Reconnection Fluxes in Eruptive and Confined Flares and Implications for Superflares on the Sun

We study the energy release process of a set of 51 flares (32 confined, 19 eruptive) ranging from GOES class B3 to X17. We use Ha filtergrams from Kanzelhohe Observatory together with Solar Dynamics Observatory HMI and Solar and Heliospheric Observatory MDI magnetograms to derive magnetic reconnection fluxes and rates. The flare reconnection flux is strongly correlated with the peak of the GOES 1-8 angstrom soft X-ray flux (c = 0.92, in log-log space) for both confined and eruptive flares. Confined flares of a certain GOES class exhibit smaller ribbon areas but larger magnetic flux densities in the flare ribbons (by a factor of 2). In the largest events, up to approximate to 50%. of the magnetic flux of the active region (AR) causing the flare is involved in the flare magnetic reconnection. These findings allow us to extrapolate toward the largest solar flares possible. A complex solar AR hosting a magnetic flux of 2 x 10(23) Mx, which is in line with the largest AR fluxes directly measured, is capable of producing an X80 flare, which corresponds to a bolometric energy of about 7 x 10(32) erg. Using a magnetic flux estimate of 6 x 10(23) Mx for the largest solar AR observed, we find that flares of GOES class approximate to X500 could be produced (E-bol approximate to 3 x 10(33) erg). These estimates suggest that the present day's Sun is capable of producing flares and related space weather events that may be more than an order of magnitude stronger than have been observed to date.