Abstract Stellar flares are sudden magnetic eruptions that release intense energy and cause detectable variations in stellar light curves. These transient events offer windows into the magnetic field dynamics, plasma behaviour and reconnection processes in active stellar regions. Despite their importance, the diversity in flare morphology and its underlying physical origins remains poorly understood. To explore this variability, we analyse 2319 white-light flares observed by Kepler, extracting a comprehensive set of 22 time- and frequency-domain features from each event. Using unsupervised clustering, we identify two distinct flare populations that are not associated with global stellar parameters such as rotation period, temperature or mass. Instead, the separation is primarily driven by differences in frequency evolution, damping persistence and local magnetic activity (LMA). Our findings demonstrate that flare light-curve morphology can be quantitatively grouped, revealing internal dynamical contrasts without reliance on host star properties. This article is part of the theme issue ‘Transferable mechanisms in the oscillatory processes of solar and stellar flares’.
Abstract Kink oscillations are transverse displacement fluctuations of coronal loops caused by perturbations, with their phase speed directly related to magnetic field strength, making them a vital tool for diagnosing coronal plasma conditions. In this study, we employ the fractional anisotropy (FA)-based video motion amplification (FA-VMM) technique, combined with the damped harmonic oscillator model and simple harmonic oscillator model, to systematically analyse the small-amplitude oscillations in three sets of coronal loops (one in a flaring active region and two in quiescent active regions) observed in the SDO/AIA 171 Å band. After 20× magnification and model fitting, we find that coronal loops in the steady state generally exhibit intrinsic decayless oscillations, with parameter variations probably reflecting differences in loop properties. Crucially, for the flare-perturbed active-region coronal loop, we document a complete oscillation state transition: the flare-induced disturbance drives a large-amplitude decaying oscillation while the period remained stable throughout; subsequently, the system recovers to its initial decayless state. These findings provide detailed observational evidence for understanding the elasticity of coronal loops and their dynamic response to transient perturbations. This article is part of the theme issue ‘Transferable mechanisms in the oscillatory processes of solar and stellar flares'.
Abstract This article is part of the theme issue ‘Transferable mechanisms in the oscillatory processes of solar and stellar flares’.
Abstract Slow magneto-acoustic and entropy waves can serve as diagnostic tools not only for probing plasma parameters but also for investigating non-adiabatic processes. Under coronal conditions, these waves are strongly affected by both thermal conduction and thermal misbalance. In this work, we obtain an exact solution to the linear evolutionary equation describing wave dynamics in plasma under the assumption of an infinitely strong magnetic field. We show that two dispersion sources significantly expand the set of parametric regions with qualitatively similar behaviour of dispersion curves and increase the number of ways an arbitrary initial disturbance can develop. The resulting solution allows for monitoring the behaviour of individual harmonics of the eigenmodes for the plasma parameter of interest, thus expanding the possibilities for fitting observations and studying the behaviour of model signals. Using the obtained solution, we show that several thermal misbalance regimes enable quasi-periodic pulsation formation. We also demonstrate that additional information about non-adiabatic processes can be gained by considering the amplitude ratios of various plasma parameters and their phase shifts for arbitrary harmonic numbers. In particular, this may be of interest for plasma diagnostics using sloshing oscillations, which involve the simultaneous development of multiple oscillations. This article is part of the theme issue ‘Transferable mechanisms in the oscillatory processes of solar and stellar flares'.
Abstract In the present work, we use a phase-resolved methodology for the detection and characterization of non-stationary quasi-periodic pulsations (QPPs) in the hard X-ray light curves observed from the High Energy L1-Orbiting X-ray Spectrometer instrument on board the Aditya-L1 mission, complemented by cross-examination using soft X-ray observations from Geostationary Operational Environmental Satellites. Our approach combines variational mode decomposition with Hilbert–Huang analysis and wavelet transforms to sharpen transient QPP signatures, extract narrowband amplitude–frequency modulated modes and track instantaneous period drift across various phases of the flares. By applying these tools to pre-flare, impulsive and decay phases and associated intervals, we identify the timing, duration and evolution of statistically significant oscillations and assess their relation to thermal and non-thermal flare components. Finally, we extend the method to the catalogue of non-stationary QPP flares reported by Mehta et al. (Mehta et al. 2023 Mon. Not. R. Astron. Soc. 523, 3689–3698. (doi:10.1093/mnras/stad1619)), where we identify statistically significant QPPs in both impulsive and decay phases for 74 out of 98 flares. Among these, 56.8% exhibit small period drifts, while 43.2% show larger deviations. Positive period drift dominates (73%), indicating a systematic increase in characteristic timescales as flares evolve. A phase–period–duration analysis reveals distinct regimes of non-stationarity, including overlapping multi-mode oscillations, intermittent same-period episodes and strongly evolving multi-modal behaviour. This article is part of the theme issue ‘Transferable mechanisms in the oscillatory processes of solar and stellar flares'.
