Challenges of Using Historical Aurora Observations for the Reconstruction of Solar Activity before the 19th Century, Especially during and near the Maunder Minimum

Abstract

In order to complement gaps in the surveillance of solar activity in historical times, various proxies are used to reconstruct past solar cycles and long-term maxima and minima of solar activity, the most famous being the Maunder Minimum (MM), which is usually defined to span the period between the years 1645 and 1715. We explain the problems within existing data bases and call upon trying to find the original sources of Schröder, since his aurorae catalog spans the whole MM and contradicts what has been deduced from more used compilations. We take a critical look at the proposed source-critical scheme introduced by Neuhäuser and Neuhäuser and show it to be counterproductive because it largely ignores the source situation, i.e., the scientific understanding of the reporters of times long past and their intentions. While historical sunspot and aurora reports can be useful to fine-tune our knowledge of solar activity in times before the onset of systematical surveillances, they should not be used as an index of solar activity, since they cannot be quantitatively expressed due to the non-scientific manner of the reports and ambiguous wording. Reconstructions based on cosmogenic isotopes are significantly preferable for establishing the level of solar activity in the past. The conclusions reached by this review should be regarded as a caution against expecting important conclusions to emerge from low quality data.