The validity of a technique developed by the authors toidentify historical occurrences of intense geomagnetic storms, which isbased on finding approximately coincident observations of sunspots andaurorae recorded in East Asian histories, is corroborated using more modernsunspot and auroral observations. Scientific observations of aurorae inJapan during the interval 1957–2004 are used to identify geomagneticstorms that are sufficiently intense to produce auroral displays at lowgeomagnetic latitudes. By examining white-light images of the Sun obtainedby the Royal Greenwich Observatory, the Big Bear Solar Observatory, theDebrecen Heliophysical Observatory and the Solar and HeliosphericObservatory spacecraft, it is found that a sunspot large enough to be seenwith the unaided eye by an "experienced" observer was located reasonablyclose to the central solar meridian immediately before all but one of the30 distinct Japanese auroral events, which represents a 97% success rate.Even an "average" observer would probably have been able to see a sunspotwith the unaided eye before 24 of these 30 events, which represents an80% success rate. This corroboration of the validity of the techniqueused to identify historical occurences of intense geomagnetic storms is important becauseearly unaided-eye observations of sunspots and aurorae provide the onlypossible means of identifying individual historical geomagnetic stormsduring the greater part of the past two millennia.
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