A Geomagnetic Precursor Technique for Predicting the Solar Activity Cycle

Abstract:

The Western hemisphere has been recording sunspot numbers since Galileo discovered sunspots in the early 17^th century, and the roughly 11-year solar cycle has been recognized since the 19^th century. However, predicting the strength of any particular cycle remains a relatively imprecise task. This project’s aim was to update and improve a forecasting technique based on geomagnetic precursors of future solar activity The model is a refinement of R. J. Thompson’s 1993 paper that relates the number of geomagnetically disturbed days, as defined by the aa and Ap indices, to the sum of the sunspot number in the current and the previous cycle, R_n + R_n-1.[1] The method exploits the fact that two cycles coexist for some period on the Sun near solar minimum and therefore that the number of sunspots and disturbed days during the declining phase of one cycle gives an indication of the following cycle’s strength. We wrote and updated IDL software procedures to define disturbed days with varying threshold values and graphed R_n + R_n-1 against them. The aa threshold was derived from the Ap threshold. After comparing the graphs for Ap values from 20 to 50, an Ap threshold of 30 and the corresponding aa threshold of 44 were chosen as yielding the best correlation. Confidence regions were computed to provide a quantitative uncertainty on future predictions. The 80% confidence region gives a range of ±40 in sunspot number.

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[1] Thompson, R. J. (1993). A technique for predicting the amplitude of the solar cycle. Solar Physics, 148, 2, 383-388.

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