page_title: Stellar corona
section_title: Thermal conduction
hierarchical_section_title: Stellar corona / Physics of the corona / Thermal conduction
caption_attribution_description: Solar Images. A mosaic of the extreme ultraviolet, first, images from STEREO-A's SECCHI/Extreme Ultraviolet Imaging Telescope
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: In the corona thermal conduction occurs from the external hotter atmosphere towards the inner cooler layers. Responsible for the diffusion process of the heat are the electrons, which are much lighter than ions and move faster, as explained above.
  When there is a magnetic field the thermal conductivity of the plasma becomes higher in the direction which is parallel to the field lines rather than in the perpendicular direction.
  A charged particle moving in the direction perpendicular to the magnetic field line is subject to the Lorentz force which is normal to the plane individuated by the velocity and the magnetic field. This force bends the path of the particle. In general, since particles also have a velocity component along the magnetic field line, the Lorentz force constrains them to bend and move along spirals around the field lines at the cyclotron frequency.
  If collisions between the particles are very frequent, they are scattered in every direction. This happens in the photosphere, where the plasma carries the magnetic field in its motion. In the corona, on the contrary, the mean free-path of the electrons is of the order of kilometres and even more, so each electron can do a helicoidal motion long before being scattered after a collision. Therefore, the heat transfer is enhanced along the magnetic field lines and inhibited in the perpendicular direction.
  In the direction longitudinal to the magnetic field, the thermal conductivity of the corona is
  where  is the Boltzmann constant,
   is the temperature in kelvin,
   the electron mass,
   the electric charge of the electron,
  is the Coulomb logarithm, and
  is the Debye length of the plasma with particle density  .
  The Coulomb logarithm  is roughly 20 in the corona, with a mean temperature of 1 MK and a density of 10¹⁵ particles/m³, and about 10 in the chromosphere, where the temperature is approximately 10kK and the particle density is of the order of 10¹⁸ particles/m³, and in practice it can be assumed constant.
  Thence, if we indicate with  the heat for a volume unit, expressed in J m⁻³, the Fourier equation of heat transfer, to be computed only along the direction  of the field line, becomes
  .
  Numerical calculations have shown that the thermal conductivity of the corona is comparable to that of copper.
caption: A mosaic of the extreme ultraviolet images taken from STEREO on December 4, 2006. These false color images show the Sun's atmospheres at a range of different temperatures. Clockwise from top left: 1 million degrees C (171 Å—blue), 1.5 million °C (195Å—green), 60000–80000°C (304 Å—red), and 2.5 million °C (286 Å—yellow).
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