Blueshifted Si IV 1402.77{\AA} line profiles in a moving flare kernel observed by IRIS

TL;DR

This study presents high-resolution IRIS spectroscopic observations of a solar flare kernel, revealing blueshifted Si IV lines and dynamic plasma motions that challenge existing flare models.

Contribution

It provides the first high-cadence, high-resolution observations of a moving flare kernel showing blueshifted lines, offering new constraints for flare modeling.

Findings

Blueshifted Si IV 1402.77 Å line observed with Doppler velocities over 50 km/s.

Concurrent blueshifts in C II and Mg II lines indicating upward plasma motions.

The blueshifted region corresponds to the kernel front accelerating through a weak magnetic field area.

Abstract

We analyze spectra of a slipping flare kernel observed during the 2015 June 22 M6.5-class flare by the Interface Region Imaging Spectrograph (IRIS). During the impulsive and peak phases of the flare, loops exhibiting an apparent slipping motion along the ribbons were observed in the 131{\AA} channel of SDO/AIA. The IRIS spectrograph slit observed a portion of the ribbons, including a moving kernel corresponding to a flare loop footpoint in Si IV, C II, and Mg II at a very-high 1 s cadence. The spectra observed in the kernel were mostly redshifted and exhibited pronounced red wings, as typically observed in large flares. However, in a small region in one of the ribbons, the Si IV 1402.77{\AA} line was partially blueshifted, with the corresponding Doppler velocity |v_{D}| exceeding 50 km s$^{-1}$. In the same region, the C II 1334.53{\AA}, 1335.66{\AA} and 1335.71{\AA} lines were weakly blueshifted (|v_{D}| < 20 km s$^{-1}$) and showed pronounced blue wings, which were observed also in the Mg II k 2796.35{\AA} as well as the Mg II triplet 2798.75{\AA} and 2798.82{\AA} lines. Using high-cadence AIA observations we found that the region where the blueshifts occurred corresponds to the accelerating kernel front as it moved through a weak-field region. The IRIS observations with high resolution allowed us to capture the acceleration of the kernel under the slit for the first time. The unique observations of blueshifted chromospheric and TR lines provide new constrains for current models of flares.