Astronomical Society of Coonabarabran

The 150' Mt Wilson telescope and the Hale classifications, (a "visit" with Tom Cragg) - Part 3

by Harry Roberts, with acknowledgment and thanks to Tom Cragg.

George Ellery Hale was a self taught astronomer, and building the great American observatory on Mt Wilson was his life's project. Explaining solar magnetism was his great triumph. Firstly in 1912 and later in 1919 he (with others) laid out a complete scheme called the Hale (or Mt Wilson) magnetic classification for sunspot groups (active regions, AR), summarised here:

α	A single dominant spot, usually connected to a plage of opposite magnetic polarity
β	A pair of dominant spots of opposite polarity
γ	Complex groups (AR) with irregular distribution of polarities
βγ	Bipolar groups (AR) with no marked north-south inversion line
δ	Umbrae of opposite polarity in a single penumbra

In addition the suffixes p and f are used when the preceding or following spot, respectively, is dominant. In the majority of groups the p spot is dominant.

Ref. Zirin, H. “Astrophysics of the Sun” p 316.

"The tendency of new dipoles (sunspots) to pop up in the middle of active regions leads to one form of the complex Gamma and Delta spots (AR) the major source of solar flares. Most solar flares result from the (magnetic) stresses produced by flux (new sunspot) emergence." Zirin, p10~.

Readers may have seen iron fillings scattered on paper below which a magnet is placed, the swirling shapes that result are a good model of simple sunspots like Hale Alpha and Beta classes (above). But big sunspots are more complex. The classes describe the different polarity mixes, and a big spot group will contain a wide range of field strengths as well. It is the task of the 150' Mt Wilson solar telescope to unscramble these variables.

A complete outline of sunspot polarity lies outside this short summary, but readers might refer to Phillips "Guide to the Sun" or Zirin's "Astrophysics of the Sun" for the details. With suitable equipment amateurs can observe sunspots as they grow on the Sun, and by consulting the Mt Wilson website for details of their polarity, you will soon get a feeling of how the spots develop, and learn to anticipate flaring. The Beta-Gamma or Beta-Gamma-Delta groups are the ones to watch. With H-alpha capability amateurs can watch the flares as they erupt.

Fig 1 shows the large group AR 10486 hosting a GOES Class X 1.3 flare that the writer observed in H-alpha, as well as the group's magnetograph from the Mt Wilson site (acknowledge), made about ten hours earlier. Note the magnetograph is reversed left to right since it's a projected image. The boundary between R (red) and V (violet) polarity is indicated with a dotted line. The numbers indicate the field strengths. We can see that the flare has erupted along that line, the magnetic shear boundary, where the fields lie horizontal and in opposite directions.

The H-alpha sketch also shows a dark curved active region filament (ARF, right hand side) that also follows the shear boundary. Amateurs without H-alpha equipment can often detect the magnetic boundary because in white light sunspot umbrae that lie along the boundary may be stretched into long thin shapes, or have their penumbra swept away to become "naked" umbrae.

Watch the shear boundary closely, it's the place where transients will occur without warning.
Enjoy Sun watching.