Sitka Magnetic Observatory July to September, 1947

(Latitude 57° 03′.0 N., longitude 135° 20′.1 or 9h 01.m3 W. of Gr.)July 17–23—Climaxing a short period of very quiet magnetic conditions a fairly severe storm began abruptly at 17h 50m GMT, July 17. During the first fourteen hours activity was limited to short‐period, low‐amplitude oscillations; however, at about 08h, July 18, this activity was replaced by rather deep bays of short duration. Between 10h and 13h a very deep bay occurred in H, which gave a K‐index of 8; however, the most violent activity of the storm took place during the fourteenth hour of July 18 when there was recorded short‐period, large‐amplitude oscillations producing a K‐index of 8. The intensity of the storm began to diminish at about 18h, July 18, but moderately disturbed conditions continued until about 18h, July 20. During the following three days only slight disturbances occurred but at about 06h, July 23, activity began to increase. For the period 09h‐12h, July 23, a K‐index of 7 was recorded. The storm may be said to have ended at 12h, July 23, but lesser disturbances were recorded for several days following.