Spectro-polarimetric Properties of White-Light Flare Kernels and Sunquake Sources

Alexander Kosovichev, Samuel Granovsky, John Stefan (NJIT), Viacheslav Sadykov (GSU), Graham Kerr, Joel Allred (GSFC), Ivan Sharykin and Ivan Zimovets (IKI)

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Excitation of helioseismic waves in solar flares – "sunquakes"

• Sunquakes are observed in Doppler velocity images in the photosphere as expanding ring waves.�These phenomena are relatively rare: out of 500 M-X-class flares observed in the 24th cycle, seismic waves were recorded in 94 cases (Sharykin and Kosovichev, 2020)�Sunquakes are often observed in confined impulsive flares without CMEs.
• The sunquake sources are observed in photospheric Dopplergrams, magnetograms and continuum images (“white-light flare kernels”) as compact (1-3 arcsec) short (1-3 min) impacts at the beginning of the impulsive phase. �The sunquakes and white-light kernels are not explained within the framework of standard models of flares with electron beams.�The new results show that such an explanation can be found in proton beam models.

X2.6 Flare Observations,

July 9, 1996 with SoHO/MDI

Statistical analysis

• Statistical analysis showed that helioseismically active solar flares are characterized by significantly larger fluxes of non-thermal X-ray emission compared to flares without photospheric impacts.
• The amplitude of sunquakes correlates with the growth rate of the X-ray flux.
• A linear relationship between helioseismic energy and the total flux of non-thermal X-ray radiation and the total energy of accelerated electrons is found.
• No correlation with the power-law index of the nonthermal X-ray spectrum was found.

Example: X1.5 white-light flare, May 10, 2022 – observations in the deep photospheric line Fe I 6173 A (SDO/HMI)

Continuum intensity

Doppler velocity

The two main sources of white light emission and seismic waves, 1-2 arcsec in size, are located along the magnetic polarity inversion line (PIL).

Spatio-temporal diagram seismic waves

Maps of intensity (top) and magnetic field changes along the line of sight (bottom)

The sources are located in areas with a strong vertical electric current in a low-lying magnetic field arcade along the PIL.

Location of sources on electric current density maps.

Flare disturbances propagate along the arcade at a speed of 50-100 km/s

Zimovets et al. (2016), Kuznetsov et al. (2016)

Magnetic field lines in a NFFF model.

The continuum emission peak was observed before the hard X-ray peaks

Spectro-polarimetric characteristics of a sunquake source (SDO/HMI): �X1.5 flare, May 10, 2022

Location of the source on the intensity maps (left top panel).

Intensity variations in the wing and core of the line (middle panel). �The line profile (left top panel) shows emission in the core of the line.

Variations of the Stokes parameter profiles I, Q, U, V show abrupt changes in the horizontal and vertical components of the magnetic field.

Stokes I Stokes Q Stokes U Stokes V

HMI observations of X9.3 flare of September 6, 2017, revealed several locations within the umbra or along the umbra/penumbra boundary where the Stokes I of Fe I 6173A line was in full emission

Modeling of photospheric sources of flares and seismic waves

Initial parameters of electron and proton beams:�Total Energy Flux Fd, the�energy spectrum index δ, the low cutoff energy Ec

Fokker-Planck code + radiative hydrodynamics RADYN (joe Allred),

3D Acoustic code

(JT Stefan) and

3D Radiative MHD (S.Granovsky, I.Kitiashvili)

Solution of NLTR Radiation Transport Equations (RH1.5D), Stokes Profile Calculation, and Simulation of SDO/HMI Observations

Comparison with observed space-time diagrams and seismic wave amplitudes

Comparison with observed Stokes parameters

RADYN models with electron beams (including FCHROMA models) did not show the inversion of the Fe I 6173A line.

RADYN models with proton beams (Graham Kerr):�Energy flux Fd=10¹¹ erg cm⁻² s⁻¹, injection time 20 s, spectral slope δ=[3, 5]�Low energy cutoff Ec=[50, 100, 150, 250, 500, 1000, 3000] keV

Find a consistent model

The modeling results show a reversal in the line core for proton beams with a high low energy cutoff, E_c> 1 Mev

High-energy proton beams can heat the lower photosphere at high temperatures and explain the emission in the line core, but not the full line inversion.�

.��Dashed line is an empirical model for the X1 flare, March 29, 2014 (Kleint et al. 2016)

Current solution: the Stokes I is inverted when the particle beam heats low-temperature regions of sunspot umbra or penumbra

Inverted synthetic Stokes I

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Modified temperature profile of a RADYN model for proton beam with Fd= 10¹¹ erg cm⁻² s⁻¹, injection time 20 s, δ=3, Ec=3 MeV.

(Granovsky et al., 2025)

Conclusions

• Strong localized disturbances at the beginning of the impulsive phase of flares excite seismic waves inside the Sun, which are observed on the surface as ring waves.�The sources of these disturbances are usually located near the PIL, coincide with white-light kernels and variations in the magnetic field. In time, they correlate with the hard X-ray impulses, which indicates the effect of accelerated particles on the lower layers of the solar atmosphere.�Such perturbations cannot be explained within the framework of the standard radiative hydrodynamics (RADYN) model with electron beams.�However, proton beams penetrate into the deeper layers of the solar atmosphere, and can explain the white-light kernels in and the inversion of the HMI Stokes I. In addition, our models show that low-energy protons carry enough momentum to excite seismic waves. �Future work: include the combined effects of electron and proton beams and 3D radiative MHD modeling.

Simulations of helioseismic waves show that waves with a large amplitude of velocity exceeding 100 m/s at a distance of 18 Mm from the source, in agreement with observations, can, on the contrary, be excited by low-energy proton beams.

Graphs of the logarithm of the velocity amplitude of seismic beams as a function of distance from the source for proton beams with different threshold energies.

In this case, the main mechanism is the direct transfer of momentum by a stream of low-energy protons.

Global Acoustic Model of the Sun

photosphere

corona

corona

chromosphere

convection zone

The interaction of the coronal wave with the chromosphere leads to the excitation of an oblique acoustic wave that propagates from the transition zone to the photosphere. Due to the large inclination, the perturbation velocity at a given altitude in the chromosphere is much higher than the local velocity of sound – observed as Morton waves observed in Hα - “the scissors effect" - the apparent supersonic motion.

In the standard model of flares, disturbances in the Sun's lower atmosphere are caused by beams of accelerated electrons

Electrons

Photospheric sources

Shibata et al. (1997)

Radiation Gas-Dynamic Model of Flares

Radiative shock.

Compression behind the shock wave front (chromospheric condensation) can explain the glow in white light, but it does not explain the perturbations observed in the lower photosphere.

(Pikelner and Kostyuk, 1975; Livshits et al., 1980)