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Ann. Geophys., 23, 945-971, 2005
www.ann-geophys.net/23/945/2005/
© European Geosciences Union 2005
Identification of possible intense historical geomagnetic storms using combined sunspot and auroral observations from East Asia
D. M. Willis1,2, G. M. Armstrong1, C. E. Ault1, and F. R. Stephenson3
1Space and Astrophysics Group, Department of Physics, University of Warwick, Coventry CV4 7AL, UK
2Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
3Department of Physics, University of Durham, Durham DH1 3LE, UK
Abstract. Comprehensive catalogues of ancient sunspot and auroral
observations from East Asia are used to identify possible intense historical
geomagnetic storms in the interval 210 BC-AD 1918. There are about 270
entries in the sunspot catalogue and about 1150 entries in the auroral
catalogue. Special databases have been constructed in which the scientific
information in these two catalogues is placed in specified fields. For the
purposes of this study, an historical geomagnetic storm is defined in terms
of an auroral observation that is apparently associated with a particular
sunspot observation, in the sense that the auroral observation occurred
within several days of the sunspot observation. More precisely, a selection
criterion is formulated for the automatic identification of such geomagnetic
storms, using the oriental records stored in the sunspot and auroral
databases. The selection criterion is based on specific assumptions about
the duration of sunspot visibility with the unaided eye, the likely range of
heliographic longitudes of an energetic solar feature, and the likely range
of transit times for ejected solar plasma to travel from the Sun to the
Earth. This selection criterion results in the identification of nineteen
putative historical geomagnetic storms, although two of these storms are
spurious in the sense that there are two examples of a single sunspot
observation being associated with two different auroral observations
separated by more than half a (synodic) solar rotation period. The literary
and scientific reliabilities of the East Asian sunspot and auroral records
that define the nineteen historical geomagnetic storms are discussed in
detail in a set of appendices. A possible time sequence of events is
presented for each geomagnetic storm, including possible dates for both the
central meridian passage of the sunspot and the occurrence of the energetic
solar feature, as well as likely transit times for the ejected solar plasma.
European telescopic sunspot drawings from the seventeenth century are also
used to assess the credibility of some of the later historical geomagnetic
storms defined solely by the East Asian sunspot and auroral records. These
drawings cast doubt on a few of the associations between sunspot and auroral
observations based entirely on the oriental records, at least to the extent
that the occidental drawings provide a more realistic date for central
meridian passage of the sunspot actually associated with a particular
auroral observation. Nevertheless, on those occasions for which European
sunspot drawings are available, the dates of all the pertinent East Asian
sunspot and auroral observations are corroborated, apart from just one
Chinese sunspot observation. The ancient historical observations of sunspots
and aurorae are discussed briefly in terms of modern observations of great
geomagnetic storms.
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