Editing Chromosphere

{{short description|Layer of a star's atmosphere}}
[[File:BBSO full-disk H-alpha 2002-07-26 153931 color.png|thumb|When observed in the ''H''<sub>α</sub> spectral line, the chromosphere appears deep red.]]

A '''chromosphere''' ("sphere of color", from the [[Ancient Greek]] words χρῶμα (''khrôma'') 'color' and σφαῖρα (''sphaîra'') '[[sphere]]') is the second layer of a [[Stellar atmosphere|star's atmosphere]], located above the [[photosphere]] and below the [[solar transition region]] and [[Stellar corona|corona]]. The term usually refers to the [[Sun]]'s chromosphere, but not exclusively, since it also refers to the corresponding layer of a [[stellar atmosphere]]. The name was suggested by the English astronomer [[Norman Lockyer]] after conducting systematic solar observations in order to distinguish the layer from the white-light emitting [[photosphere]].<ref>{{Cite journal |date=1869-12-31 |title=II. Spectroscopic observation of the sun, No. II., was resumed and concluded |url=https://doi.org/10.1098/rspl.1868.0019 |journal=Proceedings of the Royal Society of London |volume=17 |pages=131–132 |doi=10.1098/rspl.1868.0019 |issn=0370-1662}}</ref><ref>{{Cite journal |last=Lockyer |first=J. Norman |date=1868-01-01 |title=Spectroscopic Observation of the Sun, No. II. |url=https://ui.adsabs.harvard.edu/abs/1868RSPS...17..131L |journal=Proceedings of the Royal Society of London |volume=17 |pages=131–132|bibcode=1868RSPS...17..131L }}</ref>  

In the [[solar atmosphere|Sun's atmosphere]], the chromosphere is roughly {{convert|3,000|to|5,000|km|mi|abbr=off|sp=us}} in height, or slightly more than 1% of the Sun's radius at maximum thickness. It possesses a homogeneous layer at the boundary with the photosphere. Narrow jets of [[Plasma (physics)|plasma]], called [[Solar spicule|spicules]], rise from this homogeneous region and through the chromosphere, extending up to {{convert|10,000|km|mi|abbr=on|sp=us}} into the corona above.

The chromosphere has a characteristic red color due to [[electromagnetic emission]]s in the [[H-alpha|''H''<sub>α</sub>]] [[spectral line]]. Information about the chromosphere is primarily obtained by analysis of its emitted electromagnetic radiation.<ref>{{Cite journal|last1=Jess|first1=D.B|last2=Morton|first2=RJ|last3=Verth|first3=G|last4=Fedun|first4=V|last5=Grant|first5=S.T.D|last6=Gigiozis|first6=I.|date=July 2015|title=Multiwavelength Studies of MHD Waves in the Solar Chromosphere.|journal=Space Science Reviews|volume=190|issue=1–4|pages=103–161|arxiv=1503.01769|bibcode=2015SSRv..190..103J|doi=10.1007/s11214-015-0141-3|s2cid=55909887 }}</ref> The chromosphere is also visible in the light emitted by ionized calcium, Ca II, in the violet part of the solar spectrum at a wavelength of 393.4 nanometers (the [[Calcium K-line]]).<ref>[https://solarscience.msfc.nasa.gov/chromos.shtml] {{PD-notice}}</ref>

Chromospheres have also been observed on [[star]]s other than the Sun.<ref>{{cite web
 |url= http://solarscience.msfc.nasa.gov/chromos.shtml
 |title= The Chromosphere
 |access-date= 2014-04-28
 |archive-url= https://web.archive.org/web/20140404000148/http://solarscience.msfc.nasa.gov/chromos.shtml
 |archive-date= 2014-04-04
 |url-status= dead
 }}</ref>  On large stars, chromospheres sometimes make up a significant proportion of the entire star. For example, the chromosphere of [[supergiant star]] [[Antares]] has been found to be about 2.5 times larger in thickness than the star's radius.<ref>{{cite web |title=Supergiant Atmosphere of Antares Revealed by Radio Telescopes |url=https://public.nrao.edu/news/supergiant-atmosphere-of-antares-revealed-by-radio-telescopes/ |website=National Radio Astronomy Observatory |access-date=9 September 2022}}</ref>

== Physical properties ==

[[file:solar eclips 1999 5.jpg|thumb|right|The red color of the chromosphere could be seen during the [[solar eclipse of August 11, 1999]].]]

The density of the Sun's chromosphere decreases exponentially with distance from the center of the Sun by a factor of roughly 10 million, from about {{val|2|e=-4|u=kg/m<sup>3</sup>}} at the chromosphere's inner boundary to under {{val|1.6|e=-11|u=kg/m<sup>3</sup>}} at the outer boundary.<ref>
{{Citation
 | last1 = Kontar | first1 = E. P.
 | last2 = Hannah | first2 = I. G.
 | last3 = Mackinnon | first3 = A. L.
 | title = Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop
 | journal = Astronomy and Astrophysics
 | volume = 489
 | issue = 3
 | page = L57
 | year = 2008
 | arxiv = 0808.3334
 | bibcode = 2008A&A...489L..57K
 | doi = 10.1051/0004-6361:200810719
| s2cid = 1651161
 }}</ref> The temperature initially decreases from the inner boundary at about {{val|6,000|u=K}}<ref name="nasa">{{cite web
 | url=https://history.nasa.gov/SP-402/p2.htm
 | title=SP-402 A New Sun: The Solar Results From Skylab
|archive-url=https://web.archive.org/web/20041118125616/https://history.nasa.gov/SP-402/p2.htm |archive-date=2004-11-18}}</ref> to a minimum of approximately {{val|3,800|u=K}},<ref>{{Citation
 | last = Avrett | first = E. H.
 | title = The Solar Temperature Minimum and Chromosphere
 | journal = [[ASP Conference Series]]
 | volume = 286 | page = 419
 | year = 2003
 | bibcode=2003ASPC..286..419A
 | isbn = 978-1-58381-129-0
}}</ref> but then increasing to upwards of {{val|35,000|u=K|fmt=commas}}<ref name="nasa" /> at the outer boundary with the [[Solar transition region|transition layer]] of the [[Solar corona|corona]] (see {{slink|Stellar corona#Coronal heating problem}}).

The density of the chromosphere is 10<sup>−4</sup> times that of the underlying [[photosphere]] and 10<sup>−8</sup> times that of the [[Atmosphere of Earth|Earth's atmosphere]] at sea level. This makes the chromosphere normally invisible and it can be seen only during a [[total eclipse]], where its reddish colour is revealed. The colour hues are anywhere between pink and red.<ref name="Freedman">
{{cite book
 | last1 = Freedman | first1 = R. A.
 | last2 = Kaufmann III | first2 = W. J.
 | year = 2008
 | title = Universe
 | url = https://archive.org/details/universe0008edfree | url-access = registration | publisher = [[W. H. Freeman and Company|W. H. Freeman and Co.]]
 | location = New York, USA
 | page = [https://archive.org/details/universe0008edfree/page/762 762]
 | isbn = 978-0-7167-8584-2
}}</ref> Without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere.

The chromosphere's [[spectrum]] is dominated by [[Spectral line|emission lines]] when observed at the solar limb.<ref>{{Cite journal |last1=Carlsson |first1=Mats |last2=Pontieu |first2=Bart De |last3=Hansteen |first3=Viggo H. |date=2019-08-18 |title=New View of the Solar Chromosphere |url=https://www.annualreviews.org/content/journals/10.1146/annurev-astro-081817-052044 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=57 |pages=189–226 |doi=10.1146/annurev-astro-081817-052044 |bibcode=2019ARA&A..57..189C |issn=0066-4146}}</ref><ref>{{Cite journal |last=Solanki |first=Sami K. |date=June 2004 |title=Structure of the solar chromosphere |url=https://www.cambridge.org/core/product/identifier/S1743921304005587/type/journal_article |journal=Proceedings of the International Astronomical Union |language=en |volume=2004 |issue=IAUS223 |pages=195–202 |doi=10.1017/S1743921304005587 |bibcode=2004IAUS..223..195S |issn=1743-9213}}</ref> In particular, one of its strongest lines is the [[H-alpha|''H''<sub>α</sub>]] at a [[wavelength]] of {{Val|656.3|u=nm}}; this line is emitted by a [[hydrogen atom]] whenever its [[electron]] makes a transition from the ''n''=3 to the ''n''=2 [[energy level]]. A wavelength of {{Val|656.3|u=nm}} is in the red part of the spectrum, which causes the chromosphere to have a characteristic reddish colour.  

== Phenomena ==

{{further|Solar phenomena}}

[[file:the Solar Chromosphere at the highest possible resolution.png|thumb|right|High-resolution observations of the solar chromosphere show hair-like spicules, here shown in a false colored image made in borderline ultraviolet radiation of calcium K-line.]]

Many different phenomena can be observed in chromospheres.

=== Plage ===

{{main|Solar plage}}

A plage is a particularly bright region within stellar chromospheres, which are often associated with magnetic activity.<ref name="degrijs21" />

=== Spicules ===

{{main|Solar spicule}}

The most commonly identified feature in the solar chromosphere are spicules. Spicules rise to the top of the chromosphere and then sink back down again over the course of about 10 minutes.<ref>{{Cite book |last=Wilkinson |first=John |url=https://www.worldcat.org/oclc/773089685 |title=New eyes on the sun : a guide to satellite images and amateur observation |date=2012 |publisher=Springer |isbn=978-3-642-22839-1 |location=Berlin |oclc=773089685}}</ref>

=== Oscillations ===

Since the first observations with the instrument SUMER on board [[Solar and Heliospheric Observatory|SOHO]], periodic oscillations in the solar chromosphere have been found with a frequency from {{Val|3|u=mHz}} to {{Val|10|u=mHz}}, corresponding to a characteristic periodic time of three minutes.<ref>{{cite journal |author1=Carlsson, M. |author2=Judge, P. |author3=Wilhelm, K. |title= SUMER Observations Confirm the Dynamic Nature of the Quiet Solar Outer Atmosphere: The Internetwork Chromosphere | journal= The Astrophysical Journal| year= 1997 | volume= 486|issue=1 |page= L63 | bibcode = 1997ApJ...486L..63C | doi = 10.1086/310836 |arxiv = astro-ph/9706226 |s2cid=119101577 }}</ref> Oscillations of the radial component of the plasma velocity are typical of the high chromosphere. The photospheric granulation pattern usually has no oscillations above {{Val|20|u=mHz}}; however, higher frequency waves ({{Val|100|u=mHz}}, or a {{Val|10|u=s}} period) were detected in the solar atmosphere (at temperatures typical of the transition region and corona) by [[TRACE]].<ref>{{cite journal | author= De Forest, C.E. | title= High-Frequency Waves Detected in the Solar Atmosphere | journal= The Astrophysical Journal | year= 2004 | volume= 617| issue= 1 | page= L89 | bibcode=2004ApJ...617L..89D | doi=10.1086/427181 | doi-access= free }}</ref>

=== Loops ===

{{see also|Coronal loop}}

Plasma loops can be seen at the border of the solar disk in the chromosphere. They are different from [[Solar prominence|solar prominences]] because they are concentric arches with maximum temperature of the order {{Val|0.1|u=MK}} (too low to be considered coronal features). These cool-temperature loops show an intense variability: they appear and disappear in some UV lines in a time less than an hour, or they rapidly expand in 10–20 minutes. Foukal<ref>{{cite journal |author= Foukal, P.V. |title= The pressure and energy balance of the cool corona over sunspots |journal= The Astrophysical Journal |year= 1976 |volume= 210 |page= 575 |bibcode=1976ApJ...210..575F |doi=10.1086/154862 }}</ref> studied these cool loops in detail from the observations taken with the EUV spectrometer on [[Skylab]] in 1976. When the plasma temperature of these loops becomes coronal (above {{Val|1|u=MK}}), these features appear more stable and evolve over longer times.

== Network ==

Images taken in typical chromospheric lines show the presence of brighter cells, usually referred to as the ''network'', while the surrounding darker regions are named ''internetwork''. They look similar to [[Solar granule|granules]] commonly observed on the photosphere due to the heat [[convection]].

== On other stars ==

Chromospheres are present on almost all luminous stars other than [[white dwarf]]s. They are most prominent and magnetically active on lower-[[main sequence star]]s, on [[brown dwarf]]s of F and later spectral types, and on [[Giant star|giant]] and [[subgiant star]]s.<ref name="degrijs21">{{cite journal |last1=de Grijs |first1=Richard |last2=Kamath |first2=Devika |title=Stellar Chromospheric Variability |journal=Universe |date=15 November 2021 |volume=7 |issue=11 |pages=440 |doi=10.3390/universe7110440 |bibcode=2021Univ....7..440D |doi-access=free }}</ref>

A spectroscopic measure of chromospheric activity on other stars is the Mount Wilson '''S-index'''.<ref>{{Cite journal|title=Observational evidence for enhanced magnetic activity of superflare stars|first1=Christoffer|last1=Karoff|first2=Mads Faurschou|last2=Knudsen|first3=Peter|last3=De Cat|first4=Alfio|last4=Bonanno|first5=Alexandra|last5=Fogtmann-Schulz|first6=Jianning|last6=Fu|first7=Antonio|last7=Frasca|first8=Fadil|last8=Inceoglu|first9=Jesper|last9=Olsen|first10=Yong|last10=Zhang|first11=Yonghui|last11=Hou|first12=Yuefei|last12=Wang|first13=Jianrong|last13=Shi|first14=Wei|last14=Zhang|date=March 24, 2016|journal=Nature Communications|volume=7|issue=1|pages=11058|doi=10.1038/ncomms11058|doi-access=free|pmid=27009381 |pmc=4820840|bibcode=2016NatCo...711058K }}</ref><ref>[https://habitability.univie.ac.at/fileadmin/user upload/k habconf2016/pdf/poster/Mengel.pdf ''A small survey of the magnetic fields of planet-hosting stars''] ({{webarchive|url=https://web.archive.org/web/20161222081413/https://habitability.univie.ac.at/fileadmin/user upload/k habconf2016/pdf/poster/Mengel.pdf|date=2016-12-22}}) gives "Wright J. T., Marcy G. W., Butler R. P., Vogt S. S., 2004, ApJS, 152, 261" as a ref for s-index.</ref>

== See also ==

* {{annotated link|Orders of magnitude (density)}}
* {{annotated link|Moreton wave}}

== References ==

{{reflist}}

== External links ==

* [http://alienworlds.southwales.ac.uk/sunStructure.html#/transitionregion Animated explanation of the Chromosphere (and Transition Region) ] {{Webarchive|url=https://web.archive.org/web/20151116133527/http://alienworlds.southwales.ac.uk/sunStructure.html#/transitionregion |date=2015-11-16 }} (University of South Wales).

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[[category:Sun]]