Plasma Composition Evolution in a Solar Flare - The Effect of Reconnection Outflow - SSW

PLASMA COMPOSITION EVOLUTION IN A SOLAR FLARE - THE EFFECT OF RECONNECTION OUTFLOW - ANDY SHU HO TO

We analyse the coronal elemental abundances of the X8.2 flare on 2017 September 10 using spatially resolved measurements from Hinode/EIS (EUV Imaging Spectrometer). Using both the Ca XIV 193.87 Å/Ar XIV 194.40 Å and Fe XVI 262.98 Å/S XIII 256.69 Å composition diagnostic ratios, we show that the flare loops exhibit a large variation of coronal abundances, with the loop tops showing enhanced coronal abundances that decrease to photospheric values toward the footpoints. We propose that this variation is caused by two physical processes. The highly fractionated abundance at the loop top is likely associated with plasma downflow from the plasma sheet, which has coronal abundances. Meanwhile, upflows caused by chromospheric evaporation/ablation fill the loop footpoints with unfractionated photospheric composition. Mixing between these sources produces the observed gradient along the loops. Our findings provide a novel explanation for the composition discrepancy observed between Sun-as-a-star and spatially resolved flare composition measurements, and have significance for understanding loop top brightenings also known as bright knots. Spatially resolved spectroscopy proves critical for revealing these complex abundance structures.