Speakers
Description
During solar flares, nonthermal heating of the chromosphere induces overpressure, leading to explosive evaporation and consequent condensation to conserve momentum. While this phenomenon has been widely observed via Doppler shifts in various chromospheric and transition region lines, it has yet to be observed in the Si III 1206 Å line (logT≈4.7) which is set to be included in the SOLAR-C/EUVST LW3 band (1115-1275 Å). We analyse recently released spectral observations from SORCE/SOLSTICE, which rastered across the Si III 1206 Å and Lyman-alpha lines at a cadence of ~1 minute, to study the Doppler velocity in the Si III line for seven M- and C-class flares. By subtracting a pre-flare profile from disk-integrated flare spectra, we isolate the flaring component and fit Gaussians to measure Doppler shifts. Of the seven events, six exhibited redshifts in the line profile, indicative of condensation, with velocities reaching up to ~100 km/s. Intriguingly, two events show blueshifts, suggestive of upflowing material, with one transitioning from blueshift to redshift at the flare peak. These observed upflows may be chromospheric bubbles, where cooler chromospheric material is lifted by underlying evaporating material. Our findings establish the Si III line as a novel diagnostic for chromospheric mass motions, offering crucial insights into flare driven dynamics. These results should inform the interpretation of upcoming high-resolution SOLAR-C/EUVST imaging spectroscopy observations of Si III during Solar Cycle 25.
