On the use of naked-eye sunspot observations during the Maunder Minimum

TL;DR

This study analyzes naked-eye sunspot observations from the 19th century, showing they do not necessarily indicate high solar activity and support the view of the Maunder Minimum as a period of very low solar activity.

Contribution

It provides a detailed analysis of NESO data, challenging recent interpretations and reinforcing the low activity status of the Maunder Minimum.

Findings

Most NESO days had fewer than six sunspot groups.

Large sunspot groups were generally small, below 200 millionths of solar disc.

NESO records support the Maunder Minimum as a period of very low solar activity.

Abstract

Naked-eye sunspot observations (NESO, hereafter) have been recorded for last two millennia, approximately. This kind of records were made around the world, mainly in Asian civilizations, and they are compiled in several catalogues. In this work, we analyze solar activity in days of the 19th century when NESO were recorded. We found that only more than five sunspot groups were recorded in 39 % of days corresponding to these NESO events. Furthermore, regarding the largest groups observed in days when NESO were reported, we show the uncorrected areas of these groups were below 200 millionths of solar disc (msd hereafter) in 3.2 % of total cases while it is 12.9 % for areas between 200 and 499 msd. Thus, NESO records do not imply high solar activity and big sunspot groups necessarily. Therefore, these results contradict the interpretations of recent works that, using the same NESO set, suggest the solar activity level during the Maunder Minimum is still an open question. NESO records support the Maunder Minimum as a very low solar activity period.