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| {{Other uses|bow shock (aerodynamics)}}
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| [[File:52706main hstorion lg.jpg|thumb|right|<center>[[LL Ori]] bow shock in [[Orion nebula]]. The star's wind collides with the nebula flow.<br>Hubble, 1995</center>]]
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| <!-- new image needed [[ |thumb|350px|right|The locations of [[Voyager probes|Voyagers 1 and 2]] as of 2005. The "bow shock" shown is the shock caused by the sun as it moves through the [[interstellar medium]]; the Earth is too small to be seen on this scale. Note that the intensity of this shock is greatly exaggerated.]] -->
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| In [[astrophysics]], a '''bow shock''' is the area between a [[magnetosphere]] and an ambient medium. For stars, this boundary is typically the edge of the [[astrosphere]], where the [[stellar wind]] meets the [[interstellar medium]]. For a planetary [[magnetosphere]], the bow shock is the boundary at which the speed of the stellar wind abruptly drops as a result of its approach to 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> The best-studied example of a bow shock is that occurring where the [[solar wind|Sun's wind]] encounters [[Earth]]'s magnetopause, although bow shocks occur around all magnetized planets, such as [[Jupiter#Magnetosphere|Jupiter]]<ref>{{cite journal|last1=Szego|first1=Karoly|title=Cassini plasma spectrometer measurements of Jovian bow shock structure|date=18 July 2003|url=http://eprints.ucl.ac.uk/81775/}}</ref> or [[Saturn#Magnetosphere|Saturn]].<ref>{{cite news|title=Cassini encounters Saturn's bow shock|date=|year=|month=|url=http://www-pw.physics.uiowa.edu/space-audio/cassini/bow-shock/|newspaper=Department of Physics and Astronomy, University of Iowa}}</ref> Earth's bow shock is about {{convert|17|km}} 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}} from the planet.<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>
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| For several decades, the solar wind has been thought to form a bow shock at the edge of the [[heliosphere]], where it collides with the surrounding interstellar medium. This long-held belief was called into question in 2012 when data from the [[Interstellar Boundary Explorer]] (IBEX) found [[Sun|the Sun]] and heliosphere to be moving more slowly through the interstellar medium than previously believed.<ref name=bow>[http://www.nasa.gov/mission_pages/ibex/news/nobowshock.html NASA - IBEX Reveals a Missing Boundary At the Edge Of the Solar System]</ref> This new finding suggests that beyond our [[Solar System]]'s [[termination shock]] and [[Heliopause (astronomy)|heliopause]], there may be no, or very little bow shock.<ref name=bow/>
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| ==Description==
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| The defining criterion is that the bulk velocity of the fluid (in this case, the [[Plasma (physics)|plasma]] of the solar wind) drops from "[[Supersonic speed|supersonic]]" to "subsonic", where [[plasma physics|the speed of sound in a plasma]] is defined as
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| <math>c_s^2 = \gamma p/ \rho </math>
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| where ''c<sub>s</sub>'' is the speed of sound, <math> \gamma </math> is the [[Heat capacity ratio|ratio of specific heats]], ''p'' is the [[pressure]], and <math> \rho </math> is the density of the plasma.
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| The particles making up the solar wind follow spiral paths along magnetic field lines. The velocity of each particle as it gyrates around a field line can be treated similarly to a thermal velocity in an ordinary gas, and in an ordinary gas, the mean thermal velocity is roughly the speed of sound. At the bow shock, the bulk forward velocity of the wind (which is the component of the velocity parallel to the field lines about which the particles gyrate) drops below the speed at which the particles are corkscrewing.
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| Bow shocks are also a common feature in [[Herbig Haro objects]], in which a much stronger collimated outflow of gas and dust from the star interacts with the interstellar medium, producing bright bow shocks that are visible at optical wavelengths.
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| The following images show further evidence of bowshock existence from dense gases and plasma in the [[Orion Nebula]].
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| <gallery>
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| Image:Heliosheath.JPG|
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| Image:Heliosheath2.JPG|
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| Image:Heliosheath3.JPG|
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| Image:Heliosheath4.JPG|
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| </gallery>
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| ==The Sun==
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| It was hypothesised that the [[heliosphere]] also forms a bow shock as it travels through the [[interstellar medium]]. This will occur if the interstellar medium is moving supersonically ''towards'' the Sun, since the solar wind is moving supersonically ''away'' from the Sun. The point where the flow of the interstellar medium becomes subsonic is the bow shock; the point where the interstellar medium and solar wind pressures balance is at the [[heliopause (astronomy)|heliopause]]; the point where the solar wind flow becomes subsonic is the [[termination shock]]. According to [[Robert J. Nemiroff]] and Jerry Bonnell of [[NASA]], the solar bow shock would lie at a distance around 230 AU<ref>http://antwrp.gsfc.nasa.gov/apod/ap020624.html</ref> from the Sun - more than twice the distance of the termination shock as encountered by the Voyager spacecraft. However, data in 2012 from NASA's [[Interstellar Boundary Explorer]] (IBEX) and corroborated with results from the [[Voyager program|Voyager spacecraft]], has found that due to refinements in the relative speed of the heliosphere and the local interstellar magnetic field strength it is believed the heliosphere is prevented from forming a bow shock in the region of our galaxy the Sun is currently passing through.
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| ==In the infrared==
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| In 2006, a far infrared bow shock was detected near the [[asymptotic giant branch|AGB star]] [[R Hydrae]].<ref>[http://cdsads.u-strasbg.fr/cgi-bin/nph-data_query?bibcode=2006ApJ...648L..39U&db_key=AST&link_type=ABSTRACT&high=45ba3a9f6d12619 Detection of a Far-Infrared Bow Shock Nebula around R Hya: The First MIRIAD Results]</ref>[[Image:Sig06-029 medium.jpg|thumb|350px|right|The bow shock around R Hydrae<ref>[http://sscws1.ipac.caltech.edu/Imagegallery/image.php?image_name=sig06-029 Spitzer Science Center Press Release: Red Giant Plunging Through Space]</ref>]]
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| ==See also==
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| * [[Shock wave]]
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| * [[Heliosheath]]
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| * [[Fermi glow]]
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| * [[List of plasma (physics) articles]]
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| ==Notes==
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| {{reflist}}
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| ==References==
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| *{{Cite book |last=Kivelson |first=M. G. |last2=Russell |first2=C. T. |year=1995 |title=Introduction to Space Physics |location=New York |publisher=Cambridge University Press |page=129 |isbn=0-521-45104-3 }}
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| *{{Cite book |last=Cravens |first=T. E. |year=1997 |title=Physics of Solar System Plasmas |location=New York |publisher=Cambridge University Press |page=142 |isbn=0-521-35280-0 }}
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| ==External links==
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| *[http://antwrp.gsfc.nasa.gov/apod/image/0011/bzcam_wiyn.gif Bow shock image (BZ Cam)]
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| *[http://antwrp.gsfc.nasa.gov/apod/image/0010/gcenter2_gemini_big.jpg Bow shock image (IRS8)]
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| *[http://www.eso.org/public/archives/images/screen/eso9702a.jpg Bow shock image (HD77581)]
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| *[http://imgsrc.hubblesite.org/hu/db/2002/05/images/a/formats/large_web.jpg Bow shock image (LL Ori)]
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| *[http://www.atnf.csiro.au/pasa/17_1/cairns/paper/node1.html#fig3 Good diagram from a research paper on shocks]
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| *[http://www.dartmouth.edu/~hrm/R/heliosphere.html More on the ''Voyagers'']
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| *[http://www-pw.physics.uiowa.edu/space-audio/sounds/jovbow.wav Hear Jovian bow shock from the Uni. of Iowa]
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| *[http://www.physorg.com/news98367923.html Cluster spacecraft makes a shocking discovery (Planetary Bow Shock)]
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| {{The Sun}}
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| [[Category:Planetary science]]
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| [[Category:Sun]]
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| [[Category:Shock waves]]
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| [[Category:Waves in plasmas]]
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Hello. Allow me introduce the writer. Her title is Refugia Shryock. He used to be unemployed but now he is a computer operator but his marketing by no means arrives. Years ago we moved to North Dakota. One of the things she enjoys most is to do aerobics and now she is trying to earn money with it.
My web page ... meal delivery service (next)