The solar chromosphere at high resolution with IBIS. I. New insights from the Ca II 854.2 nm line

摘要

(Abridged) Aims: In this paper, we seek to establish the suitability of imagingspectroscopy performed in the Ca II 854.2 nm line as a means to investigate thesolar chromosphere at high resolution. Methods: We utilize monochromatic images obtained with the InterferometricBIdimensional Spectrometer (IBIS) at multiple wavelengths within the Ca II854.2 nm line and over several quiet areas. We analyze both the morphologicalproperties derived from narrow-band monochromatic images and the averagespectral properties of distinct solar features such as network points,internetwork areas and fibrils. Results: The spectral properties derived over quiet-Sun targets are in fullagreement with earlier results obtained with fixed-slit spectrographicobservations, highlighting the reliability of the spectral information obtainedwith IBIS. Furthermore, the very narrowband IBIS imaging reveals with muchclarity the dual nature of the Ca II 854.2 nm line: its outer wings graduallysample the solar photosphere, while the core is a purely chromosphericindicator. The latter displays a wealth of fine structures including brightpoints, akin to the Ca II H2V and K2V grains, as well as fibrils originatingfrom even the smallest magnetic elements. The fibrils occupy a large fractionof the observed field of view even in the quiet regions, and clearly outlineatmospheric volumes with different dynamical properties, strongly dependent onthe local magnetic topology. This highlights the fact that 1-D modelsstratified along the vertical direction can provide only a very limitedrepresentation of the actual chromospheric physics.