The nature of sunspots was put on firmer physical grounds at the beginning of the 20th century, when it was determined spectroscopically that sunspots were the seat of strong, concentrated magnetic fields.
The image on the right is a magnetogram. Magnetograms are synthetic images constructed by measuring the magnetic field along the line of sight at a given location on the solar surface, and assigning a color according to the field strength; the pink-red-yellow sequence corresponds to increasingly strong positive normal magnetic field component, or magnetic fieldlines pointing away from the Sun, and the purple-blue-light blue sequence to increasingly strong negative field, i.e. magnetic fieldlines pointing into the Sun. Upon comparing this magnetogram to the Calcium image on the left, one sees that regions of strongest magnetic fields (yellow and light blue on the magnetogram) always coincide with sunspots. Diffuse magnetic fields of lesser strengths are also present all over the solar surface, with moderately strong (~100 Gauss) fields most often associated with plages. Field strengths in sunspots are in the range 1000–4000 Gauss, with the stronger fields in the larger sunspots; this is much larger than the average 0.5 Gauss of the Earth's surface magnetic field. What is perhaps even more striking is the topological complexity of the solar magnetic field; Unlike on the Earth's surface where the essentially dipolar magnetic field defines rather unambiguously a magnetic north and south, on the solar surface a compass would be rather useless for orientation purposes
Written By P. Charbonneau and O.R. White–April 18, 1995