Halpha line profile asymmetries and the chromospheric flare velocity field

TL;DR

This study investigates Halpha and Ca II 8542 Å line profile asymmetries during solar flares, revealing that these asymmetries are caused by velocity gradients in the chromosphere rather than plasma flows.

Contribution

It combines high-resolution observations with RADYN simulations to clarify the physical origin of line asymmetries in solar flare chromospheres, challenging previous flow-based interpretations.

Findings

Halpha red asymmetry occurs before flare maximum.

Blue asymmetry appears after flare maximum.

Velocity gradients cause line profile asymmetries, not plasma flows.

Abstract

The asymmetries observed in the line profiles of solar flares can provide important diagnostics of the properties and dynamics of the flaring atmosphere. In this paper the evolution of the Halpha and Ca II 8542 {\AA} lines are studied using high spatial, temporal and spectral resolution ground-based observations of an M1.1 flare obtained with the Swedish 1-m Solar Telescope. The temporal evolution of the Halpha line profiles from the flare kernel shows excess emission in the red wing (red asymmetry) before flare maximum, and excess in the blue wing (blue asymmetry) after maximum. However, the Ca II 8542 {\AA} line does not follow the same pattern, showing only a weak red asymmetry during the flare. RADYN simulations are used to synthesise spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. We show that the red asymmetry observed in Halpha is not necessarily associated with plasma downflows, and the blue asymmetry may not be related to plasma upflows. Indeed, we conclude that the steep velocity gradients in the flaring chromosphere modifies the wavelength of the central reversal in the Halpha line profile. The shift in the wavelength of maximum opacity to shorter and longer wavelengths generates the red and blue asymmetries, respectively.