The Solar Corona

Click on image for full size (86K GIF) Blending of an eclipse image (from the High Altitude Observatory) with a Yohkoh X-ray image (from the Yohkoh Science Team).

Rising above the Sun's chromosphere , the temperature jumps sharply from a few tens of thousands degrees Kelvin to as much as a few million degrees in the Sun's outer atmosphere, the solar corona. Understanding the reason the Sun's corona is so hot is one of the many challenges facing solar physicists today. Because of the very high temperatures, the corona emits high energy radiation and can be observed in X-rays. The Earth's atmosphere absorbs X-rays, but satellites above the atmosphere, such as the Yohkoh spacecraft, can observe the Sun in these wavelengths. Shown on the left is a blending of a Yohkoh X-ray image (reddish colors) with an eclipse image taken by the High Altitude Observatory (gray-white colors) on November 3, 1994. Near the poles of the Sun, the corona is dark for both X-rays and white light. These regions are coronal holes and are the source of the solar wind that extends out into interplanetary space. The scattered white light shows the density of plasma in the corona. The large white regions extending out far from the Sun are helmet streamers, where the solar plasma has been trapped by the Sun's magnetic field.