SUN A STORM-CENTRE

Dominion, Volume 31, Issue 154, 26 March 1938, Page 3 (Supplement)

SUN A STORM-CENTRE

Solar Outbursts And Radio Fade-Outs AURORAL DISPLAYS The recent brilliant auroral display, so brilliant that many people were afraid, visible even from so far south as Gibraltar, occurred soon after the unusually large naked-eye sunspot visible during the previous week had disappeared over the edge of the sun’s disc, wrote Professor Sydney Chapman, F.R.S., in the “Manchester Guardian” recently. It is remarkable that at the time of the great auroral display no unusual sunspot was visible. Such briilliant aurorae are almost always accompanied by great magnetic storms, and this one was no exception. How the sun produces the aurora and the storm still remains largely mysterious. During the past year, however, a notable advance has been made in our knowledge of a less extreme solar influence upon the atmosphere and the magnetism of the earth. This influence is of considerable practical importance as well as being of great scientific interest, because one of its effects is to cause a complete fading-out of highfrequency radio transmissions, for a period ranging from a few minutes to an hour or more. These radio fadeouts, as they are termed, are a matter of serious concern to communication companies and engineers; the intensity of the signals received from distant transmitting stations may suddenly drop so low that the signals become unreadable, or completely disappear. The receiver seems to “go dead,” so that the observer may suppose either that the station from which he is receiving has ceased to transmit, or that his own station has lost its power supply, perhaps by a fuse blowing or that his receiving apparatus has developed a sudden fault. Many a receiving set has been dissected on such occasions in a vain attempt to discover the trouble.

Astronomers’ Solution Such occurrences have forced themselves' on our attention only in recent years, with the increasing development of high-frequency transmission. In October, 1935, Dr. J. H. Dellinger, of the United States Bureau of Standards, reported on fade-outs in March, May, July, and August of that year. He had already discovered that they affect only the sunlit hemisphere of the earth, so that transmissions across the night hemisphere are undisturbed. He suggested that the fade-outs are due to some solar emanation lasting only for a few minutes, and asked that astronomers Should look out for brief solar outbursts, and that workers in various branches of earth-science should ascertain whether or not they were accompanied by disturbances of the magnetic field and of earth currents.

It was soon verified by astronomers, at Mount Wilson, California, and at Greenwich, that bright eruptions on some small area of the sun are frequently observable at the time of the radio fade-outs. During, 1935 and 1936 118 fade-outs were recorded, occurring simultaneously in various parts of the earth; half of these are known to have coincided with brief solar eruptions, although the sun is not under continuous detailed observation, and many eruptions must have escaped notice. Hence the fade-outs can confidently be ascribed to solar eruptions simultaneously observable.

The eruptions are usually either photographed or visually observed by a special instrument called a spectrohelioscope; in either case a spectroscope is interposed so as to spread out the light from the sun in a spectrum or rainbow band, and light of a particular colour is photographed or observed. The increase of brightness of the eruptive area on the sun is not the same over the whole range of colour; it is mainly concentrated in a few particular colours, or “bright lines,” and notably in the ultra-violet region. The light that causes the radio fade-out is not itself observable directly, being all absorbed in the upper atmosphere. It is found that eruptions on any part of the sun’s disc can affect the earth. The cause of the eruptions'is unknown.

The lonised Layers

The ultra-violet light, thus emitted, falls on the sunlit hemisphere and is absorbed in the atmosphere, at the same time ionising the air—that is, •breaking up many neutral molecules into parts carrying opposite electric charges. Modern methods of radio research have been applied to find how the ionised regions—or the ionosphere —are affected by the eruptions. It is known that at ordinary times there are two or more ionised layers, which help radio communication by diverting outward - travelling radio waves back again to the earth, with but little loss by absorption. During the radio fade-outs it is found that a new ionised layer is formed at a level of about 50 miles height, below the ordinary ionised layers; the new layer is at a level where the density is high enough for high-frequency radio waves to be much absorbed, so that radio waves entering it are lost there and cannot reach the upper layers which would return them to the earth. The relatively low level of the new ionised layer shows that the abnormal ultra-violet light emitted during the solar eruptions is more penetrating than ffiat which produces the ordinary ionised layers. When the solar eruption is over the new ionised layer quickly “decays” through the reunion of . the positively and negatively charged particles in it, to form neutral molecules again; the layer then ceases to influence radio waves and the fade-out is over. The ordinary ionised layers appear to remain unaffected during the solar eruption. Radio transmission engineers would like to know beforehand when fadeouts will occur, so that they may provide beforehand against the interruption of radio "traffic” by changing the wave-length used, or possibly in some cases by diverting the waves to other routes across the dark hemisphere. Unfortunately, despite the rapid and great advance in our knowledge about the group of phenomena associated with the solar eruptions, it is as yet entirely beyond the powers of solar physicists to predict the eruptions. The fade-outs supply, however, a new stimulus, of an economic kind, toward the Intensification of solar research.