Dynamics of the coronal magnetic field in the 2022-10-02 X-class flare

TL;DR

This study analyzes the 2022-10-02 X1.1 solar flare seen on disk with EOVSA microwave imaging spectroscopy to map the coronal magnetic field and track its evolution during the rise phase. The authors forward-fit gyrosynchrotron spectra to derive local magnetic field strengths and electron populations, revealing a rapid decay of the coronal field (up to $10\ ext{G s}^{-1}$) co-located with a surge in nonthermal electrons, particularly at loop-top and above-loop-top regions. They build 3D flare models with NLFFF extrapolations and compare them to the microwave-derived fields, finding reasonable agreement and illustrating how magnetic energy release powers particle acceleration and flare heating. The energy-budget analysis shows the released magnetic energy is sufficient to account for the observed nonthermal and thermal components, reinforcing the pivotal role of magnetic-energy dissipation in driving eruptive flares and highlighting the cusp region as a key acceleration site.

Abstract

Solar flares are driven by release of free magnetic energy and often associated with restructurization of the magnetic field topology. Yet, observations of evolving magnetic field in the flaring volume are limited to very few cases including the 2017-09-10 X8.2 limb flare; thus, a verification of whether a similar evolution takes place in other solar flares is needed. Here we report one more, 2022-10-02, X1.1 class solar flare but seen on disk, whose microwave data permit mapping the magnetic field over the flaring source and tracking magnetic field evolution over the course of the flare. We found that the coronal magnetic field shows a prominent decay with the rate up to 10 G s$^{-1}$ in several (above) loop-top locations. The magnetic field is also confidently measured at the loop legs and the bottom part of the erupting filament. Prominent acceleration of electrons is detected where the magnetic field decays. We developed 3D models of the flare, whose magnetic field shows resemblance and also deviation from the magnetic field inferred from the microwave data. This study confirms that the coronal magnetic field decays during the rise phase of the solar flare. The amount of released magnetic energy is sufficient to support other components of the flare energy.

Figures (12)

• Figure 1: Context of the flare location and associated filament eruption. (a) AIA 1600 Å image with EOVSA multifrequency contours at 20% of the maximum brightness at each frequency, color-coded by frequency from blue (low) to red (high) as indicated by the top color bar. (b--d) EOVSA images at 3.5, 8.7, and 16.8 GHz, each overlaid with the correspondingly colored contours from panel (a). (e) AIA 1600 Å image with contours at 5, 13.6, 36.8, and 99.9% of the maximum, highlighting the flare ribbons. (f) HMI LOS magnetic field with the same contours as in panel (e). (g--h) AIA 211 Å and 94 Å images with colored and numbered regions of interest (ROIs; see Section \ref{['S_bottom']}) outlined. The dashed black curve marks the erupting filament, and the solid red lines indicate loops connecting the flare ribbons.
• Figure 2: Flare overview. First row: Dynamic microwave spectrum from EOVSA. Second row: EOVSA total power light curves of the flux density (in sfu) at several frequencies. Bottom: Normalized light curves at 304 Å, 1600 Å, 131 Å, and 94 Å (SDO/AIA) extracted from the region shown in Fig. \ref{['Fig:aia_eovsa']}, along with 7.7 GHz (EOVSA).
• Figure 3: EOVSA image on 8.71 GHz 20:25:00 UT (left) and observational spectra (symbols with error bars) from the pixel shown by red cursor (middle) in the left panel with coordinates $x=685\hbox{$^{\prime\prime}$}$, $y=247\hbox{$^{\prime\prime}$}$ and white pixel (right) with coordinates $x=675\hbox{$^{\prime\prime}$}$, $y=249\hbox{$^{\prime\prime}$}$ with the model spectral fits (blue lines). The statistical uncertainties of the measurements are given at 1$\sigma$ level as explained in Section \ref{['S_EOVSA']}. An additional uncertainty of $4\times {\rm median}(\sigma(f))\times f[{\rm GHz}]^{-1}$, clearly seen as increasing error bars towards lower frequencies, is added to the data to account for the frequency-dependent spatial resolution Gary_etal_2013.
• Figure 4: Histogram of magnetic field values determined for high-frequency source over the entire analyzed duration of the flare. The field is predominantly between 400 and 1000 G.
• Figure 5: Map of the regularized magnetic field values for the high-frequency source from two-source fitting; see Section \ref{['S_bottom']} for details. Pixels where no two-source spectra was detected during the flare evolution are shown in white. The black contour represents EOVSA emission at 5.14 GHz at the 10% level of the peak intensity at 20:20:16.0 UT, while the thick color contours indicate ROIs described in section \ref{['S_bottom']}.