20 Mar 2006

In response to Wes' question about measuring sunspot polarity, I got these two answers from my colleagues. Also see the point below about sunspot sizes. If you have trouble understanding the explanations, please ask me.

From Phil Scherrer, a solar astronomer at Stanford:

The Zeeman effect is not quite symmetrical. The line-of-sight splitting is into two components which are circularly polarized. Which component, the red or blue one, is left or right circularly polarized is governed by the direction of the field in the line-of-sight. So alternating between left and right circular analyzer (quarter wave retarder followed by linear polarizer) lets the line jump back and forth in wavelength. The distance of the jump gives field strength and the red or blue shift corresponding to left or right polarization gives the polarity.

From Barbara Thompson, a solar astronomer at NASA Goddard Space Flight Center:

Here's [a decent] discussion of how we produce magnetograms.

http://solar.physics.montana.edu/coradett/

You're right - we measure the Zeeman splitting, and that gives us the magnitude, but you need to look at the polarization to get the direction. That's the way it's done with longitudinal magnetographs. There are vector magnetographs that measure the stokes parameters too.

Regarding sunspot group sizes, there definitely is a huge variation over the cycle. There are more sunspots at solar maximum; in general there are more large sunspots than at solar minimum, and also there are more small sunspots during solar max than solar min. Sunspot number is a totally horrible indicator, because it's kind of a yucky fabricated number without any decent indication of how large a group is or how much total magnetic field there is. Averaged over many years, the sunspot number has relevance. However, you can have a *huge* sunspot in a group, or a small sunspot, and the sunspot number doesn't take this into account:

The Wolf Sunspot number R

It was introduced by Rudolf Wolf in 1848. Determining Wolf numbers is a favorite amateur observing program as it can easily be performed using small telescopes. The Wolf sunspot number includes the number of observed single spots s as well as that of the entire spot groups g. Wolf multiplied the number g of groups with a factor 10 in order to express the fact that the appearance of a new group is weighted 10 times higher than a new spot within an existing group. An isolated spot is regarded as a separate group. The Wolf number R is combined from spot and group numbers according to the relation:

R = 10g+s