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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