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How Deep Was the Maunder Minimum?

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How Deep Was the Maunder Minimum? Solar Phys DOI 10.1007/s11207-016-0908-z SUNSPOT NUMBER RECALIBRATION How Deep Was the Maunder Minimum? N.V. Zolotova1,2 · D.I. Ponyavin1 Received: 30 September 2015 / Accepted: 13 May 2016 © Springer Science+Business Media Dordrecht 2016 Abstract One of the most enigmatic features of the solar history is the Maunder min- imum (MM). We analyze reports of solar observers from the group-sunspot-number database. Particular attention is given to short notes that resulted in an underestimation of the sunspot activity. These reports by Derham, Flamsteed, Hevelius, Picard, G.D. Cassini, and Fogel are found to address the absence of sunspots of great significance, which could signify a secular minimum with a majority of small short-lived spots. Up to Schwabe’s dis- covery of the solar cycle, sunspots were considered as an irregular phenomenon; sunspot observations were not dedicated to the task of sunspot monitoring and counting. Here, we argue that the level of the solar activity in the past is significantly underestimated. Keywords Sunspots · Sun: activity 1. Introduction In view of the poor sunspot statistics in the seventeenth century in comparison to the modern age, it may be wondered whether the Sun is a constant star with a regular behavior or whether there are grand minima without a Schwabe cycle. Parker (1976) wrote: “...the number of sunspots went through two distinct maxima after 1611, and then fell to a minimum at about 1645. During the 70 years of inactivity there was occasionally a sunspot or two, but long years with none at all; there was no white-light corona visible during total eclipse by the Moon, whereas the corona is usually so conspicuous then; there were only a few significant auroral events. The occurrence of the 70-year minimum (sometimes called the Maunder minimum), indicates that there is available to the Sun a convective mode of Sunspot Number Recalibration Guest Editors: F. Clette, E.W. Cliver, L. Lefèvre, J.M. Vaquero, and L. Svalgaard B N.V. Zolotova [email protected] 1 St. Petersburg State University, 198504 St. Petersburg, Russia 2 Pulkovo Astronomical Observatory, Russian Academy of Sciences, 196140 St. Petersburg, Russia N.V. Zolotova, D.I. Ponyavin circulation different from its present state. The other mode – let us call it the Maunder mode – is such as to be less effective in the generation of magnetic field. Evidently the Sun can flip-flop back and forth between the Maunder mode and the present mode.” In the early 1970s, Parker called Maunder’s articles to Eddy’s attention (see Schröder, 2005,the interview with Eddy). Later, Eddy (1976) concluded that the “70-year period was indeed a time when solar activity all but stopped.” In contrast, a year later, Gleissberg (1977) claimed that “the occurrence of the 11-year cycle was not suspended during the Maunder minimum. However, the occurrence of a particularly low minimum of the 80-year cycle produced a sequence of weakly pronounced 11-year cycles.” Eddy commented that his own estimation (Eddy, 1976) of the magnitude of solar activ- ity in the Maunder minimum (MM) might be underestimated by a factor of two or three. Combining this comment by Eddy with the auroral data (Schove, 1979) and the loss of small spots in observations of the past (Kopecky and Kuklin, 1987), Vitinskij, Kopetskij, and Kuklin (1986, Figure 37) concluded that although the level of solar activity was low, the maximum Wolf number was hardly below 40. Eddy (1976) also stated that telescopes were good enough to resolve even small spots; however, Clerke (1894) briefly mentioned that instruments in the seventeenth century were hopelessly defective. With reference to Clerke, Eddy (1976) claimed that during the MM there was also a marked dearth of aurorae. On the other hand, Schröder (1979, 1992) stated that in almost every year – before and during the Maunder minimum – aurorae have been observed in Middle Europe. He claimed that a regular solar cycle and similar auroral and geomagnetic activity existed during the MM (for further reading see Schröder, 2005). Regarding the length of the MM, Silverman (1993) wrote “. Legrand, Le Goff, and Mazaudier (1990) modify the period of the minimum, stating that the frequency of aurora from 1645 to 1670 was only slightly less than for 1582 – 1648, but that for the interval 1671 – 1710, auroral activity decreased up to 1700, with only eight aurorae reported from 1690 – 1700, compared with twenty-eight for 1680 – 1689, and seventeen for 1670 – 1679. These results are consisted with those presented in my paper. What Legrand et al. have done, essentially, is to place the minimum between 1670 and 1703, rather than between 1645 and 1715...” VaqueroandTrigo(2015), in contrast, recently proposed that the core deep MM spanned from 1645 to 1700, and the wider extended MM for the period 1618 – 1723. They used the regional auroral series for Hungary (Réthly and Berkes, 1963) along with a reconstruction of solar activity based on the verified carbon cycle, 14C production, and an archeomagnetic field model (Usoskin et al., 2014). A lively debate on the relation of a lull in solar activity with cold conditions on Earth was launched. For instance, according to some authors (Landsberg, 1980; Legrand, Le Goff, and Mazaudier, 1990), the hypothesis that the MM was notably cold and different from other intervals in recent climatic history cannot be maintained. Other authors (Soon and Yaskell, 2003) described numerous examples of anomalies in the terrestrial climate throughout the world during the MM. On the other hand, Lockwood et al. (2010) stressed that there is only a regional and seasonal effect of solar activity related to European winters and not a global effect. Analyzing the database of the nominal number of sunspot groups [Rg]byHoytand Schatten (1998), Zolotova and Ponyavin (2015) pointed out that some observers, among whom were Marius, Picard, Siverus, Giovanni Domenico Cassini, Dechales, Maraldi, Ric- cioli (G.D. Cassini’s teacher, who never reported even a single spot from 1618 to 1661) made gaps in reports on a blank solar disk when others reported sunspots. They suggested that these gaps appeared as a result of the planetary theory of sunspots. In the same year, How Deep Was the Maunder Minimum? Usoskin et al. (2015) criticized this suggestion. They found that the Hoyt and Schatten (HS) database for Marius continuously filled in zeros in 1617 – 1618 based on his brief note that he saw fewer spots over the last one and a half years. Thus, the database for Marius is contin- uously filled in with zeros with only gaps when other observers reported sunspots. In other words, continuous zeros with gaps in the Rg-database are an extrapolation of brief notes (like “I did not see any spots several years”) as daily reports on the absence of spots. Most probably, similar periods from 1653 to 1665 for Picard and from 1675 to 1690 for Siverus and others are also extrapolations made by Hoyt and Schatten. Clette et al. (2014) found that solar-meridian observations were included in the Hoyt and Schatten database as nonactive days. However, if no sunspots were mentioned in the meridian passages, it did not necessarily mean that spots were absent. Recently, Gómez and Vaquero (2015) noted that observations by Anton Maria Schyrlaeus Rheita in 1642 were incorrectly interpreted. Instead of eight sunspot groups on 9 – 21 February 1642, he reported one group in June. Using the active-day statistics (Kovaltsov, Usoskin, and Mursula, 2004), Vaquero et al. (2015) found cyclic variability throughout the MM. The low fraction of active days has indicated that the magnitude of the sunspot cycles from 1650 to 1700 was very low. In this article, we reflect on the question “What was happening on the Sun during the Maunder minimum?”. Section 2 deals with the views of the scientific world in the seven- teenth century. Section 3 describes that the absence of the counting process of sunspots has led to a crucial suppression in the Rg-index over the MM. In the following section, we dis- cuss the underestimation of sunspot groups in textual sources. In Section 5 we compare the MM with the Dalton and Gleissberg minima. Section 7 deals with the ambiguity of the solar activity proxies. Section 8 is devoted to conclusions. 2. History With confidence, the seventeenth century can be called the era in which physics and mathe- matics were established (known as the scientific revolution). In the time of Johannes Kepler, Robert Hooke, and Isaac Newton, almost all of astronomy was reduced to astrometry. The scientific world view based on Aristotle’s geocentric cosmology and Ptolemy’s epicycles was the generally accepted doctrine for more than ten centuries. Attribution of maculae to the Sun cast doubt on the idea of the immutability of the heavens, which was a strong point in Aristotle’s canons (Mitchell, 1916). The scientific world view gradually transformed during the seventeenth to the second half of the eighteenth century. When Scheiner offered to show sunspots to his colleague Theodore Busaeus, he answered “I have read my Aristotle from end to end many times and I can assure you that I have never found in it anything similar to what you mention.” (Mitchell, 1916; Soon and Yaskell, 2003). Obviously the mention of a scientific authority like Aristotle was an incontrovertible argument at this time. As soon as spots were discovered, the controversy about the origin of maculae arose. The opponents had detected no parallax for the spots (Scheiner, 1612a; Galilei, 1613;Tarde, 1620) and concluded that the spots were not in the atmosphere of the Earth. In 1607, Johannes Kepler (Joannis Keppleri) observed a sunspot (Keppleri, 1609) and mistakenly thought that the spot was a transit of Mercury.
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