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{{Short description|Disturbance of the Earth's magnetosphere}}
{{Use dmy dates|date=June 2023}} 
{{Redirect|Magnetic storm}}

A '''geomagnetic storm''', also known as a '''magnetic storm''', is a temporary disturbance of the [[Earth's magnetosphere]] that is driven by interactions between the magnetosphere and large-scale transient [[Plasma (physics)|plasma]] and [[magnetic field]] structures that originate on or near the [[Sun]].

The structures that produce geomagnetic storms include interplanetary [[coronal mass ejections]] (CME) and [[corotating interaction region]]s (CIR). The former often originate from [[solar active region]]s, while the latter originate at the boundary between high- and low-speed streams of [[solar wind]].<ref>''Corotating Interaction Regions,'' Corotating Interaction Regions Proceedings of an ISSI Workshop, 6–13 June 1998, Bern, Switzerland, Springer (2000), Hardcover, {{ISBN|978-0-7923-6080-3}}, Softcover, {{ISBN|978-90-481-5367-1}}</ref> The frequency of geomagnetic storms increases and decreases with the [[sunspot cycle]]. During [[solar maximum|solar maxima]], geomagnetic storms occur more often, with the majority driven by CMEs.

When these structures reach Earth, the increase in the solar wind pressure initially compresses the magnetosphere. The solar wind's magnetic field interacts with the Earth's magnetic field and transfers an increased energy into the magnetosphere. Both interactions cause an increase in plasma movement 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 that pushes out the boundary between the magnetosphere and the solar wind.

Several space weather phenomena tend to be associated with geomagnetic storms. These include [[Solar particle event|solar energetic particle (SEP) events]], [[geomagnetically induced current]]s (GIC), [[ionospheric storm]]s and disturbances that cause radio and radar [[Interplanetary scintillation|scintillation]], disruption of navigation by magnetic compass and auroral displays at much lower [[magnetic latitude]]s than normal.

The largest recorded geomagnetic storm, [[Solar storm of 1859|the Carrington Event]] in September 1859, took down parts of the recently created US telegraph network, starting fires and electrically shocking telegraph operators.<ref>{{cite web |last1=Choi |first1=Charles |title=What if the Carrington Event, the largest solar storm ever recorded, happened today? |url=https://www.livescience.com/carrington-event |website=LiveScience |date=5 September 2022 |publisher=Future US |access-date=26 February 2023}}</ref> In [[March 1989 geomagnetic storm|1989, a geomagnetic storm]] energized [[geomagnetically induced current|ground induced currents]] that disrupted electric power distribution throughout most of [[Quebec]]<ref name="cbc.ca">{{cite news |title=Scientists probe northern lights from all angles |url=https://www.cbc.ca/news/science/scientists-probe-northern-lights-from-all-angles-1.552461 |publisher=[[Canadian Broadcasting Company|CBC]] |date=22 October 2005 }}</ref> and caused [[aurora (astronomy)|aurorae]] as far south as [[Texas]].<ref name="Earth dodges magnetic storm">{{cite magazine |title=Earth dodges magnetic storm |url=https://www.newscientist.com/article/mg12216702.200-earth-dodges-magnetic-storm-.html |magazine=[[New Scientist]] |date=24 June 1989 }}</ref>

==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>

== Measuring intensity ==

Geomagnetic storm intensity is reported in several different ways, including:
* [[K-index]]
* [[A-index]]
* The [[K-index#G-scale|G-scale]] used by the U.S. [[National Oceanic and Atmospheric Administration]], which rates the storm from G1 to G5 (i.e. G1, G2, G3, G4, G5 in order), where G1 is the weakest storm classification (corresponding to a [[K-index|Kp]] value of 5), and G5 is the strongest (corresponding to a Kp value of 9).<ref>{{cite web |url=https://www.swpc.noaa.gov/noaa-scales-explanation |title=NOAA Space Weather Scales |accessdate=31 May 2021}}</ref>

== History of the theory ==
In 1930, [[Sydney Chapman (mathematician)|Sydney Chapman]] and Vincenzo C. A. Ferraro wrote an article, ''A New Theory of Magnetic Storms'', that sought to explain the phenomenon.<ref>{{cite journal|author1=S. Chapman|author2=V. C. A. Ferraro|date=1930|title=A New Theory of Magnetic Storms|journal=[[Nature (journal)|Nature]]|volume=129|issue=3169|pages=129–130|bibcode=1930Natur.126..129C|doi=10.1038/126129a0|s2cid=4102736}}</ref> They argued that whenever the [[Sun]] emits a [[solar flare]] it also emits a plasma cloud, now known as a [[coronal mass ejection]]. They postulated that this [[Plasma (physics)|plasma]] travels at a velocity such that it reaches Earth within 113 days, though we now know this journey takes 1 to 5 days. They wrote that the cloud then compresses the [[Earth's magnetic field]] and thus increases this field at the Earth's surface.<ref>{{cite journal|author=V. C. A. Ferraro|date=1933|title=A New Theory of Magnetic Storms: A Critical Survey|journal=[[The Observatory (journal)|The Observatory]]|volume=56|pages=253–259|bibcode=1933Obs....56..253F}}</ref> Chapman and Ferraro's work drew on that of, among others, [[Kristian Birkeland]], who had used recently discovered [[cathode-ray tube]]s to show that the rays were deflected towards the [[Magnetic polarity|poles]] of a magnetic sphere. He theorised that a similar phenomenon was responsible for [[aurora]]s, explaining why they are more frequent in polar regions.

==Occurrences==
{{further|List of solar storms}}
The first scientific observation of the effects of a geomagnetic storm occurred early in the 19th century: from May 1806 until June 1807, [[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 [[Aurora (astronomy)|auroral event]].<ref>{{cite web| last =Russell| first =Randy| title =Geomagnetic Storms| website =Windows to the Universe| publisher =National Earth Science Teachers Association| date =March 29, 2010| url =http://www.windows2universe.org/glossary/geomagnetic_storms.html| access-date = 4 August 2013}}</ref>

On September 1–2, 1859, the largest recorded geomagnetic storm occurred. From August 28 until September 2, 1859, numerous [[sunspot]]s and [[solar flare]]s were observed on the Sun, with the largest flare 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]] was launched from the Sun and reached the Earth within eighteen hours—a trip that normally takes three to four days. The horizontal field was reduced by 1600&nbsp;nT as recorded by the [[Colaba Observatory]]. It is estimated that Dst would have been approximately −1760&nbsp;nT.<ref>{{cite journal | last1 = Tsurutani | first1 = B. T. | last2 = Gonzalez | first2 = W. D. | last3 = Lakhina | first3 = G. S. | last4 = Alex | first4 = S. | year = 2003 | title = The extreme magnetic storm of 1–2 September 1859 | url = https://zenodo.org/record/1000695| journal = J. Geophys. Res. | volume = 108 | issue = A7| page = 1268 | doi = 10.1029/2002JA009504 | bibcode=2003JGRA..108.1268T| doi-access = free }}</ref> [[Aurora (astronomy)#Historically significant events|Telegraph wires]] in both the United States and Europe experienced induced voltage increases ([[Electromotive force|emf]]), in some cases even delivering shocks to telegraph operators and igniting fires. Aurorae were seen as far south as Hawaii, Mexico, Cuba and Italy—phenomena that are usually only visible in polar regions. [[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 initiation of fires, and 1960, when widespread radio disruption was reported.<ref name="Odenwald">{{cite journal |title=Bracing the Satellite Infrastructure for a Solar Superstorm |journal=Sci. Am. |url=http://www.sciam.com/article.cfm?id=bracing-for-a-solar-superstorm |archive-url=https://web.archive.org/web/20081117113249/http://www.sciam.com/article.cfm?id=bracing-for-a-solar-superstorm |url-status=dead |archive-date=2008-11-17 }}</ref>

[[File:ExtremeEvent 19890310-00h 19890315-24h.jpg|thumb|upright=1.85|GOES-7 monitors 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]].<ref>{{cite web | title=Extreme Space Weather Events | publisher=[[National Geophysical Data Center]] | url=https://www.ngdc.noaa.gov/sxi/sxi_greatest.html}}</ref>]]

In [[Solar storm of August 1972|early August 1972]], a series of flares and solar storms peaks with a flare estimated around X20 producing the fastest CME transit ever recorded and a severe geomagnetic and proton storm that disrupted terrestrial electrical and communications networks, as well as satellites (at least one made permanently inoperative), and spontaneously detonated numerous U.S. Navy magnetic-influence sea mines in North Vietnam.<ref>{{cite journal |last = Knipp |first = Delores J. |author2 = B. J. Fraser |author3 = M. A. Shea |author-link3=Margaret Shea (scientist)|author4 = D. F. Smart |title = On the Little-Known Consequences of the 4 August 1972 Ultra-Fast Coronal Mass Ejecta: Facts, Commentary and Call to Action |journal = Space Weather |volume = 16 |issue = 11|pages = 1635–1643|date = 2018 |doi = 10.1029/2018SW002024 |doi-access = free |bibcode = 2018SpWea..16.1635K }}</ref>

The [[March 1989 geomagnetic storm]] caused the collapse of the [[Hydro-Québec]] power grid in seconds as equipment protection relays tripped in a cascading sequence.<ref name="cbc.ca"/><ref>{{harvnb|Bolduc|2002}}</ref> Six million people were [[power outage|left without power]] for nine hours. The storm caused auroras as far south as [[Texas]] and [[Florida]].<ref name="Earth dodges magnetic storm"/> The storm causing this event was the result of a coronal mass ejected from the Sun on March 9, 1989.<ref>{{cite journal|title=Geomagnetic Storms Can Threaten Electric Power Grid |journal=Earth in Space |volume=9 |issue=7 |pages=9–11 |date=March 1997 |url=http://www.agu.org/sci_soc/eiskappenman.html |url-status=dead |archive-url=https://web.archive.org/web/20080611174103/http://www.agu.org/sci_soc/eiskappenman.html |archive-date=2008-06-11 }}</ref> The minimum Dst was −589&nbsp;nT.

On July 14, 2000, an X5 class flare erupted (known as the [[Bastille Day event]]) and a coronal mass was launched directly at the Earth. A geomagnetic super storm occurred on July 15–17; the minimum of the Dst index was −301&nbsp;nT. Despite the storm's strength, no power distribution failures were reported.<ref>{{cite conference |title=High-voltage power grid disturbances during geomagnetic storms |last=Stauning |first=P. |book-title=Proceedings of the Second Solar Cycle and Space Weather Euroconference, 24–29 September 2001 |location=Vico Equense, Italy |editor=Huguette Sawaya-Lacoste |id=ESA SP-477 |publisher=Noordwijk: ESA Publications Division |isbn=92-9092-749-6 |date=2002 |pages=521–524}}</ref> The Bastille Day event was observed by ''[[Voyager 1]]'' and ''[[Voyager 2]]'',<ref>{{cite journal | last1 = Webber | first1 = W. R. | last2 = McDonald | first2 = F. B. | last3 = Lockwood | first3 = J. A. | last4 = Heikkila | first4 = B. | year = 2002 | title = The effect of the July 14, 2000 "Bastille Day" solar flare event on >70 MeV galactic cosmic rays observed at V1 and V2 in the distant heliosphere | journal = Geophys. Res. Lett. | volume = 29 | issue = 10| pages = 1377–1380 | doi = 10.1029/2002GL014729 | bibcode=2002GeoRL..29.1377W| doi-access = free }}</ref> 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 [[Geostationary Operational Environmental Satellite|GOES]] XRS sensor—a huge X28 flare,<ref>{{cite journal | last1 = Thomson | first1 = N. R. | last2 = Rodger | first2 = C. J. | last3 = Dowden | first3 = R. L. | year = 2004 | title = Ionosphere gives size of greatest solar flare | journal = Geophys. Res. Lett. | volume = 31 | issue = 6| page = L06803 | doi = 10.1029/2003GL019345 | bibcode = 2004GeoRL..31.6803T | doi-access = free }}</ref> resulting in an extreme radio blackout, on 4 November. These flares were associated with CME events that caused three geomagnetic storms between 29 October and 2 November, 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&nbsp;nT. Another storm in this sequence occurred on 4–5 November with a minimum Dst of −69&nbsp;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 became known as the [[Halloween Solar Storm]].<ref>{{cite web |url=http://www.swpc.noaa.gov/Services/HalloweenStorms_assessment.pdf |title=Halloween Space Weather Storms of 2003 |access-date=2011-05-17 |url-status=dead |archive-url=https://web.archive.org/web/20110728172705/http://www.swpc.noaa.gov/Services/HalloweenStorms_assessment.pdf |archive-date=2011-07-28 }} Halloween Space Weather Storms of 2003, NOAA Technical Memorandum OAR SEC-88, Space Environment Center, Boulder, Colorado, June 2004</ref> The [[Wide Area Augmentation System]] (WAAS) operated by the [[Federal Aviation Administration]] (FAA) was offline for approximately 30 hours due to the storm.<ref name="nas2008">{{cite report |url=http://www.nap.edu/catalog/12507.html |title=Severe Space Weather Events - Understanding Societal and Economic Impacts – Workshop Report, National Research Council of the National Academies |publisher=The National Academies Press |location=Washington, D. C. |date=2008}}</ref> The Japanese ADEOS-2 satellite was severely damaged and the operation of many other satellites were interrupted due to the storm.<ref>{{cite web |url=https://www.oecd.org/governance/risk/46891645.pdf |title=Geomagnetic Storms |publisher=CENTRA Technology, Inc. |type=report |date=14 January 2011 |archive-url=https://web.archive.org/web/20230529201915/https://www.oecd.org/governance/risk/46891645.pdf |archive-date=2023-05-29 |url-status=dead}} prepared for the Office of Risk Management and Analysis, United States Department of Homeland Security</ref>

== Interactions with planetary processes ==
[[Image:magnetopause.svg|thumb|upright=1.75|Magnetosphere in the near-Earth space environment.]]
The solar wind also carries with it the Sun's magnetic field. This field will have either a North or South orientation. If the solar wind has energetic bursts, contracting and expanding the magnetosphere, or if the solar wind takes a southward [[Polarization (waves)|polarization]], geomagnetic storms can be expected. The southward field causes [[magnetic reconnection]] of the dayside magnetopause, rapidly injecting magnetic and particle energy into the Earth's magnetosphere.

During a geomagnetic storm, the ionosphere's [[F region|F<sub>2</sub> layer]] becomes unstable, fragments, and may even disappear. In the northern and southern pole regions of the Earth, [[aurora (astronomy)|auroras]] are observable.

== Instruments ==
[[Magnetometer]]s monitor the auroral zone as well as the equatorial region. Two types of [[radar]], coherent scatter and incoherent scatter, are used to probe the auroral ionosphere. By bouncing signals off ionospheric irregularities, which move with the field lines, one can trace their motion and infer magnetospheric convection.

Spacecraft instruments include:

* Magnetometers, usually of the flux gate type. Usually these are at the end of booms, to keep them away from magnetic interference by the spacecraft and its electric circuits.<ref name="Magnetometry">{{cite web | last = Snare | first = Robert C. | title = A History of Vector Magnetometry in Space | publisher = University of California | url = http://www-ssc.igpp.ucla.edu/personnel/russell/ESS265/History.html | access-date = 2008-03-18 | archive-url = https://web.archive.org/web/20120520150421/http://www-ssc.igpp.ucla.edu/personnel/russell/ESS265/History.html | archive-date = 2012-05-20 | url-status = dead }}</ref>
* Electric sensors at the ends of opposing booms are used to measure potential differences between separated points, to derive electric fields associated with convection. The method works best at high plasma densities in low Earth orbit; far from Earth long booms are needed, to avoid shielding-out of electric forces.
* Radio sounders from the ground can [[Ionospheric sounding|bounce radio waves]] of varying frequency off the ionosphere, and by timing their return determine the electron density profile—up to its peak, past which radio waves no longer return. Radio sounders in low Earth orbit aboard the Canadian [[Alouette 1]] (1962) and [[Alouette 2]] (1965), beamed radio waves earthward and observed the electron density profile of the "topside ionosphere". Other radio sounding methods were also tried in the ionosphere (e.g. on [[IMAGE (spacecraft)|IMAGE]]).
* Particle detectors include a [[Geiger counter]], as was used for the original observations of the [[Van Allen radiation belt]]. [[Scintillator detectors]] came later, and still later "channeltron" [[electron multiplier]]s found particularly wide use. To derive charge and mass composition, as well as energies, a variety of [[Mass spectrometry|mass spectrograph]] designs were used. For energies up to about 50&nbsp;keV (which constitute most of the magnetospheric plasma) [[time-of-flight spectrometer]]s (e.g. "top-hat" design) are widely used.{{Citation needed|date=December 2008}}

Computers have made it possible to bring together decades of isolated magnetic observations and extract average patterns of electrical currents and average responses to interplanetary variations. They also run simulations of the global magnetosphere and its responses, by solving the equations of [[magnetohydrodynamics]] (MHD) on a numerical grid. Appropriate extensions must be added to cover the inner magnetosphere, where magnetic drifts and ionospheric conduction need to be taken into account. At polar regions, directly linked to the [[solar wind]], large-scale ionospheric anomalies can be successfully modeled, even during geomagnetic super-storms. <ref>{{cite journal | author=Pokhotelov D. |display-authors=et al. | title = Polar tongue of ionisation during geomagnetic superstorm |journal = Ann. Geophys. |volume=39 |pages=833–847 |year=2021 |issue=5 |doi=10.5194/angeo-39-833-2021 |bibcode=2021AnGeo..39..833P |doi-access=free |url=https://elib.dlr.de/144151/1/angeo-39-833-2021.pdf }}</ref> At smaller scales (comparable to a degree of latitude/longitude) the results are difficult to interpret, and certain assumptions about the high-latitude forcing uncertainty are needed. <ref>{{cite journal | author=Pedatella N. |display-authors=et al. | title = Effects of High-Latitude Forcing Uncertainty on the Low-Latitude and Midlatitude Ionosphere |journal = J. Geophys. Res. |volume=123 |pages=862–882 |year=2018 |issue=1 |doi=10.1002/2017JA024683 |bibcode=2018JGRA..123..862P |doi-access= |s2cid=133846779 }}</ref>

== Impacts ==

=== Infrastructure ===

It has been suggested that a geomagnetic storm on the scale of the [[solar storm of 1859]] today would cause billions or even trillions of dollars of damage to satellites, power grids and radio communications, and could cause electrical blackouts on a massive scale that might not be repaired for weeks, months, or even years.<ref name="nas2008"/> Such sudden electrical blackouts may threaten food production.<ref name="food">{{cite journal |last =Lassen | first = B |title=Is livestock production prepared for an electrically paralysed world? |journal=J Sci Food Agric |volume=93 |issue=1 |pages=2–4 | year=2013 | pmid=23111940 |doi=10.1002/jsfa.5939 | bibcode = 2013JSFA...93....2L |doi-access=free }}</ref>

==== Electrical grid ====

When [[magnetic field]]s move about in the vicinity of a conductor such as a wire, a [[geomagnetically induced current]] is produced in the conductor. This happens on a grand scale during geomagnetic storms (the same mechanism also influenced telephone and telegraph lines before fiber optics, see above) on all long transmission lines. Long transmission lines (many kilometers in length) are thus subject to damage by this effect. Notably, this chiefly includes operators in China, North America, and Australia, especially in modern high-voltage, low-resistance lines. The European grid consists mainly of shorter transmission circuits, which are less vulnerable to damage.<ref name="kappenman">{{cite news | url=https://spectrum.ieee.org/energy/the-smarter-grid/a-perfect-storm-of-planetary-proportions/0 | archive-url=https://web.archive.org/web/20120127072117/http://spectrum.ieee.org/energy/the-smarter-grid/a-perfect-storm-of-planetary-proportions/0 | url-status=dead | archive-date=27 January 2012 | title=A Perfect Storm of Planetary Proportions | work=[[IEEE Spectrum]] |date=February 2012 | access-date=2012-02-13 }}</ref><ref>Natuurwetenschap & Techniek Magazine, June 2009</ref>

The (nearly direct) currents induced in these lines from geomagnetic storms are harmful to electrical transmission equipment, especially [[transformer]]s—inducing core [[saturation (magnetic)|saturation]], constraining their performance (as well as tripping various safety devices), and causing coils and cores to heat up. In extreme cases, this heat can disable or destroy them, even inducing a chain reaction that can overload transformers.<ref>{{cite web |url=http://192.211.16.13/curricular/ENERGY/0708/articles/solar/SolarForecast07SkyTel.pdf|title=Solar Forecast: Storm AHEAD |archive-url=https://web.archive.org/web/20080911192808/http://192.211.16.13/curricular/ENERGY/0708/articles/solar/SolarForecast07SkyTel.pdf |archive-date=2008-09-11 |url-status=dead}}</ref><ref>{{Cite web|url=http://science.nasa.gov/headlines/y2009/21jan_severespaceweather.htm|title=NASA - Severe Space Weather--Social and Economic Impacts|date=24 January 2009|archive-url=https://web.archive.org/web/20090124153337/http://science.nasa.gov/headlines/y2009/21jan_severespaceweather.htm |accessdate=27 June 2023|archive-date=24 January 2009 |url-status=dead}}</ref> Most generators are connected to the grid via transformers, isolating them from the induced currents on the grid, making them much less susceptible to damage due to [[geomagnetically induced current]]. However, a transformer that is subjected to this will act as an unbalanced load to the generator, causing negative sequence current in the stator and consequently rotor heating.

A 2008 study by Metatech corporation concluded that a storm with a strength comparable to that of 1921 would destroy more than 300 transformers and leave over 130 million people without power in the United States, costing several trillion dollars.<ref>{{cite book |title=Severe Space Weather Events: Understanding Societal and Economic Impacts : a Workshop Report |location=Washington, D.C. |publisher=National Academies, 2008 |url=http://www.nap.edu/openbook.php?record_id=12507&page=1 |date=15 November 2011 |pages=78,105,106|doi=10.17226/12507 |isbn=978-0-309-12769-1 }}</ref> The extent of the disruption is debated, with some congressional testimony indicating a potentially indefinite outage until transformers can be replaced or repaired.<ref>{{cite web |title=Testimony of the Foundation For Resilient Societies before the Federal Energy Regulatory Commission | url=https://www.resilientsocieties.org/uploads/5/4/0/0/54008795/resilient_societies_testimony_rm15-11-00_march_1_2016_tech_conference_final_feb_23_2016.pdf}}</ref> These predictions are contradicted by a [[North American Electric Reliability Corporation]] report that concludes that a geomagnetic storm would cause temporary grid instability but no widespread destruction of high-voltage transformers. The report points out that the widely quoted Quebec grid collapse was not caused by overheating transformers but by the near-simultaneous tripping of seven relays.<ref>{{cite web |url=https://www.frcc.com/Public%20Awareness/Lists/Announcements/Attachments/105/GMD%20Interim%20Report.pdf |title=2012 Special Reliability Assessment Interim Report: Effects of Geomagnetic Disturbances on the Bulk Power System |publisher=North American Electric Reliability Corporation |date=February 2012 |access-date=2013-01-19 |url-status=dead |archive-url=https://web.archive.org/web/20150908075507/https://www.frcc.com/Public%20Awareness/Lists/Announcements/Attachments/105/GMD%20Interim%20Report.pdf |archive-date=2015-09-08 }}</ref> In 2016, the United States [[Federal Energy Regulatory Commission]] adopted NEARC rules for  equipment testing for electric utilities. Implementation of any upgrades needed to protect against the effects of geomagnetic storms was required within four years, and the regulations also directed further research.<ref>{{cite web |url=https://www.federalregister.gov/documents/2016/09/30/2016-23441/reliability-standard-for-transmission-system-planned-performance-for-geomagnetic-disturbance-events |title=Rule Reliability Standard for Transmission System Planned Performance for Geomagnetic Disturbance Events |date=September 30, 2016 |author=Federal Energy Regulatory Commission}}</ref>

Besides the transformers being vulnerable to the effects of a geomagnetic storm, electricity companies can also be affected indirectly by the geomagnetic storm. For instance, Internet service providers may go down during geomagnetic storms (and/or remain non-operational long after). Electricity companies may have equipment requiring a working Internet connection to function, so during the period the Internet service provider is down, the electricity too may not be distributed.<ref>Kijk magazine 6/2017, mentioned by Marcel Spit of Adviescentrum Bescherming Vitale Infrastructuur]</ref>

By receiving geomagnetic storm alerts and warnings (e.g. by the [[Space Weather Prediction Center]]; via Space Weather satellites as SOHO or ACE), power companies can minimize damage to power transmission equipment, by momentarily disconnecting transformers or by inducing temporary blackouts. Preventive measures also exist, including preventing the inflow of GICs into the grid through the neutral-to-ground connection.<ref name="kappenman"/>

==== Communications ====

[[High frequency]] (3–30&nbsp;MHz) communication systems use the ionosphere to reflect radio signals over long distances. Ionospheric storms can affect radio communication at all latitudes. Some frequencies are absorbed and others are reflected, leading to rapidly fluctuating signals and unexpected [[radio propagation|propagation]] paths. TV and commercial radio stations are little affected by solar activity, but ground-to-air, ship-to-shore, [[shortwave]] [[Broadcasting|broadcast]] and [[amateur radio]] (mostly the bands below 30&nbsp;MHz) are frequently disrupted. Radio operators using HF bands rely upon solar and geomagnetic alerts to keep their communication circuits up and running.

Military detection or early warning systems operating in the high frequency range are also affected by solar activity. The ''[[over-the-horizon radar]]'' bounces signals off the ionosphere to monitor the launch of aircraft and missiles from long distances. During geomagnetic storms, this system can be severely hampered by radio clutter. Also some submarine detection systems use the magnetic signatures of submarines as one input to their locating schemes. Geomagnetic storms can mask and distort these signals.

The [[Federal Aviation Administration]] routinely receives alerts of solar radio bursts so that they can recognize communication problems and avoid unnecessary maintenance. When an aircraft and a ground station are aligned with the Sun, high levels of noise can occur on air-control radio frequencies.{{citation needed|date=August 2016}} This can also happen on [[UHF]] and [[Super high frequency|SHF]] satellite communications, when an Earth station, a satellite and the Sun are in [[Sun outage|alignment]]. In order to prevent unnecessary maintenance on satellite communications systems aboard aircraft AirSatOne provides a live feed for geophysical events from NOAA's [[Space Weather Prediction Center]].<ref>{{cite web |url=https://www.airsatone.com//gams |title=AirSatOne's Live Feed}}</ref> allows users to view observed and predicted space storms. Geophysical Alerts are important to flight crews and maintenance personnel to determine if any upcoming activity or history has or will have an effect on satellite communications, GPS navigation and HF Communications.

[[electrical telegraph|Telegraph]] lines in the past were affected by geomagnetic storms. Telegraphs used a single long wire for the data line, stretching for many miles, using the ground as the return wire and fed with [[Direct current|DC]] power from a battery; this made them (together with the power lines mentioned below) susceptible to being influenced by the fluctuations caused by the [[ring current]]. The voltage/current induced by the geomagnetic storm could have diminished the signal, when subtracted from the battery polarity, or to overly strong and spurious signals when added to it; some operators learned to disconnect the battery and rely on the induced current as their power source. In extreme cases the induced current was so high the coils at the receiving side burst in flames, or the operators received electric shocks. Geomagnetic storms affect also long-haul telephone lines, including undersea cables unless they are [[fiber optic]].<ref>{{Cite web|url=http://image.gsfc.nasa.gov/poetry/storm/storms.html|archive-url=https://web.archive.org/web/20050911073432/http://image.gsfc.nasa.gov/poetry/storm/storms.html|url-status=dead|title=image.gsfc.nasa.gov|archive-date=11 September 2005}}</ref>

Damage to communications satellites can disrupt non-terrestrial telephone, television, radio and Internet links.<ref>{{cite news|title=Solar Storms Could Be Earth's Next Katrina|newspaper = NPR.org|url=https://www.npr.org/templates/story/story.php?storyId=124125001|access-date=2010-03-04}}</ref> The [[United States National Academy of Sciences|National Academy of Sciences]] reported in 2008 on possible scenarios of widespread disruption in the 2012–2013 solar peak.<ref>{{cite book |title=Severe Space Weather Events—Understanding Societal and Economic Impacts: Workshop Report |publisher=National Academies Press |location=Washington, D.C. |date=2008 |doi=10.17226/12507 |isbn=978-0-309-12769-1 |url=http://www.nap.edu/catalog.php?record_id=12507#toc}}</ref> A solar superstorm could cause large-scale global months-long [[Internet outage]]s. A study describes potential mitigation measures and exceptions – such as user-powered [[Wireless mesh network|mesh networks]], related [[peer-to-peer]] applications and new protocols – and analyzes the robustness of the current [[Internet infrastructure]].<ref>{{cite news |title=Computer scientist warns global internet is not prepared for a large solar storm |url=https://techxplore.com/news/2021-08-scientist-global-internet-large-solar.html |access-date=22 September 2021 |work=techxplore.com |language=en}}</ref><ref>{{cite news |title=A Bad Solar Storm Could Cause an 'Internet Apocalypse' |url=https://www.wired.com/story/solar-storm-internet-apocalypse-undersea-cables/ |access-date=22 September 2021 |magazine=Wired}}</ref><ref>{{cite conference |last1=Jyothi |first1=Sangeetha Abdu |book-title=Proceedings of the 2021 ACM SIGCOMM 2021 Conference |title=Solar superstorms: Planning for an internet apocalypse |date=9 August 2021 |pages=692–704 |doi=10.1145/3452296.3472916 |publisher=Association for Computing Machinery|isbn=9781450383837 |doi-access=free }}</ref>

==== Navigation systems ====

[[Satellite navigation|Global navigation satellite systems (GNSS)]], and other navigation systems such as [[LORAN]] and the now-defunct [[Omega Navigation System|OMEGA]] are adversely affected when solar activity disrupts their signal propagation. The OMEGA system consisted of eight transmitters located throughout the world. Airplanes and ships used the very low frequency signals from these transmitters to determine their positions. During solar events and geomagnetic storms, the system gave navigators information that was inaccurate by as much as several miles. If navigators had been alerted that a proton event or geomagnetic storm was in progress, they could have switched to a backup system.

GNSS signals are affected when solar activity causes sudden variations in the density of the ionosphere, causing the satellite signals to [[Scintillation (astronomy)|scintillate]] (like a twinkling star). The scintillation of satellite signals during ionospheric disturbances is studied at [[High Frequency Active Auroral Research Program|HAARP]] during ionospheric modification experiments. It has also been studied at the [[Jicamarca Radio Observatory]].

One technology used to allow GNSS receivers to continue to operate in the presence of some confusing signals is [[Receiver Autonomous Integrity Monitoring]] (RAIM), used by GPS. However, RAIM is predicated on the assumption that a majority of the GPS constellation is operating properly, and so it is much less useful when the entire constellation is perturbed by global influences such as geomagnetic storms. Even if RAIM detects a loss of integrity in these cases, it may not be able to provide a useful, reliable signal.

==== Satellites ====

Geomagnetic storms and increased solar [[ultraviolet]] emission heat Earth's upper atmosphere, causing it to expand. The heated air rises, and the density at the orbit of [[satellite]]s up to about {{convert|1000|km|-2|abbr=on}} increases significantly. This results in increased [[Drag (physics)|drag]], causing satellites to slow and change [[orbit]] slightly. [[Low Earth orbit]] satellites that are not repeatedly boosted to higher orbits slowly fall and eventually burn up. [[Skylab]]'s 1979 destruction is an example of a spacecraft [[Atmospheric reentry|reentering]] Earth's atmosphere prematurely as a result of higher-than-expected solar activity.<ref>{{cite book|last1=Benson|first1=Charles Dunlap|first2=William David|last2=Compton|name-list-style=amp |year=1983 |url=https://history.nasa.gov/SP-4208/contents.htm|title=Living and Working in Space: A History of Skylab |publisher=NASA Scientific and Technical Information Office|id=SP-4208|oclc=8114293}}</ref> During the great geomagnetic storm of March 1989, four of the [[U.S. Navy]]'s navigational satellites had to be taken out of service for up to a week, the [[U.S. Air Force Space Command|U.S. Space Command]] had to post new [[orbital elements]] for over 1000 objects affected, and the [[Solar Maximum Mission]] satellite fell out of orbit in December the same year.<ref>{{cite journal |url=ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/Publications/March1989Events/EffectsoftheMarch1989SolarActivity-Allen%20et%20al%20-%201989.pdf |title=Effects of the March 1989 Solar Activity |author1=Joe Allen |author2=Lou Frank |author3=Herb Sauer |author4=Patricia Reiff |journal=Eos |date=November 14, 1989 |archive-url=https://web.archive.org/web/20200208014913/ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/Publications/March1989Events/EffectsoftheMarch1989SolarActivity-Allen%20et%20al%20-%201989.pdf |archive-date=2020-02-08 |url-status=dead |page=1488}}</ref>

The vulnerability of the satellites depends on their position as well. The [[South Atlantic Anomaly]] is a perilous place for a satellite to pass through, due to the unusually weak geomagnetic field at low Earth orbit.<ref name=NYT>{{cite news |url=https://www.nytimes.com/1990/06/05/science/dip-on-earth-is-big-trouble-in-space.html |title='Dip' on Earth is Big Trouble in Space |work=[[The New York Times]] |last=Broad |first=William J. |date=5 June 1990 |access-date=31 December 2009}}</ref>

==== Pipelines ====

Rapidly fluctuating geomagnetic fields can produce [[geomagnetically induced current]]s in [[Pipeline transport|pipeline]]s. This can cause multiple problems for pipeline engineers. Pipeline flow meters can transmit erroneous flow information and the [[corrosion]] rate of the pipeline can be dramatically increased.<ref>{{Cite journal |last1 = Gummow |first1 = R |title = GIC effects on pipeline corrosion and corrosion control systems |journal = Journal of Atmospheric and Solar-Terrestrial Physics |volume = 64 |page = 1755 |date = 2002 |doi = 10.1016/S1364-6826(02)00125-6|bibcode = 2002JASTP..64.1755G |last2 = Eng |first2 = P |issue = 16 }}</ref><ref>{{Cite journal |last1 =Osella |first1 =A |last2 =Favetto |first2 =A |last3 =López |first3 =E |title =Currents induced by geomagnetic storms on buried pipelines as a cause of corrosion |journal =Journal of Applied Geophysics |volume =38 |page =219 |date =1998 |doi =10.1016/S0926-9851(97)00019-0|bibcode = 1998JAG....38..219O |issue =3 }}</ref>

=== Radiation hazards to humans ===

Earth's atmosphere and magnetosphere allow adequate protection at ground level, but [[astronaut]]s are subject to potentially lethal [[radiation poisoning]]. The penetration of high-energy particles into living cells can cause [[chromosome]] damage, [[cancer]] and other health problems. Large doses can be immediately fatal. Solar [[proton]]s with energies greater than 30&nbsp;[[MeV]] are particularly hazardous.<ref>{{Cite book|url=https://books.google.com/books?id=tjacAgAAQBAJ&pg=PA9|title=Radiation and the International Space Station: Recommendations to Reduce Risk|last1=Council|first1=National Research|last2=Sciences|first2=Division on Engineering and Physical|last3=Board|first3=Space Studies|last4=Applications|first4=Commission on Physical Sciences, Mathematics, and|last5=Research|first5=Committee on Solar and Space Physics and Committee on Solar-Terrestrial|date=2000|publisher=National Academies Press|isbn=978-0-309-06885-7|page=9}}</ref>

[[Solar proton event]]s can also produce elevated radiation aboard [[Jet airliner|aircraft]] flying at high altitudes. Although these risks are small, [[flight crew]]s may be exposed repeatedly, and monitoring of solar proton events by satellite instrumentation allows exposure to be monitored and evaluated, and eventually flight paths and altitudes to be adjusted to lower the absorbed dose.<ref>{{Cite web|url=https://cordis.europa.eu/docs/projects/files/FIGM/FIGM-CT-2000-00068/75331981-6_en.pdf|title=Evaluation of the Cosmic Radiation Exposure of Aircraft Crew|access-date=19 May 2024}}</ref><ref>{{cite web |url=http://www.unscear.org/docs/reports/2008/09-86753_Report_2008_GA_Report_corr2.pdf |title=Sources and Effects of Ionizing Radiation, UNSCEAR 2008}}</ref><ref>{{cite web| last =Phillips| first =Tony| title =The Effects of Space Weather on Aviation| website =Science News| publisher =NASA| date =25 October 2013| url =https://science.nasa.gov/science-news/science-at-nasa/2013/25oct_aviationswx/| access-date =12 July 2017| archive-date =28 September 2019| archive-url =https://web.archive.org/web/20190928003535/https://science.nasa.gov/science-news/science-at-nasa/2013/25oct_aviationswx/| url-status =dead}}</ref>

[[Ground level enhancement]]s, also known as ground level events or GLEs, occur when a [[solar particle event]] contains particles with sufficient energy to have effects at ground level, mainly detected as an increase in the number of [[neutrons]] measured at ground level. These events have been shown to have an impact on radiation dosage, but they do not significantly increase the risk of cancer.<ref name="BritishGovSpaceWeather">{{cite web |url=https://www.gov.uk/guidance/space-weather-and-radiation|title=British Government: Space Weather and radiation guidance, Public Health England|access-date=6 January 2022}}</ref>

===Animals===

There is a large but controversial body of scientific literature on connections between geomagnetic storms and human health. This began with Russian papers, and the subject was subsequently studied by Western scientists. Theories for the cause include the involvement of [[cryptochrome]], [[melatonin]], the [[pineal gland]], and the [[circadian rhythm]].<ref>{{cite journal |last1=James Close |title=Are stress responses to geomagnetic storms mediated by the cryptochrome compass system? |journal=Proc Biol Sci |date=Jun 7, 2012 |volume=279 |issue=1736 |pages=2081–2090 |doi=10.1098/rspb.2012.0324 |pmid=22418257 |pmc=3321722 }}</ref>

Some scientists suggest that solar storms induce [[Cetacean stranding|whales to beach]] themselves.<ref>{{Cite web|url=https://www.sciencedaily.com/releases/2017/02/170206083827.htm|title=Scientist studies whether solar storms cause animal beachings|website=ScienceDaily|accessdate=27 June 2023}}</ref><ref>{{cite news|url=https://www.bbc.com/news/science-environment-41110082|title=Northern lights link to whale strandings|first=Matt|last=McGrath|work=BBC News|date=5 September 2017}}</ref> Some have speculated that migrating animals which use [[magnetoreception]] to navigate, such as birds and honey bees, might also be affected.<ref>{{cite web | title=Solar Storms May Ignite South-Reaching Auroras Wednesday | website=US News & World Report | date=6 September 2017 | url=http://www.usnews.com/news/national-news/articles/2017-09-06/solar-storms-may-ignite-south-reaching-auroras-wednesday | access-date=27 June 2023}}</ref>

== See also ==
{{cmn|
* [[Advanced Composition Explorer]]
* [[Electromagnetic pulse]]
* [[List of solar storms]]
* [[Magnetar]]
* [[Solar and Heliospheric Observatory]]
* [[STEREO]]
* [[Van Allen Probes]]
}}

== References ==
{{reflist|30em}}

== Further reading ==
{{refbegin}}
* {{cite journal |doi=10.1016/S1364-6826(02)00128-1 |last=Bolduc |first=L. |title=GIC observations and studies in the Hydro-Québec power system |journal=J. Atmos. Sol.-Terr. Phys. |volume=64 |issue=16 |pages=1793–1802 |date=2002 |bibcode = 2002JASTP..64.1793B }}
* {{cite book |author=Campbell, W.H. |title=Earth Magnetism: A Guided Tour Through Magnetic Fields |publisher=Harcourt Sci. & Tech. |location=New York |date=2001 |isbn=978-0-12-158164-0 }}
* Carlowicz, M., and R. Lopez, [http://www.stormsfromthesun.net Storms from the Sun], Joseph Henry Press, 2002, www.stormsfromthesun.net 
* {{cite book |author=Davies, K. |title=Ionospheric Radio |publisher=Peter Peregrinus |location=London, UK |date=1990 |pages=331–345 |isbn=978-0-86341-186-1 |series=IEE Electromagnetic Waves Series }}
* {{cite book |author=Eather, R.H. |title=Majestic Lights |publisher=AGU |location=Washington DC |date=1980 |isbn=978-0-87590-215-9 }}
* {{cite book |editor1=Garrett, H.B. |editor2=Pike, C.P. |title=Space Systems and Their Interactions with Earth's Space Environment |publisher=American Institute of Aeronautics and Astronautics |location=New York |date=1980 |isbn=978-0-915928-41-5 }}
* {{cite book | author=Gauthreaux, S. Jr. |chapter=Ch. 5 |title=Animal Migration: Orientation and Navigation |publisher=Academic Press |location=New York |date=1980 |isbn=978-0-12-277750-9 |url-access=registration |url=https://archive.org/details/animalmigrationo0000unse }}
* {{cite book |author=Harding, R. |title=Survival in Space |publisher=Routledge |location=New York |date=1989 |isbn=978-0-415-00253-0 |url-access=registration |url=https://archive.org/details/survivalinspacem0000hard }}
* {{cite journal |doi=10.1029/91EO00062 |author=Joselyn J.A. |title=The impact of solar flares and magnetic storms on humans |journal=EOS |volume=73 |issue=7 |pages=81, 84–5 |date=1992 |bibcode = 1992EOSTr..73...81J }}
* {{cite book |author1=Johnson, N.L. |author2=McKnight, D.S. |title=Artificial Space Debris |publisher=Orbit Book  |location=Malabar, Florida |date=1987 |isbn=978-0-89464-012-4 }}
* {{cite book |author=Lanzerotti, L.J. |chapter=Impacts of ionospheric / magnetospheric process on terrestrial science and technology |editor1=Lanzerotti, L.J. |editor2=Kennel, C.F. |editor3=Parker, E.N. |title=Solar System Plasma Physics, III |publisher=North Holland |location=New York |date=1979 }}
* {{cite book |author=Odenwald, S. |title=The 23rd Cycle:Learning to live with a stormy star |publisher=Columbia University Press |date=2001 |isbn=978-0-231-12079-1 }}
* Odenwald, S., 2003, [http://www.solarstorms.org "The Human Impacts of Space Weather"].
*  Stoupel, E., (1999) [http://www.breadandbutterscience.com/SSTA.pdf Effect of geomagnetic activity on cardiovascular parameters], Journal of Clinical and Basic Cardiology, 2, Issue 1, 1999, pp 34–40. IN James A. Marusek (2007) Solar Storm Threat Analysis, ''Impact, Bloomfield, Indiana 47424'' 
* Volland, H., (1984), "Atmospheric Electrodynamics", Kluwer Publ., Dordrecht
{{refend}}

== External links ==
* [http://www.spaceweatherlive.com/ Live solar and geomagnetic activity data at Spaceweather]
* [https://web.archive.org/web/20120806142735/http://www.swpc.noaa.gov/index.html NOAA Space Weather Prediction Center]
* [http://www.bcmt.fr/data_plot_realtime.php Real time magnetograms]
* [http://aurorawatch.lancs.ac.uk/ Aurora Watch] at Lancaster University
* [http://geomag.usgs.gov USGS Geomagnetism program]
* [https://www.parliament.uk/globalassets/documents/commons-committees/defence/121220-PM-to-Chair-re-EMP.pdf Paper on the risk of a repeat of 1859 Carrington Event] by [[John Beddington]]
Links related to power grids:
* [http://hireme.geek.nz/solar-storm-hvac-transformer-avoidable-failure.html Geomagnetic Storm Induced HVAC Transformer Failure is Avoidable] {{Webarchive|url=https://web.archive.org/web/20130517120625/http://hireme.geek.nz/solar-storm-hvac-transformer-avoidable-failure.html |date=2013-05-17 }}
* [https://web.archive.org/web/20100325191721/http://www.economics.noaa.gov/?goal=commerce NOAA Economics – Geomagnetic Storm datasets and Economic Research]
* [https://web.archive.org/web/20080611174103/http://www.agu.org/sci_soc/eiskappenman.html Geomagnetic Storms Can Threaten Electric Power Grid]

{{Solar storms}}
{{Magnetospherics}}
{{Natural disasters}}
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[[Category:Geomagnetic storms| ]]
[[Category:Astronomical events of the Solar System]]
[[Category:Geomagnetism]]
[[Category:Ionosphere]]
[[Category:Natural disasters]]
[[Category:Solar phenomena]]
[[Category:Sources of electromagnetic interference]]
[[Category:Space hazards]]
[[Category:Space weather]]