Solar cycle 15 was the fifteenth solar cycle since 1755, when extensive recording of solar sunspot activity began.[1][2] The solar cycle lasted 10 years, beginning in August 1913 and ending in August 1923. The maximum smoothed sunspot number (monthly number of sunspots averaged over a twelve-month period) observed during the solar cycle was 105.4 (August 1917), and the minimum was 5.6.[3] There were a total of 534 days with no sunspots during this cycle.[4][5][6] A major geomagnetic storm during 13–15 May 1921 caused damage in communication systems and aurora displays in much of the eastern United States. [7]
Geomagnetic storms in March 1918, August 1919, October 1919, and March 1920 affected telegraph lines, while a solar flare on 13 May 1921 also affected rail signal and switching equipment, in what was known as the "New York Railroad Storm."[8]
