Unreleased sunspot records by Hacki in 1671 during the Maunder Minimum

The Maunder Minimum (1645–1715) was a period characterized by an abnormally low number of sunspots observed on the solar disc. This work focuses on solar activity in 1671, analyzing sunspot number data from a new historical documentary source: five letters exchanged between Hacki and Hevelius, which have been translated from Latin to English. These new records provide an independent confirmation of the sunspot groups observed in 1671, complementing previous works based on observations by other astronomers. Moreover, they highlight some corrections for future versions of the group number database. On the other hand, by compiling all sunspot data for 1671 published so far, we improve our knowledge of the solar activity level in that year. We obtained that the percentage of active days (days when sunspots were observed on the solar disc) was 21.3 %, although with a high uncertainty ranging between 6.3 % and 76.7 % considering days without any record as spotless or active days, respectively. Despite being one of the more active years in the Maunder Minimum, we conclude that the solar activity level in 1671 remains low in comparison to other periods of reduced solar activity, such as the Dalton Minimum. Thus, our study also impacts on the reconstruction of the solar cycle during the Maunder Minimum.