Empirical flare energy limits for the largest historical sunspots

Abstract

Extreme solar particle events reveal that the Sun can occasionally produce eruptions significantly more energetic than those observed in the modern era. These events are thought to originate from powerful coronal mass ejections, typically associated with large solar flares triggered by magnetic field reconnection in complex active regions. Stellar observations indicate that Sun-like stars can host superflares exceeding 10^34 erg roughly once per century, yet it remains uncertain whether the Sun can reach such flare energies. We empirically estimate the upper limit of solar flare energies using statistical relations between flare - ribbon areas and released energy derived from modern observations. By extrapolating these upper-envelope relations to the largest sunspot group recorded since 1859 - the Great Sunspot of 8 April 1947 - we find that exceptionally large and complex solar active regions could, in principle, produce flares with bolometric energies of a few 10^34 erg.

Figures (4)

• Figure 1: The great sunspot of 1947. Shown are Mt Wilson sunspot drawing (top), Kodaikanal Ca ii K observation (bottom left), and reconstructed unsigned magnetogram from the Kodaikanal Ca ii K observation (bottom right) for 7 April 1947. The Ca ii K observation is saturated at contrast values of $\pm$1, while the magnetogram is saturated at 100 G. The black rectangle roughly marks the AR.
• Figure 2: Sunspot groups on the visible solar hemisphere during the Halloween event in 2003. Shown are a continuum observation (left) and an unsigned LoS magnetogram (right) from SoHO/MDI on 29 October 2003. The magnetogram is saturated at 100 G. The black rectangle roughly marks the larger AR, which reached a maximum spot area of $\sim 3500$ msh. Another large AR ($\sim 2500$ msh) was present at nearly the same longitude northwards of the largest group during this period (outlined with the blue rectangle), see Sect. \ref{['sec:results']}\ref{['sec:nesting']} for a discussion.
• Figure 3: Ribbon area versus active region area for flares in the K2017 catalogue kazachenko_database_2017. Black points: all events; blue points: top 5% envelope in ribbon-to-AR area ratio; red points: top 1% envelope. The solid blue and red lines are log–log linear fits to the 5% and 1% envelope subsets, respectively (that is representing the 95th- and 99th-percentile relations. Blue and red shaded regions: prediction intervals for the 5% and 1% envelope subsets, respectively.
• Figure 4: Bolometric flare energy as a function of active-region area. Solid curves show the empirically derived 95% (blue) and 99% (red) envelope relations between AR area and bolometric flare energy, obtained from Eqs. \ref{['eq:Srbn']} and \ref{['eq:energy']}. Dashed curves indicate the corresponding upper prediction intervals for each envelope. Shaded regions highlight the range between the envelope and its prediction interval. Vertical dotted lines mark AR areas of the Carrington (1859) and 1947 event. For the 1947 event, both image-based and empirically inferred estimates are shown, with the corresponding range indicated by hatching.