The 12 x-ray images of the Sun's atmosphere, obtained between 1991 and 1995 at 120 day increments, provide a dramatic view of how the corona changes during the waning part of the solar cycle. The x-ray Sun appears completely different from the Sun we see in the sky. Only very hot gases can emit x rays; the Sun's atmosphere, at millions of degrees, is hot enough to emit x rays, while the much cooler surface of the sun, at 6000 degrees C., is not. As a result, an x-ray image reveals a bright glow for the corona and a black disk for the surface of the Sun. In the corona, the shape and character of the hot gases are controlled by the magnetic fields, just as beads move with string upon which they are threaded. As the solar activity cycle progresses from maximum to minimum, the Sun's magnetic field changes from a complex structure to a simpler configuration with fewer fields. Since the Sun's hot gases are controlled by these fields, the x-ray images reflect this gloabl change, with an overall decrease in brightness by 100 times. (G.L. Slater and G.A. Linford)

The solar x-ray images are from the Yohkoh mission of ISAS, Japan. The x-ray telescope was prepared by the Lockheed Palo Alto Research Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo with the support of NASA and ISAS.

The "Changing Sun" image is used courtesy of Lockheed Martin. For more on the Yohkoh Soft X-Ray Telescope, see its home page.


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