Editing Earth's magnetic field (section)

== Time dependence ==
=== Short-term variations ===
[[File:Magnetic Storm Oct 2003.jpg|thumb|'''Background''': a set of traces from magnetic observatories showing a [[magnetic storm]] in 2000.<br /> '''Globe''': map showing locations of observatories and contour lines giving horizontal magnetic intensity in [[Micro-|μ]] [[Tesla (unit)|T]].]]

The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere ([[ionospheric dynamo region]]) and magnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more mostly reflect changes in the [[Earth's interior]], particularly the iron-rich [[structure of the Earth|core]].<ref name="MMMch2" />

Frequently, the Earth's magnetosphere is hit by [[solar flare]]s causing geomagnetic storms, provoking displays of aurorae. The short-term instability of the magnetic field is measured with the [[K-index]].<ref>{{cite web |url=http://www.swpc.noaa.gov/info/Kindex.html |title=The K-index |publisher=Space Weather Prediction Center |access-date=20 October 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131022183358/http://www.swpc.noaa.gov/info/Kindex.html |archive-date=22 October 2013 }}</ref>

Data from [[THEMIS]] show that the magnetic field, which interacts with the solar wind, is reduced when the magnetic orientation is aligned between Sun and Earth – opposite to the previous hypothesis. During forthcoming solar storms, this could result in [[Power outage|blackouts]] and disruptions in [[artificial satellite]]s.<ref>{{cite web |first=Bill |last=Steigerwald |url=http://www.nasa.gov/mission_pages/themis/news/themis_leaky_shield.html |title=Sun Often "Tears Out A Wall" In Earth's Solar Storm Shield |work=THEMIS: Understanding space weather |publisher=NASA |date=16 December 2008 |access-date=20 August 2011 |archive-date=16 March 2010 |archive-url=https://web.archive.org/web/20100316055408/http://www.nasa.gov/mission_pages/themis/news/themis_leaky_shield.html |url-status=dead }}</ref>

=== Secular variation ===
{{Main|Geomagnetic secular variation}}
[[File:Earth Magnetic Field Declination from 1590 to 1990.gif|thumb|Estimated declination contours by year, 1590 to 1990 (click to see variation)]]
[[File:Geomagnetic axial dipole strength.svg|thumb|Strength of the axial dipole component of Earth's magnetic field from 1600 to 2020]]

Changes in Earth's magnetic field on a time scale of a year or more are referred to as ''secular variation''. Over hundreds of years, magnetic declination is observed to vary over tens of degrees.<ref name="MMMch2" /> The animation shows how global declinations have changed over the last few centuries.<ref name="declination">{{cite journal |last1=Jackson |first1=Andrew |last2=Jonkers |first2=Art R. T. |last3=Walker |first3=Matthew R. |title=Four centuries of Geomagnetic Secular Variation from Historical Records |journal=Philosophical Transactions of the Royal Society A |volume=358 |pages=957–990 |year=2000 |jstor=2666741 |issue=1768 |bibcode=2000RSPTA.358..957J |doi=10.1098/rsta.2000.0569|citeseerx=10.1.1.560.5046 |s2cid=40510741 }}</ref>

The direction and intensity of the dipole change over time. Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century.<ref name="MMMch2" /> At this rate of decrease, the field would be negligible in about 1600 years.<ref name="GSC">{{cite web |url=http://nrhp.focus.nps.gov/ |title=Secular variation |work=Geomagnetism |publisher=Canadian Geological Survey |year=2011 |access-date=18 July 2011 |archive-date=25 July 2008 |archive-url=https://web.archive.org/web/20080725123211/http://nrhp.focus.nps.gov/ |url-status=dead }}</ref> However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual.<ref name="Constable_dipole">{{Cite book|last=Constable|first=Catherine|title=Encyclopedia of Geomagnetism and Paleomagnetism|publisher=Springer-Verlag|year=2007|isbn=978-1-4020-3992-8|editor-last=Gubbins|editor-first=David|location=|pages=159–161|chapter=Dipole Moment Variation|doi=10.1007/978-1-4020-4423-6_67|author-link=Catherine Constable|editor2-last=Herrero-Bervera|editor2-first=Emilio}}</ref>

A prominent feature in the non-dipolar part of the secular variation is a ''westward drift'' at a rate of about 0.2° per year.<ref name="GSC" /> This drift is not the same everywhere and has varied over time. The globally averaged drift has been westward since about 1400 AD but eastward between about 1000 AD and 1400 AD.<ref name="Dumberry">{{cite journal |last1=Dumberry |first1=Mathieu |last2=Finlay |first2=Christopher C. |title=Eastward and westward drift of the Earth's magnetic field for the last three millennia |journal=Earth and Planetary Science Letters |volume=254 |issue=1–2 |pages=146–157 |year=2007 |doi=10.1016/j.epsl.2006.11.026 |bibcode=2007E&PSL.254..146D |url=http://www.epm.geophys.ethz.ch/~cfinlay/publications/dumberry_finlay_epsl07.pdf |access-date=2013-10-22 |archive-url=https://web.archive.org/web/20131023062443/http://www.epm.geophys.ethz.ch/~cfinlay/publications/dumberry_finlay_epsl07.pdf |archive-date=2013-10-23 |url-status=dead }}</ref>

Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to as ''paleomagnetic secular variation'' or ''paleosecular variation (PSV)''. The records typically include long periods of small change with occasional large changes reflecting [[geomagnetic excursion]]s and reversals.<ref name="TauxeCh1">{{harvnb|Tauxe|1998|loc=Chapter 1}}</ref>

A 1995 study of lava flows on [[Steens Mountain]], Oregon appeared to suggest the magnetic field once shifted at a rate of up to 6° per day at some time in Earth's history, a surprising result.<ref name="nature.com" /> However, in 2014 one of the original authors published a new study which found the results were actually due to the continuous thermal demagnitization of the lava, not to a shift in the magnetic field.<ref name="epsl.com" />

In July 2020 scientists report that analysis of simulations and a recent observational field model show that maximum rates of directional change of Earth's magnetic field reached ~10° per year – almost 100 times faster than current changes and 10 times faster than previously thought.<ref>{{cite news|title=Simulations show magnetic field can change 10 times faster than previously thought|language=en|work=phys.org|url=https://phys.org/news/2020-07-simulations-magnetic-field-faster-previously.html|access-date=16 August 2020}}</ref><ref>{{cite journal|last1=Davies|first1=Christopher J.|last2=Constable|first2=Catherine G.|date=6 July 2020|title=Rapid geomagnetic changes inferred from Earth observations and numerical simulations|journal=Nature Communications|language=en|volume=11|issue=1|page=3371|doi=10.1038/s41467-020-16888-0|pmid=32632222|pmc=7338531|bibcode=2020NatCo..11.3371D|issn=2041-1723|doi-access=free}}</ref>

=== Magnetic field reversals ===
[[File:Geomagnetic polarity late Cenozoic.svg|thumb|upright|Geomagnetic polarity during the late [[Cenozoic Era]]. Dark areas denote periods where the polarity matches today's polarity, light areas denote periods where that polarity is reversed.]]
{{Main|Geomagnetic reversal}}

Although generally Earth's field is approximately dipolar, with an axis that is nearly aligned with the rotational axis, occasionally the North and South [[geomagnetic poles]] trade places. Evidence for these ''geomagnetic reversals'' can be found in [[basalt]]s, sediment cores taken from the ocean floors, and seafloor magnetic anomalies.<ref>{{cite book |last=Vacquier |first=Victor |title=Geomagnetism in marine geology |year=1972 |publisher=Elsevier Science |location=Amsterdam |isbn=978-0-08-087042-7 |page=38 |edition=2nd}}</ref> Reversals occur nearly [[Poisson distribution|randomly]] in time, with intervals between reversals ranging from less than 0.1 million years to as much as 50 million years. The most recent geomagnetic reversal, called the [[Brunhes–Matuyama reversal]], occurred about 780,000 years ago.<ref name="inconstant">{{cite news |url=https://science.nasa.gov/science-news/science-at-nasa/2003/29dec_magneticfield/ |title=Earth's Inconstant Magnetic Field |work=Science@Nasa |last=Phillips |first=Tony |date=29 December 2003 |access-date=27 December 2009 |archive-date=1 November 2022 |archive-url=https://web.archive.org/web/20221101165248/https://science.nasa.gov/science-news/science-at-nasa/2003/29dec_magneticfield/ |url-status=dead }}</ref><ref name="MMMch5">{{harvnb|Merrill|McElhinny|McFadden|1996|loc=Chapter 5}}</ref> A related phenomenon, a geomagnetic ''excursion'', takes the dipole axis across the equator and then back to the original polarity.<ref name="MMMexcursion">{{harvnb|Merrill|McElhinny|McFadden|1996|pp=148–155}}</ref><ref>{{cite journal |url=https://www.sciencedaily.com/releases/2012/10/121016084936.htm |title=Ice Age Polarity Reversal Was Global Event: Extremely Brief Reversal of Geomagnetic Field, Climate Variability, and Super Volcano |doi=10.1016/j.epsl.2012.06.050 |journal=Earth and Planetary Science Letters |date=16 October 2012 |access-date=21 March 2013|bibcode=2012E&PSL.351...54N |last1=Nowaczyk |first1=N. R. |last2=Arz |first2=H. W. |last3=Frank |first3=U. |last4=Kind |first4=J. |last5=Plessen |first5=B. |volume=351 |page=54 }}</ref> The [[Laschamp event]] is an example of an excursion, occurring during the last ice age (41,000 years ago).

The past magnetic field is recorded mostly by [[Magnetic mineralogy#Strongly magnetic minerals|strongly magnetic minerals]], particularly [[iron oxides]] such as [[magnetite]], that can carry a permanent magnetic moment. This [[remanent magnetization]], or ''remanence'', can be acquired in more than one way. In [[lava flow]]s, the direction of the field is "frozen" in small minerals as they cool, giving rise to a [[thermoremanent magnetization]]. In sediments, the orientation of magnetic particles acquires a slight bias towards the magnetic field as they are deposited on an ocean floor or lake bottom. This is called ''detrital remanent magnetization''.<ref name="McElhinny2000" />

Thermoremanent magnetization is the main source of the magnetic anomalies around mid-ocean ridges. As the seafloor spreads, [[magma]] wells up from the [[Mantle (geology)|mantle]], cools to form new basaltic crust on both sides of the ridge, and is carried away from it by seafloor spreading. As it cools, it records the direction of the Earth's field. When the Earth's field reverses, new basalt records the reversed direction. The result is a series of stripes that are symmetric about the ridge. A ship towing a magnetometer on the surface of the ocean can detect these stripes and infer the age of the ocean floor below. This provides information on the rate at which seafloor has spread in the past.<ref name="McElhinny2000" />

[[Radiometric dating]] of lava flows has been used to establish a ''geomagnetic polarity time scale'', part of which is shown in the image. This forms the basis of [[magnetostratigraphy]], a geophysical correlation technique that can be used to date both sedimentary and volcanic sequences as well as the seafloor magnetic anomalies.<ref name="McElhinny2000" />

=== Earliest appearance ===
Paleomagnetic studies of [[Paleoarchean]] lava in Australia and [[Conglomerate (geology)|conglomerate]] in South Africa have concluded that the magnetic field has been present since at least about {{Ma|3450}}.<ref>{{cite journal |first1=T. N. W. |last1=McElhinney |first2=W. E. |last2=Senanayake |title=Paleomagnetic Evidence for the Existence of the Geomagnetic Field 3.5 Ga Ago |journal=Journal of Geophysical Research |volume=85 |issue = B7|page=3523 |year=1980 |bibcode=1980JGR....85.3523M |doi=10.1029/JB085iB07p03523 |doi-access=free }}</ref><ref name="Tarduno2009">{{cite journal |last1=Usui |first1=Yoichi |last2=Tarduno |first2=John A. |last3=Watkeys |first3=Michael |last4=Hofmann |first4=Axel |last5=Cottrell |first5=Rory D. |title=Evidence for a 3.45-billion-year-old magnetic remanence: Hints of an ancient geodynamo from conglomerates of South Africa |journal=Geochemistry, Geophysics, Geosystems |year=2009 |volume=10 |issue=9 |pages=n/a |doi=10.1029/2009GC002496 |bibcode= 2009GGG....10.9Z07U|doi-access=free }}</ref><ref>{{cite journal |last1=Tarduno |first1=J. A. |last2=Cottrell |first2=R. D. |last3=Watkeys |first3=M. K. |last4=Hofmann |first4=A. |last5=Doubrovine |first5=P. V. |last6=Mamajek |first6=E. E. |last7=Liu |first7=D. |last8=Sibeck |first8=D. G. |last9=Neukirch |first9=L. P. |last10=Usui |first10=Y. |s2cid=23162882 |title=Geodynamo, Solar Wind, and Magnetopause 3.4 to 3.45 Billion Years Ago |journal=Science |date=4 March 2010 |volume=327 |issue=5970 |pages=1238–1240 |doi=10.1126/science.1183445 |bibcode=  2010Sci...327.1238T|pmid=20203044 }}</ref> In 2024 researchers published evidence from Greenland for the existence of the magnetic field as early as 3,700 million years ago.<ref>{{Cite journal |last1=Nichols |first1=Claire I. O. |last2=Weiss |first2=Benjamin P. |last3=Eyster |first3=Athena |last4=Martin |first4=Craig R. |last5=Maloof |first5=Adam C. |last6=Kelly |first6=Nigel M. |last7=Zawaski |first7=Mike J. |last8=Mojzsis |first8=Stephen J. |last9=Watson |first9=E. Bruce |last10=Cherniak |first10=Daniele J. |date=2024 |title=Possible Eoarchean Records of the Geomagnetic Field Preserved in the Isua Supracrustal Belt, Southern West Greenland |journal=Journal of Geophysical Research: Solid Earth |language=en |volume=129 |issue=4 |doi=10.1029/2023JB027706 |issn=2169-9313|doi-access=free |bibcode=2024JGRB..12927706N }}</ref>

=== Future ===
[[File:Brunhes geomagnetism western US.png|thumb|Variations in virtual axial dipole moment since the last reversal]]
Starting in the late 1800s and throughout the 1900s and later, the overall geomagnetic field has become weaker; the present strong deterioration corresponds to a 10–15% decline and has accelerated since 2000; geomagnetic intensity has declined almost continuously from a maximum 35% above the modern value, from circa year 1 AD. The rate of decrease and the current strength are within the normal range of variation, as shown by the record of past magnetic fields recorded in rocks.

The nature of Earth's magnetic field is one of [[heteroscedastic]] (seemingly random) fluctuation. An instantaneous measurement of it, or several measurements of it across the span of decades or centuries, are not sufficient to extrapolate an overall trend in the field strength. It has gone up and down in the past for unknown reasons. Also, noting the local intensity of the dipole field (or its fluctuation) is insufficient to characterize Earth's magnetic field as a whole, as it is not strictly a dipole field. The dipole component of Earth's field can diminish even while the total magnetic field remains the same or increases.

The Earth's magnetic north pole is drifting from northern [[Canada]] towards [[Siberia]] with a presently accelerating rate—{{convert|10|km}} per year at the beginning of the 1900s, up to {{convert|40|km}} per year in 2003,<ref name="inconstant" /> and since then has only accelerated.<ref>{{cite web |url=http://news.nationalgeographic.com/news/2009/12/091224-north-pole-magnetic-russia-earth-core.html |archive-url=https://web.archive.org/web/20091228073445/http://news.nationalgeographic.com/news/2009/12/091224-north-pole-magnetic-russia-earth-core.html |url-status=dead |archive-date=December 28, 2009 |title=North Magnetic Pole Moving Due to Core Flux |last=Lovett |first=Richard A. |date=December 24, 2009}}</ref><ref>{{cite journal |last1=Witze |first1=Alexandra |title=Earth's magnetic field is acting up and geologists don't know why |journal=Nature |date=9 January 2019 |volume=565 |issue=7738 |pages=143–144 |doi=10.1038/d41586-019-00007-1|pmid=30626958 |bibcode=2019Natur.565..143W |doi-access=free }}</ref>