page_title: Coronal radiative losses
section_title: Processes of radiation of the solar corona
hierarchical_section_title: Coronal radiative losses / Processes of radiation of the solar corona
caption_attribution_description: Fraunhofer Lines Diagram. SPECTRUM OF VISIBLE LIGHT from the Sun shows a continuum of colors crossed by dark absorption lines; the light was white when it entered the narrow slit of a spectrograph. Wavelength decreases from bottom to top and left to right. Darkest red at bottom left is the border of the infrared; at top right the ultraviolet begins. Hundreds of dark absorption lines cross the stacked rainbow of colors: each line is the distinctive mark of a particular ion of an individual chemical element. The first dark red line at bottom left is o~ of hydrogen. The dark pair of broader lines in violet in the top row at center are Franhofer's H and K lines of singly ionized calcium. Polski: Linie Fraunhofera Português: Diagrama de Fraunhofer, é um exemplo típico de análise espectrográfica Svenska: Fraunhoferska linjerna English: Fraunhofer lines Română: Spectrul soarelui
context_page_description: In astronomy and in astrophysics, for radiative losses of the solar corona, it is meant the energy flux radiated from the external atmosphere of the Sun, and, in particular, the processes of production of the radiation coming from the solar corona and transition region, where the plasma is optically-thin. On the contrary, in the chromosphere, where the temperature decreases from the photospheric value of 6000 K to the minimum of 4400 K, the optical depth is about 1, and the radiation is thermal.
  The corona extends much further than a solar radius from the photosphere and looks very complex and inhomogeneous in the X-rays images taken by satellites.
  The structure and dynamics of the corona are dominated by the solar magnetic field. There are strong evidences that even the heating mechanism, responsible for its high temperature of million degrees, is linked to the magnetic field of the Sun.
  The energy flux irradiated from the corona changes in active regions, in the quiet Sun and in coronal holes; actually, part of the energy is irradiated outwards, but approximately the same amount of the energy flux is conducted back towards the chromosphere, through the steep transition region.
context_section_description: The electromagnetic waves coming from the solar corona are emitted mainly in the X-rays. This radiation is not visible from the Earth because it is filtered by the atmosphere. Before the first rocket missions, the corona could be observed only in white light during the eclipses, while in the last fifty years the solar corona has been photographed in the EUV and X-rays by many satellites (Pioneer 5, 6, 7, 8, 9, Helios, Skylab, SMM, NIXT, Yohkoh, SOHO, TRACE, Hinode).
  The emitting plasma is almost completely ionized and very light, its density is about 10⁻¹⁶ - 10⁻¹⁴ g/cm³. Particles are so isolated that almost all the photons can leave the Sun's surface without interacting with the matter above the photosphere: in other words, the corona is transparent to the radiation and the emission of the plasma is optically-thin. The Sun's atmosphere is not the unique example of X-ray source, since hot plasmas are present wherever in the Universe: from stellar coronae to thin galactic halos. These stellar environments are the subject of the X-ray astronomy.
  In an optically-thin plasma the matter is not in thermodynamical equilibrium with the radiation, because collisions between particles and photons are very rare, and, as a matter of fact, the square root mean velocity of photons, electrons, protons and ions is not the same: we should define a temperature for each of these particle populations. The result is that the emission spectrum does not fit the spectral distribution of a blackbody radiation, but it depends only on those collisional processes which occur in a very rarefied plasma.
  While the Fraunhofer lines coming from the photosphere are absorption lines, principally emitted from ions which absorb photons of the same frequency of the transition to an upper energy level, coronal lines are emission lines produced by metal ions which had been excited to a superior state by collisional processes. Many spectral lines are emitted by highly ionized atoms, like calcium and iron, which have lost most of their external electrons; these emission lines can be formed only at certain temperatures, and therefore their individuation in solar spectra is sufficient to determine the temperature of the emitting plasma.
  Some of these spectral lines can be forbidden on the Earth: in fact, collisions between particles can excite ions to metastable states; in a dense gas these ions immediately collide with other particles and so they de-excite with an allowed transition to an intermediate level, while in the corona it is more probable that this ion remains in its metastable state, until it encounters a photon of the same frequency of the forbidden transition to the lower state. This photon induces the ion to emit with the same frequency by stimulated emission. Forbidden transitions from metastable states are often called as satellite lines.
  The Spectroscopy of the corona allows the determination of many physical parameters of the emitting plasma. Comparing the intensity in lines of different ions of the same element, temperature and density can be measured with a good approximation: the different states of ionization are regulated by the Saha equation.
  The Doppler shift gives a good measurement of the velocities along the line of sight but not in the perpendicular plane.
  The line width should depend on the Maxwell–Boltzmann distribution of velocities at the temperature of line formation (thermal line broadening), while it is often larger than predicted.
  The widening can be due to pressure broadening, when collisions between particles are frequent, or it can be due to turbulence: in this case the line width can be used to estimate the macroscopic velocity also on the Sun's surface, but with a great uncertainty.
  The magnetic field can be measured thanks to the line splitting due to the Zeeman effect.
caption: Fraunhofer lines in the solar spectrum.
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