Spectroscopic diagnosis of a B-class flare and an associated filament eruption

Abstract

The flare ribbon and an associated filament eruption are diagnosed using O IV 1401.16 Å, Si IV 1402.77 Å, and Mg II k 2796.35 Å spectral lines provided by Interface Region Imaging Spectrograph (IRIS). The flare ribbons have downflow (redshifts) in all these lines, and this redshift decreases from the transition region to the chromosphere. While the overlapping region (flare-ribbon+filament rise/eruption) is dominated by upflows (blueshifts) in all three spectral lines. We found an extremely blueshifted Si IV profile (i.e., blueshift around ‑180 km/s) in the overlapping region. The mean non-thermal velocity (v_nt) in the flare ribbons is higher in O IV than Si IV. While, in the overlapping region, O IV have lower v_nt than Si IV. Note that very high v_nt around 80 km/s (in Si IV) exists in this weak B-class flare. The Mg II k line widths are almost the same in the flare ribbon and overlapping region but, they are extremely broad than previously reported. We found double peak profiles of Si IV and O IV in the overlapping region. Most probably, one peak is due to downflow (flare ribbon) and another due to upflow (filament rise/eruption). We report a high redshift of more than 150 km/s in the weak B-class flare. In some cases, both peaks show upflows which might be the result of the superposition of two different sources, i.e., overlapping of two different velocity distributions in the line of sight.