The presence of large sunspots near the central solar meridian at the times of modern Japanese auroral observations

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.

The presence of large sunspots near the central solar meridian at the times of major geomagnetic storms

“…However, the method has to be partly automated because of the large number (1074) of major geomagnetic storms in the interval 1874 April 17-1976 December 31. In particular, it is impracticable to show all the solar images for the six-day intervals immediately before these major storms, contrary to the situation in the paper by Willis et al (2006), so statistical results have to be presented instead. The main statistical results, which are given in Table 2, are discussed in Sect.…”

“…Even an "average" observer would probably have been able to see at least one sunspot with the unaided eye before 24 of these 30 events, which represents 186 D. M. Willis et al: Large sunspots and major geomagnetic storms an 80% success rate. As noted by Willis et al (2006), this corroboration of the validity of the technique used to identify historical occurrences of intense geomagnetic storms (Willis et al, 2005) is important because early unaided-eye observations of sunspots and aurorae provide the only possible means of identifying individual geomagnetic storms during the greater part of the past two millennia. Indeed, this identification of geomagnetic storms is only feasible because the dates of many unaided-eye observations of sunspots and aurorae during the last 2000 years are known precisely (year, month and day all recorded exactly).…”

“…The method employed in this paper follows closely that developed by Willis et al (2006). However, the method has to be partly automated because of the large number (1074) of major geomagnetic storms in the interval 1874 April 17-1976 December 31.…”

“…Similarly, the historical auroral observations from East Asia extend throughout the interval 210 BC-AD 1911, although in this case many of the early auroral observations and also the single observation in AD 1911 do not have precise dates. The modern Japanese auroral observations used to validate the technique employed in the identification of historical occurrences of intense geomagnetic storms (Willis et al, 2006) lie within the much shorter interval AD 1957AD -2004. However, the great merit of using modern Japanese auroral observations is that these observations are from the same region of the World as the historical auroral observations (China, Japan and Korea).…”

“…In particular, Willis et al (2006) used scientific observations of aurorae in Japan during the interval 1957-2004(Shiokawa et al, 2005 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 (RGO), the Big Bear Solar Observatory (BBSO), the Debrecen Heliophysical Observatory (DHO) and the Solar and Heliospheric Observatory (SOHO) spacecraft, it was found that at least one 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 in the interval ; this represents a 97% success rate.…”