Physical properties of bright Ca II K fibrils in the solar chromosphere^⋆

Institute for Solar Physics, Department of Astronomy, Stockholm University, Albanova University Centre, 10691 Stockholm, Sweden
e-mail: sepideh.kianfar@astro.su.se

Received: 24 January 2020
Accepted: 16 March 2020

Abstract

Context. Broad-band images of the solar chromosphere in the Ca II H&K line cores around active regions are covered with fine bright elongated structures called bright fibrils. The mechanisms that form these structures and cause them to appear bright are still unknown.

Aims. We aim to investigate the physical properties, such as temperature, line-of-sight velocity, and microturbulence, in the atmosphere that produces bright fibrils and to compare those to the properties of their surrounding atmosphere.

Methods. We used simultaneous observations of a plage region in Fe I 6301-2 Å, Ca II 8542 Å, Ca II K, and Hα acquired by the CRISP and CHROMIS instruments on the Swedish 1 m Solar Telescope. We manually selected a sample of 282 Ca II K bright fibrils. We compared the appearance of the fibrils in our sample to the Ca II 8542 Å and Hα data. We performed non-local thermodynamic equilibrium inversions using the inversion code STiC on the Fe I 6301-2 Å, Ca II 8542 Å, and Ca II K lines to infer the physical properties of the atmosphere.

Results. The line profiles in bright fibrils have a higher intensity in their K₂ peaks compared to profiles formed in the surrounding atmosphere. The inversion results show that the atmosphere in fibrils is on average  −100 K hotter at an optical depth log(τ_500 nm) = −4.3 compared to their surroundings. The line-of-sight velocity at chromospheric heights in the fibrils does not show any preference towards upflows or downflows. The microturbulence in the fibrils is on average 0.5 km s⁻¹ higher compared to their surroundings. Our results suggest that the fibrils have a limited extent in height, and they should be viewed as hot threads pervading the chromosphere.