Superflare
Since superflares can occur on stars apparently equivalent in every way to the Sun, it is natural to ask if they can occur on the Sun itself. An estimate based on the original Kepler photometric studies suggested a frequency on solar-type stars (early G-type and rotation period more than 10 days) of once every 800 years for an energy of 10 erg and every 5000 years at 10 erg. One-minute sampling provided statistics for less energetic flares and gave a frequency of one flare of energy 10 erg every 5–600 years for a star rotating as slowly as the Sun; this would be rated as X100 on the solar flare scale. This is based on a straightforward comparison of the number of stars studied with the number of flares observed. An extrapolation of the empirical statistics for solar flares to an energy of 10 erg suggests a frequency of one in 10,000 years.
Solar storm of August 1972
The August 4 flare which triggered the extreme solar particle event (SPE) around and intense geomagnetic storm on Earth was among the largest documented by science. This flare saturated the Solrad 9 X-ray sensor at approximately X5.3 but was estimated to be in the vicinity of X20, the threshold of the very rarely reached R5 on the NOAA radio blackout space weather scale. A radio burst of 76,000 sfu was measured at 1 GHz. This was an exceptionally long duration flare, generating X-ray emissions above background level for more than 16 hours. Rare emissions in the gamma ray (formula_1-ray) spectrum were detected for the first time, on both August 4 and August 7, by the Orbiting Solar Observatory (OSO 7). The broad spectrum electromagnetic emissions of the largest flare are estimated to total 1-5 x 10 ergs in energy released.