Spectropolarimetric inversions of the Ca ii 8542 Å line in a M-class solar flare

Abstract

We study the M1.9 class solar flare SOL2015-09-27T10:40 UT using high-resolution full-Stokes imaging spectropolarimetry of the Ca ii 8542 Å line obtained with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the non-LTE code NICOLE are used to construct semi-empirical models of the flaring atmosphere to investigate the structure and evolution of the flare temperature and magnetic field. A comparison of the temperature stratification in flaring and non-flaring areas reveals strong heating of the flare ribbon during the flare peak. The polarization signals of the ribbon in the chromosphere during the flare maximum become stronger when compared to its surroundings and to pre- and post- flare profiles. Furthermore, a comparison of the response functions to perturbations in the line-of-sight magnetic field and temperature in flaring and non-flaring atmospheres shows that during the flare the Ca ii 8542 Å line is more sensitive to the lower atmosphere where the magnetic field is expected to be stronger. The chromospheric magnetic field was also determined with the weak-field approximation which led to results similar to those obtained with the NICOLE inversions.

arXiv comment11 pages, 9 figures, accepted in ApJ

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• Spectral Characteristics and Formation Height of Off-Limb Flare Ribbons
• Chromospheric velocities in an M3.2 flare using He I 1083.0 nm and Ca II 854.2 nm
• Semi-empirical models of spicule from inversion of Ca II 8542 Å line
• Measurement of Vector Magnetic Field in a Flare kernel with a Spectropolarimetric Observation in He I 10830 A
• Limitations of the Ca II 8542 Å line for the determination of magnetic field oscillations
• Impact of opacity effects on chromospheric oscillations inferred from NLTE inversions
• On the Importance of Ca II Photoionisation by the Hydrogen Lyman Transitions in Solar Flare Models
• On the effect of oscillatory phenomena on Stokes inversion results
• Designing wavelength sampling for Fabry-Pérot observations. Information-based spectral sampling
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• Stratification of canopy magnetic fields in a plage region. Constraints from a spatially-regularized weak-field approximation method
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