Localising pulsations in the hard X-ray and microwave emission of an X-class flare

Abstract

The aim of this work is to identify the mechanism driving pulsations in hard X-ray (HXR) and microwave emission during solar flares. Using combined HXR and microwave observations from Solar Orbiter/STIX and EOVSA, we investigate an X1.3 GOES class flare, 2022-03-30T17:21:00, which displays pulsations on timescales evolving from $ 7$ s in the impulsive phase to $ s later in the flare. We analysed the temporal, spatial, and spectral evolution of the HXR and microwave pulsations during the impulsive phase of the flare. We reconstructed images for individual peaks in the impulsive phase and performed spectral fitting at high cadence throughout the first phase of pulsations. Our imaging analysis demonstrates that the HXR and microwave emission originates from multiple sites along the flare ribbons. The brightest sources and the location of the emission change in time. Through HXR spectral analysis, the electron spectral index is found to be anti-correlated with the HXR flux, showing a `soft-hard-soft' spectral index evolution for each pulsation. The timing of the associated filament eruption coincides with the early impulsive phase. Our results indicate that periodic acceleration and/or injection of electrons from multiple sites along the flare arcade is responsible for the pulsations observed in HXR and microwave emission. The evolution of pulsation timescales is likely a result of changes in the 3D magnetic field configuration over time related to the associated filament eruption.

Localizing Quasiperiodic Pulsations in Hard X-Ray, Microwave, and Lyα Emissions of an X6.4 Flare

“…Based on the observation from EOVSA, the flare QPPs with varying periods of ∼10-20 s and ∼30-60 s were seen in the microwave emission (see Kou et al 2022). Combined with the observations from STIX and EOVSA, the flare QPPs with fast variations on timescales evolving from ∼7 to ∼35 s were found in HXR and microwave channels (see Collier et al 2024). Using the observations from HXI, STIX, and the Chashan Broadband Solar millimeter spectrometer, the flare QPP with a period of ∼27 s is simultaneously found in HXR and microwave channels (Shi et al 2024).…”

“…Recently, the localizing QPPs were simultaneously observed in HXR and microwave emissions, but the measured quasiperiods were always shorter than 60 s (e.g., Kou et al 2022;Collier et al 2024;Shi et al 2024). In this study, we localize the flare QPPs at a long period of about 200 s, which are simultaneously seen in wavelengths of HXR, microwave, and Lyα during an X-class flare.…”

Solar Flare Ribbon Fronts. II. Evolution of Heating Rates in Individual Flare Footpoints

“…These timings (i.e., around 200 s total heating) are rather long compared to what has become the general community's consensus, which more typically assumes bombardment by electron beams into individual locations on the order of a few seconds to tens of seconds (see, e.g., discussions in the review by Kerr 2022). This is based on both observations of the chromospheric response (i.e., the rapid apparent motion of bright flare ribbons as new field lines reconnect) as well as HXR footpoint motion, and the appearance of structure in HXR lightcurves which can be interpreted as elementary bursts of energy injection (see, e.g., the recent example by Collier et al 2024).…”

Dynamical Evolution of Newly Formed Structures After Magnetic Reconnection

“…During this process, magnetic energy is released into thermal and kinetic energy of the plasmas (Phan et al 2020;Fleishman et al 2023). In the solar atmosphere, kinds of activities and phenomena, e.g., flares (Fleishman et al 2020;Collier et al 2024), jets (Shibata et al 1992;Sterling et al 2024), brightenings (Berghmans et al 2021;Zhang et al 2023), and coronal mass ejections (CMEs; Green et al 2018;O'Kane et al 2021), are associated with this mechanism. The observational signatures of inflows and outflows (Reeves et al 2020;Yan et al 2022) are thought to be a direct consequence of the reconnection process.…”