An Optically Thin View of the Flaring Chromosphere: Nonthermal widths in a chromospheric condensation during an X-class solar flare

TL;DR

This study uses IRIS observations of an X-class solar flare to analyze optically thin chromospheric lines, constraining nonthermal widths and testing flare models, revealing limitations in current simulations of line broadening.

Contribution

It provides the first detailed analysis of nonthermal widths in optically thin lines during a flare, challenging the assumption of high microturbulence in models.

Findings

Nonthermal widths of about 10 km/s were observed in flare chromosphere lines.

Current models can reproduce some line features but underestimate line widths.

Microturbulence of 10 km/s alone does not fully explain observed line broadening.

Abstract

The bulk of solar flare energy is deposited in the chromosphere. Flare ribbons and footpoints in the chromosphere therefore offer great diagnostic potential of flare energy release and transport processes. High quality observations from the IRIS spacecraft have transformed our view of the Sun's atmospheric response to flares. Since most of the chromospheric lines observed by IRIS are optically thick, forward modelling is required to fully appreciate and extract the information they carry. Reproducing certain aspects of the Mg II lines remain frustratingly out of reach in state-of-the-art flare models, which are unable to satisfactorily reproduce the very broad line profiles. A commonly proposed resolution to this is to assert that very large values of `microturbulence' is present. We asses the validity of that approach by analysing optically thin lines in the flare chromosphere from the X-class flare SOL2014-10-25T17:08:00, using the derived value of nonthermal width as a constraint to our numerical models. A nonthermal width of the order 10~km~s$^{-1}$ was found within the short-lived red wing components of three spectral lines, with relatively narrow stationary components. Simulations of this flare were produced, and in the post-processing spectral synthesis we include within the downflows a microturbulence of 10~km~s$^{-1}$. While we can reproduce the O I 1355.598~\AA\ line rather well, and we can capture the general shape and properties of the Mg II line widths, the synthetic lines are still too narrow.