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{{Short description|High-altitude atmospheric effect}}
[[Image:Plasma fountain.gif|thumb|200px|right|The Earth's plasma fountain, showing oxygen, helium, and hydrogen ions which gush into space from regions near the Earth's poles. The faint yellow area shown above the north pole represents gas lost from Earth into space; the green area is the [[aurora borealis]]
The '''polar wind''' or '''plasma fountain''' is a permanent outflow of [[plasma (physics)|plasma]] from the polar regions of Earth's [[magnetosphere]].<ref name=SchunkNagy2000>{{Cite book |last1=Schunk |first1=R. W. |title=Ionospheres: physics, plasma physics, and chemistry |last2=Nagy |first2=Andrew |date=2000 |publisher=Cambridge University Press |isbn=978-0-521-63237-9 |series=Cambridge atmospheric and space science series |location=New York}}</ref>{{rp|29}}
==History==
In 1966 Bauer<ref>{{Cite book |last=Bauer |first=S.J. |title=Electron Density Profiles in Ionosphere and Exosphere |date=1966 |publisher=North-Holland |editor-last=Frihagen |editor-first=J. |chapter=The structure of the topside ionosphere}}</ref> and, separately, Dessler ahd Michel<ref>{{Cite journal |last=Dessler |first=A. J. |last2=Michel |first2=F. C. |date=1966-03-01 |title=Plasma in the geomagnetic tail |url=http://doi.wiley.com/10.1029/JZ071i005p01421 |journal=Journal of Geophysical Research |language=en |volume=71 |issue=5 |pages=1421–1426 |doi=10.1029/JZ071i005p01421}}</ref> noted that since the Earth's geomagnetic field above the poles forms a long tail away from the Sun out beyond the Moon's orbit, ions should flow from the higher pressure region in the ionosphere out into space.<ref>{{Cite journal |last=Schunk |first=R. W. |last2=Sojka |first2=J. J. |date=June 1997 |title=Global ionosphere‐polar wind system during changing magnetic activity |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/97JA00292 |journal=Journal of Geophysical Research: Space Physics |language=en |volume=102 |issue=A6 |pages=11625–11651 |doi=10.1029/97JA00292 |issn=0148-0227}}</ref>
The term "polar wind" was coined<ref name=YauAbePeterson2007/>{{rp|1937|q=Axford (1968) coined the term ‘‘polar wind’’ to describe the supersonic nature of the thermal plasma expansion and outflow, in analogy to the supersonic expansion of the solar wind plasma from the solar corona into interplanetary space.}} in 1968 in a pair of articles by Banks and Holzer<ref name="BanksHolzer1968">{{cite journal |doi=10.1029/JA073i021p06846 |last1=Banks |first1=Peter M. |last2=Holzer |first2=Thomas E. |year=1968 |title=The Polar Wind |journal=Journal of Geophysical Research |volume=73 |pages=6846–6854 |url=http://europa.agu.org/?view=article&uri=/journals/ja/JA073i021p06846.xml&t=polar%20wind |bibcode=1968JGR....73.6846B |issue=21}}</ref> and by [[Ian Axford]].<ref name="Axford1968">{{cite journal |doi=10.1029/JA073i021p06855 |last=Axford |first=W. Ian |year=1968 |title=The Polar Wind and the Terrestrial Helium Budget |journal=Journal of Geophysical Research |volume=73 |pages=6855–6859 |bibcode=1968JGR....73.6855A |issue=21}}</ref> Since the process by which the ionospheric plasma flows away from the Earth along magnetic field lines, is similar to the flow of solar plasma away from the Sun's corona (the [[solar wind]]), Axford suggested the term "polar wind."
The earliest experimental characterization of the polar wind came from the 1966 [[Explorer 33]] and especially the 1974 [[ISIS (satellite)|ISIS-2]] satellite projects.<ref name=Hoffman1974>{{Cite journal |last=Hoffman |first=J. H. |last2=Dodson |first2=W. H. |last3=Lippincott |first3=C. R. |last4=Hammack |first4=H. D. |date=1974-10-01 |title=Initial ion composition results from the Isis 2 satellite |url=http://doi.wiley.com/10.1029/JA079i028p04246 |journal=Journal of Geophysical Research |language=en |volume=79 |issue=28 |pages=4246–4251 |doi=10.1029/JA079i028p04246}}</ref><ref name=Lemaire2007>{{Cite journal |last=Lemaire |first=J. F. |last2=Peterson |first2=W. K. |last3=Chang |first3=T. |last4=Schunk |first4=R. W. |last5=Barakat |first5=A. R. |last6=Demars |first6=H. G. |last7=Khazanov |first7=G. V. |date=2007-11-01 |title=History of kinetic polar wind models and early observations |url=https://linkinghub.elsevier.com/retrieve/pii/S1364682607002404 |journal=Journal of Atmospheric and Solar-Terrestrial Physics |series=Recent Advances in the Polar Wind Theories and Observations |volume=69 |issue=16 |pages=1901–1935 |doi=10.1016/j.jastp.2007.08.011 |issn=1364-6826}}</ref> Additional data from the 1981 [[Dynamics Explorer]] led to some uncertainty in the theoretical models about the role of cool O<sup>+</sup> ions. This issue was cleared up with the more comprehensive data from 1989 [[Akebono (satellite)|Akebono ]] satellite,<ref name=YauAbePeterson2007/>{{rp|1955}} and the 1996 [[Polar (satellite)| Polar satellite]].<ref name=YauAbePeterson2007/>{{rp|1966}}
The idea for the polar wind originated with the desire to solve the paradox of the terrestrial [[helium]] budget. This paradox consists of the fact that helium in the Earth's atmosphere seems to be produced (via radioactive decay of [[uranium]] and [[thorium]]) faster than it is lost by escaping from the upper atmosphere. The realization that some helium could be ionized, and therefore escape the Earth along open magnetic field lines near the magnetic poles (the 'polar wind'), is one possible solution to the paradox.
==Causes==
[[File:Discovering Earth’s Third Global Energy Field (SVS14628 - 2B).jpg |thumb |right|Conceptual diagram of the two main effects of the ambipolar electric field: inflating the ionosphere and generating the polar wind.The sparkling blue haze surrounding Earth represents the plasma in the ionosphere. The sparkling lines represent polar wind flowing up and out.<ref name=Endurance/> ]]
After 30 years of research, the "classical" cause of the polar wind has been shown to be ambipolar outflow of thermal plasma: ion acceleration by a polarization [[electric field]] at high altitudes.<ref name=SchunkNagy2000/>{{rp|451}}
One known or "classical" cause of the polar wind is ion acceleration by a polarization [[electric field]] at high altitudes. In this region the [[ionosphere|ionospheric]] plasma expands and the low density allows gravity to pull ions down relative to the electrons in the plasma. The charge separation results in the electric field which then sends some of the ions up and out of the atmosphere.<ref name=WellingReview2015>{{Cite journal |last1=Welling |first1=Daniel T. |last2=André |first2=Mats |last3=Dandouras |first3=Iannis |last4=Delcourt |first4=Dominique |last5=Fazakerley |first5=Andrew |last6=Fontaine |first6=Dominique |last7=Foster |first7=John |last8=Ilie |first8=Raluca |last9=Kistler |first9=Lynn |last10=Lee |first10=Justin H. |last11=Liemohn |first11=Michael W. |last12=Slavin |first12=James A. |last13=Wang |first13=Chih-Ping |last14=Wiltberger |first14=Michael |last15=Yau |first15=Andrew |date=2015 |title=The Earth: Plasma Sources, Losses, and Transport Processes |url=http://link.springer.com/10.1007/s11214-015-0187-2 |journal=Space Science Reviews |language=en |volume=192 |issue=1–4 |pages=145–208 |doi=10.1007/s11214-015-0187-2 |bibcode=2015SSRv..192..145W |issn=0038-6308}}</ref>{{rp|147}} This mechanism is known as "ambipolar outflow"<ref name=Schunk2007/> and the field as "ambipolar electric field" or "polarization electric field". Additional mechanisms including ion acceleration by solar [[photoelectrons]] escaping along [[magnetosphere|magnetic field lines]].<ref name=Schunk2007>{{Cite journal |last=Schunk |first=R. W. |date=2007-11-01 |title=Time-dependent simulations of the global polar wind |url=https://linkinghub.elsevier.com/retrieve/pii/S136468260700243X |journal=Journal of Atmospheric and Solar-Terrestrial Physics |series=Recent Advances in the Polar Wind Theories and Observations |volume=69 |issue=16 |pages=2028–2047 |doi=10.1016/j.jastp.2007.08.009 |bibcode=2007JASTP..69.2028S |issn=1364-6826}}</ref>▼
The polarization or ambipolar electric field was originally proposed in the 1920s for ionized [[Stellar atmosphere|stellar atmospheres]].<ref name=Lemaire2007/>{{rp|1927}} Gravitational charge separation creates a field amounting to
<math display="block"> E = -\mu \cdot \vec{g} / e </math>
where <math>\vec{g}</math> is the gravitational field and <math>\mu</math> is the mean ionic mass, half the difference between the mass of the singly charged ions and the electron. This simple formula is only applicable in a plasma in hydrostatic equilibrium. More complex models applicable to real plasmas show larger field strength. In any case the field is very small but, unlike other forces, it points away from gravity.<ref name=Lemaire2007/>{{rp|1927}} In low density plasma at high altitude is overwhelms gravity for light ions.
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The outflow of ions due to the ambipolar electric field end up accumulating in the [[plasmasphere]] if they follow closed magnetic field lines but ions following open magnetic field lines exit the Earth system.<ref name=WellingReview2015/>{{rp|167}} Ions following open magnetic field lines are push away from the Sun by forces of the solar wind (anti-solar convection).<ref name=WellingReview2015/>{{rp|149}}
==Measurements==
Numerous investigations of the polar wind have launched, including [[ISIS (satellite)|ISIS-2]],
* the primary ingredients in the polar wind are electrons,
* O<sup>+</sup> ions dominate at below 4000km,
* all three ion species reach [[supersonic]] velocities above 7000km and velocities increase to over [[Mach number]] 2 above 50,000km.<ref name=YauAbePeterson2007/>{{rp|1976|q=All three ion species are supersonic by the time they reach 7000 km, the ion Mach number of H+ , He+ , and O + being 1.5, 1.5, and 5, respectively, at this altitude and increasing to 4.6, 2.5, and 3.5 at 50,500 km}}
* polar wind velocity increases with altitude, and is higher on the dayside of the Earth,
The polarization or ambipolar electric field was directly measured in 2022 by a [[sounding rocket]] launched from [[Svalbard]]. This NASA mission was called Endurance.<ref name=Endurance>{{citation |url=https://svs.gsfc.nasa.gov/14628 |title=Discovering Earth’s Third Global Energy Field |date=28 August 2024 |author=Lacey Young |publisher=NASA}}</ref> Comparing the electrical potential at altitude of 250 km to that at 768 km gave a difference of +0.55 [[volt]] with an [[error bar|uncertainty]] of 0.09 volt.<ref name=Nature>{{
== See also ==
* [[Ambipolar diffusion]]
==References==
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