The polarization of HeI 10830A is generated by the joint action of scattering processes and the Zeeman and Hanlé effects ( Trujillo Bueno et al., 2002). The HeI 10830A absorption line originates in the upper chromosphere beneath the intense corona or in dark filaments (low-lying prominences) seen on the solar disk. The lower level of this line is a metastable ground state of the orthohelium its population is maintained by the ionization of neutral helium by extreme UV (EUV) irradiation from the overlying corona and subsequent recombination. One important line preferred for measuring the chromospheric magnetic field is the HeI triplet at 10830A. The reason is that the Zeeman signal of the chromospheric lines is much weaker than that of photospheric lines because of the broadness of the spectral lines and the weakness of the magnetic field in the chromosphere. However, the measurement is less successful than in the photosphere. Therefore, measurement of the magnetic field in the chromosphere is crucial for understanding the fundamental physical processes that generate the hot and dynamic outer solar atmosphere. It is a region where the energy generated in the photosphere is transported to the corona and converted to heat or bulk kinetic energy by fascinating dynamical phenomena. On the other hand, in the chromosphere, the interface region connecting the photosphere and the corona, the magnetic pressure becomes dominant over the gas pressure. The magnetic field in the photosphere can be determined relatively well using the Zeeman effect. In the photosphere, the gas pressure dominates the magnetic pressure and the kinetic energy of convection is transferred to the magnetic field in the form of magnetohydrodynamic waves or magnetic stresses (electric currents). Kiyoshi Ichimoto, in The Sun as a Guide to Stellar Physics, 2019 5 Magnetic Fields in the Chromosphere The open magnetic fields in coronal holes do not return directly to another place on the Sun, allowing charged particles to escape the Sun's magnetic grasp and flow outwards into surrounding space. Coronal holes are nearly always present at the Sun's poles, and are sometimes found at lower solar latitudes. These extended regions have so little hot material in them that they appear as large dark areas seemingly devoid of radiation at X-ray wavelengths. Some of the magnetic fields extend outward, within regions called coronal holes. Not all magnetic fields on the Sun are closed loops. Courtesy of Gregory L Slater, Gary A Linford, and Lawrence Shing, NASA, ISAS, Lockheed-Martin Solar and Astrophysics Laboratory, National Astronomical Observatory of Japan, and University of Tokyo. This image of the Sun’s corona was recorded by the Soft X-ray Telescope (SXT) aboard the Japanese Yohkoh satellite on 1 February 1992, near the maximum of the 11-year cycle of solar magnetic activity. The brightest features are called active regions and correspond to the sites of the most intense magnetic field strength. It shows magnetic coronal loops which thread the corona and hold the hot gases in place. The bright glow seen in this X-ray image of the Sun is produced by ionized gases at a temperature of a few million degrees kelvin. As a result, the million-degree corona can be seen all across the Sun's face, with high spatial and temporal resolution, in X-rays.įigure 1. Also, the photosphere is too cool to emit intense radiation at these wavelengths, so it appears dark under the hot gas. Very hot material-such as that within the corona-emits most of its energy at X-ray wavelengths.
The solar corona has a temperature of millions of degrees kelvin, hundreds of times hotter than the underlying visible solar disk whose effective temperature is 5780 K. Modern solar satellites, such as the Solar and Heliospheric Observatory ( SOHO), use coronagraphs to get clear, edge-on views of the corona. Telescopes called coronagraphs allow us to see the corona by using occulting disks to mask the Sun's face and block out the photosphere's glare. They can both be seen during a total solar eclipse, when the Moon blocks the intense light of the photosphere. The thin chromosphere and extensive corona lie above the visible sharp edge of the photosphere. The visible photosphere, or sphere of light, is the level of the solar atmosphere from which we get our light and heat, and it is the part that we can see with our eyes. Lang, in Encyclopedia of Geology, 2005 The Outer Solar Atmosphere
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