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Ann. Geophys., 24, 2743-2758, 2006
www.ann-geophys.net/24/2743/2006/
© European Geosciences Union 2006
The presence of large sunspots near the central solar meridian at the times of modern Japanese auroral observations
D. M. Willis1,2, R. Henwood3, and F. R. Stephenson4
1Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
2Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick, Coventry CV4 7AL, UK
3UK Solar System Data Centre, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
4Department of East Asian Studies, University of Durham, Durham DH1 3TH, UK
Abstract. The validity of a technique developed by the authors to
identify historical occurrences of intense geomagnetic storms, which is
based on finding approximately coincident observations of sunspots and
aurorae recorded in East Asian histories, is corroborated using more modern
sunspot and auroral observations. Scientific observations of aurorae in
Japan during the interval 1957–2004 are used to identify geomagnetic
storms that are sufficiently intense to produce auroral displays at low
geomagnetic latitudes. By examining white-light images of the Sun obtained
by the Royal Greenwich Observatory, the Big Bear Solar Observatory, the
Debrecen Heliophysical Observatory and the Solar and Heliospheric
Observatory spacecraft, it is found that a sunspot large enough to be seen
with the unaided eye by an "experienced" observer was located reasonably
close to the central solar meridian immediately before all but one of the
30 distinct Japanese auroral events, which represents a 97% success rate.
Even an "average" observer would probably have been able to see a sunspot
with the unaided eye before 24 of these 30 events, which represents an
80% success rate. This corroboration of the validity of the technique
used to identify historical occurences of intense geomagnetic storms is important because
early unaided-eye observations of sunspots and aurorae provide the only
possible means of identifying individual historical geomagnetic storms
during the greater part of the past two millennia.
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