The Sun – Corona and Solar Wind

If one artificially covers the solar sphere, one sees the corona as bright, luminous rays of light. A comet that passes by the sun creates the curved stripe in the left part of the image. (Source: SOHO (ESA&NASA))
Structure of the corona's magnetic fields (Source: SOHO (ESA&NASA))
Stored energy is released in explosive flares over magnetically active areas. Together with the permanently radiated solar winds, the flares distort the earth's magnetic field. (Source: SOHO (ESA&NASA))

If one covers the solar sphere, just like the moon does during a solar eclipse, one observes bright, luminous rays of light – the corona. Such images can also be produced with special devices, called coronographs, which mask the solar disk. The corona is the outer atmosphere of the sun. It starts a few hundred kilometres above the chromosphere, and extends far into the interplanetary space. In the right image the size of the masked sun is marked with the white circle.

The corona distinguishes itself through a very high temperature of about 1-2 million kelvins, which leads to an almost total ionization of the plasma. A picture of the LASCO coronagraph on the space probe SOHO is shown. The image was captured on 23 December 1996. Until today we do not exactly know why the corona is so hot.

The corona has a multifaceted structure. Tube-like and outwardly closed magnetic-field structures form in active areas, which concentrate radiated plasma in the UV and X-ray band. These can be observed as magnetic-field waves (seen as bright emissions in the image). In contrast to those are the coronal holes, which are magnetically open regions in which the magnetic field lines extend far and which contain plasma that radiates considerably less. They are mostly found over the poles and can be seen there as dark areas.

A lot of magnetic energy is stored in the corona, which can be released as explosive flares in active areas. In this context, coronal mass ejections occur, by which plasma clouds can be discharged at high speeds. This can be seen in the adjacent image. The sun thus functions as a giant particle accelerator, whose effects we can observe in the form of radio, X-ray, and gamma radiation. The detection of this cosmic radiation allows one to draw conclusions about the processes that take place in the corona. Because of the high temperatures in the corona, there is no pressure equilibrium between the coronal plasma and the interstellar medium, which leads to (in addition to the coronal mass ejections) constant matter dispersal from the sun – the solar wind. This leads to a permanent distortion of our earth's magnetic field.