On the magnetic and thermodynamic properties of dark fibrils in the chromosphere

TL;DR

This study uses advanced spectropolarimetric inversions across multiple spectral lines to analyze the thermodynamic and magnetic properties of dark fibrils in the solar chromosphere, revealing temperature and magnetic field variations along fibrils.

Contribution

The paper introduces a multi-line, multi-atom inversion approach with the STiC code to better constrain fibril properties in the solar chromosphere.

Findings

Fibrils have hot footpoints around 5900 K.

Temperature decreases by about 250 K along fibrils.

Magnetic field strength is nearly twice as large at footpoints.

Abstract

Fibrillar structures are ubiquitous in the solar chromosphere and their potential for mediating the mass and energy transport in the solar atmosphere is undeniable. An accurate determination of their properties requires the use of advanced high-resolution observations which are now becoming broadly available from different observatories. We exploit the capabilities of multi-atom, multi-line spectropolarimetric inversions using the Stockholm Inversion Code (STiC). Non-local thermodynamic equilibrium inversions of a fibril-rich area are performed using spectropolarimetric observations in the Ca II 854.2 nm line obtained with the CRISP imaging spectropolarimeter and spectroscopic observations in the Ca II H line obtained with the CHROMospheric Imaging Spectrometer (CHROMIS) at the Swedish 1-meter Solar Telescope (SST). Additionally, coobservations in the Mg II h & k lines obtained with the Interface Region Imaging Spectrograph (IRIS) are used in the inversions to better constrain the thermodynamic properties of the fibrils. The incorporation of multiple atomic species and spectral lines proves to better constrain the properties of the plasma constituting the fibrils. In particular, the tracing of a large number of fibrils allowed for the study of the variation of the temperature and magnetic field along their projected length over the field of view. The results provide a view of fibrils possessing hot footpoints of about 5 900 K. The temperature drop away from the footpoints is on average 250 K, with a larger drop of around 500 K for the longer fibrils. The magnetic field is also reported to be larger at the footpoints, being almost twice as large as the minimum value reported at the middle point of the fibrils.