Geomagnetic activity and the solar wind during the Maunder Minimum

We used a strong (r = 0.96) correlation between Ii-year averages of sunspot number (SSN) and the geomagnetic aa index to infer that the mean level of geomagnetic activity during the Maunder Minimum (1645-1715) was approximately a third of that observed for recent solar cycles (similar to 7 nT vs. similar to 24 nT). This result is consistent with reported radiocarbon measurements. We determined the variation of Ii-year averages of solar wind speed (v) and the southward component of the interplanetary magnetic field (B-s) with cycle-averaged SSN for the two most recent cycles and also compared cycle-averaged variations of v(2)B(s)) and aa for the same interval. We then extrapolated these observed solar wind variations to Maunder Minimum conditions (mean SSN of similar to 2 and mean aa value of similar to 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 (B-s = 0.3 +/- 0.1 nT),;than at present (440 km s(-1) and 1.2 nT). Various lines of evidence (including Be-10 data) suggest that, despite the virtual absence of sunspots that characterized the Maunder Minimum, the Ii-year geomagnetic (solar wind) cycle persisted throughout this period. During the last half of the 17th Century, it appears that yearly aa values varied between NO nT at minima of Ii-year cycles and similar to 10-15 nT at maxima.