Zeeman Effect in Sunspots (2) ...

The locations of sunspots on the solar surface exhibit certain regularities that are simply displayed by constructing a Maunder butterfly diagram in the following way.
The Maunder Butterfly Diagram
First, the solar disk is divided into strips of latitude having consant surface area. In each such strip, at a given time, the percentage of areas covered by sunspots and active regions is determined and a color is assigned representing the percentage coverage (for example, 0-10 percent gets one color, 11-20 percent gets a second color, and so on). When the color-coded points are plotted versus latitude on the vertical axis and time on the horizontal axis, the following diagram results.

This type of plot was first constructed by E. W. Maunder in 1904 and looks like a row of butterflies. Hence the name "Maunder butterfly diagram." The diagram shows us immediately some important properties of sunspots that provide clues to their nature:

  • There are essentially no sunspots at high latitudes (more than about 40 degrees from the equator)
  • The almost empty times between two successive butterflies is sunspot minimum; sunspot maximum is about midway on each butterfly. The approximately eleven-year sunspot cycle is clearly indicated by the spacing of butterflies and gaps.
  • The distribution of sunspots starts at higher latitudes in the beginning of a cycle and drifts continuously toward the equator as the cycle proceeds from one minimum to the next (the slant of the butterfly wings). However, almost no sunspots occur at any time within a few degrees of the equator.
  • Sunspots in a new cycle begin appearing at higher latitudes at solar minima while spots from the old cycle can still be seen near the equator (the vertical alignment of the tip of a butterfly's wing with the nose of the butterfly behind it).

    • The information in the butterfly diagram, coupled with the preceding observations concerning the magnetic fields of sunspots and the polarities of sunspot pairs, suggest a possible explanation for sunspots that invokes the solar magnetic field and the differential rotation of the Sun.