Because of its high, million-degree temperature, the corona emits most of its energy, and its most intense radiation, as X-rays, ttey can be used to image the hot corona all across the Sun's face with high spatial and temporal resolution, ttis is because the Sun's visible photosphere, being so much cooler, produces negligible amounts of X-ray radiation, and appears dark under the million-degree corona. In contrast, the faint visible light of coronal emission lines is only detected at the edge of the Sun during a total solar eclipse, and cannot be seen in the intense glare of sunlight outside eclipse.
Since the Sun's X-ray radiation is totally absorbed in the Earth's atmosphere, it must be observed with telescopes lofted into space by rockets or in satellites. Herbert Friedman (1916-2000) and his colleagues at the Naval Research Laboratory obtained the first X-ray pictures of the Sun in 1960, during a brief 5-minute rocket flight, ttese crude, early images were replaced with high-resolution X-ray photographs taken and returned to Earth by astronauts from NASA's Skylab thirteen years later, during a 9-month period in 1973-74. Most recently, the Yohkoh, or "sunbeam" spacecraft, launched on 30 August 1991 by the Japanese Institute of Space and Astronautical Science, provided millions of X-ray images of the Sun for a ten year period, until 2001. Yohkoh's images at invisible X-ray wavelengths are almost as sharp and clear as pictures made in visible wavelengths from the ground (Fig. 6.9).
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