Reconnection-driven Decaying Pulsations Modulated by Slow Magnetoacoustic Waves

TL;DR

The paper investigates rapidly decaying quasi-periodic pulsations (QPPs) observed in high-energy X-ray and microwave emissions during two major solar flares. Using Empirical Mode Decomposition, the authors extract decaying QPPs and fit them with a damped harmonic function, obtaining quasi-periods of about $P\approx177\pm8\ \mathrm{s}$ for the M1.4 flare and $P\approx118\pm4\ \mathrm{s}$ for the X8.7 flare, with decay times $\tau$ of $\approx249\pm25\ \mathrm{s}$ and $\approx124\pm5\ \mathrm{s}$ respectively. Spatial imaging reveals hot loops with double footpoints, and coronal diagnostics yield loop-top temperatures $T_h$ and densities $n_e$ that imply slow-mode waves with phase speeds $v_{ph}$ of roughly $400$ km s$^{-1}$ and $670$ km s$^{-1}$, below local sound speeds $v_s$, supporting standing slow MHD waves. The analysis shows weak thermal conduction, negligible radiative losses, and dominant viscous damping ($\epsilon$) in combination with oscillatory magnetic reconnection modulated by slow-mode waves, providing a coherent mechanism for the observed rapidly decaying QPPs and linking flare energetics to particle acceleration processes.

Abstract

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 rapidly decaying remains unknown. We present the detection of quasi-periodic pulsations (QPPs) with rapidly decaying in high-energy emissions produced in two major flares on 10 January and 14 May 2024. 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$\pm$8 s (249$\pm$25 s) and 118$\pm$4 s (124$\pm$5 s), respectively. The restructured X-ray images reveal double footpoints connected by hot flare loops. Their phase speeds are estimated to about 400 km/s and 670 km/s, 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 that the decaying QPPs are triggered by oscillatory magnetic reconnection that is modulated by standing slow magnetoacoustic waves, with their rapid decay attributable to a co-effect of viscous damping and localized magnetic reconnection rate.

Figures (6)

• Figure 1: Overview of two major flares on 10 January and 14 May 2024. (a1 & b1): Light curves recorded by GOES 1-8 Å and LYRA 1$-$200 Å. The vertical dotted lines marks their start, peak and stop times. (a2-a4): Normalized time series measured by MSS-1, GOES, STIX, and HXI. (b2-b4): Normalized time series captured by MSS-1, GOES, STIX, and EOVSA. The context image is the radio dynamic spectrum measured by EOVSA.
• Figure 2: EMD analysis of the M1.4 flare. (a1 & a2): Normalized intensity curves in wavebands of MSS-1 1-24 keV and STIX 4-25 keV. The tomato line is the trend signal obtained from EMD . (b1 & b2): Detrended time series (original signal minus trend). The red curve represents residual signal containing several IMFs. (c1 & c2): Damped oscillatory IMFs and their best fitting with a damped harmonic function. (d1 & d2): FFT power spectra of detrended signals. The cyan line is MCMC-optimized fit, and the hot pink line represents a confidence level at 99%. The magenta vertical line marks the period derived from least-squares fitting.
• Figure 3: Similar to Figure \ref{['emd1']}, but the EMD and FFT analyses are preformed for the X8.7 flare in channels of EOVSA 12 GHz and STIX 25-150 keV.
• Figure 4: (a1-b2): AIA EUV maps in wavelengths of 94 Å and 131 Å captured during the M1.4 flare, with a FOV of about 90$^{\prime\prime}$$\times$90$^{\prime\prime}$. The tomato contours represent the HXR radiation measured by HXI at levels of 20%, 50%, and 80%. (a3): Narrow-band EM maps integrated over temperature ranges of 5-10 MK and 10-30 MK. The hot pink box outlines the flare loop-top source, used for estimating the plasma temperature. (d): Temporal evolutions with error bars of the plasma temperature, as measured by MSS-1, STIX, GOES, and AIA, respectively. (e): The X-ray spectrum with error bars in the energy range of 4-80 keV, the fitted thermal and nonthermal components, and their sum. Some fit parameters such as EM, $T$, $E_{\rm c}$, and $\delta$ are labeled.
• Figure 5: (a1 & a2): AIA EUV maps in wavelengths of 94 Å and 131 Å during the X8.7 flare, with a FOV of about 90$^{\prime\prime}$$\times$90$^{\prime\prime}$. (a3): Narrow-band EM map integrated over the temperature range of 5-30 MK. The hot pink box outlines the flare loop-top region, used to estimate the plasma temperature. (b1): Sketch plot of the spatial locations of STIX and its connection with the Sun and Earth. (b2-b3): Reconstructed STIX maps in energy ranges of 25-150 keV, and 4-25 keV. (c): Temporal evolutions with error bars of the plasma temperature, as measured by MSS-1, GOES, and AIA. (d): The X-ray spectrum with error bars in the energy range of 4-150 keV, the fitted thermal and nonthermal components, and their sum. Some fit parameters such as EM, $T$, $E_{\rm c}$, and $\delta$ are labeled.