The chromosphere ("sphere of color") is the second of the three main layers in the Sun's atmosphere and is roughly 3,000 to 5,000 kilometers deep. Its rosy red color is only apparent during eclipses. The chromosphere sits just above the photosphere and below the solar transition region. The layer of the chromosphere atop the photosphere is homogeneous. A forest of hairy-appearing spicules rise from the homogeneous layer, some of which extend 10,000 km into the corona above.

The density of the chromosphere is only 10−4 times that of the photosphere, the layer beneath, and 10−8 times that of the atmosphere of Earth at sea level. This makes the chromosphere normally invisible and it can be seen only during a total eclipse, where its reddish color is revealed. The color hues are anywhere between pink and red.[1] Without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere beneath.

The density of the chromosphere decreases with distance from the center of the Sun. This decreases exponentially from 1017 particles per cubic centimeter, or approximately 2×10−4 kg/m3 to under 1.6×10−11 kg/m3 at the outer boundary.[2] The temperature decreases from the inner boundary at about 6,000 K[3] to a minimum of approximately 3,800 K,[4] before increasing to upwards of 35,000 K[3] at the outer boundary with the transition layer of the corona.

Chromospheres have been observed also in stars other than the Sun.[5] The Sun's chromosphere has been hard to examine and decipher, although observations continue with the help of the electromagnetic spectrum.[6]


Comparing chromosphere and photosphere

Whilst the photosphere has an absorption line spectrum, the chromosphere's spectrum is dominated by emission lines. In particular, one of its strongest lines is the Hα at a wavelength of 656.3 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 656.3 nm is in the red part of the spectrum, which causes the chromosphere to have its characteristic reddish colour.

By analysing the spectrum of the chromosphere, it was found that the temperature of this layer of the solar atmosphere increases with increasing height in the chromosphere itself. The temperature at the top of photosphere is only about 4,400 K, while at the top of chromosphere, some 2,000 km higher, it reaches 25,000 K.[1][7] This is however the opposite of what we find in the photosphere, where the temperature drops with increasing height. It is not yet fully understood what phenomenon causes the temperature of the chromosphere to paradoxically increase further from the Sun's interior. However, it seems likely to be explained, partially or totally, by magnetic reconnection.


Many interesting phenomena can be observed in the chromosphere, which is very complex and dynamic:

See the flash spectrum of the solar chromosphere (Eclipse of March 7, 1970).

On other stars

A spectroscopic measure of chromospheric activity on other stars is the Mount Wilson S-index.[11][12] See also Superflare#Spectroscopic observations of superflare stars.

See also


  1. ^ a b Freedman, R. A.; Kaufmann III, W. J. (2008). Universe . New York, USA: W. H. Freeman and Co. p. 762 . ISBN 978-0-7167-8584-2.
  2. ^ Kontar, E. P.; Hannah, I. G.; Mackinnon, A. L. (2008), "Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop", Astronomy and Astrophysics, 489 (3): L57, arXiv:0808.3334 , Bibcode:2008A&A...489L..57K , doi:10.1051/0004-6361:200810719
  3. ^ a b "SP-402 A New Sun: The Solar Results From Skylab" . Archived from the original on 2004-11-18.
  4. ^ Avrett, E. H. (2003), "The Solar Temperature Minimum and Chromosphere", ASP Conference Series, 286: 419, Bibcode:2003ASPC..286..419A , ISBN 978-1-58381-129-0
  5. ^ "The Chromosphere" . Archived from the original on 2014-04-04. Retrieved 2014-04-28.
  6. ^ Jess, D.B; Morton, RJ; Verth, G; Fedun, V; Grant, S.T.D; Gigiozis, I. (July 2015). "Multiwavelength Studies of MHD Waves in the Solar Chromosphere". Space Science Reviews. 190 (1–4): 103–161. arXiv:1503.01769 . Bibcode:2015SSRv..190..103J . doi:10.1007/s11214-015-0141-3 .
  7. ^ "World Book at NASA – Sun" .[dead link]
  8. ^ Carlsson, M.; Judge, P.; Wilhelm, K. (1997). "SUMER Observations Confirm the Dynamic Nature of the Quiet Solar Outer Atmosphere: The Internetwork Chromosphere". The Astrophysical Journal. 486 (1): L63. arXiv:astro-ph/9706226 . Bibcode:1997ApJ...486L..63C . doi:10.1086/310836 .
  9. ^ De Forest, C.E. (2004). "High-Frequency Waves Detected in the Solar Atmosphere" . The Astrophysical Journal. 617 (1): L89. Bibcode:2004ApJ...617L..89D . doi:10.1086/427181 .
  10. ^ Foukal, P.V. (1976). "The pressure and energy balance of the cool corona over sunspots". The Astrophysical Journal. 210: 575. Bibcode:1976ApJ...210..575F . doi:10.1086/154862 .
  11. ^ Observational evidence for enhanced magnetic activity of superflare stars
  12. ^ A small survey of the magnetic fields of planet-hosting stars gives "Wright J. T., Marcy G. W., Butler R. P., Vogt S. S., 2004, ApJS, 152, 261" as a ref for s-index.

External links

Categories: Sun

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