Heliosphere 1131151 223971585 2008-07-06T19:31:44Z 70.239.214.182 Better words for Bowshock section {{Refimprove|date=January 2008}} [[Image:Heliosphere drawing.gif|thumb|350px|Diagram of features of the heliosphere.]] The '''heliosphere''' is a bubble in [[outer space|space]] "blown" into the [[interstellar medium]] (the hydrogen and helium gas that permeates the [[galaxy]]) by the [[solar wind]]. Although electrically neutral atoms from interstellar space can penetrate this bubble, virtually all of the material in the heliosphere emanates from the [[Sun]] itself. For the first ten billion kilometres of its radius, the solar wind travels at over a million kilometres per hour.<ref name=speed>{{cite web|author=Dr. David H. Hathaway|date=January 18, 2007|title=The Solar Wind|publisher=NASA|url=http://solarscience.msfc.nasa.gov/SolarWind.shtml|accessdate=2007-12-11}}</ref><ref>{{cite news | first=Robert Roy | last=Britt | title=A Glowing Discovery at the Forefront of Our Plunge Through Space | publisher=SPACE.com | date=[[March 15]], [[2000]] | url=http://www.space.com/scienceastronomy/solarsystem/heliosphere_shock_000315.html | accessdate=2006-05-24 }}</ref> As it begins to collide with the [[interstellar medium]], it slows to subsonic speeds before finally ceasing altogether. The point where the solar wind becomes subsonic is the '''termination shock'''; the point where the interstellar medium and solar wind pressures balance is called the '''heliopause'''; the point where the interstellar medium, travelling in the opposite direction, becomes subsonic as it collides with the heliosphere is the '''bow shock'''. ==Solar wind== {{main|Solar wind|interplanetary medium}} [[Image:I screenimage 30579.jpg|thumb|200 px|Coronal mass ejections sending material out into the heliosphere.]] The solar wind consists of particles, [[ion]]ized atoms from the solar [[corona]], and fields, in particular [[magnetic field]]s. As the Sun [[rotation|rotate]]s once in approximately 27 days, the magnetic field transported by the solar wind gets wrapped into a spiral. Variations in the Sun's magnetic field are carried outward by the solar wind and can produce [[Geomagnetic storm|magnetic storm]]s in the Earth's own [[magnetosphere]]. In March 2005, it was reported that measurements by the Solar Wind Anisotropies (SWAN) instrument onboard the [[Solar and Heliospheric Observatory]] (SOHO) have shown that the heliosphere, the solar wind-filled volume which prevents the solar system from becoming embedded in the local (ambient) interstellar medium, is not axisymmetrical, but is distorted, very likely under the effect of the local galactic magnetic field.<ref name="esaDistortionHeliosphere">{{cite journal | author=Lallement, R.; Quémerais, E.; Bertaux, J. L.; Ferron, S.; Koutroumpa, D.; Pellinen, R. | title=Deflection of the Interstellar Neutral Hydrogen Flow Across the Heliospheric Interface | url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36805 | journal=Science | year=2005 | volume=307 | issue=5714 | pages=1447–1449 | accessdate=2007-05-25 | doi=10.1126/science.1107953 | pmid=15746421 }}</ref> ==Structure== ===Heliospheric current sheet=== [[Image:Heliospheric-current-sheet edit.jpg|thumb|200 px|The heliospheric current sheet out to the orbit of Jupiter]] {{main|Heliospheric current sheet}} The [[heliospheric current sheet]] is a ripple in the heliosphere created by the Sun's rotating magnetic field. Extending throughout the heliosphere, it is considered the largest structure in the Solar System and is said to resemble a "ballerina's skirt"<ref>{{cite journal |author=Mursula, K.; Hiltula, T., |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003GeoRL..30vSSC2M&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=42ca922c9c03266 |title=Bashful ballerina: Southward shifted heliospheric current sheet |year=2003 |journal=Geophysical Research Letters |volume=30 |issue=22 |pages=2135 |doi=10.1029/2003GL018201}}</ref> ===Outer structure=== [[Image:Solarmap.png|thumb|400 px|The solar system, in logarithmic scale, showing the outer extent of the heliosphere, the [[Oort cloud]] and [[Alpha Centauri]]]] The heliosphere's outer structure is determined by the interactions between the solar wind and the winds of interstellar space. The solar wind streams away from the Sun in all directions at speeds of several hundred km/s (about 1,000,000 mph) in the Earth's vicinity. At some distance from the Sun, well beyond the orbit of [[Neptune]], this supersonic wind must slow down to meet the gases in the [[interstellar medium]]. This takes place in several stages: * The solar wind is traveling at [[supersonic]] speeds within the solar system. At the '''termination shock''', a standing [[shock wave]], the solar wind falls below its speed of sound and becomes [[subsonic]]. * Once subsonic, the solar wind may be affected by the ambient flow of the interstellar medium. Pressures cause the wind to form a comet-like tail behind the Sun, called the '''heliosheath'''. * The outer surface of the heliosheath, where the heliosphere meets the interstellar medium, is called the '''heliopause'''. This is the edge of the entire heliosphere. * The heliopause causes turbulence in the interstellar medium as the sun orbits the [[Galactic Center]]. The '''[[bow shock]]''', outside the heliopause, is a turbulent region caused by the pressure of the advancing heliopause against the interstellar medium. ==Termination shock== [[Image:termination shock in sink.png|thumb|An example of the termination shock, relating it to a sink basin.]] The '''termination shock''' is the point in the heliosphere where the solar wind slows down to [[subsonic]] speed (with respect to the star) due to interactions with the local [[interstellar medium]]. This causes [[Physical compression|compression]], heating, and a change in the [[magnetic field]]. In our [[solar system]] the termination shock is believed to be 75 to 90 [[astronomical unit]]s<ref name="apod_heliosphere">{{cite web | author=Nemiroff, R.; Bonnell, J. | date =[[June 24]], [[2002]] | url =http://antwrp.gsfc.nasa.gov/apod/ap020624.html | title =The Sun's Heliosphere & Heliopause | publisher =Astronomy Picture of the Day | accessdate = 2007-05-25 }}</ref> from the Sun. The termination shock boundary fluctuates in its distance from the sun as a result of fluctuations in [[solar flare]] activity; i.e., changes in the ejections of gas and dust from the sun. The shock arises because solar wind particles are emitted from stars at about 400 km/s, while the speed of sound (in the interstellar medium) is about 100 km/s. (The exact speed depends on the density, which fluctuates considerably.) The interstellar medium, although very low in density, nonetheless has a constant pressure associated with it; the pressure from the solar wind decreases with the square of the distance from the star. As one moves far enough away from the star, the pressure from the interstellar medium becomes sufficient to slow the solar wind down to below its speed of sound; this causes a [[shock wave]]. Other termination shocks can be seen in terrestrial systems; perhaps the easiest may be seen by simply running a water [[Tap (valve)|tap]] into a [[sink]] creating a [[Hydraulic jump]]. Upon hitting the floor of the sink, the flowing water spreads out at a speed that is higher than the local [[wave speed]], forming a disk of shallow, rapidly diverging flow (analogous to the tenuous, supersonic solar wind). Around the periphery of the disk, a shock front or wall of water forms; outside the shock front, the water moves slower than the local wave speed (analogous to the subsonic interstellar medium). {{See|Hydraulic jump}} Going outward from the sun, the termination shock is followed by the [[Heliopause]] where solar wind particles are stopped by the interstellar medium, then the [[bow shock]] past which particles from the interstellar medium are no longer excited. <!-- [http://www.agu.org/cgi-bin/sessionssm05?meeting=sm05&part=SH22A&maxhits=200 Evidence] :: broken link --> Evidence presented at a meeting of the [[American Geophysical Union]] in May 2005 by Dr. [[Ed Stone]] suggests that the ''[[Voyager 1]]'' spacecraft passed termination shock in December 2004, when it was about 94 AU from the sun, by virtue of the change in magnetic readings taken from the craft. In contrast, ''[[Voyager 2]]'' began detecting returning particles when it was only 76 AU from the sun, in May 2006. This implies that the heliosphere may be irregularly shaped, bulging outwards in the sun's northern hemisphere and pushed inward in the south.<ref name="voyager_II_cnn">{{cite news | first=Ker | last=Than | title=Voyager II detects solar system's edge | publisher=CNN | date=[[May 24]], [[2006]] | url=http://www.cnn.com/2006/TECH/space/05/23/voyager.2/index.html | accessdate=2007-05-25 }}</ref> The [[Interstellar Boundary Explorer]] (IBEX) mission will attempt to gather more data on the solar system's termination shock. ==Heliosheath== [[Image:Voyager 1 entering heliosheath region.jpg|thumb|250px|A diagram depicting ''Voyager 1'' at its relative position in the [[heliosheath]]. Since then, ''Voyager 2'' has joined it in the heliosheath.]] The '''heliosheath''' is the region of the heliosphere beyond the termination shock. Here the wind is slowed, compressed and made turbulent by its interaction with the interstellar medium. Its distance from the [[Sun]] is approximately 80 to 100 [[astronomical unit]]s (AU) at its closest point; however, the heliosheath is shaped like the [[coma (cometary)|coma]] of a comet, and trails several times that distance in the direction opposite to the Sun's path through space. At its windward side, its thickness is estimated to be between 10 and 100 AU.<ref>{{cite conference | first=Pontus | last=Brandt | title=Imaging of the Heliospheric Boundary | booktitle=NASA Advisory Council Workshop on Science Associated with the Lunar Exploration Architecture: White Papers | publisher=Lunar and Planetary Institute | date=February 27–March 2, 2007 | location =Tempe, Arizona | url = http://www.lpi.usra.edu/meetings/LEA/whitepapers/Brandt_whitepaper_heliospheric_imaging.pdf | format=PDF | accessdate=2007-05-25 }}</ref> The current mission of the ''[[Voyager 1]]'' and ''[[Voyager 2]]'' space probes includes studying the heliosheath. ===Detection by spacecraft=== The precise distance to, and shape of the heliopause is still uncertain. Interplanetary/interstellar [[spacecraft]] such as ''[[Pioneer 10]]'', ''[[Pioneer 11]]'', ''[[Voyager 1]]'' and ''[[Voyager 2]]'' are traveling outward through the [[solar system]] and will eventually pass through the heliopause. *It is believed that [[Voyager 1]] crossed the [[termination shock]] and entered the [[heliosheath]] in the middle of December 2004, at a distance of 94 [[Astronomical unit|AU]].<ref name=Voy1-term2004>{{cite web |date=1 June 2005 |title=Voyager Termination Shock |publisher=Department of Physics and Astronomy (University of Iowa) |author=Donald A. Gurnett |url=http://www-pw.physics.uiowa.edu/space-audio/voyager/termination-shock/ |accessdate=2008-02-06}}</ref> An earlier report that this had occurred as early as August 2002 (at 85 AU) is now generally believed to have been premature.<ref name=Voy1-term2005>{{cite web |date=25 May 2005 |title=Voyager 1 reaches the edge of the solar system |publisher=NewScientist |author=Celeste Biever |url=http://space.newscientist.com/article/mg18625015.000-voyager-1-reaches-the-edge-of-the-solar-system.html |accessdate=2008-02-06}}</ref> *However, ''[[Voyager 2]]'' crossed the termination shock on [[August 30]] [[2007]] at 84 AU,<ref name=Voy2-term2007>{{cite web |date=10 December 2007 |title=Voyager 2 probe reaches solar system boundary |publisher=NewScientist |author=David Shiga |url=http://space.newscientist.com/article/dn13029-voyager-2-probe-reaches-solar-system-boundary.html?feedId=online-news_rss20 |accessdate=2008-02-06}}</ref> showing evidence of denting in the heliosphere, believed to be caused by an interstellar magnetic field.<ref>[http://www.nasa.gov/mission_pages/voyager/voyager-20071210.html Voyager 2 Proves Solar System Is Squashed NASA.gov #2007-12-10]</ref> Since the [[Pioneer 10]] and [[Pioneer 11| 11]] probes have both stopped communicating, it will never be known where they pass into the heliosheath. ==Heliopause== [[Image:Heliopause diagram.png|right|thumb|300px|The '''heliopause''' is the boundary between the heliosphere and the [[interstellar medium]] outside the [[solar system]]. As the solar wind approaches the heliopause, it slows suddenly, forming a [[shock wave]] called the [[termination shock]] of the solar wind.]] The '''heliopause''' is the theoretical boundary where the [[Sun]]'s [[solar wind]] is stopped by the [[interstellar medium]]; where the solar wind's strength is no longer great enough to push back the [[stellar wind]]s of the surrounding stars. === Hypotheses === According to one [[hypothesis]],<ref>{{cite web | author=Wood, B. E.; Alexander, W. R.; Linsky, J. L. | date = [[July 13]], [[2006]] | url =http://www.aas.org/publications/baas/v27n4/aas187/S045002.html | title =The Properties of the Local Interstellar Medium and the Interaction of the Stellar Winds of \epsilon Indi and \lambda Andromedae with the Interstellar Environment | publisher =American Astronomical Society | accessdate = 2007-05-25 }}</ref> there exists a region of hot hydrogen known as the '''hydrogen wall''' between the bow shock and the heliopause. The wall is composed of interstellar material interacting with the edge of the heliosphere. Another hypothesis suggests that the heliopause could be smaller on the side of the solar system facing the Sun's orbital motion through the galaxy. It may also vary depending on the current velocity of the solar wind and the local density of the interstellar medium. It is known to lie far outside the orbit of [[Neptune]]. The current mission of the ''[[Voyager 1]]'' and ''[[Voyager 2|2]]'' spacecraft is to find and study the termination shock, heliosheath, and heliopause''.[[Voyager 1]]'' reached the termination shock on [[May 23]]-24, 2005,<ref>{{cite web | first=Bill | last=Steigerwald | date=[[May 24]], [[2005]] | url=http://www.nasa.gov/vision/universe/solarsystem/voyager_agu.html | title=Voyager Enters Solar System's Final Frontier | publisher=American Astronomical Society | accessdate= 2007-05-25 }}</ref> and ''[[Voyager 2]]'' reached it on [[August 30]], [[2007]] according to [[NASA]].<ref>{{cite web | first= | last= | date=[[December 10]], [[2007]] | url=http://voyager.jpl.nasa.gov/news/voyager_squashed.html | title=Voyager 2 Proves Solar System Is Squashed | publisher=Jet Propulsion Labratory | accessdate= 2007-05-25 }}</ref> It is anticipated that both missions may ultimately reach the heliopause itself. In the mean time, the [[Interstellar Boundary Explorer]] (IBEX) mission will attempt to image the heliopause from Earth orbit within two years of its 2008 launch. When particles emitted by the sun bump into the interstellar ones, they slow down while releasing energy. Many particles accumulate in and around the heliopause, highly energised by their negative acceleration, creating a shock wave. An alternative definition is that the heliopause is the [[magnetopause]] between the solar system's [[magnetosphere]] and the galaxy's plasma currents. [[Image:Sig06-029 medium.jpg|thumb|200 px|Image and artist's impression of the bow shock left by the star [[R Hydrae]].]] ==Bowshock== {{main|Bow shock}} It is hypothesized that the [[Sun]] also has a [[bow shock|bowshock]] produced in its travels within the [[interstellar medium]], as shown in the figure. The shock is named from its resemblance to the wake left by a ship's bow and is formed for similar reasons, though of plasma instead of water. Bowshocks will occur if the interstellar medium is moving supersonically 'toward' the sun, since its solar wind moves 'away' from the sun supersonically. When the interstellar wind hits the heliosphere it slows and creates a region of turbulence. [[NASA]]'s Robert Nemiroff and Jerry Bonnell believe the solar bowshock may lie at around 230&nbsp;AU<ref name="apod_heliosphere" /> from the Sun. This phenomenon has been observed by NASA's orbital [[GALEX]] telescope. The red giant star [[Mira]] in the constellation [[Cetus]] has been shown to have both a cometlike debris tail of ejecta from the star, and a distinct bowshock preceding it in movement through space at over 130 kilometers per second. == See also == * [[Heliospheric current sheet]] * [[Space weather]] * [[Coronal mass ejection]] * [[Solar flare]] * [[Fermi glow]] * [[Interstellar medium]] * [[Stellar system]] * [[Voyager program|Voyager Program]] == Notes == {{reflist}} ==References and further reading== * [http://helios.gsfc.nasa.gov/heliosph.html The Heliosphere (Cosmicopia)] * [http://web.mit.edu/space/www/helio.review/axford.suess.html The Heliosphere], MIT Space Plasma Group * [http://voyager.jpl.nasa.gov/mission/interstellar.html Voyager Interstellar Mission Objectives] * {{cite news | title=Space probes reveal Solar System's bullet shape | url=http://www.cosmosmagazine.com/node/1296 |publisher=COSMOS magazine |date=[[May 11]], [[2007]] | accessdate= 2007-05-12 }} * [http://www.uiowa.edu/~ournews/2003/december/120803gurnett.html UI's Don Gurnett Says Voyager 1 Is Approaching Edge Of Solar System] [[December 8]], [[2003]] Univ. of Iowa Press release * {{cite news |url=http://www.universetoday.com/am/publish/size_of_heliopause.html |title=Heliopause Seems to Be 23 Billion Kilometres |publisher=Universe Today |date=[[December 9]], [[2003]] |accessdate=2007-08-08}} == External links == *[http://solar-heliospheric.engin.umich.edu The Solar and Heliospheric Research Group at the University of Michigan] * [http://science.nasa.gov/ssl/pad/solar/suess/Interstellar_Probe/ISP-ObservObj.html Observing objectives] of [[NASA]]'s [http://interstellar.jpl.nasa.gov/ Interstellar Probe]. * [http://www.cnn.com/2005/TECH/space/05/25/voyager.space/index.html CNN: NASA: Voyager I enters solar system's final frontier] – [[May 25]], [[2005]] * [http://www.newscientist.com/article.ns?id=mg18625015.000 ''New Scientist'': Voyager 1 reaches the edge of the solar system] – [[May 25]], [[2005]] * [http://science.nasa.gov/headlines/y2006/21sep_voyager.htm?list33024 Surprises from the Edge of the Solar System] – Voyager 1 Newest Findings as of September 2006 *[http://adsabs.harvard.edu/abs/2004AdSpR..34...66W The heliospheric hydrogen wall and astrospheres] *[http://www.dartmouth.edu/~hrm/R/heliosphere.html Heliosphere], has a diagram. *[http://www.igpp.ucr.edu/global_heliosphere.htm Global Structure of the Heliosphere] *[http://www.dartmouth.edu/~hrm/pubabs.html Publications in Refereed Journals] *[http://www.galex.caltech.edu/ NASA GALEX (Galaxy evolution Explorer) homepage at Cal Tech] *[http://www.astronomycast.com/astronomy/episode-65-the-end-of-our-tour-through-the-solar-system/ Heliosphere] [[Astronomy Cast]] episode #65, includes full transcript. {{The Sun}} [[Category:Space plasmas]] [[Category:Plasma physics]] [[Category:Sun]] [[Category:Voyager program]] [[ar:مجال شمسي]] [[ca:Heliosfera]] [[cs:Heliosféra]] [[da:Heliosfære]] [[de:Heliosphäre]] [[es:Heliosfera]] [[fr:Héliosphère]] [[gl:Heliosfera]] [[ko:태양권]] [[lt:Heliosfera]] [[nl:Heliosfeer]] [[pl:Heliosfera]] [[pt:Heliosfera]] [[sk:Heliosféra]] [[fi:Heliosfääri]] [[zh:太陽圈]]