Editing Interplanetary medium

{{short description|Material which fills the Solar System}}
{{More citations needed|date=July 2007}}

[[File:Heliospheric-current-sheet.gif|thumb|upright=1.3|The [[heliospheric current sheet]] results from the influence of the [[Sun]]'s [[rotating magnetic field]] on the [[Plasma (physics)|plasma]] in the interplanetary medium.<ref>{{cite web|url=http://quake.stanford.edu/~wso/gifs/HCS.html|archive-url=https://web.archive.org/web/20060901124602/http://quake.stanford.edu/~wso/gifs/HCS.html|url-status=dead|archive-date=1 September 2006|title=Heliospheric Current Sheet|date=1 September 2006}}</ref>]]

The '''interplanetary medium''' ('''IPM''') or '''interplanetary space''' consists of the mass and energy which fills the [[Solar System]], and through which all the larger Solar System bodies, such as [[planet]]s, [[dwarf planet]]s, [[asteroid]]s, and [[comet]]s, move. The IPM stops at the [[Heliopause (astronomy)|heliopause]], outside of which the [[interstellar medium]] begins. Before 1950, interplanetary space was widely considered to either be an empty vacuum, or consisting of "[[Aether theories|aether]]".

==Composition and physical characteristics==
The interplanetary medium includes [[Interplanetary dust cloud|interplanetary dust]], [[cosmic ray]]s, and hot [[Plasma (physics)|plasma]] from the [[solar wind]].<ref name="EA-20190312">{{cite news |author=NASA |title=What scientists found after sifting through dust in the solar system |url=https://www.eurekalert.org/pub_releases/2019-03/nsfc-wsf031219.php |date=12 March 2019 |work=[[EurekAlert!]] |access-date=12 March 2019 }}</ref>{{Failed verification|date=June 2019}} The density of the interplanetary medium is very low, decreasing in inverse proportion to the square of the distance from the Sun. It is variable, and may be affected by [[magnetic field]]s and events such as [[coronal mass ejection]]s.   Typical particle densities in the interplanetary medium are about 5-40&nbsp;particles/cm{{sup|3}}, but exhibit substantial variation.<ref name=":0">{{Cite tech report|number=NASA-TM-X-55995|title=Micro-scale structures in the interplanetary medium|author-first=Leonard&nbsp;F.|author-last=Burlaga|date=September 1967|institution=[[NASA]] [[Goddard Space Flight Center]]|url=https://ntrs.nasa.gov/api/citations/19680000537/downloads/19680000537.pdf|access-date=17 August 2023}}</ref>{{Rp|location=Figure 1}}  In the vicinity of the [[Earth]], it contains about 5&nbsp;particles/cm{{sup|3}},<ref name=":2">{{Cite journal |last1=Eviatar |first1=Aharon |last2=Schulz |first2=Michael |date=1970 |orig-date=7 July 1969 |title=Ion-temperature anisotropies and the structure of the solar wind |journal=Planetary and Space Science |location=Northern Ireland |publisher=Pergamon Press |volume=18 |issue=3 |pages=321–332 |doi=10.1016/0032-0633(70)90171-6|bibcode=1970P&SS...18..321E }}</ref>{{Rp|page=326}} but values as high as 100&nbsp;particles/cm{{sup|3}} have been observed.<ref name=":0" />{{Rp|location=Figure 2}}

The temperature of the interplanetary medium varies through the solar system.  [[Joseph Fourier]] estimated that interplanetary medium must have temperatures comparable to those observed at [[Earth's poles]], but [[Right for the wrong reason|on faulty grounds]]: lacking modern estimates of [[Atmospheric circulation|atmospheric heat transport]], he saw no other means to explain the relative consistency of [[Earth's climate]].<ref>{{Cite journal |last=Fourier |first=Jean-Baptiste&nbsp;Joseph |author-link=Joseph Fourier |date=1 September 2004 |orig-date=1827 |title=Mémoire sur les Températures du Globe Terrestre et des Espaces Planétaires |trans-title=On the Temperatures of the Terrestrial Sphere and Interplanetary Space |url=https://geosci.uchicago.edu/~rtp1/papers/Fourier1827Trans.pdf |journal=Mémoires D l'Académie Royale des Sciences de l'Institute de France |volume=VII |pages=570–604 |translator-last=Pierrehumbert |translator-first=R.&nbsp;T.}}</ref>  A very hot interplanetary medium remained a minor position among geophysicists as late as 1959, when Chapman proposed a temperature on the order of 10000&nbsp;K,<ref name=":3">{{Cite journal |last=Chapman |first=S. |date=1959 |title=Interplanetary Space and the Earth's Outermost Atmosphere |url=https://www.jstor.org/stable/100693 |journal=Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences |volume=253 |issue=1275 |pages=462–481 |doi=10.1098/rspa.1959.0208 |jstor=100693 |bibcode=1959RSPSA.253..462C |s2cid=95492893 |issn=0080-4630|url-access=subscription }}</ref> but observation in [[Low Earth orbit]] of the [[exosphere]] soon contradicted his position.{{Citation needed|date=August 2023}}  In fact, both Fourier and Chapman's final predictions were correct: because the interplanetary medium is so [[rarefied]], it does not exhibit [[thermodynamic equilibrium]].  Instead, different components have different temperatures.<ref name=":0" />{{Rp|page=4}}<ref name=":2" /><ref name=":1">{{Cite journal |last1=Sittler |first1=Edward&nbsp;C. |last2=Guhathakurta |first2=Madhulika |date=1 October 1999 |orig-date=20 March 1998 |title=Semi&shy;empirical two-dimensional magneto&shy;hydro&shy;dynamic model of the solar corona and interplanetary medium |journal=The Astrophysical Journal |publisher=[[American Astronomical Society]] |publication-place=USA |volume=523 |pages=812–826 |doi=10.1086/307742|doi-access=free }}  Corrected in [[Digital object identifier|doi]]:[http://dx.doi.org/10.1086/324303 10.1086/324303].</ref>  The solar wind exhibits temperatures consistent with Chapman's estimate in [[cislunar space]],<ref name=":2" />{{Rp|pages=326,329}}<ref name=":1" /><ref>{{Cite tech report|last1=Burlaga|first1=L.&nbsp;F.|last2=Ogilvie|first2=K.&nbsp;W.|title=Solar wind temperature and speed|date=October 1972|publisher=[[US Department of Commerce]] [[National Technical Information Service]]|publication-place=Springfield, VA|url=https://ntrs.nasa.gov/api/citations/19730003092/downloads/19730003092.pdf|access-date=17 August 2023|number=NASA-TM-X-66091}}</ref> and dust particles near Earth's orbit exhibit temperatures {{Cvt|257|-|298|K|F}},<ref name=":4">{{Cite journal |last1=Dumont |first1=R. |last2=Levasseur-Regourd |first2=A.-C. |date=Feb 1998 |orig-date=16 December 1986 |title=Properties of interplanetary dust from infrared and optical observations I: Temperature, global volume intensity, albedo and their heliocentric gradients |url=https://adsabs.harvard.edu/full/1988A%26A...191..154D |journal=Astronomy and Astrophysics |volume=191 |issue=1 |pages=154–160 |bibcode=1988A&A...191..154D |issn=0004-6361 |via=[[NASA]] [[Astrophysics Data System]]}}</ref>{{Rp|page=157}} averaging about {{Cvt|283|K|F}}.<ref>{{Cite web |last=Libal |first=Angela |date=1 June 2023 |title=The Temperatures of Outer Space Around the Earth |url=https://sciencing.com/temperatures-outer-space-around-earth-20254.html |access-date=2023-08-18 |website=Sciencing |publisher=[[Leaf Group]] Media |language=en |publication-place=Santa Monica, CA}}</ref>  In general, the solar wind temperature decreases [[Inverse-square law|proportional to the inverse-square]] of the distance to the Sun;<ref name=":3" /> the temperature of the dust decreases proportional to the inverse [[cube root]] of the distance.<ref name=":4" />{{Rp|page=157}}  For dust particles within the [[asteroid belt]], typical temperatures range from {{Cvt|200|K|F}} at 2.2&nbsp;AU down to {{Cvt|165|K|F}} at 3.2&nbsp;AU.<ref>{{cite journal
  | author=Low, F. J.
  | display-authors=etal
  | title=Infrared cirrus&nbsp;– New components of the extended infrared emission
  | journal=Astrophysical Journal Letters
  | date=1984
  | volume=278
  | pages=L19–L22
  | bibcode=1984ApJ...278L..19L
  | doi=10.1086/184213 }}</ref>

Since the interplanetary medium is a [[Plasma (physics)|plasma]], or gas of [[ion]]s, the interplanetary medium has the characteristics of a plasma, rather than a simple gas. For example, it carries the Sun's magnetic field with it, is highly electrically conductive (resulting in the [[heliospheric current sheet]]), forms plasma [[Double layer (plasma)|double layer]]s where it comes into contact with a planetary magnetosphere or at the [[Heliopause (astronomy)|heliopause]], and exhibits filamentation (such as in [[Aurora (astronomy)|aurorae]]).

The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth being over 100 times greater than originally anticipated. If space were a vacuum, then the Sun's {{10^|−4}} tesla magnetic dipole field would reduce with the cube of the distance to about {{10^|−11}} tesla. But satellite observations show that it is about 100 times greater at around {{10^|−9}} tesla. [[Magnetohydrodynamic]] (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field induces electric currents which in turn generate magnetic fields, and in this respect it behaves like an [[MHD dynamo]].

==Extent of the interplanetary medium==
The outer edge of the [[heliosphere]] is the boundary between the flow of the solar wind and the [[interstellar medium]]. This boundary is known as the [[Heliopause (astronomy)|heliopause]] and is believed to be a fairly sharp transition of the order of 110 to 160 [[astronomical unit]]s from the Sun. The interplanetary medium thus fills the roughly spherical volume contained within the heliopause.

==Interaction with planets==
How the interplanetary medium interacts with planets depends on whether they have [[magnetic field]]s or not. Bodies such as the [[Moon]] have no magnetic field and the [[solar wind]] can impact directly on their surface. Over billions of years, the [[lunar regolith]] has acted as a collector for solar wind particles, and so studies of rocks from the [[geology of the Moon|lunar surface]] can be valuable in studies of the solar wind.

High-energy particles from the solar wind impacting on the lunar surface also cause it to emit faintly at [[X-ray]] wavelengths.

Planets with their own magnetic field, such as the Earth and [[Jupiter]], are surrounded by a [[magnetosphere]] within which their magnetic field is dominant over the [[Sun]]'s. This disrupts the flow of the solar wind, which is channelled around the magnetosphere. Material from the solar wind can "leak" into the magnetosphere, causing [[aurora (astronomy)|aurorae]] and also populating the [[Van Allen radiation belts]] with ionised material.

==Observable phenomena of the interplanetary medium==
[[File:Night Sky from Hawai‘i and Chile (iotw2225c).jpg|thumb|upright=2|The [[interplanetary dust cloud]] illuminated and visible as [[zodiacal light]], with its parts the ''false dawn'',<ref>{{cite web|title=False Dawn|url=http://www.eso.org/public/images/potw1707a/|website=www.eso.org|access-date=14 February 2017}}</ref> ''[[gegenschein]]'' and the rest of its band, which is visually crossed by the [[Milky Way]], in this composite image of the night sky above the northern and southern hemisphere]]
The interplanetary medium is responsible for several optical phenomena visible from Earth. [[Zodiacal light]] is a broad band of faint light sometimes seen after sunset and before sunrise, stretched along the [[ecliptic]] and appearing brightest near the horizon. This glow is caused by sunlight [[light scattering by particles|scattered]] by [[interplanetary dust cloud|dust particles]] in the interplanetary medium between Earth and the Sun.

A similar phenomenon centered at the [[antisolar point]], [[gegenschein]] is visible in a naturally dark, moonless [[night sky]]. Much fainter than zodiacal light, this effect is caused by sunlight [[backscatter]]ed by [[cosmic dust|dust particles]] beyond Earth's orbit.

==History==
The term "interplanetary" appears to have been first used in print in 1691 by the scientist [[Robert Boyle]]: "The air is different from the æther (or vacuum) in the... interplanetary spaces" Boyle ''Hist. Air''. In 1898, American astronomer [[Charles Augustus Young]] wrote: "Inter-planetary space is a vacuum, far more perfect than anything we can produce by artificial means..." (''The Elements of Astronomy'', Charles Augustus Young, 1898).

The notion that space is considered to be a [[vacuum]] filled with an "[[Aether (classical element)|aether]]", or just a cold, dark vacuum continued up until the 1950s. Tufts University Professor of astronomy, Kenneth R. Lang, writing in 2000 noted, "Half a century ago, most people visualized our planet as a solitary sphere traveling in a cold, dark vacuum of space around the Sun".<ref>{{cite book|author=Kenneth R. Lang|title=The Sun from Space|url=https://books.google.com/books?id=Sn5gZ6gHKakC&pg=PA17|year=2000|publisher=Springer Science & Business Media|isbn=978-3-540-66944-9|page=17}}</ref> In 2002, Akasofu stated "The view that interplanetary space is a vacuum into which the Sun intermittently emitted corpuscular streams was changed radically by [[Ludwig Biermann]] (1951, 1953) who proposed on the basis of comet tails, that the Sun continuously blows its atmosphere out in all directions at supersonic speed" ([[Syun-Ichi Akasofu]], ''Exploring the Secrets of the Aurora'', 2002)

==See also==
{{div col|colwidth=30em}}
* [[Interplanetary dust cloud]]
* [[Interplanetary magnetic field]]
* [[Interstellar space]]
* [[Interstellar medium]]
* [[Interstellar dust]]
* [[Intergalactic space]]
* [[Intergalactic medium]]
* [[Intergalactic dust]]
* [[Space physics]]
{{div col end}}

==References==
{{reflist|2}}

==External links==
* [http://www.nineplanets.org/medium.html Bill Arnett's ''The Nine Planets'' page about the interplanetary medium]

{{Molecules detected in outer space}}
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[[Category:Outer space]]
[[Category:Planetary systems]]
[[Category:Solar System]]
[[Category:Space plasmas]]