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Composition and Characteristics of the Chromosphere
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 sunspots 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 spectroscope at any clear time of day was developed, and in 1930 the invention of the coronagraph allowed them to be continuously photographed.
chromosphere(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).