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 ~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 ~8 × 1010?proton?cm–2, ~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 ~X230 (~9 × 1033?erg). For comparison, the flares on 2003 November 4 and 1859 September 1 had observed/inferred values of ~X35 (~1033?erg) and ~X45 (~2 × 1033?erg), respectively. The estimated size of the source active region for a ~1034?erg flare is ~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 ~1945-1995 interval, the most active of the last ~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.
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