ON A SOLAR ORIGIN FOR THE COSMOGENIC NUCLIDE EVENT OF 775 A.D.

We explore requirements for a solar particle event (SPE) and flare capable of producing the cosmogenic nuclide event of 775 A.D., and review solar circumstances at that time. A solar source for 775 would require a >1 GV spectrum similar to 45 times stronger than that of the intense high-energy SPE of 1956 February 23. This implies a > 30 MeV proton fluence (F-30) of similar to 8 x 10(10) proton cm(-2), similar to 10 times larger than that of the strongest 3 month interval of SPE activity in the modern era. This inferred F-30 value for the 775 SPE is inconsistent with the occurrence probability distribution for > 30 MeV solar proton events. The best guess value for the soft X-ray classification (total energy) of an associated flare is similar to X230 (similar to 9 x 10(33) erg). For comparison, the flares on 2003 November 4 and 1859 September 1 had observed/inferred values of similar to X35 (similar to 10(33) erg) and similar to X45 (similar to 2 x 10(33) erg), respectively. The estimated size of the source active region for a similar to 10(34) erg flare is similar to 2.5 times that of the largest region yet recorded. The 775 event occurred during a period of relatively low solar activity, with a peak smoothed amplitude about half that of the second half of the 20th century. The similar to 1945-1995 interval, the most active of the last similar to 2000 yr, failed to witness a SPE comparable to that required for the proposed solar event in 775. These considerations challenge a recent suggestion that the 775 event is likely of solar origin.