Editing Solar wind (section)

== Limits ==

===Alfvén surface===
{{Main|Alfvén surface}}
[[File:Parker Solar Probe touches the Sun.webm|thumb|NASA animation of the [[Parker Solar Probe]] passing through the Sun's corona. Inside the corona's boundary, its Alfvén surface, plasma waves travel back and forth to the Sun's surface.]]
The Alfvén surface is the boundary separating the corona from the solar wind defined as where the coronal plasma's [[Alfvén speed]] and the large-scale solar wind speed are equal.<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><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>

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. On April 28, 2021, during its eighth flyby of the Sun, NASA's [[Parker Solar Probe]] encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated the Alfvén surface.<ref>{{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}}{{PD-notice}}</ref>

===Outer limits===
{{Main|Heliosphere}}
[[File:Transitional regions.jpg|thumb|upright=1.2|An infographic featuring the outer regions of the heliosphere based on results from the Voyager spacecraft]]
The solar wind "blows a bubble" in the [[interstellar medium]] (the rarefied hydrogen and helium gas that permeates the galaxy). The point where the solar wind's strength is no longer great enough to push back the interstellar medium is known as the [[Heliopause (astronomy)|heliopause]] and is often considered to be the outer border of the Solar System. The distance to the heliopause is not precisely known and probably depends on the current velocity of the solar wind and the local density of the interstellar medium, but it is far outside [[Pluto]]'s orbit. Scientists hope to gain perspective on the heliopause from data acquired through the [[Interstellar Boundary Explorer]] (IBEX) mission, launched in October 2008.

The heliopause is noted as one of the ways of defining the extent of the Solar System, along with the [[Kuiper belt|Kuiper Belt]] and the radius at which the Sun's gravitational influence is matched by other stars.<ref>{{Cite web|url=https://svs.gsfc.nasa.gov/12639|title=GMS: Where is the Edge of the Solar System?|website=svs.gsfc.nasa.gov|date=September 5, 2017|access-date=2019-09-22}}</ref> The maximum extent of that influence has been estimated at between 50,000 AU and 2 light-years, compared to the heliopause (the outer boundary of the heliosphere), which has been detected at about 120 AU by the ''Voyager 1'' spacecraft.<ref name="voyager.jpl.nasa.gov">{{Cite web|url=https://voyager.jpl.nasa.gov/frequently-asked-questions/|title=Voyager – Frequently Asked Questions|website=voyager.jpl.nasa.gov|language=en|access-date=2019-09-22}}</ref>

The ''[[Voyager 2]]'' spacecraft crossed the [[termination shock]] more than five times between August 30 and December 10, 2007.<ref>{{Cite web|url=https://www.nasa.gov/mission_pages/voyager/voyager-20071210.html|title=NASA – Voyager 2 Proves Solar System Is Squashed|website=www.nasa.gov|access-date=2022-07-31|archive-date=2020-04-13|archive-url=https://web.archive.org/web/20200413080741/https://www.nasa.gov/mission_pages/voyager/voyager-20071210.html|url-status=dead}}</ref> ''Voyager 2'' crossed the shock about a [[Orders of magnitude (length)|Tm]] closer to the Sun than the 13.5 Tm distance where ''[[Voyager 1]]'' came upon the termination shock.<ref>{{cite news|date=December 11, 2016|title=Voyager 2 finds solar system's shape is 'dented'|newspaper=Reuters|url=https://www.reuters.com/article/us-space-voyager-idUSN1044867120071211}}</ref><ref>{{cite web|last=Tobin|first=Kate|title=Spacecraft reaches edge of solar system – Nov. 5, 2003|url=http://edition.cnn.com/2003/TECH/space/11/05/voyager.solar.boundary/|publisher=CNN}}</ref> The spacecraft moved outward through the termination shock into the [[heliosheath]] and onward toward the [[interstellar medium]].