On the Radial Appearance of the Corona

Abstract

WOULD an indefinitely extending solar atmosphere, if its existence could be proved, be in itself sufficient to explain the appearance of the solar corona? Should we not still have to explain the apparent radiation which is so distinctly part of the phenomenon?—If the light or heat of the sun which radiates symmetrically outwards as from a point at its centre be the cause of the illumination, surely the figure of the corona would bear some relation to the figure of the atmosphere or medium on which the light or heat acts? Yet I think I may say that it is quite impossible to conceive a medium so distributed and arranged as to form rays such as those seen in the corona. If the recent photographs had not shown beyond a doubt that this irregular radiating appearance belongs to the corona and the neighbourhood of the sun,^* it would have gone a long way to prove that the corona is at least partly due to the earth's atmosphere or mere optical effect. But, as it is, I think this radiation clearly proves that the corona cannot be due to the direct aci ion of the light and heat of the sun on any surrounding matter. In fact, I cannot conceive an atmosphere the character of which varies in a radial manner, however rapidly either its nature or density may vary with the distance from the surface of the sun. If, instead of an atmosphere, we try to conceive a ring of meteors, still the radial gaps so clearly marked on the photographs present insurmountable difficulty. This, moreover, is impossible on other grounds. It is impossible that there can be an almost homogeneous mass of meteoric matter circulating round the sun in the form of an outer sphere, and if it circulated in the ecliptic or any other plane, it would present the appearance of Saturn's belt, whereas the corona appears altogether different from this, and cannot possibly be a film of light in any plane but that of the sun's limb.