Abstract
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.
Highlights
In a detailed investigation, Willis et al (2005) used comprehensive catalogues of ancient sunspot and auroral observations from East Asia (China, Japan and Korea) to identify possible historical occurrences of intense geomagnetic storms in the interval 165 BC–AD 1910
This condition is based on three assumptions: (i) a sunspot large enough to be seen with the unaided eye could have been observed continually by the ancient East Asian observers if it was within ±5 days of crossing the central solar meridian; (ii) the energetic solar feature producing the historical geomagnetic storm occurred when the associated sunspot was within ±4 days of the central meridian; and (iii) the time delay between the energetic solar feature and the onset of the historical geomagnetic storm was in the range 1 to 6 days
An “average” sunspot observer would still have been able to see a sunspot with the unaided eye before 24 of the 30 distinct Japanese auroral observations listed in Table 1, which represents an 80% success rate
Summary
In a detailed investigation, Willis et al (2005) used comprehensive catalogues of ancient sunspot and auroral observations from East Asia (China, Japan and Korea) to identify possible historical occurrences of intense geomagnetic storms in the interval 165 BC–AD 1910. The paper by Willis et al (2005) provides further details on the three crucial assumptions that result in the condition –8≤T ≤+15 for the identification of historical geomagnetic storms This discussion need not be repeated here, certain key facts regarding the ancient East Asian sunspot and auroral observations should be emphasised before discussing the modern observations. As a result of this limited temporal resolution, the shortest time interval that can be realistically considered in any investigation of the ancient sunspot and auroral observations is just one day (24 h) It is those specific sunspot and auroral observations, for which an exact date is known (year, month and day all recorded precisely), that are crucially important in the identification of historical geomagnetic storms. The great advantage of using modern solar observations is that the positions of sunspots (and sunspot groups) on the solar surface are essentially known at all times, as a result of routine monitoring-type observations of the Sun, which obviates the need for the assumption that sunspots can be observed continually within ±5 days of the central solar meridian
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