THE GREAT MAGNETIC STORM OF MARCH 24, 1940

Abstract

The greatest magnetic storm of recent years occurred on Sunday, March 24, 1940. For a few hours it completely interrupted all long-distance communication, war reports and Easter greetings alike. Had it occurred on a business day the financial loss would have been even greater with garbled stock reports adding to the confusion. A magnetic storm interrupts communication by inducing, in long telegraph and cable circuits, electric currents strong enough to change regular messages beyond recognition. Radio communication suffered in much the same way. The static was very bad and the announcers talked a language no one could understand. Only far-distant stations were affected however. Local broadcasts, which are not reflected from the ionosphere, were just as distinct as ever. The earth's magnetic field at Mount Wilson was first disturbed at 10:17 p.m. on March 23 (6h 17m G.C.T.), but the storm did not begin in earnest until March 24 at 5 : 49 a.m. (13h49m G.C.T.). The most disturbed period was between 8 : 00 and 11 : 00 a.m. (15h 40ra to 18h 50m G.C.T.). The variations were so large and so rapid that the spot of light from the magnetometer sometimes moved too fast to register and occasionally was off the record. The exact range in the horizontal intensity is therefore uncertain, but at 8 : 00 a.m. it dropped in about twenty minutes from at least 200 gammas above normal to 600 below. This was a decrease of more than 3 per cent of the normal horizontal intensity on Mount Wilson, which is about 26,000 gammas (0.26 C.G.S. units). An aurora of moderate intensity was observed by Anthony Wausnock at the Mount Wilson Observatory on the night of March 24 just before moonrise (March 25, 3h SO111 G.C.T.). The aurora was not visible in the light of the moon which was one day past full phase. During March 25 the horizontal intensity was from 200 to 120 gammas below normal. On March 26 it began to recover