Temperature Diagnostics of Chromospheric Fibrils using DKIST/ViSP Observations: K-Means Clustering Approach

Abstract

The chromosphere is a critical layer of the solar atmosphere situated between the photosphere and the corona. Studying its temperature structure is important to understand the complex dynamics and energy-transfer processes between these layers. We investigate the thermodynamic properties of chromospheric fibrils adjacent to a plage region using high-resolution DKIST/ViSP observations of the Ca II 854.2 nm spectral line. We analyze the spectral profiles with the non-LTE inversion code NICOLE combined with K-means clustering. The high spectral and spatial resolution of the DKIST observations allows us to trace thermodynamic properties temperature, density, line-of-sight velocity, and microturbulent velocity along and across the fibrils. We note that while the thermodynamic parameters are retrieved under the assumption of hydrostatic equilibrium, the resulting density and temperature values should be interpreted with the caveat that dynamic and magnetic terms are neglected. The temperature along the fibril length decreases smoothly by about 1000 K from the hotter footpoints toward the mid-axis. The temperature variation across the lateral boundary of fibrils is more abrupt and can vary by several hundreds of degree Kelvin across a megameter. Denser fibrils tend to be associated with cooler, downflowing plasma, while less-dense fibrils do not show this trend. Furthermore, the hotter parts of the fibrils tend to exhibit higher microturbulent velocities than the cooler parts.