The most plainly apparent of solar variations is the cycle of the sun spots. Sun spots appear through the telescope, or through good binoculars when large, as dark spots on the photosphere. Most of them are roughly circular in shape, and they tend to occur in groups. The center of a spot (called the umbra) is darker than the outer ring (penumbra) (Kedzie 17). A sun spot usually begins as a small black point which grows into the umbra; then the penumbra fills out. The spot appears to travel slowly from left to right on the sun’s disk, of course, with the solar rotation.
After a few days, or more likely after a few weeks, the spot slowly decreases in size and finally disappears. As the temperature inside a sun spot is about 4000° F. (about 2000° C. ) below that of the surrounding atmosphere, it may be called the greatest refrigerator known to man! (MacRobert 4) Though smaller spots are only a few hundred miles in diameter, larger spots or groups may be fifty thousand miles wide – holes into which the whole earth could be dropped like a golf ball into a hat.
These larger spots are visible to the unaided (but protected) eye. Naked-eye sun spots were noticed many centuries ago by the Chinese, who regarded them with vague fear. The first man to examine sun spots in some detail through a telescope was questionably established the eleven-odd year cycle in the increase and decrease of sun spots. This cycle stands today as one of the basic facts in solar variation (Kedzie 20). With each new cycle, sun spots begin to break out about 36° north and south of the sun’s equator.
In a few years, with maximum activity, most of the spots are much nearer the equator; and the old cycle finally dies out between 5° and 10° north and south as the next cycle is getting under way in higher latitudes (Kedzie 21). The sun bursts into activity with comparative suddenness – in about four years; and simmers down more slowly through about seven years. The last sun-spot maximum was in 2001; the next is scheduled for about 2011 -2012 (Bond 3). In addition to the eleven-year periodicity of sun spots, overtones of from six to fifteen months appear to be superimposed on the larger and slower fundamental.
Also, of course, there is a twenty-seven-day period imposed by the sun’s rotation. In the year 1908, George Ellery Hale made in California a significant discovery that revealed the true nature of sun spots. Ordinary visual observations, it is true, had occasionally shown a vortical or whirling motion of gases in the neighborhood of the spots. When photographic plates sensitive to red light became available, Hale photographed the sun in the light of one spectral color only. He was enabled to exclude all other colors from his photographs by means of an ingenious instrument known as the spectroheliograph.
As the single color he chose was emitted by a single gas, hydrogen, he secured in effect a picture of the sun’s surface at one particular level – the level where hydrogen gas predominated. The photographs plainly showed gigantic whirling currents about the sun spots, similar in principle to earthly cyclones. And like earthly cyclones, the solar vortices whirl in opposite directions on opposite hemispheres (Hoyt and Schatten 43-44). Hale showed further, by intricate light measurements, that a sun spot is a great electro-magnet having an intense magnetic field, which is probably induced by the free electrons and ions in its rotating gases.
A sun-spot magnetic field probably extends outward in a more or less directed way like the field of a bar magnet; and interferes, locally, with the main magnetic field of the sun (which in itself is much like the earth’s field that guides all our compasses, but about one hundred times as strong). Hale found also that the sun spots change magnetic polarity from one cycle to another, so that the complete magnetic cycle of the sun averages twice eleven-odd years, or twentytwo to twenty-three years (Hoyt and Schatten 43-44).