Discontinuous Change in Earth's Spin Rate following Great Solar Storm of August 1972

Abstract

THE question of a link between changes in the Earth's spin rate and the activity of the Sun is of topical interest, and there is good evidence that the changing length of day is influenced by the mean level of solar activity^1,2. The possibility of a one-to-one correlation between specific events on the Sun and specific changes in the length of day has remained more controversial, however, although there was a suggestion of such an effect associated with the great solar storm of 1959 (refs. 3–5) Specifically, Danjon suggested^3–5 that there was an increase in the length of day when the nucleonic component of solar cosmic rays increased ; this was in addition to the usual steady increase in the length of day. Other observers questioned the reality of this effect (for a discussion of the controversy see ref. 6), and because the 1959 solar storm was the greatest recorded since the time of Galileo, there was no immediate hope of an independent test of Danjon's claim. In August 1972, however, an even greater disturbance occurred on the Sun^7–9. It seemed to us that this might provide the ideal opportunity to resolve the controversy, and we have indeed found a discontinuous change in the length of day, and a change in the rate of change of the length of day (a glitch) immediately after that event. Changes in the length of day, and thus in the spin rate of the Earth, are revealed by regular measurements of Universal Time (UT) carried out at many observatories around the world. For our purpose, we are interested in UT2, the version of Universal Time with the effects of the Chandler Wobble and seasonal variations removed. The difference between Atomic Time (AT) and UT2 shows, on average, a monotonic increase as the Earth's spin slows down and the length of day increases.