Heliopause - Wikiparency

# Heliopause  **Microsim — three.js (Wikitube framework):** *The edge of the heliosphere in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=heliosphere&embed=1" data-title="The edge of the heliosphere in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=heliosphere`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: under show, choose clouds and boundaries to hide the planets and keep the heliosphere's shells, then pick out the middle one, the heliopause at 121 AU, between the termination shock at 94 AU and the bow shock near 230 AU; scroll in and compare the 27 AU gap between the inner two shells with Neptune's orbit, about 30 AU in radius; drag the view round and note that the shells are drawn as simple nested surfaces, not the blunt-nosed, tailed shape of the real boundary.* The **heliopause** is the outer boundary of the [[Heliosphere|heliosphere]], the surface at which the outward pressure of the [[Solar_wind|solar wind]] is balanced by the pressure of the [[Interstellar_medium|interstellar medium]]. Inside it the [[Plasma_(physics)|plasma]] and magnetic field come from the [[Sun]]; outside it they come from the rest of the [[Milky_Way|Milky Way]].[^nasa-vim][^richardson2019] It is the usual definition of where interstellar space begins, although the Sun's gravity reaches much farther, to the [[Oort_cloud|Oort cloud]].[^nasa2018-v2] Only two spacecraft have crossed it: [[Voyager_1|Voyager 1]] on 25 August 2012 at 121.6 [[Astronomical_unit|AU]] and Voyager 2 on 5 November 2018 at about 119 AU.[^burlaga2019] Their measurements show a boundary thinner and sharper than most models expected, a jump in plasma [[Density|density]] by a factor of about twenty, and interstellar gas hotter than predicted, while its overall shape and its motion with the solar cycle are still argued over.[^gurnett2019][^richardson2019][^opher2020] The explorer at the top of this page opens on the edge of the heliosphere, where the heliopause is the middle of three shells, placed at Voyager 1's crossing distance. ## Definition and pressure balance The solar wind leaves the Sun faster than the [[Speed_of_sound|speed of sound]] in it and stays supersonic for most of its journey. Its pressure falls with the square of distance as the flow spreads out, so at some distance it can no longer push back the surrounding interstellar gas. The balance happens in stages. At the termination shock the wind drops abruptly to subsonic speed; it then flows, slowed, compressed and heated, through the heliosheath; and at the heliopause the solar material finally meets interstellar material and turns aside instead of mixing.[^nasa-vim][^stone2005] Before any spacecraft reached it, the heliopause was expected at roughly 110 AU.[^apod2002] A rough estimate shows why the distance is of order a hundred AU. A typical solar wind at 1 AU, with about 5 [[Proton|protons]] per cubic centimetre moving at 400 km/s, carries a dynamic pressure ρv² of about 1.3 × 10⁻⁹ Pa; at 120 AU that falls by a factor of 14,400, to about 9 × 10⁻¹⁴ Pa (derived). That is comparable with the combined thermal and magnetic pressure of the local interstellar gas, which is why the boundary sits well beyond the planets but far inside the Oort cloud (derived). The same reasoning explains why the boundary moves: its distance follows the solar wind's pressure and the pressure and density of the interstellar gas outside, and both change with time.[^nasa-vim][^frisch2011] ## Voyager 1 crossing Voyager 1 crossed the termination shock in December 2004 at 94 AU and spent the next eight years in the heliosheath.[^stone2005] In the summer of 2012 its particle detectors registered a sharp and distinct boundary crossed five times in about thirty days: low-energy ions from inside the heliosphere nearly disappeared, and galactic cosmic rays rose.[^krimigis2013][^stone2013] The magnetometer, however, recorded no change in the field's direction at any of these crossings. Burlaga, Ness and Stone read that as evidence that the probe had entered a "heliosheath depletion region", a magnetic highway along which particles from inside stream out and cosmic rays stream in, rather than interstellar space itself.[^burlaga2013] The question was settled by the plasma. Voyager 1's plasma instrument had failed in 1980, so the density could be measured only indirectly, when the Sun sent out a burst that set the surrounding plasma oscillating. Its plasma-wave instrument recorded such oscillations in 2012 and 2013, and their frequency gave an electron density of about 0.08 per cubic centimetre, some forty times that expected inside the heliosphere.[^gurnett2013] Working back through the record fixed the crossing at 25 August 2012, at 121.6 AU and about 18 billion km from the Sun.[^nasa-vim][^burlaga2019] ## Voyager 2 crossing Voyager 2, travelling south of the plane of the planets, crossed the termination shock in 2007 at about 84 AU, some 10 AU closer to the Sun than its twin, which showed that the heliosphere is not symmetric.[^stone2008] It reached the heliopause on 5 November 2018 at about 119 AU.[^burlaga2019][^nasa2018-v2] Unlike Voyager 1 it had a working plasma instrument, so this crossing was the first in which the solar wind's speed, density and temperature were measured directly through the boundary.[^richardson2019] The particle signature began early. Particles of solar origin started a gradual decline on 7 August 2018 at 118.2 AU, while galactic cosmic rays rose by about 20 percent over a few weeks; on 5 November the change became abrupt, with energetic solar particles falling and cosmic rays above 213 MeV rising.[^krimigis2019] The magnetic field direction turned smoothly from the approach, through a "magnetic barrier" in the heliosheath next to the boundary, into the interstellar medium, with little or no change across the heliopause itself, and the interstellar field was stronger than Voyager 1 had found.[^burlaga2019] Voyager 2 did not see the flux-tube structure that Voyager 1 had met just inside the boundary.[^stone2019] ## Structure of the boundary Voyager 2's plasma data give the most detailed picture of what the boundary looks like from inside. A boundary region about 1.5 AU wide lies before the heliopause, in which the plasma slows, heats up and is about twice as dense as typical heliosheath plasma. A much thinner layer begins about 0.06 AU inside the heliopause, where the outward speed falls and the density and magnetic field rise, and the final transition took less than a day of the spacecraft's travel.[^richardson2019] At Voyager 2's speed of about 15 km/s, one day corresponds to about 1.3 million km, less than 0.01 AU (derived). The density contrast is the clearest marker. The outer heliosphere holds about 0.002 [[Electron|electrons]] per cubic centimetre. Voyager 2's plasma-wave instrument measured 0.039 cm⁻³ at 119.7 AU in January 2019, and Voyager 1 had measured 0.055 cm⁻³ at 122.6 AU in October 2013, a rise by a factor of about 20 to 30 (derived), in line with pressure-balance estimates of a factor of 20 to 50.[^gurnett2019] The two crossings differ in detail: Voyager 2 found a thinner and simpler heliopause than Voyager 1, and the authors of the magnetic-field study describe the barrier, the heliopause and the gas just beyond as one interconnected, changing system rather than a single sharp wall.[^burlaga2019] ## Distance, shape and variability The two crossings, 121.6 and 119 AU, at different latitudes and six years apart, are the only direct measurements of the heliopause's distance.[^burlaga2019] Between them the heliosheath was about 28 AU thick along Voyager 1's path and about 35 AU along Voyager 2's (derived from the shock and heliopause distances).[^stone2005][^stone2008] Everything else about the boundary's shape comes from models and from remote sensing. The upstream side, facing the Sun's motion through the local gas, is blunt; whether the downstream side is drawn out into a long tail like a comet's, or the whole heliosphere is small and nearly round, remains disputed.[^opher2020][^krimigis2009] Energetic neutral atoms imaged from Saturn orbit by Cassini suggested a bubble-like heliosphere without a long tail, and magnetohydrodynamic models that include pick-up ions point the same way.[^krimigis2009][^opher2020] The boundary also moves with the Sun's activity. When the solar wind strengthens, the heliosphere is pushed outward, and when it weakens, the boundary retreats; the distance depends as well on the density and pressure of the interstellar gas the Sun happens to be passing through.[^nasa-vim][^frisch2011] The explorer's shells are ILLUSTRATIVE: they are spheres at single distances, the termination shock at 94 AU, the heliopause at 121 AU and a bow shock near 230 AU, and they show neither the boundary's asymmetry nor its motion. ## Beyond the heliopause Just outside the heliopause lies the very local interstellar medium. Voyager 2 found it variable near the boundary and hotter than predicted, at 30,000 to 50,000 K against the 15,000 to 30,000 K that models and earlier observations had suggested.[^richardson2019] Farther upstream, neutral interstellar [[Hydrogen|hydrogen]] is thought to pile up into a "hydrogen wall", and the flow probably forms a gentle bow wave rather than the bow shock once expected near 230 AU, because the Interstellar Boundary Explorer measured the Sun's speed through the local gas at only about 23 km/s.[^zank2013][^mccomas2012] IBEX also mapped a narrow ribbon of energetic neutral atoms across the sky that no model had predicted, which appears to be ordered by the interstellar magnetic field draped over the heliosphere.[^mccomas2009] The gas the Sun moves through is the [[Local_Interstellar_Cloud|Local Interstellar Cloud]], itself inside the larger [[Local_Bubble|Local Bubble]].[^frisch2011] The heliopause is where these structures begin to act directly on the Solar System: galactic cosmic rays that reach the inner planets have all crossed it, and the magnetic barrier Voyager 2 found just inside it helps set how many do.[^burlaga2019][^krimigis2019] ## See also - [[Heliosphere]] - [[Solar_wind]] - [[Voyager_1]] - [[Interstellar_medium]] - [[Local_Interstellar_Cloud]] - [[Timeline_of_Solar_System_exploration]] ## References [^nasa-vim]: NASA Science. "Voyager: interstellar mission" (fetched 2026-09-18). https://science.nasa.gov/mission/voyager/interstellar-mission/ [^richardson2019]: Richardson, J. D.; Belcher, J. W.; Garcia-Galindo, P.; Burlaga, L. F. (2019). "Voyager 2 plasma observations of the heliopause and interstellar medium". *Nature Astronomy* 3: 1019–1023. https://doi.org/10.1038/s41550-019-0929-2 [^burlaga2019]: Burlaga, L. F.; Ness, N. F.; Berdichevsky, D. B.; et al. (2019). "Magnetic field and particle measurements made by Voyager 2 at and near the heliopause". *Nature Astronomy* 3: 1007–1012. https://doi.org/10.1038/s41550-019-0920-y [^krimigis2019]: Krimigis, S. M.; Decker, R. B.; Roelof, E. C.; et al. (2019). "Energetic charged particle measurements from Voyager 2 at the heliopause and beyond". *Nature Astronomy* 3: 997–1006. https://doi.org/10.1038/s41550-019-0927-4 [^gurnett2019]: Gurnett, D. A.; Kurth, W. S. (2019). "Plasma densities near and beyond the heliopause from the Voyager 1 and 2 plasma wave instruments". *Nature Astronomy* 3: 1024–1028. https://doi.org/10.1038/s41550-019-0918-5 [^nasa2018-v2]: Potter, S. (2018). "NASA's Voyager 2 probe enters interstellar space". NASA. https://www.nasa.gov/news-release/nasas-voyager-2-probe-enters-interstellar-space/ [^apod2002]: Nemiroff, R.; Bonnell, J. (2002). "The Sun's heliosphere & heliopause". *Astronomy Picture of the Day*, NASA, 24 June 2002. https://apod.nasa.gov/apod/ap020624.html [^stone2005]: Stone, E. C.; Cummings, A. C.; McDonald, F. B.; et al. (2005). "Voyager 1 explores the termination shock region and the heliosheath beyond". *Science* 309: 2017–2020. https://doi.org/10.1126/science.1117684 [^krimigis2013]: Krimigis, S. M.; Decker, R. B.; Roelof, E. C.; et al. (2013). "Search for the exit: Voyager 1 at heliosphere's border with the galaxy". *Science* 341: 144–147. https://doi.org/10.1126/science.1235721 [^burlaga2013]: Burlaga, L. F.; Ness, N. F.; Stone, E. C. (2013). "Magnetic field observations as Voyager 1 entered the heliosheath depletion region". *Science* 341: 147–150. https://doi.org/10.1126/science.1235451 [^stone2013]: Stone, E. C.; Cummings, A. C.; McDonald, F. B.; et al. (2013). "Voyager 1 observes low-energy galactic cosmic rays in a region depleted of heliospheric ions". *Science* 341: 150–153. https://doi.org/10.1126/science.1236408 [^gurnett2013]: Gurnett, D. A.; Kurth, W. S.; Burlaga, L. F.; Ness, N. F. (2013). "In situ observations of interstellar plasma with Voyager 1". *Science* 341: 1489–1492. https://doi.org/10.1126/science.1241681 [^stone2008]: Stone, E. C.; Cummings, A. C.; McDonald, F. B.; et al. (2008). "An asymmetric solar wind termination shock". *Nature* 454: 71–74. https://doi.org/10.1038/nature07022 [^stone2019]: Stone, E. C.; Cummings, A. C.; Heikkila, B. C.; Lal, N. (2019). "Cosmic ray measurements from Voyager 2 as it crossed into interstellar space". *Nature Astronomy* 3: 1013–1018. https://doi.org/10.1038/s41550-019-0928-3 [^frisch2011]: Frisch, P. C.; Redfield, S.; Slavin, J. D. (2011). "The interstellar medium surrounding the Sun". *Annual Review of Astronomy and Astrophysics* 49: 237–279. https://doi.org/10.1146/annurev-astro-081710-102613 [^opher2020]: Opher, M.; Loeb, A.; Drake, J.; Toth, G. (2020). "A small and round heliosphere suggested by magnetohydrodynamic modelling of pick-up ions". *Nature Astronomy* 4: 675–683. https://doi.org/10.1038/s41550-020-1036-0 [^krimigis2009]: Krimigis, S. M.; Mitchell, D. G.; Roelof, E. C.; Hsieh, K. C.; McComas, D. J. (2009). "Imaging the interaction of the heliosphere with the interstellar medium from Saturn with Cassini". *Science* 326: 971–973. https://doi.org/10.1126/science.1181079 [^zank2013]: Zank, G. P.; Heerikhuisen, J.; Wood, B. E.; et al. (2013). "Heliospheric structure: the bow wave and the hydrogen wall". *The Astrophysical Journal* 763: 20. https://doi.org/10.1088/0004-637X/763/1/20 [^mccomas2012]: McComas, D. J.; Alexashov, D.; Bzowski, M.; et al. (2012). "The heliosphere's interstellar interaction: no bow shock". *Science* 336: 1291–1293. https://doi.org/10.1126/science.1221054 [^mccomas2009]: McComas, D. J.; Allegrini, F.; Bochsler, P.; et al. (2009). "Global observations of the interstellar interaction from the Interstellar Boundary Explorer (IBEX)". *Science* 326: 959–962. https://doi.org/10.1126/science.1180906 ## External links - NASA Science: Voyager interstellar mission. https://science.nasa.gov/mission/voyager/interstellar-mission/ - NASA JPL: Voyager mission status. https://voyager.jpl.nasa.gov/mission/status/ - Nature Astronomy (2019): collection of five Voyager 2 heliopause papers, volume 3, issue 11. https://www.nature.com/natastron/volumes/3/issues/11 ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Heliopause) : [Wikitube](https://en.wikitube.io/wiki/Heliopause) · pinned revision [1343318230](https://en.wikipedia.org/w/index.php?oldid=1343318230) · 2026-09-18 *At this revision Wikipedia's "Heliopause" is a disambiguation page; the astronomical subject is the Heliopause subsection of Wikipedia's Heliosphere article. With no pair skeleton to mirror, this article's sections are Wikitube's own (approved by MTN, 2026-09-18).* ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-087 · explorer state `?obj=heliosphere`.*