AURORA POLARIS

Wanganui Chronicle, Volume 80, Issue 92, 20 April 1938, Page 6

AURORA POLARIS

r J _ 'HE delightful spectacle which was visible from Wanganui in the southern portion of the sky on Saturday night, is a natural phenomenon concerning which man knows very little indeed. The lighting effect appears in both northern and southern hemispheres, being known as Aurora Borealis and Aurora Australis respectively. In consequence of civilisation having developed in the northern hemisphere, Aurora Borealis has been subjected to much more study than has Aurora Australis. At one time laymen thought that the aurora was related to th. polar icefields; and that the rays which moved upward through the sky were rays of light reflected from the icefields. The explanation, however, was inadequate because it gave no cause for the fanshape of the lights nor their movement sweeping across the heavens. From observations made over a period of two centuries it has been found that numbers of auroras have been found to follow the number of sunspots, and these have a maximum approximately every eleven years. The series of observations revealed also that more auroras appear in March and September, when the earth is more directly opposite the spot zones in the sun, than in June and December, when such is not the ease. The report that the Aurora Australis was accompanied by Aurora Borealis, and also that a new and extensive sunspot had been observed, should occasion no surprise, nor should the subsequent inclement weather which marred Easter be regarded as other than in the natural order of events, for it is found that whenever a brilliant aurora appears there is almost sure to be a magnetic storm.

What causes the aurora is still a mystery: it appears to be a by-product of these tremendous upheavals in the sun which are revealed by the appearance of sunspots. For the nineteen great magnetic storms between 1875 and 1903 there was an average delay of twenty-five hours between the ) assage of the spots over the central line (meridian) of the. sun and the storm on the earth. From this it is inferred that the action, whatever its nature, was not propagated with the speed of light, which takes roughly eight and a-half minutes to travel from the sun to the earth. Two theories have been advanced to explain aurora: one, that negative particles arc shot off from the sun and are caught in the earth’s magnetic field; and two, that particles with a plus charge come to the earth from radio-active substances in the sun, but it has been pointed out that, even were these particles Io move with the velocity of light they could not penetrate the earth’s atmosphere to within a distance of sixteen miles of the earth’s surface. The second theory is therefore, as yet. not tenable.