Editing Stellar corona (section)

===Thermal conduction===
[[File:STEREO-A first images.jpg|thumb|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 ({{gaps|195|Å—green}}), {{gaps|60|000}}–{{gaps|80|000|°C}} (304 Å—red), and 2.5 million °C (286 Å—yellow).]]
[[File:STEREO-A first images slow anim.gif|thumb|[[STEREO]]&nbsp;– First images as a slow animation]]
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.<ref name="Spitzer">{{cite book
|last=Spitzer
|first= L.
|year=1962
|title= Physics of fully ionized gas
|publisher= Interscience tracts of physics and astronomy
}}</ref>
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<ref name="Spitzer" />
<math display="block">
k = 20 \left(\frac{2}{\pi}\right)^{3/2}\frac{\left(k_\text{B} T \right)^{5/2}k_\text{B}}{m_e^{1/2} e^4 \ln \Lambda} \approx \frac{T^{5/2}}{\ln \Lambda} \times 1.8 \times 10^{-10}~ \mathrm{W m^{-1}K^{-1}}
</math>
where <math>k_\text{B}</math> is the [[Boltzmann constant]], <math>T</math> is the temperature in kelvin, <math>m_e</math> is the electron mass, <math>e</math> is the electric charge of the electron,
<math display="block"> \ln \Lambda = \ln \left(12\pi n \lambda_D^3 \right) </math>
is the Coulomb logarithm, and
<math display="block">\lambda_D = \sqrt{ \frac{k_\text{B} T }{4 \pi n e^2 }}</math>
is the [[Debye length]] of the plasma with particle density  <math>n</math>. The Coulomb logarithm <math> \ln \Lambda </math> is roughly 20 in the corona, with a mean temperature of 1 MK and a density of 10<sup>15</sup> particles/m<sup>3</sup>, and about 10 in the chromosphere, where the temperature is approximately 10kK and the particle density is of the order of 10<sup>18</sup> particles/m<sup>3</sup>, and in practice it can be assumed constant.

Thence, if we indicate with <math>q</math> the heat for a volume unit, expressed in J m<sup>−3</sup>, the Fourier equation of heat transfer, to be computed only along the direction <math>x</math> of the field line, becomes
<math display="block"> \frac{\partial q}{\partial t} = 0.9 \times 10^{-11}~ \frac{\partial^2  T^{7/2}}{\partial x ^2 }.</math>

Numerical calculations have shown that the thermal conductivity of the corona is comparable to that of copper.