Alexander G. Kosovichev^1,2
1. Department of Physics, New Jersey Institute of Technology, Newark, NJ 07102
2. NASA Ames Research Center, Moffett Field, Mountain View, CA 94040

Summary: A spectro-polarimetric analysis of the sunquake sources observed during the X1.5 flare of May 10, 2022, revealed transient emission in the FeI 6173Å line core, indicating intense, impulsive heating in the lower photosphere at the beginning of the flare impulsive phase. In addition, the observed variations of the Stokes profiles provide unambiguous evidence of fast permanent changes in the magnetic field strength and geometry in the sunquake sources. The results suggest that proton beams, along with electron beams, play a key role in solar flares.

The first significant sunquake of Solar Cycle 25 observed during the X1.5 flare of May 10, 2022, revealed intriguing features that shed light on the origin of sunquakes^[1]. The HMI Dopplergrams revealed two sets of expanding ripples, indicating the presence of two sunquake sources separated in space and time (Figure 1). Each source had a double impact structure corresponding to the footpoints of low-lying magnetic flux tubes across the polarity inversion line.

Figure 1| The photospheric flare impacts observed by the HMI instrument around 13:56:20 UT (Source 1) and 13:57:50 UT (Source 2). Panels a) and c) show the relative continuum intensity enhancements during 45 sec of the impacts; panels b) and d) show the line-of-sight magnetograms during the flare impacts. The contour lines show the corresponding Doppler velocity signals for 3, 4, 5, 6, and 7 km/s. Solid lines show the positive (red-shifted) velocity, and dashed lines show the negative velocity; e-f) the time-distance diagrams obtained by the angular averaging of the frequency-filtered Dopplergrams with the central points located in the Source 1 and 2 areas.

The Stokes profiles of the Fe I 6173Å line (Figure 2) show substantial rapid variations during two or three 90-second observing sequences. The line intensity increases by 10-30% in the line continuum (far line wings) and by 30-50% in the line core. In the line core, the peak intensity is reached earlier than in the line continuum, indicating that the impact affected higher layers of the solar photosphere earlier than the lower layers. At the peak values, the line core was in emission. After the impulsive variations, the Stokes I profile quickly returns to the pre-flare values, indicating the rapid cooling of the photosphere. However, the Stokes Q, U, and V profiles, characterizing linear and circular polarization, show permanent changes, indicating changes in the magnetic field geometry and strength.

Figure 2| Source 1a: a) image of the Fe I 6173Å line continuum intensity time difference during the flare impact; b) image of the line core intensity time difference during the flare impact; c) the temporal behavior of the line continuum (solid curve) and core (dashed curve) averaged over the black box in panels a) and b); d) variations of the line profile at different time moments before, during and after the impact; e) the corresponding variations of Stokes I, Q, U, and V line profiles.

The results suggests that the flare energy release, which occurred in low-lying magnetic loops, produced high-energy electrons and protons. The electrons, precipitating into the high chromosphere, produced hard X-ray emission at the loop footpoints. The high-energy protons penetrating deeper into the photosphere heated it to high temperatures, which produced the flare white light and emission in the core of the HMI line, while the low-energy protons delivered the momentum sufficient for the generation of sunquakes.

Figure 3|(a) Variation of the Stokes I, (b) line profiles at various time moments before, during and after the impact, and (c) the corresponding temperature profiles in the low atmosphere, calculated in the radiative hydrodynamics (RADYN) model driven by a proton with the energy flux of 10¹¹ erg cm^-2 s^-1, the power law index γ=3, and the low energy cut-off of E_c=1 MeV; (d-f) the same as panels (a-c) for E_c=3 MeV (Ref. [2]). The dashed line shows the semi-empirical model of a white-light flare, obtained by Kleint et al.^[3].

References

[1] Kosovichev, A. G., Sadykov, V. M., & Stefan, J. T. 2023, ApJ, 958, 160
[2] Sadykov, V. M., Stefan, J. T., Kosovichev, A. G., et al. 2024, ApJ, 960, 80
[3] Kleint, L., Heinzel, P., Judge, P., & Krucker, S. 2016, ApJ, 816, 88