In 1908, Hale discovered Zeeman splitting of spectral lines in the radiation from sunspots indicating they possess strong magnetic fields. In addition, they form in pairs of opposite polarity. Note that the polarities of east- west pairs in the southern hemisphere are reversed from those in the north. At the left is a visible-light photograph of the Sun taken at the same time, so that the correspondence between the surface magnetic field and the visible sunspots can easily be seen. . The sense of this polarity swaps every 11 years, making an underlying 22 year magnetic cycle.
Sunspot pairs represent loops of magnetic field breaking through the surface.
The very outer layers of the sun can be seen visibly during eclipse. Centred below is a photograph taken by ARI members from a boat in the Channel during the 1999 eclipse. The red chromosphere and beyond the white corona extending millions of miles into space. The chromosphere emits light mainly in the hydrogen alpha line and therefore appears red. Its temperature rises from about 4,500 K to 8,500 K at the top.