The Magnetic Storm of June 17, and Aurora

Abstract

PROF. BARNARD'S interesting letter dated June 25, in NATURE of July 15, on what is termed “The Great Aurora of June 16, 1915,” is at first sight rather puzzling to the non-astronomical reader. The large magnetic storm began about 1.50 a.m. on June 17. On June 16, it is true, there was a magnetic disturbance, but not such as to suggest a striking auroral display. The explanation presumably is that Prof. Barnard is referring to an astronomical day, commencing at Greenwich noon on June 16. This, at least, would explain his statement that at Wisconsin(about 90° W.) at 21h. 25m. “the sky was bright with dawn.” This one would expect between 3 and 4 a.m. local time. If this is correct, then the first auroral appearance chronicled by Prof. Barnard was at 3.30 a.m. on June 17, Greenwich civil time, and the maximum brilliancy about 8.15 a.m. It was principally during these morning hours that the Kew magnetic curves had the rapid oscillatory character usually associated with aurora and earth currents. The newspaper reports quoted by Prof. Barnard seem to fit this explanation.