The Magnetic Storm and Solar Disturbance of June 17, 1915

Abstract

THE greatest magnetic disturbance of the present cycle of sun-spot activity, which commenced in March, 1914, and the most violent since that of September-25, 1909, occurred on June 17, 1915. It commenced G.M.T. 1.50 a.m. with a sudden increase of H.F., and a corresponding sharp, though slight, movement of the declination needle towards the west. The greatest angular range in declination was 91.5â of arc, which occurred at 6 p.m. The spot of light on the recording drum of the H.F. gradually swung downwards with decreasing force, until at 7.35 a.m. it passed beyond the limits of record, and remained off for thirty-seven minutes. It then returned for a moment, when a further sharp decrease took it beyond the limits of record until 11.30 a.m. Then with a succession of oscillations it increased, attaining a maximum of angular displacement of 100 prime; at 4.15 p.m. (1â=0.44—10^â5 C.G.S. units). The total range exceeded 130â. The V.F. also attained its maximum value of increasing force at 4.15 p.m. In all the elements the disturbance was most intense between 4 and 6 p.m., although it did not exhibit any of the very rapid oscillations sometimes characteristic of such movements. A second phase, or repetition of the storm, consisted, as so often happens, of a few isolated well-marked swings in the form of peaks on the photographic records of all the elements. One occurred between 10.30 and 11.25 p.m. on the declination, the magnet swinging east, and the range being 38â. This was preceded at 11.5 p.m. by an increase of H.F. of 31â. A second peak was recorded on this element at 1.45 a.m. on June 18, the force decreasing by 11â. A small decrease of V.F. accompanied this movement of the H.F.