Similarities and differences between solar and stellar flare pulsation processes

Quasi-periodic pulsations (QPPs) are oscillatory signatures commonly detected in the light curves of solar and stellar flares, offering valuable diagnostics of the underlying magnetic and plasma processes. This review compares the observational characteristics, detection methods, and physical interpretations of QPPs in both solar and stellar contexts. Solar flare QPPs, extensively studied in X-rays and EUV bands using instruments such as GOES, STIX, and Fermi, display typical periods of tens of seconds and show correlations with flare duration and magnetic loop length. Stellar QPPs, observed in X-rays and white light by missions such as Kepler, TESS, and XMM-Newton, exhibit much longer periods - ranging from minutes to hours - consistent with larger-scale magnetic structures in more active stars. Despite differences in scale and observing band, statistical and comparative studies reveal common scaling relations and damping behaviors, suggesting that both solar and stellar QPPs are manifestations of the same fundamental mechanisms, likely magnetohydrodynamic oscillations or oscillatory reconnection within flare loops. The comparison underscores a continuity between solar and stellar magnetic activity, linking the solar detailed physical processes to stellar-scale phenomena and providing constraints for future models and surveys.

💡 Research Summary

This review paper provides a comprehensive comparison of quasi‑periodic pulsations (QPPs) observed in solar and stellar flares, focusing on their observational characteristics, detection techniques, statistical properties, and physical interpretations. Solar flares have been extensively studied in soft X‑ray (GOES 1–8 Å), hard X‑ray (Fermi/GBM, STIX), and EUV (SDO/AIA, Solar Orbiter/EUI) bands. Large‑scale statistical analyses of more than 5 000 GOES events reveal that QPPs are detected in 7 % of C‑class, 29 % of M‑class, and 46 % of X‑class flares, with a log‑normal period distribution centred at ≈22 s (range 12–40 s). Crucially, the QPP period correlates with flare duration (P ∝ τ^0.7) and with ribbon separation distance (P ∝ d^0.6), suggesting a link to the length of the magnetic loop that hosts the oscillation. No significant dependence on flare magnitude or CME occurrence is found.

A suite of detection methods is described: classical Fourier and Lomb‑Scargle periodograms, wavelet transforms, the Automated Flare Inference of Oscillations (AFINO) which fits a power‑law background model to the Fourier spectrum, and newer machine‑learning approaches such as Fully Convolutional Networks (FCNs). Each method has different sensitivities to background coloured noise and to the stationarity of the signal, which explains the spread in reported detection rates across instruments. For example, GOES data, dominated by thermal emission, are more sensitive to QPPs than the higher‑energy 15–25 keV channel of Fermi/GBM.

Stellar QPPs have been identified in the white‑light light curves of Kepler and TESS, as well as in X‑ray observations from XMM‑Newton and Chandra. Their periods are typically much longer, ranging from a few minutes to several hours, reflecting the larger magnetic structures and higher activity levels of the host stars. Detection techniques mirror those used for the Sun, with Fourier, wavelet, Empirical Mode Decomposition (EMD), and FCNs all applied. EMD is highlighted for its ability to detrend the data and assess statistical significance against coloured noise in a self‑consistent way.

Statistical studies of stellar flares show that QPP occurrence correlates with flare energy and stellar rotation/activity indicators, supporting the idea that loop length (or magnetic scale height) governs the period, analogous to the solar ribbon‑separation scaling. Damping of QPPs is observed in both solar and stellar events; the damping time to period ratios are compatible with theoretical expectations for MHD wave dissipation (thermal conduction, viscosity, radiative losses) or for non‑linear energy release cycles.

The paper surveys candidate physical mechanisms, grouping them into three categories: (i) direct modulation of emission by MHD wave modes (kink, sausage, torsional, acoustic), (ii) modulation of the reconnection or particle‑acceleration efficiency, and (iii) spontaneous, quasi‑periodic energy release (e.g., oscillatory reconnection, LRC‑type circuit oscillations, Kelvin‑Helmholtz instability). The observed scaling between period and loop length, together with the prevalence of harmonics, favours resonant MHD modes or oscillatory reconnection as the most plausible drivers. The authors note that different mechanisms may dominate in the impulsive rise phase (fast external wave triggering) versus the decay phase (formation of longer loops and slower reconnection).

In conclusion, despite differences in observational bandpasses and temporal scales, solar and stellar QPPs share fundamental properties and likely arise from the same underlying physics—magnetohydrodynamic oscillations or oscillatory reconnection within flare loops. This continuity bridges detailed solar diagnostics with large‑scale stellar flare statistics, offering a powerful avenue to test flare models across many orders of magnitude in energy and size. The authors advocate for coordinated high‑resolution solar observations (e.g., DKIST, Solar Orbiter) together with upcoming stellar surveys (e.g., PLATO) to refine QPP detection, constrain damping mechanisms, and ultimately improve space‑weather forecasting and stellar activity modelling.