View source for Alfvén surface

{{Short description|Boundary between solar corona and wind}}
[[File:Parker Solar Probe touches the Sun.webm|thumb|NASA animation showing [[Parker Solar Probe]] passing through the Sun's outer atmosphere, its [[Stellar corona|corona]], in April 2021. The boundary at the edge of the corona is the Alfvén critical surface. Inside that surface (circle at left), plasma connects to the Sun by waves traveling back and forth to the surface. Beyond it (circle at right), the Sun's magnetic fields and gravity are too weak to contain the plasma and it becomes the [[solar wind]], racing across the [[Solar System]] so fast that waves within the wind cannot make it back to the Sun.<ref name=animation/>]]
The '''Alfvén surface''' is the boundary separating a star's [[Stellar corona|corona]] from the [[stellar wind]] defined as where the coronal plasma's [[Alfvén speed]] and the large-scale stellar wind speed are equal. It is named after [[Hannes Alfvén]], and is also called '''Alfvén critical surface''', '''Alfvén point''', or '''Alfvén radius'''. In 2018, the [[Parker Solar Probe]] became the first spacecraft that crossed Alfvén surface of the [[Sun]].

== Definition ==
[[File:Simulated stellar wind environment for AU Mic driven by the different ZDI maps of the star.jpg|thumb|Simulated stellar wind environment for [[AU Microscopii]]. A translucent shade shows the resulting Alfvén surface of the stellar wind.<ref name="Alvarado-Gómez"/>]]
[[File:Effect of magnetic field strength and geometry on the AS.jpg|thumb|Effect of magnetic field strength and geometry on the Alfvén surface. The AS is highly affected by the change in the surface geometry.<ref name="Chebly"/>]]
[[File:Simulated stellar wind environment for the Proxima Cen system.jpg|thumb|Simulated stellar wind environment for the [[Proxima Centauri]] system. The purple [[isosurface]] corresponds to the Alfvén surface of the stellar wind.<ref>{{cite journal |last1=Alvarado-Gómez |first1=Julián D. |last2=Drake |first2=Jeremy J. |last3=Garraffo |first3=Cecilia |last4=Cohen |first4=Ofer |last5=Poppenhaeger |first5=Katja |last6=Yadav |first6=Rakesh K. |last7=Moschou |first7=Sofia P. |title=An Earth-like Stellar Wind Environment for Proxima Centauri c |journal=The Astrophysical Journal Letters |date=1 October 2020 |volume=902 |issue=1 |pages=L9 |doi=10.3847/2041-8213/abb885 |issn=2041-8205|arxiv=2009.07266 |doi-access=free }}</ref>]]

Stars do not have a solid surface. However, they have a superheated atmosphere, made of solar material bound to the star by gravity and magnetic forces.<ref name="touching">{{citation-attribution|1={{cite web |last1=Hatfield |first1=Miles |title=NASA Enters the Solar Atmosphere for the First Time |url=https://www.nasa.gov/feature/goddard/2021/nasa-enters-the-solar-atmosphere-for-the-first-time-bringing-new-discoveries |website=NASA |date=13 December 2021 |access-date=30 July 2022 |archive-date=15 December 2021 |archive-url=https://archive.today/20211215144804/https://www.nasa.gov/feature/goddard/2021/nasa-enters-the-solar-atmosphere-for-the-first-time-bringing-new-discoveries |url-status=live }}}}</ref> The [[stellar corona]] extends far beyond the solar surface, or [[photosphere]], and is considered the outer boundary of the star. It marks the transition to the [[solar wind]] which moves through the [[planetary system]]. This limit is defined by the distance at which disturbances in the solar wind cannot propagate back to the solar surface. Those disturbances cannot propagate back towards a star if the outbound solar wind speed exceeds Mach one, the speed of 'sound' as defined for the solar wind. This distance forms an irregular 'surface' around a star is called the Alfvén surface.<ref name=NASA>{{citation-attribution|1={{cite web |title=SVS: Parker Solar Probe: Crossing the Alfven Surface |url=https://svs.gsfc.nasa.gov/4958 |website=svs.gsfc.nasa.gov |access-date=30 July 2022 |language=en |date=14 December 2021 |archive-date=8 August 2022 |archive-url=https://web.archive.org/web/20220808233611/https://svs.gsfc.nasa.gov/4958 |url-status=live }}}}</ref> It can also be described as a point where gravity and magnetic fields are too weak to contain heat and pressure that push the material away from a star. This is the point where solar atmosphere ends and where solar wind begins.<ref name="touching"/>

Adhikari, Zank, & Zhao (2019) define the Alfvén surface as:<ref>{{cite journal |last1=Adhikari |first1=L. |last2=Zank |first2=G. P. |last3=Zhao |first3=L.-L. |title=Does Turbulence Turn off at the Alfvén Critical Surface? |journal=The Astrophysical Journal |date=30 April 2019 |volume=876 |issue=1 |pages=26 |doi=10.3847/1538-4357/ab141c|bibcode=2019ApJ...876...26A |s2cid=156048833 |doi-access=free }}</ref>
<blockquote>
the location at which the large-scale bulk [[solar wind]] speed <math>U</math> and the [[Alfvén speed]] <math>V_{\text{A}}</math> are equal, and thus it separates sub-Alfvénic coronal flow |<math>U</math>|≪|<math>V_{\text{A}}</math>| from super-Alfvénic solar wind flow |<math>U</math>|≫|<math>V_{\text{A}}</math>|
</blockquote>

DeForest, Howard, & McComas (2014) define the Alfvén surface as:<ref>{{cite journal |last1=DeForest |first1=C. E. |last2=Howard |first2=T. A. |last3=McComas |first3=D. J. |title=Inbound waves in the solar corona: a direct indicator of Alfvén Surface location |journal=The Astrophysical Journal |date=12 May 2014 |volume=787 |issue=2 |pages=124 |doi=10.1088/0004-637X/787/2/124|arxiv=1404.3235 |bibcode=2014ApJ...787..124D |s2cid=118371646 }}
</ref><blockquote>
a natural boundary that marks the causal disconnection of individual packets of plasma and magnetic flux from the Sun itself. The Alfvén surface is the locus where the radial motion of the accelerating solar wind passes the radial [[Alfvén speed]], and therefore any displacement of material cannot carry information back down into the corona. It is thus the natural outer boundary of the solar corona, and the inner boundary of interplanetary space.
</blockquote>

Alfvén surface separates the sub- and super-Alfvénic regimes of the stellar wind, which influence the structure of any [[magnetosphere]]/[[ionosphere]] around an orbiting planet in the system.<ref name="Alvarado-Gómez">{{cite journal |last1=Alvarado-Gómez |first1=Julián D. |last2=Cohen |first2=Ofer |last3=Drake |first3=Jeremy J. |last4=Fraschetti |first4=Federico |last5=Poppenhaeger |first5=Katja |last6=Garraffo |first6=Cecilia |last7=Chebly |first7=Judy |last8=Ilin |first8=Ekaterina |last9=Harbach |first9=Laura |last10=Kochukhov |first10=Oleg |title=Simulating the Space Weather in the AU Mic System: Stellar Winds and Extreme Coronal Mass Ejections |journal=The Astrophysical Journal |date=1 April 2022 |volume=928 |issue=2 |pages=147 |doi=10.3847/1538-4357/ac54b8 |issn=0004-637X|doi-access=free |arxiv=2202.07949 }} [[File:CC BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0] {{Webarchive|url=https://web.archive.org/web/20171016050101/https://creativecommons.org/licenses/by/4.0/ |date=2017-10-16 }}</ref> Characterization of the Alfvén surface can serve as an inner-boundary of the [[habitable zone]] of the star.<ref name="Chebly">{{cite journal |last1=Chebly |first1=Judy J. |last2=Alvarado-Gómez |first2=Julián D. |last3=Poppenhaeger |first3=Katja |title=Destination exoplanet: Habitability conditions influenced by stellar winds properties |journal=Astronomische Nachrichten |date=May 2022 |volume=343 |issue=4 |doi=10.1002/asna.20210093 |url=https://onlinelibrary.wiley.com/doi/full/10.1002/asna.20210093 |access-date=18 May 2023 |language=en |issn=0004-6337 |arxiv=2111.09707 |s2cid=238922661 |archive-date=18 May 2023 |archive-url=https://web.archive.org/web/20230518070205/https://onlinelibrary.wiley.com/doi/full/10.1002/asna.20210093 |url-status=live }} [[File:CC BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0] {{Webarchive|url=https://web.archive.org/web/20171016050101/https://creativecommons.org/licenses/by/4.0/ |date=2017-10-16 }}</ref> Alfvén surface can be found "nominally" at 10–30&nbsp;star radii.<ref>{{cite journal |last1=Goelzer |first1=Molly L. |last2=Schwadron |first2=Nathan A. |last3=Smith |first3=Charles W. |title=An analysis of Alfvén radius based on sunspot number from 1749 to today |journal=Journal of Geophysical Research: Space Physics |date=January 2014 |volume=119 |issue=1 |pages=115–120 |doi=10.1002/2013JA019420 |language=en|doi-access=free }}</ref>

== Research ==
Researchers were unsure exactly where the Alfvén critical surface of the Sun lay. Based on remote images of the corona, estimates had put it somewhere between 10 and 20 [[solar radii]] from the surface of the Sun.<ref name="touching"/> On April 28, 2021, during its eighth flyby of the Sun, NASA's [[Parker Solar Probe]] (PSP) encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated the Alfvén surface;<ref name="touching"/><ref name=animation>{{cite web |title=GMS: Animation: NASA's Parker Solar Probe Enters Solar Atmosphere |url=https://svs.gsfc.nasa.gov/14036 |website=svs.gsfc.nasa.gov |access-date=30 July 2022 |language=en |date=14 December 2021 |archive-date=4 October 2022 |archive-url=https://web.archive.org/web/20221004004943/https://svs.gsfc.nasa.gov/14036 |url-status=live }}</ref> the probe measured the solar wind plasma environment with its [[FIELDS]] and [[SWEAP]] instruments.<ref name=NASA/> This event was described by NASA as "touching the Sun".<ref name="touching"/> During the flyby, Parker Solar Probe passed into and out of the corona several times. This proved the predictions that the Alfvén critical surface is not shaped like a smooth ball, but has spikes and valleys that wrinkle its surface.<ref name="touching"/>

At 09:33 UT on 28 April 2021 Parker Solar Probe entered the magnetized atmosphere of the Sun {{convert|13|e6km|e6mi|abbr=off}} above the photosphere, crossing below the Alfvén critical surface for five hours into plasma in causal contact with the Sun with an Alfvén Mach number of 0.79 and magnetic pressure dominating both ion and electron pressure. Magnetic mapping suggests the region was a steady flow emerging on rapidly expanding coronal magnetic field lines lying above a [[pseudostreamer]]. The sub-Alfvénic nature of the flow may be due to suppressed magnetic reconnection at the base of the pseudostreamer, as evidenced by unusually low densities in this region and the magnetic mapping.<ref>{{cite journal |last1=Kasper |first1=J. C. |last2=Klein |first2=K. G. |last3=Lichko |first3=E. |last4=Huang |first4=Jia |last5=Chen |first5=C. H. K. |last6=Badman |first6=S. T. |last7=Bonnell |first7=J. |last8=Whittlesey |first8=P. L. |last9=Livi |first9=R. |last10=Larson |first10=D. |last11=Pulupa |first11=M. |last12=Rahmati |first12=A. |last13=Stansby |first13=D. |last14=Korreck |first14=K. E. |last15=Stevens |first15=M. |last16=Case |first16=A. W.|author-link16=Anthony W. Case |last17=Bale |first17=S. D. |last18=Maksimovic |first18=M. |last19=Moncuquet |first19=M. |last20=Goetz |first20=K. |last21=Halekas |first21=J. S. |last22=Malaspina |first22=D. |last23=Raouafi |first23=Nour E. |last24=Szabo |first24=A. |last25=MacDowall |first25=R. |last26=Velli |first26=Marco |last27=Dudok De Wit |first27=Thierry |last28=Zank |first28=G. P. |title=Parker Solar Probe Enters the Magnetically Dominated Solar Corona |journal=Physical Review Letters |date=14 December 2021 |volume=127 |issue=25 |pages=255101 |doi=10.1103/PhysRevLett.127.255101|pmid=35029449 |doi-access=free }} [[File:CC BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0] {{Webarchive|url=https://web.archive.org/web/20171016050101/https://creativecommons.org/licenses/by/4.0/ |date=2017-10-16 }}</ref>

== Further reading ==
* {{cite journal |last1=Kasper |first1=Justin C. |last2=Klein |first2=Kristopher G. |title=Strong Preferential Ion Heating is Limited to within the Solar Alfvén Surface |journal=The Astrophysical Journal Letters |date=1 June 2019 |volume=877 |issue=2 |pages=L35 |doi=10.3847/2041-8213/ab1de5|arxiv=1906.02763 |bibcode=2019ApJ...877L..35K |s2cid=174801124 |doi-access=free }}
* {{cite journal |last1=Guilet |first1=Jerome |last2=Foglizzo |first2=Thierry |last3=Fromang |first3=Sebastien |title=Dynamics of an Alfven surface in core collapse supernovae |journal=The Astrophysical Journal |date=2010 |volume=729 |issue=1 |page=71 |doi=10.1088/0004-637X/729/1/71 |arxiv=1006.4697|s2cid=118461285 }}
* {{cite journal |last1=Fionnagáin |first1=D ó |last2=Vidotto |first2=A A |last3=Petit |first3=P |last4=Folsom |first4=C P |last5=Jeffers |first5=S V |last6=Marsden |first6=S C |last7=Morin |first7=J |last8=do Nascimento |first8=J-D |title=The Solar Wind in Time II: 3D stellar wind structure and radio emission |journal=Monthly Notices of the Royal Astronomical Society |date=22 November 2018 |doi=10.1093/mnras/sty3132 |url=https://academic.oup.com/mnras/article/483/1/873/5199230 |access-date=18 May 2023|doi-access=free |arxiv=1811.05356 }}
* {{cite journal |last1=Garraffo |first1=Cecilia |last2=Drake |first2=Jeremy J. |last3=Cohen |first3=Ofer |title=The dependence of stellar mass and angular momentum losses on latitude and on active region and dipolar magnetic fields |journal=The Astrophysical Journal |date=27 October 2015 |volume=813 |issue=1 |pages=40 |doi=10.1088/0004-637X/813/1/40|arxiv=1509.08936 |s2cid=118740200 }}
* {{cite journal |last1=Vidotto |first1=A. A. |last2=Jardine |first2=M. |last3=Morin |first3=J. |last4=Donati |first4=J. F. |last5=Opher |first5=M. |last6=Gombosi |first6=T. I. |title=M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planets |journal=Monthly Notices of the Royal Astronomical Society |date=21 February 2014 |volume=438 |issue=2 |pages=1162–1175 |doi=10.1093/mnras/stt2265 |doi-access=free |url=https://academic.oup.com/mnras/article/438/2/1162/1006909 |access-date=18 May 2023|arxiv=1311.5063 }}
* {{cite journal |last1=Vidotto |first1=Aline A. |title=The evolution of the solar wind |journal=Living Reviews in Solar Physics |date=26 April 2021 |volume=18 |issue=1 |pages=3 |doi=10.1007/s41116-021-00029-w |pmid=34722865 |language=en |issn=1614-4961|pmc=8550356 }}

== References ==
{{reflist}}

== External links ==
* {{cite web |title=Solving the sun's super-heating mystery with Parker Solar Probe |url=https://news.umich.edu/solving-the-suns-super-heating-mystery-with-parker-solar-probe/ |website=University of Michigan News |access-date=30 July 2022 |date=4 June 2019}}

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[[Category:Solar phenomena]]