  Miyake et al. (henceforth M12) recently reported, based on 14C data, an
extreme cosmic event ca. AD775. Using a simple model, M12 claimed that the
event was too strong to be caused by a solar flare within the standard theory.
This implied a new paradigm of either an impossibly strong solar flare or a
very strong cosmic ray event of unknown origin occurred ca. AD775. We show that
the strength of the event was significantly overestimated by M12. Several
subsequent works have attempted to find a possible exotic source for such an
event, but they are all based on incorrect estimates by M12. We revisit this
event with analysis of new datasets and consistent theoretical modelling. We
verified the experimental result for the AD775 event using independent datasets
including 10Be series and newly measured 14C annual data. We surveyed available
historical chronicles for astronomical observations for the AD770s to identify
potential sightings of aurorae or supernovae. We interpreted the 14C
measurements using an appropriate carbon cycle model. We show that: (1) The
reality of the AD775 event is confirmed by new measurements of 14C; (2) by
using an inappropriate carbon cycle model, M12 strongly overestimated the
event's strength; (3) The revised magnitude of the event is consistent with
different independent datasets (14C, 10Be, 36Cl) and can be associated with a
strong, but not inexplicably strong, SEP event (or a sequence of events), and
provides the first evidence for an event of this magnitude (the fluence >30 MeV
was about 4.5*10^{10} /cm2) in multiple datasets; (4) This is in agreement with
increased auroral activity identified in historical chronicles. This point to
the likely solar origin of the event, which is the greatest solar event on a
multi-millennial time scale, placing a strong observational constraint on the
theory of explosive energy releases on the Sun and cool stars.
