Can Superflares Occur on Our Sun?

Abstract

Abstract Recent observations of Sun-like stars, similar to our Sun in their surface temperature (5600–6000 K) and slow rotation (rotational period $ >$ 10 d), using the Kepler satellite by Maehara et al. (2012, Nature, 485, 478) have revealed the existence of superflares (with energy of 10$ ^{33}$ –10$ ^{35}$ erg). From statistical analyses of these superflares, it was found that superflares with energy of 10$ ^{34}$ erg occur once in 800 yr, and superflares with 10$ ^{35}$ erg occur once in 5000 yr. In this paper, we examine whether superflares with energy of 10$ ^{33}$ –10$ ^{35}$ erg could occur on the present Sun through the use of simple order-of-magnitude estimates based on current ideas related to the mechanisms of the solar dynamo. If magnetic flux is generated by differential rotation at the base of the convection zone, as assumed in typical dynamo models, it is possible that the present Sun would generate a large sunspot with a total magnetic flux of $ \sim$ 2 $ \times$ 10$ ^{23}$ Mx ($ =$ G cm$ ^{2}$ ) within one solar cycle period, and lead to superflares with an energy of 10$ ^{34}$ erg. To store a total magnetic flux of $ \sim$ 10$ ^{24}$ Mx, necessary for generating 10$ ^{35}$ erg superflares, it would take $ \sim$ 40 yr. Hot Jupiters have often been argued to be a necessary ingredient for the generation of superflares, but we found that they do not play any essential role in the generation of magnetic flux in the star itself, if we consider only the magnetic interaction between the star and the hot Jupiter. This seems to be consistent with Maehara et al.'s finding of 148 superflare-generating solar-type stars that do not have a hot Jupiter-like companion. Altogether, our simple calculations, combined with Maehara et al.'s analysis of superflares on Sun-like stars, show that there is a possibility that superflares of 10$ ^{34}$ erg would occur once in 800 yr on our present Sun.