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[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).
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prominencesClouds of gas in the Sun's upper chromosphere/inner corona, with a higher density and at a lower temperature than their surroundings, that are visible (ordinarily only in the monochromatic light of certain strong Fraunhofer lines) as bright projections beyond the limb. When viewed against the brighter disk they appear as dark absorption features and are termed filaments. They exhibit a great diversity of structure and are most conveniently classified according to their behavior, as either quiescent or active.
Quiescent prominences are particularly long-lived and are among the most stable of all solar features. They may persist for several months before breaking up or, less frequently, blowing up and have been known to reform at the same location with an almost identical configuration. Typically they are a couple of hundred thousand kilometers long, several tens of thousands of kilometers high, and several thousand kilometers thick. They occur in high heliographic latitudes, where they are supported by the horizontal magnetic field separating the polar field from the adjacent fields of opposite polarity that were formerly associated with the f -spots of active regions (see sunspot cycle). They attain their greatest frequency a few years after the minimum of the sunspot cycle, when their average latitude is around ±50°. Thereafter they appear in increasingly high latitudes, reaching the polar regions shortly after sunspot maximum. Then, after a brief discontinuity, they reappear around latitude ±50° and remain there in small numbers until a few years after the next minimum, when they again progress poleward.
Active prominences are relatively short-lived and may alter their structure appreciably over a matter of minutes. There are many characteristic types, for example surges and sprays, in which chromospheric material is ejected into the inner corona, and loop prominences and coronal rain, in which the reverse occurs. Loop prominences are impulsive events that often accompany flares, whereas coronal rain represents the return of flare-ejected material. In developing active regions arch filaments are usually present. These tend to connect regions of opposite polarity across the line of inversion and gradually ascend while material descends along both sides of the arch.
Intermediate between quiescent and active prominences are the so-called active-region filaments. These invariably lie along the line of inversion between the vertical magnetic fields of opposite polarity in well-developed active regions. Though long-lived, they may be distinguished from quiescent filaments by an almost continuous flow of material along their axis.
With the exception of surges and sprays, prominences may be regarded as an efficient (though limited) heat sink for the highly ionized corona, their material condensing out of it and then descending along the predominantly vertical magnetic field lines.