Editing Chromosphere (section)

== 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.