Solar Flare Ribbon Fronts I: Constraining flare energy deposition with IRIS spectroscopy

Abstract

Lower atmospheric lines show peculiar profiles at the leading edge of ribbons during solar flares. In particular, increased absorption of the BBSO/GST \hei~10830~Å line \citep[e.g.][]{Xu2016}, as well as broad and centrally reversed profiles in the spectra of the \mgii~and \cii~lines observed by the \iris~satellite \citep[e.g.][]{Panos2018,Panos2021a} have been reported. In this work, we aim to understand the physical origin of the \iris\ ribbon front line profiles, which seem to be common of many, if not all, flares. To achieve this, we quantify the spectral properties of the \iris~\mgii~ribbon front profiles during four large flares and perform a detailed comparison with a grid of radiative hydrodynamic models using the \radynfp~code. We also studied their transition region counterparts, finding that these ribbon front locations are regions where transition region emission and chromospheric evaporation are considerably weaker compared to other parts of the ribbons. Based on our comparison between the \iris~observations and modelling, our interpretation is that there are different heating regimes at play in the leading and trailing regions of the ribbons. More specifically, we suggest that bombardment of the chromosphere by more gradual and modest non-thermal electron energy fluxes can qualitatively explain the \iris~observations at the ribbon front, while stronger and more impulsive energy fluxes are required to drive chromospheric evaporation and more intense TR emission. Our results provide a possible physical origin for the peculiar behaviour of the \iris~chromospheric lines in the ribbon leading edge and new constraints for the flare models.

References in corpus (18)

• CHIANTI -- an atomic database for emission lines -- Paper XVI: Version 10, further extensions
• Impulsive phase flare energy transport by large-scale Alfven waves and the electron acceleration problem
• Evidence of Non-Thermal Particles in Coronal Loops Heated Impulsively by Nanoflares
• Temporal evolution of multiple evaporating ribbon sources in a solar flare
• The 2014 March 29 X-flare: sub-arcsecond resolution observations of Fe XXI 1354.1
• Spectral signatures of chromospheric condensation in a major solar flare
• Recovering Thermodynamics from Spectral Profiles observed by IRIS: A Machine and Deep Learning Approach
• Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE): II. Flares and Eruptions
• The development of lower-atmosphere turbulence early in a solar flare
• Modeling Mg II h, k and Triplet Lines at Solar Flare Ribbons
• He I 10830 Å Dimming During Solar Flares, I. The Crucial Role of Non-Thermal Collisional Ionisations
• Modelling Mg II During Solar Flares. I. Partial Frequency Redistribution, Opacity, and Coronal Irradiation
• Correlated Spatiotemporal Evolution of Extreme-Ultraviolet Ribbons and Hard X-rays in a Solar Flare
• Exploring mutual information between IRIS spectral lines. I. Correlations between spectral lines during solar flares and within the quiet Sun
• Probing Current Sheet Instabilities from Flare Ribbon Dynamics
• Exploring mutual information between IRIS spectral lines. II. Calculating the most probable response in all spectral windows
• Multi-Passband Observations of A Solar Flare over the He I 10830 Å line
• Comparison of Enhanced Absorption in He I 10830 Å in Observations and Modeling During the Early Phase of a Solar Flare

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• Solar flare ribbons structured by uncombed chromospheric loops
• Evolution of the Ratio of Mg II Intensities During Solar Flares
• Small-Scale and Transient EUV Kernels in Solar Flare Ribbons
• Statistical Study of Balmer Continuum Enhancement in Solar Flares