Observations and Modeling of Line Asymmetries in Chromospheric Flares

TL;DR

This paper reviews observations and models of spectral line asymmetries in solar chromospheric flares, emphasizing the importance of non-LTE radiative transfer techniques for interpreting plasma flows and chromospheric evaporation.

Contribution

It provides a comprehensive overview of observational data, critiques classical methods, and introduces advanced non-LTE modeling approaches for analyzing line asymmetries in flares.

Findings

Line asymmetries are caused by vertical plasma motions in flares.

Non-LTE modeling improves interpretation of chromospheric line shifts.

Reconnection and hydrodynamical models explain observed plasma flows.

Abstract

For many years various asymmetrical profiles of different spectral lines emitted from solar flares have been frequently observed. These asymmetries or line shifts are caused predominantly by vertical mass motions in flaring layers and they provide a good diagnostics for plasma flows during solar flares. There are many controversial results of observations and theoretical analysis of plasma flows in solar chromospheric flares. The main difficulty is the interpretation of line shifts or asymmetries. For many years, methods based on bisector techniques were used but they give a reliable results only for some specific conditions and in most cases cannot be applied. The most promising approach is to use the non-LTE techniques applied for flaring atmosphere. The calculation of synthetic line profiles is performed with the radiative transfer techniques and the assumed physical conditions correspond to flaring atmosphere. I will present an overview of different observations and interpretations of line asymmetries in chromospheric flares. I will explain what we have learnt about the chromospheric evaporation in the frame of hydrodynamical models as well as reconnection models. A critical review will be done on the classical methods used to derive Doppler-shifts for optically thick chomospheric lines. In particular, details on the new approach for interpreting chromospheric line asymmetries based on the non-LTE techniques will be presented.