THE SOLAR CORONA

Te Awamutu Courier, Volume 59, Issue 4221, 8 December 1939, Page 6

THE SOLAR CORONA

IMPROVEMENTS IN PHOTOGRAPHY. “The statement made in a recent cable message from New York that the Bell Telephone Co. announces the invention of a camera using television principles enabling a study to be made of the corona of the sun on any clear day, and that “the hope is held out that these studies will enable predictions to be made about transmission conditions” may perhaps puzzle some who read it by reason of its seeming incongruities. It is almost as if we were told that the Australian Wool Growers’ Association had devised a sewing machine based on the pricipke of relativity for the purpose of ascertaining the influence of the Martian canals upon the potato crop. Almost, but not quite. For, in the first place, the terms in the cabled statement form a chain whose last link connects back to the first—radio communication in all its blanches Ibeing one of the most important departments in the Bell Telephone Co’s vast industrial and technical organisation; and in the second each term is linked with its neighbours in a way which it will be the purpose of this brief article to partially to elucidate.

It is common knowledge to-day that long-distance wireless communication, whether by telegraph or telephone, is subject to large and sometimes vexations fludtuations of effici-

ency. The reason for these fluctuations is known to reside in changes that take place in layers of the upper atmosphene extending upwards from a heighjt of about 100 miles. The highly rarefied air of these regions absorbs strongly the short wave or ultra-violet constituents of the light received from the sun, and. in doing so, is converted into a, condition in which it becomes electrically conducting. AN ELECTRICAL BOUNDARY. It is thus made capable of acting as a sort of electrical boundary between our planet and outer space, confining to some extent at least the trains of electrical waves sent out by ( wireless transmitting stations within an atmospheric shell and returning them to the earth’s surface sonnytimes ten thousand miles from their starting point. Now, the ultra-violet light, which alone is competent to produce conducting state in air, seems to be much more variable in quantity and ra’diant heat and light which penetrates the atmosphere and reaches ground level. The surface of the inner body of the sun. which emits this radiation, must be likened not to that of a white-hot solid ball, but rather to a tumultuous ocean of incandescent gas and vapor siibject to cyclonic storms vast enough to engulf our whole planet in their gigantic vortices, and to explosive eruptions of flaming gases of tremendous violence. In recent years it has been found that these eruptions are the cause of remarkable terrestrial effects, such as violent magnetic storms, brilliant

auroral displays, add, in line -with our present topic, serious interference with long-range radio transmission. Now, these gaseous outbursts, terrific as they are—.their fiery flames sometimes mount mallions of miles above the surface of the sun—cannot ordinarily be seen with the naked eye, nor even with a telescope .They can, however, be Observed as bloodred “prominences” protruding from the edge of the sun’s disc during those rare and brief intervals when it is hidden from view in total eclipse by the moon.

Only then, too, is to be seen that remarkable hajo of pearly-white plumes and streamers of light, called the corona, the precise nature and mode of origin of which is still not quite settled.

Since the invention by the American astronomer Hale, in 1893, of the spdtetro-helioseope, an instrument which sorts out (the characteristic light emitted by the glowing gas (mostly hydrogen) of the prominences from the general stream of radiation, these prominences ean naw be seen or photographed by this light whenever the sun itself is visible; but not, in spite of many attempts, until 1931 was the first partial success attained in the photography of the corona under non-eclipse conditions. SCATTERED SUNUGHT. The difficulty here lies not so much in the overwhelming brightness of the solar disc or photosphere around which, the corona' extends as a faintly luminous fringe—-for the image of this central disc formed by a lens could easily be blocked out but in the scattering which sunlight under-

goes in passing through our imperfectly clear atmosphere, a scattering which renders the sky around the sun far brighter than the corona. Also, the glass of ordinary lenses scatters enough light to swamp the coronal image.

These obstacles were finally overcome by the persistent genius of M. Bernard Lyot, of the Paris Observatory, the first by choosing for the sight of his “coronagraph” a station

near the top of the Pic du Midi in the Pyrenees at a height of nearly 9000 feet, the second by making the lenses of the telescope of a specially clear glass, and by so designing the instrument as to block out the image of the sun itself from the photographic plate while avoiding all internal reflections. . Not only do phonographs taken with the Lyot coronagraph ghow clearly the shape and structure of the inner corona, both of which continuously change, following the sunspot cycle of 22 years, but the red flames of the eruptive prominence responsible for our beam Wireless troubles are also clearly shown. Those concerned in the . development and use of the new televisioncoronagraph have presumably followed the lead of Lyot in choosing a site for it in the clear air of some high mountain peak, and j.lso in constructing the instrument in such a way as to minimise internal scattering of light. In the .absence of all details regarding the construction and tech-

nique of operating the camera we can do little more than speculate upon the nature of these and their possible advantages over the more conventional (type of instrument.

It may be that the point-by-point method of reproduction involved in the “scanning” process of television may permit of the elimination of the uniformly bright background due to atmospheric haze leaving only the fainter pattern of the coronal streamers and arches and the brighter prominences. A

If it is so, the new method will be incontestably superior to the old. Whether or not the expectation of valuable practical results is realised, this new astronomical achievement affords another interesting illustration of the facility with which advances made in one. branch of science can be turned to account in overcoming obstacles to progress in another.