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(krō`məsfēr') [Gr.,=color sphere], layer of rarefied, transparent gases in the solar atmosphere; it measures 6,000 mi (9,700 km) in thickness and lies between the photospherephotosphere,
luminous, apparently opaque layer of gases that forms the visible surface of the sun or any other star. The photosphere lies between the dense interior gases and the more attenuated gases of the chromosphere.
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 (the sun's visible surface) and the coronacorona,
luminous envelope surrounding the sun, outside the chromosphere. Its density is less than one billionth that of the earth's atmosphere. The corona is visible only at the time of totality during a total eclipse of the sun.
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 (its outer atmosphere).

Composition and Characteristics of the Chromosphere

The flash spectrum has been a valuable tool in the study of the chromosphere. This spectrumspectrum,
arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph).
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 is obtained before a solar eclipse reaches totality and is formed from the thin arc of the sun disappearing behind the moon's disk. An analysis of the emission lines gives information about the heights of the chromosphere and the heights at which various elements exist in it. Using the flash spectrum, scientists have found that the chromosphere is composed primarily of hydrogen, which causes its visible pinkish tint, and of sodium, magnesium, helium, calcium, and iron in lesser amounts. The chromosphere consists of three distinct layers that, moving outward from the sun's surface, decrease in density and increase abruptly in temperature. The lower chromosphere is about 10,800°F; (6,000°C;), the middle rises to 90,000°F; (50,000°C;), and the upper part, merging into the lower corona, reaches 1,800,000°F; (1,000,000°C;).

Solar Activity Originating in the Chromosphere

Spicules and Plages

At 600 mi (1,000 km) above the photosphere, the chromosphere separates into cool, high-density columns, called spicules, and hot, low-density material. The spicules, each about 500 mi (800 km) in diameter, shoot out at 20 mi per sec (32 km per sec) and rise as high as 10,000 mi (16,000 km) before falling back. Any point on the sun will erupt a spicule about once every 24 hr and there may be up to 250,000 of them at any instant.

Other types of solar activity are found to occur in the chromosphere. The elements of each layer are sometimes distributed in bright, cloudlike patches called plages, or flocculi, and in general are located along the same zones as sunspotssunspots,
dark, usually irregularly shaped spots on the sun's surface that are actually solar magnetic storms. The spots are darker because the temperature of the spots is lower than that of the surrounding photosphere (the visible surface of the sun).
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 and fluctuate with the same 11-yr cycle; the relationship between the two is not yet understood.

Quiescent and Eruptive Prominences

Most spectacular of the solar features are the streams of hot gas, called prominences, that shoot out thousands or even hundreds of thousands of miles from the sun's surface at velocities as great as 250 mi per sec (400 km per sec). Two major classifications are the quiescent and the eruptive prominences. Quiescent prominences bulge out from the surface about 20,000 mi (32,000 km) and can last days or weeks. Eruptive prominences are thin flames of gas often reaching heights of 250,000 mi (400,000 km); they occur most frequently in the zones containing sunspots. Dark strandlike objects called filaments were discovered on the disk and were originally thought to be a special kind of feature. These are now known to be prominences seen against the bright background of the photosphere.

Until the middle of the 19th cent. prominences could be viewed extending from the edge of the sun's disk only during a solar eclipse. However, in 1868 a method of observing them with a spectroscopespectroscope,
optical instrument for producing spectral lines and measuring their wavelengths and intensities, used in spectral analysis (see spectrum). When a material is heated to incandescence it emits light that is characteristic of the atomic makeup of the material.
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 at any clear time of day was developed, and in 1930 the invention of the coronagraphcoronagraph
, device invented by the French astronomer B. Lyot (1931) for the purpose of observing the corona of the sun and solar prominences occurring in the chromosphere.
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 allowed them to be continuously photographed.

Solar Flares

Another phenomenon occurring in the chromosphere is the solar flare, a sudden and intense brightening in a plage that rises to great brilliance in a few minutes, then fades dramatically in a half hour to several hours. This feature is also associated with sunspots and is thought to be triggered by the sudden collapse of the magnetic field in the plage. A flare releases the energy equivalent of a billion hydrogen bombs and is the most energetic of solar events. The ultraviolet and X-ray radiation from larger flares can disrupt magnetic compasses and navigation and radio signals as well as affect the electrical grid on the earth and can damage satellites and space probes. Cosmic rays and solar wind particles from some flares interact in the polar regions, creating brilliant auroral displays (see auroraaurora borealis
and aurora australis
, luminous display of various forms and colors seen in the night sky. The aurora borealis of the Northern Hemisphere is often called the northern lights, and the aurora australis of the Southern Hemisphere is known as the southern
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(kroh -mŏ-sfeer) The stratum of a star's atmosphere immediately above the photosphere and below the corona. The chromosphere is considerably less dense than the photosphere, and its gases are characterized by an emission rather than an absorption spectrum. The best-studied chromosphere is that of the Sun.

In the solar chromosphere the temperature rises over a few thousand kilometers from about 4000 K at the temperature minimum to around 50 000 K at the transition region (see Sun). The rise in temperature (which continues in the transition region and inner corona) was once thought to be the result of ascending shock waves, but this mechanism does not tally with detailed observations of the coronae of the Sun and other stars. It is now believed that magnetic heating is responsible (see corona).

The solar chromosphere is visible under natural circumstances only when the photosphere is totally eclipsed by the Moon (see eclipse). It is then seen in profile at the Sun's limb. It may, however, be observed at times other than totality with the aid of a spectroheliograph/spectrohelioscope or a telescope equipped with a suitable narrow-band interference filter. See also chromospheric network; flash spectrum; spicules.

Chromospheres of other stars are studied by observing their strong ultraviolet emission lines or the narrow optical emission lines seen in the center of their photospheric broad absorption lines. In many stars of similar spectral type to the Sun, the chromospheric emission changes with a period of several years, indicating the presence of a cycle of activity akin to the solar cycle (see sunspot cycle). Chromospheric brightness is related to speed of rotation, being greater for stars that rotate rapidly (either because they are young or because of the effect of a companion – see RS Canum Venaticorum star).

Collins Dictionary of Astronomy © Market House Books Ltd, 2006


A transparent, tenuous layer of gas that rests on the photosphere in the atmosphere of the sun.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


Astronomy a gaseous layer of the sun's atmosphere extending from the photosphere to the corona and visible during a total eclipse of the sun
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
The weak continuous spectrum of the chromosphere [1518] has drawn the attention of solar observers for over 100 years [19-22].
In fact, as one proceeds out from the photosphere to the top of the chromosphere, the density was hypothesized to be changing from ~10-7g/[cm.sup.3] to ~10-15 g/[cm.sup.3], respectively [33].
Conversely, the position is now adopted that the presence of a continuous spectrum in the visible range within the chromosphere [15-18] represents a direct manifestation of condensed matter in this region of the solar atmosphere.
The chromosphere corresponds to a region of the Sun where hydrogen atoms are re-entering the condensed state, prior to their recombination with photospheric material.
Hence, the spatial extent of the chromosphere constitutes one of the most elegant observations relative to the existence of a condensed solar photosphere.
Gas pressure can simply account for the spatial extend of the chromosphere in condensed solar models [35,39].
Currently, it is well established that the dimensions of the chromosphere are perceived as vastly different, whether it is studied in H[alpha], or using the HeII line at 30.4 nm [243, Fig.
The prolate nature of the chromosphere and the extended structure which the Sun manifests above the polar axis cannot be easily explained by the gaseous models.
The author has already addressed the chromosphere in detail, as a region of hydrogenre-condensation, superimposed on the corona in the lower portion of the solar atmosphere [28,29].
The tremendous height, 5 000 to 10 000 km, of the chromosphere has posed a longstanding problem for the gaseous models of the Sun [3, p, 140-142].
Furthermore, unlike the case with the gaseous Sun, the chromosphere can now be easily supported using gas pressure.
The author has already suggested that the chromosphere is a region of hydrogen recondensation where hydrides play an important role [28,29].