The most powerful explosions on the Sun [...] drive the most severe
space-weather storms. Proxy records of flare energies based on SEPs in
principle may offer the longest time base to study infrequent large events. We
conclude that one suggested proxy, nitrate concentrations in polar ice cores,
does not map reliably to SEP events. Concentrations of select radionuclides
measured in natural archives may prove useful in extending the time interval of
direct observations up to ten millennia, but as their calibration to solar
flare fluences depends on multiple poorly known properties and processes, these
proxies cannot presently be used to help determine the flare energy frequency
distribution. Being thus limited to the use of direct flare observations, we
evaluate the probabilities of large-energy solar explosions by combining solar
flare observations with an ensemble of stellar flare observations. We conclude
that solar flare energies form a relatively smooth distribution from small
events to large flares, while flares on magnetically-active, young Sun-like
stars have energies and frequencies markedly in excess of strong solar flares,
even after an empirical scaling with the mean activity level of these stars. In
order to empirically quantify the frequency of uncommonly large solar flares
extensive surveys of stars of near-solar age need to be obtained, such as is
feasible with the Kepler satellite. Because the likelihood of flares larger
than approximately X30 remains empirically unconstrained, we present indirect
arguments, based on records of sunspots and on statistical arguments, that
solar flares in the past four centuries have likely not substantially exceeded
the level of the largest flares observed in the space era, and that there is at
most about a 10% chance of a flare larger than about X30 in the next 30 years.Comment: 14 pages, 3 figures (in press as of 2012/06/18); Journal of
Geophysical Research (Space Physics), 201