The Magnetic Storm of August 11–12, 1919

Abstract

THE earlier months of 1919 showed a great deal of magnetic disturbance, but for some two months past conditions have been unusually quiet. On the morning of August 11, at about 7h. G.M.T., there was a “sudden commencement”, followed by the largest magnetic storm experienced for some years at Kew Observatory. Conditions remained highly disturbed until near 10h. on August 12, when, the photographic sheets were changed. The range in declination was 2° 5′, and that in vertical force 935γ. The horizontal-force trace was twice beyond the limits of registration, on each occasion for more than ten minutes; thus the range shown, 840γ, may have been considerably exceeded. Many of the movements were too rapid to be shown clearly in the trace. Rapid oscillations were especially in evidence between 7h. and 10h., and again between 14h. and 18h., on August 11. The declination curve also showed smaller but very rapid oscillations from midnight to 9h. of August 12. The extreme easterly reading, 13° 44′ W., was recorded at about 8h. of August 11, and the extreme westerly reading, 15° 49′ W., at about 16h. 32m.