250&nbsp;nT).<ref>{{Cite journal|last1=Cander|first1=L. R.|last2=Mihajlovic|first2=S. J.|date=1998-01-01|title=Forecasting ionospheric structure during the great geomagnetic storms|journal=Journal of Geophysical Research: Space Physics|language=en|volume=103|issue=A1|pages=391–398|doi=10.1029/97JA02418|issn=2156-2202|bibcode = 1998JGR...103..391C |doi-access=free}}</ref>