Editing Earth's magnetic field (section)

== Characteristics ==
At any location, the Earth's magnetic field can be represented by a three-dimensional vector. A typical procedure for measuring its direction is to use a compass to determine the direction of magnetic North. Its angle relative to true North is the ''declination'' ({{math|<var>D</var>}}) or ''variation''. Facing magnetic North, the angle the field makes with the horizontal is the ''inclination'' ({{math|<var>I</var>}}) or ''[[magnetic dip]]''. The ''intensity'' ({{math|<var>F</var>}}) of the field is proportional to the force it exerts on a magnet. Another common representation is in {{math|<var>X</var>}} (North), {{math|<var>Y</var>}} (East) and {{math|<var>Z</var>}} (Down) coordinates.<ref name="MMMch2" />
[[File:XYZ-DIS magnetic field coordinates.svg|500px|thumb|center|Common coordinate systems used for representing the Earth's magnetic field]]

=== Intensity ===
The intensity of the field is often measured in [[Gauss (unit)|gauss (G)]], but is generally reported in [[Tesla (unit)|microteslas]] (μT), with 1&nbsp;G = 100&nbsp;μT. A nanotesla is also referred to as a gamma (γ). The Earth's field ranges between approximately {{cvt|22 and 67|μT|G}}.<ref>{{Cite web |title=An Overview of the Earth's Magnetic Field |url=http://www.geomag.bgs.ac.uk/education/earthmag.html#_Toc2075549 |access-date=2024-03-02 |website=www.geomag.bgs.ac.uk}}</ref> By comparison, a strong [[refrigerator magnet]] has a field of about {{cvt|10000|μT|G}}.<ref>{{cite web |url=http://www.magnet.fsu.edu/education/tutorials/magnetminute/tesla-transcript.html |last1=Palm |first1=Eric |title=Tesla |publisher=National High Magnetic Field Laboratory |year=2011 |access-date=20 October 2013 |archive-url=https://web.archive.org/web/20130321234403/http://www.magnet.fsu.edu/education/tutorials/magnetminute/tesla-transcript.html |archive-date=21 March 2013 |url-status=dead }}</ref>

A map of intensity contours is called an ''isodynamic chart''. As the [[#Geographical variation|World Magnetic Model]] shows, the intensity tends to decrease from the poles to the equator. A minimum intensity occurs in the [[South Atlantic Anomaly]] over South America while there are maxima over northern Canada, Siberia, and the coast of Antarctica south of Australia.<ref name="renamed_from_2010_on_20131022170733" />

The intensity of the magnetic field is subject to change over time. A 2021 paleomagnetic study from the [[University of Liverpool]] contributed to a growing body of evidence that the Earth's magnetic field cycles with intensity every 200 million years. The lead author stated that "Our findings, when considered alongside the existing datasets, support the existence of an approximately 200-million-year-long cycle in the strength of the Earth's magnetic field related to deep Earth processes."<ref>{{Cite web|date=2021-08-31|title=Ancient lava reveals secrets of Earth's magnetic field cycle|url=https://cosmosmagazine.com/science/ancient-lava-reveals-secrets-of-earths-magnetic-field-cycle/|access-date=2021-09-03|website=Cosmos Magazine|language=en-AU}}</ref>

=== Inclination ===
{{Main|Magnetic dip}}
The inclination is given by an angle that can assume values between &minus;90° (up) to 90° (down). In the northern hemisphere, the field points downwards. It is straight down at the [[North Magnetic Pole]] and rotates upwards as the latitude decreases until it is horizontal (0°) at the magnetic equator. It continues to rotate upwards until it is straight up at the South Magnetic Pole. Inclination can be measured with a [[dip circle]].

An ''isoclinic chart'' (map of inclination contours) for the Earth's magnetic field is shown [[#Geographical variation|below]].

=== Declination ===
{{Main|Magnetic declination}}
Declination is positive for an eastward deviation of the field relative to true north. It can be estimated by comparing the magnetic north–south heading on a compass with the direction of a [[celestial pole]]. Maps typically include information on the declination as an angle or a small diagram showing the relationship between magnetic north and true north. Information on declination for a region can be represented by a chart with isogonic lines (contour lines with each line representing a fixed declination).

=== Geographical variation ===
{{center|
Components of the Earth's magnetic field at the surface from the [[World Magnetic Model]] for 2020.<ref name="renamed_from_2010_on_20131022170733">{{Cite report |last1=Chulliat |first1=A. |last2=Brown |first2=W. |last3=Alken |first3=P. |last4=Beggan |first4=C. |last5=Nair |first5=M. |last6=Cox |first6=G. |last7=Woods |first7=A. |last8=Macmillan |first8=S. |last9=Meyer  |first9=B. |last10=Paniccia |first10=M. |date=2020 |title=The US/UK World Magnetic Model for 2020–2025 |url=https://repository.library.noaa.gov/view/noaa/24390 |access-date=30 August 2023}}</ref>
}}
<gallery mode="packed" align="center" heights="115">
File:World Magnetic Field Model Total Intensity.jpg|Intensity
File:World Magnetic Field Model Inclination.jpg|Inclination
File:World Magnetic Field Model 2025.jpg|Declination
</gallery>

=== Dipolar approximation ===
[[File:Geographical and Magnetic Poles.png|thumb|Relationship between Earth's poles. A1 and A2 are the geographic poles; B1 and B2 are the geomagnetic poles; C1 (south) and C2 (north) are the magnetic poles.]]
{{See also|Dipole model of the Earth's magnetic field}}
Near the surface of the Earth, its magnetic field can be closely approximated by the field of a magnetic dipole positioned at the center of the Earth and tilted at an angle of about 11° with respect to the rotational axis of the Earth.<ref name="NGDC">{{cite web |title=Geomagnetism Frequently Asked Questions |url=https://www.ncei.noaa.gov/products/geomagnetism-frequently-asked-questions |access-date=21 October 2013 |publisher=National Geophysical Data Center}}</ref> The dipole is roughly equivalent to a powerful bar [[magnet]], with its south pole pointing towards the geomagnetic North Pole.<ref>{{cite news |first=Anne |last=Casselman |title=The Earth Has More Than One North Pole |newspaper=Scientific American |date=28 February 2008 |url=http://www.scientificamerican.com/article.cfm?id=the-earth-has-more-than-one-north-pole |access-date=21 May 2013}}</ref> This may seem surprising, but the north pole of a magnet is so defined because, if allowed to rotate freely, it points roughly northward (in the geographic sense). Since the north pole of a magnet attracts the south poles of other magnets and repels the north poles, it must be attracted to the south pole of Earth's magnet. The dipolar field accounts for 80–90% of the field in most locations.<ref name="MMMch2" />
{{clear}}

=== Magnetic poles{{anchor|Poles}} ===
{{Main|Geomagnetic pole}}
[[File:Magnetic North Pole Positions 2015.svg|thumb|upright=1.1|The movement of Earth's North Magnetic Pole across the Canadian arctic]]
Historically, the north and south poles of a magnet were first defined by the Earth's magnetic field, not vice versa, since one of the first uses for a magnet was as a compass needle. A magnet's  North pole is defined as the pole that is attracted by the Earth's North Magnetic Pole, in the arctic region, when the magnet is suspended so it can turn freely. Since opposite poles attract, the North Magnetic Pole of the Earth is really the south pole of its magnetic field (the place where the field is directed downward into the Earth).<ref name="Serway">{{cite book  | last = Serway  | first = Raymond A. |author2=Chris Vuille | title = Essentials of college physics | publisher = Cengage Learning | year = 2006 | location = USA | page = 493 | url = https://books.google.com/books?id=8n4NCyRgUMEC&pg=PA493 | isbn = 978-0-495-10619-7}}</ref><ref name="Emiliani">{{cite book  | last = Emiliani | first = Cesare | title = Planet Earth: Cosmology, Geology, and the Evolution of Life and Environment | publisher = Cambridge University Press | year = 1992 | location = UK | page = 228 | url = https://books.google.com/books?id=MfAGpVq8gpQC&pg=PA228 | isbn = 978-0-521-40949-0}}</ref><ref name="Manners">{{cite book  | last = Manners | first = Joy | title = Static Fields and Potentials | publisher = CRC Press | year = 2000 | location = USA | page = 148 | url = https://books.google.com/books?id=vJyqbRPsXYQC&pg=PA148 | isbn = 978-0-7503-0718-5}}</ref><ref name="Hyperphysics">{{cite web | last = Nave | first = Carl R. | title = Bar Magnet | work = Hyperphysics | publisher = Dept. of Physics and Astronomy, Georgia State Univ.  | year = 2010 | url = http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html | access-date = 2011-04-10}}</ref>

The positions of the magnetic poles can be defined in at least two ways: locally or globally.<ref>{{cite journal |first1=Wallace A. |last1=Campbell |s2cid=128421452 |title="Magnetic" pole locations on global charts are incorrect |journal=Eos, Transactions American Geophysical Union |volume=77 |issue=36 |page=345 |year=1996 |doi=10.1029/96EO00237 |bibcode=1996EOSTr..77..345C }}</ref> The local definition is the point where the magnetic field is vertical.<ref>{{cite web |url=http://deeptow.whoi.edu/northpole.html |title=The Magnetic North Pole |publisher=Woods Hole Oceanographic Institution |access-date=21 October 2013 |archive-url=https://web.archive.org/web/20130819061122/http://deeptow.whoi.edu/northpole.html |archive-date=19 August 2013 |url-status=dead }}</ref> This can be determined by measuring the inclination. The inclination of the Earth's field is 90° (downwards) at the North Magnetic Pole and –90° (upwards) at the South Magnetic Pole. The two poles wander independently of each other and are not directly opposite each other on the globe.  Movements of up to {{convert|40|km}} per year have been observed for the North Magnetic Pole. Over the last 180 years, the North Magnetic Pole has been migrating northwestward, from Cape Adelaide in the [[Boothia Peninsula]] in 1831 to {{convert|600|km}} from [[Resolute Bay]] in 2001.<ref name="inconstant" /> The ''magnetic equator'' is the line where the inclination is zero (the magnetic field is horizontal).

The global definition of the Earth's field is based on a mathematical model. If a line is drawn through the center of the Earth, parallel to the moment of the best-fitting magnetic dipole, the two positions where it intersects the Earth's surface are called the North and South geomagnetic poles. If the Earth's magnetic field were perfectly dipolar, the geomagnetic poles and magnetic dip poles would coincide and compasses would point towards them. However, the Earth's field has a significant [[Multipole expansion|non-dipolar]] contribution, so the poles do not coincide and compasses do not generally point at either.