A solar cycle clock for extreme space weather

Abstract

Abstract The variable solar cycle of activity is a long-standing problem in physics. It modulates the overall level of space weather activity at earth, which in turn can have significant societal impact. The Hilbert transform of the sunspot number is used to map the variable length, approximately 11 year Schwabe cycle onto a uniform clock. The clock is used to correlate extreme space weather seen in the aa index, the longest continuous geomagnetic record at earth, with the record of solar active region areas and latitudes since 1874. This shows that a clear switch-off of the mostextreme space weather events occurs when the solar active regions move to within 15 degrees of the solar equator, from regions of high gradient in solar differential rotation which can power coronal mass ejections, to a region where solar differential rotation is almost constant with latitude. This overlaps with the onset of more moderate space weather events which coincide with 27 day solar rotation recurrences in the aa index, consistent with stable, persistent source regions.This offers a physical explanation for the longstanding identification of a two component cycle of activity in the aa index.