Geomagnetic activity and the solar wind during the Maunder Minimum

Abstract

We used a strong (r = 0.96) correlation between 11-year averages of sunspot number (SSN) and the geomagnetic aa index to infer that the mean level of geomagnetic activity during the Maunder Minimum (16.45-1715) was aproximately a third of that observed for recent solar cycles (~7 nT vs. ~24 nT). We determined the variation of 11-year averages of solar wind speed (v) and the southward component of the interplanetary magnetic field (Bs) with cycle-averaged SSN for the two most recent cycles and also compared cycle-averaged variations of v2Bs and aa for the same interval. We then extrapolated these observed solar wind variations to Maunder Minimum conditions (mean SSN of ~2 and mean aa value of ~7 nT) to deduce that, on average, the solar wind during that period was somewhat slower (v = 340 ± 50 km s-1), and the interplanetary magnetic field much smoother (Bs = 0.3 ± 0.1 nT), than at present (~440 km s-1 and ~1.2 nT). Various lines of evidence (including 10Be data) suggest that, despite the virtual absence of sunspots that characterized the Maunder Minimum, the 11-year geomagnetic (solar wind) cycle persisted throughout this period.