The magnetic storm of December 16-17, 1917, as recorded at Kew and Eskdalemuir observatories

Abstract

1. In a recent paper, I described a magnetic storm occurring on August 22, 1916, in which the disturbances recorded at Kew and Eskdalemuir Observatories were closely similar in type. The magnetic storm on December 16-17, 1917, affords in many ways a remarkable contrast, the changes recorded at the two observatories differing notably in type. The disturbance was the largest of the year. In the Kew declination (D) and horizontal force (H) curves, a prominent feature was a succession of oscillations with periods of about 20 minutes, and it was the hope of identifying these with corresponding oscillations at Eskdalemuir that suggested the present investigation. Dr. Crichton Mitchell, the Superintendent of Eskdalemuir Observatory, kindly sent the original curves, so the comparison was made under favourable conditions. One of the obstacles to the identification of corresponding movements at the two stations is that, while H and D are recorded at Kew, the north (N) and west (W) components are recorded at Eskdalemuir. On instituting a minute comparison between the Kew H and D curves, it was found that, while oscillations with periods not far from 20 minutes were prominent in both, the times of the turning points by no means always agreed. A turning point in the H curve, for instance, might be represented by a short arrest or slight temporary reversal of movement in the D curve, the original direction of the D movement being almost immediately resumed. Again, towards the end of an oscillation, the trace might be nearly level for some minutes, or there might be a small short period oscillation, so that more than one choice was possible when assigning a time for the end of one major oscillation or the beginning of the next. Thus, when one had to compare Kew H with Eskdalemuir N curves, and Kew D with Eskdalemuir W curves, the fact that corresponding movements were difficult to identify is not surprising. Both observatories record vertical force (V), but the Kew V curves are so much disturbed by artificial electrical currents that they cannot be relied on for minor details. There were also difficulties special to the particular storm. Some of the N and W movements at Eskdalemuir were so rapid that the photographic trace was in places almost invisible. While the time marks occur on the base lines as well as on the curve lines, there is sensible parallax, and when the marks on the curve lines become invisible, the time cannot be fixed with quite the usual accuracy.