How energetic can solar flares become?

Nugget
Number:	520
1st Author:	Natalie KRIVOVA
2nd Author:
Published:	March 23, 2026
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Previous Nugget:	Hinode EIS Observations of Plasma Composition Evolution and Radiative Cooling of Flare Loops

Objective

Solar flares release magnetic energy stored in active regions. The largest flares directly measured during the space era reach [ihttps://astronomy.swin.edu.au/cosmos/b/Bolometric+Magnitude bolometric] energies of about 10^{^32-33} erg. At the same time, cosmogenic radionuclide records suggest that the Sun occasionally produces extreme solar particle events far stronger than those observed in recent decades (e.g., [1]). Observations of Sun-like stars may also reveal superflares reaching energies of 10^{^34-35} erg, orders of magnitude above typical solar flares. This raises a natural question: could the Sun also produce such events?

To address this question, we considered [2] several exceptionally large sunspot groups in historical records, including the region responsible for the Carrington event of 1859, often regarded as the strongest solar eruption in the telescopic record. We also describe the very large sunspot group of April 1947. These examples allow us to estimate the flare energies that such regions could potentially produce using empirical relations derived from modern solar observations.

Methodology

It was previously shown [3] that the bolometric energy of a flare correlates with the area swept by flare ribbons during magnetic reconnection. Because ribbon area traces the amount of magnetic flux involved in the eruption, it provides a useful observational proxy for the total energy released.

Using space-era flares from the "RibbonDB" catalogue [3], we compiled an empirical relation between active-region area and ribbon area (Figure 1) and identified the upper envelope of this distribution, corresponding to the 5% and 1% of the most energetic events in [3]. We then used these relationships to estimate the ribbon areas and then the flare energies that could realistically - under exceptionally plausible conditions - be reached for the largest sunspot regions recorded in historical observations (Figure 2).

File:520f1.png

Figure 1: Ribbon area vs. active-region area for flares from the RibbonDB catalogue [3].

File:520f2.png

Figure 2: Estimated bolometric flare energy as a function of active-region area.

Results

Applying this scaling to the April 1947 region (Fig. 3) - the largest directly measured sunspot group - suggests that the Sun could in principle produce flares with energies of a few 10^{34} erg. Such events would exceed the largest modern solar flares by more than an order of magnitude and would overlap with the lower end of stellar superflares.

File:520f3.png

Figure 3: The great sunspot of 1947. Shown are the Mt Wilson sunspot drawing (top), Kodaikanal Ca II K line observation (bottom left), and unsigned magnetogram reconstructed from the Kodaikanal Ca II K observation (bottom right) for 7 April 1947. The Ca II K observation is saturated at contrast values of 1, while the magnetogram is saturated at 100 G. The black rectangle roughly marks the AR.

Not every large active region produces such an extreme eruption. The actually realised flare energy depends on magnetic topology and eruption dynamics. Nevertheless, the largest historical sunspot groups demonstrate that the Sun can in principle store enough magnetic energy to power flares approaching the stellar superflare regime

References

[1] "Linking Solar Magnetism, Extreme Solar Particle Events and Stellar Superflares"

[2] "Empirical flare energy limits for the largest historical sunspots"

[3] "A Database of Flare Ribbon Properties from the Solar Dynamics Observatory. I. Reconnection Flux"