Spectral Characteristics and Formation Height of Off-Limb Flare Ribbons

TL;DR

This study combines high-resolution imaging spectroscopy and radiative hydrodynamic simulations to analyze the spectral characteristics and formation heights of off-limb solar flare ribbons, revealing their distinct observational features and energy deposition locations.

Contribution

First detailed comparison of off-limb flare ribbon observations with radiative hydrodynamic models, elucidating formation heights and spectral signatures in Hβ and Ca II lines.

Findings

Hβ ribbons appear as bright horizontal layers near flare footpoints.

Ca II 8542 Å ribbons are fainter and located at the limb edge.

Hβ line wing intensities effectively trace flare energy deposition.

Abstract

Flare ribbons are bright manifestations of flare energy dissipation in the lower solar atmosphere. For the first time, we report on high-resolution imaging spectroscopy observations of flare ribbons situated off-limb in the H$\beta$ and Ca II 8542 {\AA} lines and make a detailed comparison with radiative hydrodynamic simulations. Observations of the X8.2-class solar flare SOL2017-09-10T16:06 UT obtained with the Swedish Solar Telescope reveal bright horizontal emission layers in H$\beta$ line wing images located near the footpoints of the flare loops. The apparent separation between the ribbon observed in the H$\beta$ wing and the nominal photospheric limb is about 300 - 500 km. The Ca II 8542 {\AA} line wing images show much fainter ribbon emissions located right on the edge of the limb, without clear separation from the limb. RADYN models are used to investigate synthetic spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. The simulations show that, towards the limb, where the line of sight is substantially oblique with respect to the vertical direction, the flaring atmosphere model reproduces the high contrast of the off-limb H$\beta$ ribbons and their significant elevation above the photosphere. The ribbons in the Ca II 8542 {\AA} line wing images are located deeper in the lower solar atmosphere with a lower contrast. A comparison of the height deposition of electron beam energy and the intensity contribution function shows that the H$\beta$ line wing intensities can be an useful tracer of flare energy deposition in the lower solar atmosphere