chromosphere [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 photosphere (the sun's visible surface) and the corona (its outer atmosphere).

Composition and Characteristics of the Chromosphere

The flash spectrum has been a valuable tool in the study of the chromosphere. This spectrum 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 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.

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 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 aurora).

The chromosphere (literally, "colour sphere") is a thin layer of the Sun's atmosphere just above the photosphere, roughly 10,000 kilometres deep (approximating to, if a little less than, the diameter of the Earth). The chromosphere is more visually transparent than the photosphere. The name comes from the fact that it has a reddish colour, as the visual spectrum of the chromosphere is dominated by the deep red H-alpha spectral line of hydrogen. The coloration may be seen directly with the naked eye only during a total solar eclipse, where the chromosphere is briefly visible as a flash of colour just as the visible edge of the photosphere disappears behind the Moon.

For reasons not fully understood the temperature of the chromosphere is hotter than that of the photosphere. The photosphere is closer to the surface of the sun and its temperature is around 4000 K to 6400 K but the chromosphere is about 4500 K to as high as 20,000 K. One theory is that sonic turbulence is the source of this higher temperature.

Without special equipment the chromosphere cannot normally be seen due to its being washed out by the overwhelming brightness of the photosphere. It can be seen clearly through special narrow-band optical filters tuned to the H-alpha spectral line, and many observatories routinely observe the chromosphere using this technique, which displays filaments quite clearly. Filaments (and prominences, which are filaments viewed from the side) are the source of many coronal mass ejections and hence are important to prediction of space weather.

The most common solar feature within the chromosphere are spicules, long thin fingers of luminous gas which appear like the blades of a huge field of fiery grass growing upwards from the photosphere below. Spicules rise to the top of the chromosphere and then sink back down again over the course of about 10 minutes.

Another feature found in the chromosphere are fibrils, horizontal wisps of gas similar in extent to spicules but with about twice the duration.

Finally, solar prominences rise up through the chromosphere from the photosphere, sometimes reaching altitudes of 150,000 kilometers. These gigantic plumes of gas are the most spectacular of solar phenomena, aside from the less frequent solar flares. Above the chromosphere of some stars there is a so-called transition region, where the temperature increases rapidly to the hot corona, which forms the outermost part of the atmosphere.

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


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