page_title: Stellar corona
section_title: Physics of the corona
hierarchical_section_title: Stellar corona / Physics of the corona
caption_attribution_description: English: Taken by Hinode's Solar Optical Telescope on Jan. 12, 2007, this image of the sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity. Hinode captures these very dynamic pictures of the chromosphere. The chromosphere is a thin "layer" of solar atmosphere "sandwiched" between the visible surface, photosphere, and corona. Русский: Изображение поверхности и короны Солнца, полученное Солнечным оптическим телескопом (SOT) на борту спутника Hinode. Получено 12 января 2007 года.
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: The matter in the external part of the solar atmosphere is in the state of plasma, at very high temperature (a few million kelvin) and at very low density (of the order of 10¹⁵ particles/m³).
  According to the definition of plasma, it is a quasi-neutral ensemble of particles which exhibits a collective behaviour.
  The composition is similar to that in the Sun's interior, mainly hydrogen, but with much greater ionization than that found in the photosphere. Heavier metals, such as iron, are partially ionized and have lost most of the external electrons. The ionization state of a chemical element depends strictly on the temperature and is regulated by the Saha equation in the lowest atmosphere, but by collisional equilibrium in the optically-thin corona. Historically, the presence of the spectral lines emitted from highly ionized states of iron allowed determination of the high temperature of the coronal plasma, revealing that the corona is much hotter than the internal layers of the chromosphere.
  The corona behaves like a gas which is very hot but very light at the same time: the pressure in the corona is usually only 0.1 to 0.6 Pa in active regions, while on the Earth the atmospheric pressure is about 100 kPa, approximately a million times higher than on the solar surface. However it is not properly a gas, because it is made of charged particles, basically protons and electrons, moving at different velocities. Supposing that they have the same kinetic energy on average
  (for the equipartition theorem), electrons have a mass roughly 1800 times smaller than protons, therefore they acquire more velocity. Metal ions are always slower. This fact has relevant physical consequences either on radiative processes (that are very different from the photospheric radiative processes), or on thermal conduction.
  Furthermore, the presence of electric charges induces the generation of electric currents and high magnetic fields.
  Magnetohydrodynamic waves (MHD waves) can also propagate in this plasma, even if it is not still clear how they can be transmitted or generated in the corona.
caption: This image, taken by Hinode on 12 January 2007, reveals the filamentary nature of the corona.
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