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Silverman 1986
Silverman, S.M. (1986). Magnetic activity and solar wind velocity at and preceding solar minimum for the past 500 years. Journal of Geophysical Research 91: doi: 10.1029/JA091iA09p10157. issn: 0148-0227.

The magnetic activity and solar wind speed at and preceding sunspot minimum has been estimated for the past 5 centuries. Empirical equations relating aa, solar wind speed, auroras, or the southward component of the interplanetary field to sunspot numbers at solar activity maxima and/or minima were used to derive aa and solar wind speed estimates. Observed and estimated sunspot numbers for the period 1500--1976 were used in the calculations. The estimated magnetic activity was much more sensitive than the solar wind speeds to the prolonged solar activity minima around the beginnings of the eighteenth and nineteenth centuries. The aa varied from about 3--5 during these solar activity minima to 27 for the period around 1954. The solar wind speed, on the other hand, varied only between 300 and 600 km/s over the 5-century period. The lower speed was appoximately the same for the two prolonged solar activity minima at the beginnings of the eighteenth and nineteenth centuries. This result implies that the coronal holes continued to send out strong solar wind streams even during periods of extended low sunspot production.

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Journal of Geophysical Research
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