Cheltenham Magnetic Observatory July to September, 1947

(Latitude 38° 44′.0 N., longitude 76° 50′.5, or 5h 07m.4 W. of Gr.)July 17–20—A moderately severe magnetic storm began with a sudden commencement in all three elements at 17h 49m GMT, July 17, the severest part of the storm occurring within the first eight hours. Initial H‐motion was rapid and of large amplitude. After soon reaching a low value of 18,1777γ at 18h 06m, H then increased 265γ in a rapid although erratic manner until 20h 22m. After about 03h, July 18, H exhibited rapid pulsations of moderate amplitude. After 03h, July 19, long‐period motion of comparatively small amplitude predominated until the end of the storm at 17h July 20. Minimum H of 18,074γ occurred at 14h 36m, July 18. Following the sudden commencement Z decreased until 18h 01m, after which a rapid steady increase set in until 00h 08m, July 18, a change of 288γ being recorded. Z then decreased until 04h, when normal values were resumed. Z showed a rather rapid rise between 16h and 20h, July 18, and was erratic between 01h and 07h, July 19.