Measuring Magnetic fields on the Sun

How can one test the theory that strong magnetic fields are responsible for sunspots?

One can measure the magnetic field near a sunspot.

The emission/absorption lines from atoms are altered when the atoms are in a strong magnetic field.
Thus one can deduce strength of the magnetic field at a certain place on the Sun by measuring the spectrum of the light coming from there.
One can also measure the direction by measuring the "polarization" of the light. (However, we will not discuss polarization in any depth).

Here is a picture (a magnetogram) in which the dark regions indicate where the magnetic field is big and coming out of the photosphere, while the white regions indicate where the magnetic field is big and going into the photosphere.

Here is a magnetogram of the whole Sun taken 1995 Jul 09:

Note the strong white and black areas.

Here is a magnetogram of the whole Sun taken yesterday, 2007 October 7:

Not much was happening yesterday.

ASTR 122 course home page

Updated 8 October 2007

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA