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| [[File:Rattling Earth's Force Field.ogv|thumb|A rendering of the [[magnetic field lines]] of the Earth's magnetosphere]]
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| A '''magnetosphere''' is the area of [[outer space|space]] near an [[astronomical object]] in which [[charged particle]]s are controlled by that object's [[magnetic field]].<ref name=NASA>{{cite web|title=Magnetospheres|url=http://science.nasa.gov/heliophysics/focus-areas/magnetosphere-ionosphere/|work=NASA Science|publisher=NASA}}</ref><ref name=Ratcliffe>{{cite book|last=Ratcliffe|first=John Ashworth|title=An Introduction to the Ionosphere and Magnetosphere|year=1972|publisher=CUP Archive|isbn=9780521083416|url=http://books.google.com/books?id=uVA4AAAAIAAJ}}</ref> Near the surface of the object, the [[magnetic field lines]] resemble those of a [[magnetic dipole]]. Farther away from the surface, the field lines are significantly distorted by electric currents flowing in the plasma (e.g. in [[ionosphere]] or [[solar wind]]).<ref name=Britannica /><ref name="Van Allen">{{cite book|last=Van Allen|first=James Alfred|title=Origins of Magnetospheric Physics|year=2004|publisher=University of Iowa Press|location=Iowa City, Iowa, USA|isbn=9780877459217|oclc=646887856}}</ref> When speaking about the Earth, ''magnetosphere'' is typically used to refer to the outer layer of the [[ionosphere]],<ref name="Britannica">{{cite encyclopedia |year=2012 |title =Ionosphere and magnetosphere|encyclopedia=Encyclopedia Britannica |publisher=Encyclopedia Britannica, Inc. |url=http://www.britannica.com/EBchecked/topic/1369043/ionosphere-and-magnetosphere }}</ref> although some sources consider the ionosphere and magnetosphere to be separate.<ref name=Ratcliffe />
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| ==History==
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| {{main|Magnetosphere chronology}}
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| Study of the Earth's magnetosphere began in 1600, when [[William Gilbert (astronomer)|William Gilbert]] discovered that the magnetic field on the surface of the earth resembled that on a [[terrella]], a small, magnetized sphere. In the 1940s, [[Walter M. Elsasser]] proposed the model of [[dynamo theory]], which attributes the [[Earth's magnetic field]] to the motion of the Earth's [[iron]] [[outer core]]. Through the use of [[magnetometer]]s, scientists were able to study the variations in the Earth's magnetic field as functions of both time and latitude and longitude. Beginning in the late 1940s, rockets were used to study [[cosmic rays]]. In 1958, [[Explorer 1]], the first of the [[Explorer series]] of space missions, was launched to study the intensity of cosmic rays above the atmosphere and measure the fluctuations in this activity. This mission observed the existence of the [[Van Allen radiation belt]] (located in the inner region of the Earth's magnetosphere), with the [[Explorer 3]] mission later that year definitively proving its existence. Also in 1958, [[Eugene Parker]] proposed the idea of the [[solar wind]]. In 1959, the term magnetosphere was proposed by [[Thomas Gold]]. The Explorer 12 mission in 1961 led to the observation by Cahill and Amazeen in 1963 of a sudden decrease in the strength of the magnetic field near the noon meridian, later named the [[magnetopause]]. In 1983, the [[International Cometary Explorer]] observed the magnetotail, or the distant magnetic field.<ref name="Van Allen" />
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| ==Types of magnetospheres== | |
| The structure and behavior of magnetospheres is dependent on several variables: the type of astronomical object, the nature of sources of plasma and momentum, the [[frequency|period]] of the object's spin, the nature of the axis whereabout the object spins, the axis of the magnetic dipole, and the magnitude and direction of the [[velocity]] of the flow of solar wind.
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| The distance at which a planet can withstand the solar wind pressure is called the Chapman–Ferraro distance. This is modeled by a formula wherein <math>R_P</math> represents the radius of the planet, <math>B_{surf}</math> represents the magnetic field on the surface of the planet at the equator, and <math>V_{SW}</math> represents the velocity of the solar wind.
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| <math>R_{CF}=R_{P} \left( \frac{B_{surf}^2}{\mu_{0} \rho V_{SW}^2} \right) ^{\frac{1}{6}}</math>
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| A magnetosphere is classified as "intrinsic" when <math>R_{CF} \gg R_{P}</math>, or when the primary opposition to the flow of solar wind is the magnetic field of the object. [[Mercury (planet)|Mercury]], Earth, [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]] exhibit intrinsic magnetospheres. A magnetosphere is classified as "induced" when <math>R_{CF} \ll R_P</math>, or when the solar wind is not opposed by the object's magnetic field. In this case, the solar wind interacts with the atmosphere or ionosphere of the planet (or surface of the planet, if the planet has no atmosphere). [[Venus]] has an induced magnetic field. What this means is that since Venus appears to have no [[Dynamo theory|internal dynamo effect]], the only magnetic field present is the one that solar wind's wrapping around the physical obstacle of Venus (see also [[Atmosphere of Venus#Induced magnetosphere|Venus' Induced Magnetosphere]]) forms. When <math>R_{CF} \approx R_P</math>, the planet itself and its magnetic field both contribute. It is possible that [[Mars]] is of this type.<ref>{{cite journal|last1=Blanc|first1=M.|last2=Kallenbach|first2=R.|last3=Erkaev|first3=N.V.|title=Solar System Magnetospheres|journal=Space Science Reviews|year=2005|issue=116|pages=227–298|doi=10.1007/s11214-005-1958-y|bibcode = 2005SSRv..116..227B }}</ref>
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| ==Structure==
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| [[File:Magnetosphere Levels.svg|thumb|An artist's rendering of the structure of a magnetosphere. 1) Bow shock. 2) Magnetosheath. 3) Magnetopause. 4) Magnetosphere. 5) Northern tail lobe. 6) Southern tail lobe. 7) Plasmasphere.]]
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| ===Bow shock===
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| [[File:Sig06-029.jpg|thumb|[[Thermographic camera|Infrared image]] and artist's concept of the bow shock around [[R Hydrae]]]]
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| {{main|Bow shock}}
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| The bow shock forms the outermost layer of the magnetosphere: the boundary between the magnetosphere and the ambient medium. For stars, this is usually the boundary between the [[stellar wind]] and [[interstellar medium]]; for planets, the speed of the solar wind there so plummets as it approaches the magnetopause.<ref>{{cite journal|last1=Sparavigna|first1=A.C.|last2=Marazzato|first2=R.|title=Observing stellar bow shocks|date=10 May 2010|url=http://arxiv.org/ftp/arxiv/papers/1005/1005.1527.pdf|format=PDF}}</ref>
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| ===Magnetosheath===
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| {{main|Magnetosheath}}
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| The magnetosheath is the region of the magnetosphere between the bow shock and the magnetopause. It is formed mainly from shocked solar wind, though it contains a small amount of [[plasma (physics)|plasma]] from the magnetosphere.<ref name=cluster>{{cite journal|editor1-last=Paschmann|editor1-first=G.|editor2-last=Schwartz|editor2-first=S.J.|editor3-last=Escoubet|editor3-first=C.P.|editor4-last=Haaland|editor4-first=S.|title=Outer Magnetospheric Boundaries: Cluster Results|journal=Space Science Reviews|year=2005|volume=118|issue=1-4|publisher=Springer|location=Dordrecht, The Netherlands|isbn=1-4020-3488-1}}</ref> It is an area exhibiting high particle [[energy flux]], where the direction and magnitude of the magnetic field varies erratically. This is caused by the collection of solar wind gas that has effectively undergone [[thermalization]]. It acts as a cushion that transmits the pressure from the flow of the solar wind and the barrier of the magnetic field from the object.<ref name="Van Allen" />
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| ===Magnetopause===
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| {{main|Magnetopause}}
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| The magnetopause is the area of the magnetosphere wherein the pressure from the planetary magnetic field is balanced with the pressure from the solar wind.<ref name=Britannica /> It is the convergence of the shocked solar wind from the magnetosheath with the magnetic field of the object and plasma from the magnetosphere. Because both sides of this convergence contain magnetized plasma, the interactions between them are very complex. The structure of the magnetopause depends upon the [[Mach number]] and [[Beta (plasma physics)|beta]] of the plasma, as well as the magnetic field.<ref>{{cite journal|last=Russell|first=C.T.|title=The Magnetopause|journal=Physics of Magnetic Flux Ropes|year=1990|pages=439–453|url=http://www-ssc.igpp.ucla.edu/ssc/tutorial/magnetopause.html|publisher=American Geophysical Union|location=Washington, D.C., USA}}</ref> The magnetopause changes size and shape as the pressure from the solar wind fluctuates.<ref>{{cite web|title=The Magnetopause|url=http://www-spof.gsfc.nasa.gov/Education/wmpause.html|publisher=NASA}}</ref>
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| ===Magnetotail===
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| Opposite the compressed magnetic field is the magnetotail, where the magnetosphere extends far beyond the astronomical object. It contains two lobes, referred to as the northern and southern tail lobes. The northern tail lobe points towards the object and the southern tail lobe points away. The tail lobes are almost empty, with very few charged particles opposing the flow of the solar wind. The two lobes are separated by a plasma sheet, an area where the magnetic field is weaker and the density of charged particles is higher.<ref name="tail">{{cite web|title=The Tail of the Magnetosphere|url=http://www-spof.gsfc.nasa.gov/Education/wtail.html|publisher=NASA}}</ref>
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| ===Earth's magnetosphere=== | |
| [[File:Magnetosphere rendition.jpg|thumb|Artist's rendition of the Earth's magnetosphere]]
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| [[File:Structure of the magnetosphere mod.svg|thumb|Diagram of the Earth's magnetosphere]]
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| Over the Earth's [[equator]], the magnetic field lines become almost horizontal, then return to connect back again at high latitudes. However, at high altitudes, the magnetic field is significantly distorted by the solar wind and its solar magnetic field. On the dayside of the Earth, the magnetic field is significantly compressed by the solar wind to a distance of approximately {{convert|65,000|km|sp=us}}. The Earth's bow shock is about {{convert|17|km|sp=us}} thick<ref>{{cite news|title=Cluster reveals Earth's bow shock is remarkably thin|url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=49637|newspaper=European Space Agency|date=16 November 2011}}</ref> and located about {{convert|90,000|km|sp=us}} from the Earth.<ref>{{cite news|title=Cluster reveals the reformation of the Earth's bow shock|url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=40994|newspaper=European Space Agency|date=11 May 2011}}</ref> The magnetopause exists at a distance of several hundred kilometers off earth's surface. The Earth's magnetopause has been compared to a [[sieve]] because it allows solar wind particles to enter. [[Kelvin–Helmholtz instability|Kelvin–Helmholtz instabilities]] occur when large swirls of plasma travel along the edge of the magnetosphere at a different velocity from the magnetosphere, causing the plasma to slip past. This results in [[magnetic reconnection]], and as the magnetic field lines break and reconnect, solar wind particles are able to enter the magnetosphere.<ref>{{cite news|title=Cluster observes a 'porous' magnetopause|url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50977|newspaper=European Space Agency|date=24 October 2012}}</ref> On the nightside of the earth, the magnetic field extends in the magnetotail, which lengthwise exceeds {{convert|6,300,000|km|sp=us}}.<ref name="Britannica" /> The Earth's magnetotail is the primary source of the [[Aurora (astronomy)|polar aurora]].<ref name=tail /> Also, NASA scientists have suggested or "speculated" that the Earth's magnetotail can cause "dust storms" on the moon by creating a potential difference between the day side and the night side.<ref>http://www.nasa.gov/topics/moonmars/features/magnetotail_080416.html NASA, ''The Moon and the Magnetotail''</ref>
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| ===Other objects===
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| The [[magnetosphere of Jupiter]] is the largest planetary magnetosphere in the Solar System, extending up to {{convert|7,000,000|km|sp=us}} on the dayside and almost to the orbit of [[Saturn]] on the nightside.<ref>{{cite encyclopedia|url=http://www.igpp.ucla.edu/people/mkivelson/Publications/279-Ch24.pdf | format=PDF| title= The configuration of Jupiter's magnetosphere | first=K.K. | last=Khurana | coauthors=Kivelson, M.G. et al.| isbn= 0-521-81808-7| encyclopedia=Jupiter: The Planet, Satellites and Magnetosphere|publisher=Cambridge University Press|editor= Bagenal, F.; Dowling, T.E.; McKinnon, W.B.|year=2004}}</ref> Jupiter's magnetosphere is stronger than the Earth's by an [[order of magnitude]], and its [[magnetic moment]] is approximately 18,000 times larger.<ref>{{cite journal|last=Russell|first=C.T.|title= Planetary Magnetospheres |journal=Reports on Progress in Physics|volume=56|issue=6| pages=687–732|year=1993 |url=http://www.iop.org/EJ/article/0034-4885/56/6/001/rp930601.pdf|format=PDF|doi= 10.1088/0034-4885/56/6/001|bibcode=1993RPPh...56..687R}}</ref>
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| ==See also==
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| {{books-inline|Geomagnetism}}
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| * [[Solar wind]]
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| * [[Plasma physics]]
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| ==References==
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| {{reflist}}
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| {{Magnetospherics}}
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| [[Category:Geomagnetism]]
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| [[Category:Ionosphere]]
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| [[Category:Planetary science]]
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| [[Category:Space plasmas]]
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