A note on auroras during the so-called Maunder-Minimum

By Wilfried Schroeder

Summary: The statistic of the observed auroras during the time of the so-called Maunder-Minimum shows a regular solar cyclus and similar auroral and geomagnetic activity.

Keywords: Maunder-Minimum, auroras, solar cycle, geomagnetic activity

Recently Letfus (2000) presents a review of auroral data during the so-called Maunder-Minimum (1645-1715) which needs some additional modification. His limited knowledge of international literature gives a result which cannot be accepted in detail. Because of his interpretation of auroral data during the so-called Maunder- Minimum some clarifications seems to be useful. First we must remember that the Thirty-year war (1618-1648) ravaged great parts of middle Europe, and destroyed libraries, churches, archives, and other possible sources of scientific records. During these times the people looked to the skies and its fearful portents such as great meteors, halos, or remarkabie auroras. Often these remarkable, unusual events were described in printed colour or black-white pamphlets or leaflets. These must be examined and analyzed for interpretation of auroral statistics in this time. - Also, it must be pointed out that the aurora and sunspots were not subjects of prime interest for natural researchers; they had other fields of scientific interest.

In this connection we must consider the real chance to observe an aurora in middle latitudes (54-48 N). Frorn long-term German data for the years 1882-1966 the annual variation for middle latitude in general are well described, and can be used for comparison for the data of other centuries.

This showed that mostly great auroras were noted with intense colours and variable forms, these similar to the examples which have been observed of the time but were not published in a newspaper, book or leaflet. From the German statistics it is also clear that during low geomagnetic activity (Kp 5) auroras can sometimes appear in these latitudes. Thus, in middle latitudes during solar maxima, up to 15 auroras can be observed but during decreased solar activity only a few.

With this background we can look at auroras in the so-called Maunder minimum. Letfus, discussion was based mainly on the tables published by Krivsky who collected data from different papers to prepare a catalogue mostly without any examination of the vents, their description or sources in original.

A more detailed source analysis has been presented by Schroeder (1984, 2000) who has collected data from 1615-1715. From this a chronology of realistic auroral data can be constructed and examined for possible solar cycle effects (Table 1). The times with maximal and minimal auroral reports are indicated together with years of maxima or minima in solar activity (Table 2). This data are mostly from sporadic observation and usually relate to the more spectular events which people described at that time. On the other hand, it is well-known that we can conclude that the sun was active in years of maximal auroral data and showed lower activity during other years (those of minimal or zero auroras). The great auroras are thus indicators of the active sun during the time of the so-called Maunder-minimum. From the available auroral data we can conclude a) large auroras were frequently observed and described in the period between 1645-1715, and b) these auroras can be understood as indicators of an active sun and related geomagnetic disturbances. This must be emphasised more clearly than Letfus has done, and these carefully checked auroral data confirm the existence of a regular solar cycle during the so-called Maunder minimum.

References

Attolini, M., R. S. Cecchini, M. Galli and T. Nanni, Solar activity variations in historical aurorae records and tree radiocarbons. In: Schroeder, W. (Ed.) Advances in Geosciences. Bremen, 28-35.
Letfus, V. (2000), Sunspot and auroral activity during Maunder minimum. Solar Phys. 197, 203-213.
Schove, D. J. (1983), Sunspot Cycles. Stroudburg and New York, Ross and Comp.
Schroeder, W. (1997), Catalogue of auroras. Bremen, Science Edition.
Schroeder, W. (2000), Changes in the interpretation of aurora. Bremen, Science Edition, 100 p.
Waldmeier, M. (1955), Ergebnisse und Probleme der Sonnenforschung. Leipzig, Akad. Verlagsgesellschaft.
Wittmann, A. (1978a), Sterne und Weltraum 12, 412.
Wittmann, A. (1978b), The sunspot cycle before the Maunder Minimum. Astron. Astrophys. 66, 93-97.  

 ^a) see SCHOVE (1979); ^b) VVITTMANN and XU's catalogue;
Maxima given with p: 11.116 years by WITTMANN/XU.
Aurora Maxima determined with 5 auroras and more.