Linking Solar Magnetism, Extreme Solar Particle Events and Stellar Superflares

TL;DR

This work assesses whether extreme solar particle events (ESPEs) and stellar superflares are two faces of the same magnetic-energy phenomena. It integrates millennial cosmogenic-isotope records with Kepler/TESS observations of Sun-like stars to place ESPEs and superflares within a shared framework of solar-type magnetism, energy storage, and reconnection-driven energy release. The analysis shows ESPEs are rare but energetically extreme, while superflares occur on Sun-like stars at higher rates when scaled appropriately, yet there is no simple one-to-one correspondence between ESPEs and superflares due to magnetic topology and particle transport effects. The study highlights a unifying magnetic-energy paradigm across solar and stellar regimes and calls for millennial isotopic datasets and refined stellar samples to constrain the coupling of dynamo action, reconnection, and energy partitioning.

Abstract

The magnetic field of the Sun drives a wide range of eruptive phenomena, from small-scale nanoflares to large flares and coronal mass ejections (CMEs). While direct observations of solar activity cover only the past few decades, indirect evidence indicates that the Sun can occasionally produce events orders of magnitude stronger than any recorded ones in the modern era. Two complementary lines of evidence exist. First, extreme solar particle events (ESPEs) have been inferred from prominent spikes in cosmogenic isotope concentrations preserved in precisely dated natural archives such as tree rings and ice cores over the past 15 millennia. Second, high-precision space-borne photometry has revealed superflares on thousands of stars similar to the Sun. Whether these solar and stellar extremes are physically related remains an open question. We summarise the present state of understanding and discuss physical mechanisms that may link them. Although superflares and ESPEs are both extremely energetic manifestations of magnetic energy storage and release, their relationship does not appear to be one-to-one. Their occurrence and energetics likely depend on how magnetic flux and topology govern the partitioning of released energy between radiation, mass ejection, and particle acceleration.

Figures (3)

• Figure 1: ESPE and sunspot activity.(a) Time profile of decadal sunspot numbers $\langle{\rm SN}\rangle$ (red-filled curve, right axis) reconstructed from cosmogenic isotopes wu18; Occurrence time of known ESPEs (blue stars, left axis, blue ticks on the left and top) quantified in the fluence (event-integrated flux) of SEPs with energy above 200 MeV, $F_{200}$ (data from golubenko25 and koldobskiy25). Error bars are shown in panel (b). The blue dash-dotted line denotes an estimate of the mean annual fluence $F_{200}=3.2\cdot 10^6$ cm$^{-2}$ over the past solar cycle 24 (2009 -- 2019) raukunen22, multiplied by 1000. Note the break in the time axis. (b) Distribution histogram of the $\langle{\rm SN}\rangle$ values shown in panel (a) (red horizontal bars, top and left axes); ESPE strength ($F_{200}$, bottom axis, blue stars with 1$\sigma$ error bars) as a function of $\langle{\rm SN}\rangle$ (left axis) during the decade of the ESPE occurrence. The horizontal red dashed line denotes the median of the $\langle{\rm SN}\rangle$ distribution. A similar figure but for $F_{\rm 800}$ can be found in koldobskiy25.
• Figure 2: Integral energy spectra of ESPEs and strongest directly observed SEP events. Solid curves with shaded areas denote multi-proxy reconstructions, with 68% confidence intervals, of the reconstructed ESPE fluences. Dashed curves denote the integral spectra for three very strong SEP events: the hard-spectrum GLE#5 (23-Feb-1956), a soft-spectrum GLE#24 (04-Aug-1972) and a typical-spectrum series of GLE#42–45 (October–November 1989), as denoted in the legend. Figure adopted from usoskin_LR_23.
• Figure 3: Occurrence rate vs. energy distributions of solar and stellar flares.(a) Flare occurrence rate per star per year per unit energy for all stars in the Kepler sample (black) and for a restricted subset of Sun-like stars with effective temperatures of 5500--6000 K and photometric variability $R_\mathrm{var} < 0.3\%$ (red). Blue symbols show representative results from earlier studies Notsu2019Okamoto2021. (b) Cumulative flare occurrence rate (number of flares per star per year above a given energy $E$) for the two stellar samples shown in panel (a) (red and black curves). The black dashed line represents a power-law fit to the distribution of stellar flares with $E \gtrsim 10^{34}$ erg. For comparison, the green curve shows the solar flare occurrence rate distribution derived from soft X-ray observations of 334,122 flares between 1986 and 2020 Plutino2023. Green symbols mark the inferred frequencies of ESPEs inferred from cosmogenic isotope records Cliver2022. The horizontal error bar below the legend indicates the mean uncertainty in stellar flare energy. Figure adapted from Vasilyev2024.