Earth

=== Magnetic field ===
{{Main|Earth's magnetic field}}
[[File:Magnetosphere Levels-en.svg|alt=Diagram showing the magnetic field lines of Earth's magnetosphere. The lines are swept back in the anti-solar direction under the influence of the solar wind.|thumb|A schematic view of Earth's magnetosphere with [[solar wind]] flowing from left to right]]
The main part of Earth's magnetic field is generated in the core, the site of a [[Dynamo theory|dynamo]] process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth's surface, where it is, approximately, a [[dipole]]. The poles of the dipole are located close to Earth's geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is {{nowrap|3.05{{e|−5}} [[Tesla (unit)|T]]}}, with a [[magnetic dipole moment]] of {{nowrap|7.79{{e|22}} Am{{sup|2}}}} at epoch 2000, decreasing nearly 6% per century (although it still remains stronger than its long time average).<ref name="dipole">{{cite journal |last1=Olson |first1=Peter |last2=Amit |first2=Hagay |title=Changes in earth's dipole |url=https://pages.jh.edu/~polson1/pdfs/ChangesinEarthsDipole.pdf |journal=Naturwissenschaften |volume=93 |issue=11 |year=2006 |pages=519–542 |doi=10.1007/s00114-006-0138-6 |pmid=16915369 |bibcode=2006NW.....93..519O |s2cid=22283432}}</ref> The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes [[Geomagnetic secular variation|secular variation]] of the main field and [[geomagnetic reversal|field reversals]] at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.<ref name="fitzpatrick2006" /><ref name="campbelwh" />

The extent of Earth's magnetic field in space defines the [[magnetosphere]]. Ions and electrons of the solar wind are deflected by the magnetosphere; solar wind pressure compresses the dayside of the magnetosphere, to about 10 Earth radii, and extends the nightside magnetosphere into a long tail.<ref>{{Cite journal|last1=Ganushkina|first1=N. Yu|last2=Liemohn|first2=M. W.|last3=Dubyagin|first3=S.|date=2018|title=Current Systems in the Earth's Magnetosphere|url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017RG000590|journal=Reviews of Geophysics|language=en|volume=56|issue=2|pages=309–332|doi=10.1002/2017RG000590|bibcode=2018RvGeo..56..309G|hdl=2027.42/145256|s2cid=134666611|issn=1944-9208|hdl-access=free|access-date=24 October 2020|archive-date=31 March 2021|archive-url=https://web.archive.org/web/20210331100349/https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017RG000590|url-status=dead}}</ref> Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind, a supersonic [[bow shock]] precedes the dayside magnetosphere within the solar wind.<ref>{{cite web |url=http://sci.esa.int/jump.cfm?oid=40994 |title=Cluster reveals the reformation of the Earth's bow shock |publisher=European Space Agency |first=Arnaud |last=Masson |date=11 May 2007 |access-date=16 August 2016}}</ref> [[Charged particle]]s are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates.<ref>{{cite web |url=http://plasmasphere.nasa.gov/ |title=The Earth's Plasmasphere |publisher=NASA/Marshall Space Flight Center |last=Gallagher |first=Dennis L. |date=14 August 2015 |access-date=16 August 2016}}</ref><ref>{{cite web |url=http://plasmasphere.nasa.gov/formed.html |title=How the Plasmasphere is Formed |publisher=NASA/Marshall Space Flight Center |last=Gallagher |first=Dennis L. |date=27 May 2015 |access-date=16 August 2016 |archive-date=15 November 2016 |archive-url=https://web.archive.org/web/20161115064232/http://plasmasphere.nasa.gov/formed.html |url-status=dead }}</ref> The ring current is defined by medium-energy [[particle]]s that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field,<ref name="BaumjohannTreumann1997">{{cite book |title=Basic Space Plasma Physics |publisher=World Scientific |first1=Wolfgang |last1=Baumjohann |first2=Rudolf A. |last2=Treumann |pages=8, 31 |year=1997 |isbn=978-1-86094-079-8}}</ref> and the [[Van Allen radiation belt]]s are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.<ref name="Britannica">{{cite encyclopedia |url=https://www.britannica.com/science/ionosphere-and-magnetosphere/Magnetosphere |title=Ionosphere and magnetosphere |encyclopedia=Encyclopædia Britannica |publisher=Encyclopædia Britannica, Inc. |first=Michael B. |last=McElroy |year=2012}}</ref><ref name="Van Allen">{{cite book |title=Origins of Magnetospheric Physics |publisher=University of Iowa Press |last=Van Allen |first=James Alfred |date=2004 |isbn=978-0-87745-921-7 |oclc=646887856}}</ref>

During [[magnetic storm]]s and [[substorm]]s, charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth's ionosphere, where atmospheric atoms can be excited and ionized, causing the [[Aurora (astronomy)|aurora]].<ref name="stern2005" />