Classification of the magnetic character of sunspots according to rules set forth by the Mount Wilson Observatory in California.
alpha (a). A unipolar sunspot group.
beta (B). A sunspot group having both positive and negative magnetic polarities (bipolar), with a simple and distinct division between the polarities.
gamma (G). A complex active region in which the positive and negative polarities are so irregularly distributed as to prevent classification as a bipolar group.
beta-gamma (BG). A sunspot group that is bipolar but which is sufficiently complex that no single, continuous line can be drawn between spots of opposite polarities.
delta (D). A qualifier to magnetic class (see below) indicating that umbrae separated by less than 2 degrees within one penumbra have opposite polarity.
beta-delta (BD). A sunspot group of general beta magnetic classification but containing one (or more) delta spot(s).
beta-gamma-delta (BGD). A sunspot group of beta-gamma magnetic classification but containing one or more delta spot(s).
gamma-delta (GD). A sunspot group of gamma magnetic classification but containing one or more delta spot(s).