Temperature diagnostics of chromospheric fibrils

TL;DR

This study investigates the temperature structure of chromospheric fibrils using spectroscopic data, revealing temperature variations along fibrils and highlighting the limitations of current lines and techniques for accurate thermodynamic inference.

Contribution

It demonstrates the temperature variation along fibrils and discusses the limitations of Ca ii 854.2 nm line inversions, emphasizing the need for more temperature-sensitive lines like Mg ii h & k.

Findings

Fibril footpoints are on average 300 K hotter than midpoints.

Current spectral lines are insufficient to fully resolve temperature structures.

Adding lines like Mg ii h & k improves thermodynamic inferences.

Abstract

Context. Chromospheric fibrils are thin and elongated structures that connect nearby photospheric magnetic field concentrations of opposite polarities. Aims. We assess the possibilities and drawbacks related to the use of current instrumentation and inversion techniques to infer the thermodynamic structure of chromospheric fibrils. Methods. We employed spectroscopic observations obtained in the Ca ii 854.2 nm line with the CRISP instrument at the Swedish 1-m Solar Telescope and in coordination with observations in the ultraviolet Mg ii h & k lines taken with the IRIS satellite. We studied the temperature sensitivity of these chromospheric lines to properly invert their spectral profiles with the Stockholm inversion Code and determine the temperature, line-of-sight velocity, and microturbulent velocity of manually traced chromospheric fibrils present in the field of view. Results. Fibril-like structures show a very particular dependence of their temperature as a function of the position along their length. Their temperatures at the detected footpoints are, on average, 300 K higher than the temperature at the midpoint. The temperature variation appears to be almost symmetrical in shape, with partially traced fibrils showing a similar trend for the temperature variation. Additionally, the response of the Ca ii 854.2 nm line core to variations of the temperature for the inverted models of the atmosphere in fibril areas seems to be insufficient to properly resolve the aforementioned temperature structure. Only the addition of more temperature sensitive lines such as the Mg ii h & k lines would make it possible to properly infer the thermodynamic properties of chromospheric fibrils. Comparisons between the results obtained here and in previous studies focused on bright Ca ii K fibrils yield great similarities between these structures in terms of their temperature.