Strong Photospheric Heating Indicated By Fe I 6173 Å Line Emission During White-Light Solar Flares

The origin of WLFs, particularly their compact and transient cores, has been a subject of long-standing debates. Spectroscopic and photometric studies suggest that the observed white-light emission originates from heating of the lower chromosphere and upper photosphere. Traditional thick-target electron beam models of solar flares struggle to explain WLFs, as most energy is deposited in the upper chromosphere. Reconciling observations with models that include electron and proton beams has proven challenging, as current models often require unrealistic particle energies or fail to match the observed phenomena. SDO HMI observations have revealed that WLF events coincide with significant variations, including line-core emission, in the Fe I 6173 Å line profile, a deep photospheric line, along with excitation of helioseismic acoustic waves (sunquakes). This provides unambiguous evidence that the flare hydrodynamic impacts affect photospheric and subphospheric layers on the Sun. While RADYN proton beam models suggest that strong line core emission can be explained by proton beams with cut-off energies Ec ≥ 500 keV, stronger helioseismic impacts are driven by proton beams with Ec ≤ 100 keV. A unified model explaining both X-ray and optical observations of WLF cores remains undeveloped.