page_title: Stellar corona
section_title: Filament cavities
hierarchical_section_title: Stellar corona / Physical features / Active regions / Filament cavities
caption_attribution_description: English: Fast-growing sunspot 1112 is crackling with solar flares. So far, none of the blasts has hurled a substantial CME, or coronal mass ejection, toward Earth. In addition, a vast filament of magnetism is cutting across the sun's southern hemisphere. This filament is so large it spans a distance greater than the separation of Earth and the moon. A bright 'hot spot' just north of the filament's midpoint is UV radiation from sunspot 1112. The proximity is no coincidence; the filament appears to be rooted in the sunspot below. If the sunspot flares, it could cause the entire structure to erupt. Thus far, none of the flares has destabilized the filament.
context_page_description: A corona is an aura of plasma that surrounds the Sun and other stars. The Sun's corona extends millions of kilometres into outer space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. Spectroscopy measurements indicate strong ionization in the corona and a plasma temperature in excess of 1000000 kelvin, much hotter than the surface of the Sun.
  Light from the corona comes from three main sources, from the same volume of space:
  The K-corona is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines spreads them so greatly as to completely obscure them, giving the spectral appearance of a continuum with no absorption lines.
  The F-corona is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the zodiacal light.
context_section_description: Filament cavities are zones which look dark in the X-rays and are above the regions where Hα filaments are observed in the chromosphere. They were first observed in the two 1970 rocket flights which also detected coronal holes.
  Filament cavities are cooler clouds of gases (plasma) suspended above the Sun's surface by magnetic forces. The regions of intense magnetic field look dark in images because they are empty of hot plasma. In fact, the sum of the magnetic pressure and plasma pressure must be constant everywhere on the heliosphere in order to have an equilibrium configuration: where the magnetic field is higher, the plasma must be cooler or less dense. The plasma pressure  can be calculated by the state equation of a perfect gas: , where  is the particle number density,  the Boltzmann constant and  the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases with respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect renders sunspots apparently dark in the photosphere.
caption: Image taken by the Solar Dynamics Observatory on Oct 16 2010. A very long filament cavity is visible across the Sun's southern hemisphere.
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