Fig. 30.—Photograph of the solar corona of 1900. (After Langley and Abbot.) Illustrating the appearance of the corona in years of minimum sun-spot frequency
The light coming from the deeper portions is distinctly absorbed to a large degree by the higher-lying strata. The sun-spots also appear to become narrower in their lower parts, owing to the compression of the gases at greater depths, and one may regard their funnel-shaped cloud-walls as "half-shadows," which appear darker than the surroundings, but brighter than the so-called core of the spot. The weakening of the violet end of the spectrum is probably due to the presence of fine particles of dust in the solar gases, just as they cause the corresponding weakening of the violet end of the spectrum of the sun’s limb. The bands in the red parts of the sun-spot spectrum may originate from the deeper portions of the spot, because all the higher parts of the solar atmosphere yield simple, sharp lines. The bands suggest that chemical compounds can exist at the higher pressure of the inner portions of the sun, and that these compounds are decomposed in the outer parts of the sun, to give the line spectra of chemical elements.
Fig. 31.—Photograph of the solar corona of 1870. (After Davis.) The year 1870 was one of maximum sun-spot frequency
The enigmatical corona lies farther out in the atmosphere of the sun. It consists of streamers which may extend beyond the disk of the sun to the length of several solar diameters. The corona can only be observed at total eclipses of the sun. Figs. 30 to 32 illustrate the appearance of this very peculiar phenomenon.
Fig. 32.—Photograph of the solar corona of 1898. (After Maunder.) 1898 was a year of average solar activity
When the number of sun-spots is small, the corona streamers extend like huge brooms from the equatorial parts, and the feebler rays of the corona near the solar poles are then bent downward to the equator, just like the lines of force about the poles of a magnet (Fig. 30).
We suppose, for this reason, that the sun acts like a strong magnet, whose poles are situated near the geographical poles of the sun. In years which are richer in sun-spots the distribution of the streamers of the corona is more uniform. At moderate sun-spot frequency, large numbers of rays seem to emanate from the neighborhood of the maximum belt of sun-spots, so that the corona often assumes a quadrangular shape (compare Fig. 32).