Reconnection-driven Decaying Pulsations Modulated by Slow Magnetoacoustic Waves

Decaying pulsations have been simultaneously detected in the low-energy X-rays of solar/stellar flares, which are supposed to be associated with standing slow magnetoacoustic or kink-mode waves. The physical mechanism behind rapid decay remains unknown. We present the detection of quasiperiodic pulsations (QPPs) with rapid decay in high-energy emissions produced in two major flares on 2024 January 10 and May 14. Using empirical mode decomposition, decaying QPPs are identified in hard X-ray and microwave emissions during the flare-impulsive phase, suggesting a process of oscillatory magnetic reconnection. The quasi-periods and decay times are determined by a damped harmonic function, which are approximately 177 ± 8 s (249 ± 25 s) and 118 ± 4 s (124 ± 5 s), respectively. The restructured X-ray images reveal double footpoints connected by hot flare loops. Their phase speeds are estimated to be about 400 and 670 km s −1 , both below the local sound speed in high-temperature plasmas, indicating the presence of slow-mode waves in hot flare loops. We perform coronal diagnostics based on standing slow-mode waves and derive key physical parameters, including the polytropic index, the thermal ratio, viscous ratio and radiation ratio, which are consistent with previous results. Our observations support the conclusion that decaying QPPs are triggered by oscillatory magnetic reconnection that is modulated by standing slow magnetoacoustic waves, with their rapid decay attributable to a coeffect of viscous damping and localized magnetic reconnection rate.