chromosphere

file:solar eclips 1999 5.jpg.]]

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/m³}} at the chromosphere's inner boundary to under {{val|1.6|e=-11|u=kg/m³}} at the outer boundary.

{{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

}} The temperature initially decreases from the inner boundary at about {{val|6,000|u=K}}{{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}} to a minimum of approximately {{val|3,800|u=K}},{{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

}} but then increasing to upwards of {{val|35,000|u=K|fmt=commas}} at the outer boundary with the transition layer of the corona (see {{slink|Stellar corona#Coronal heating problem}}).

The density of the chromosphere is 10⁻⁴ times that of the underlying photosphere and 10⁻⁸ times that of the 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.

{{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 Co.

| location = New York, USA

| page = [https://archive.org/details/universe0008edfree/page/762 762]

| isbn = 978-0-7167-8584-2

}} Without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere.

The chromosphere's spectrum is dominated by emission lines when observed at the solar limb.{{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|url-access=subscription }}{{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}} In particular, one of its strongest lines is the H_α 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.

{{further|Solar phenomena}}

file:the Solar Chromosphere at the highest possible resolution.png

Many different phenomena can be observed in chromospheres.

{{main|Solar plage}}

A plage is a particularly bright region within stellar chromospheres, which are often associated with magnetic activity.

{{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.{{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}}

Since the first observations with the instrument SUMER on board 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.{{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 }} 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.{{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 }}

{{see also|Coronal loop}}

Plasma loops can be seen at the border of the solar disk in the chromosphere. They are different from 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{{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 }} 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.

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 granules commonly observed on the photosphere due to the heat convection.

Chromospheres are present on almost all luminous stars other than white dwarfs. They are most prominent and magnetically active on lower-main sequence stars, on brown dwarfs of F and later spectral types, and on giant and subgiant stars.{{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 }}

A spectroscopic measure of chromospheric activity on other stars is the Mount Wilson S-index.{{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 }}[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.

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

{{reflist}}

• [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