The solar chromosphere at high resolution with IBIS^*

I. New insights from the Ca II 854.2 nm line

¹ INAF – Osservatorio Astrofisico di Arcetri, 50125 Firenze, Italy e-mail: gcauzzi@arcetri.astro.it
² National Solar Observatory, PO Box 62, Sunspot NM 88349, USA
³ Dipartimento di Astronomia, Università di Firenze, 50125 Firenze, Italy
⁴ Kipenheuer Institute für Sonnenphysics, 79104 Freiburg, Germany

Received: 10 September 2007
Accepted: 30 November 2007

Abstract

Context.The chromosphere remains a poorly understood part of the solar atmosphere, as current modeling and observing capabilities are still ill-suited to investigating its fully 3-dimensional nature in depth. In particular, chromospheric observations that can preserve high spatial and temporal resolution while providing spectral information over extended fields of view are still very scarce.

Aims.In this paper, we seek to establish the suitability of imaging spectroscopy performed in the Ca II 854.2 nm line as a means of investigating the solar chromosphere at high resolution.

Methods.We utilize monochromatic images obtained with the Interferometric BIdimensional Spectrometer (IBIS) at multiple wavelengths within the Ca II 854.2 nm line and over several quiet areas. We analyze both the morphological properties derived from narrow-band monochromatic images and the average spectral properties of distinct solar features such as network points, internetwork areas, and fibrils.

Results.The spectral properties derived over quiet-Sun targets are in full agreement with earlier results obtained with fixed-slit spectrographic observations, highlighting the reliability of the spectral information obtained with IBIS. Furthermore, the very narrowband IBIS imaging reveals very clearly the dual nature of the Ca II 854.2 nm line. Its outer wings gradually sample the solar photosphere, while the core is a purely chromospheric indicator. The latter displays a wealth of fine structures including bright points akin to the Ca II H and K grains, and as fibrils originating from even the smallest magnetic elements. The fibrils occupy a large fraction of the observed field of view, even in the quiet regions, and clearly outline atmospheric volumes with different dynamical properties, strongly dependent on the local magnetic topology. This highlights how 1D models stratified along the vertical direction can provide only a very limited representation of the actual chromospheric physics.

Conclusions.Imaging spectroscopy in the Ca II 854.2 nm line currently represents one of the best observational tools for investigating the highly structured and highly dynamical chromospheric environment. A high-performance instrument such as IBIS is crucial in achieving the necessary spectral purity and stability, spatial resolution, and temporal cadence.