The solar chromosphere at high resolution with IBIS: III. Comparison of Ca II K and Ca II 854.2 nm imaging

TL;DR

This study compares Ca II K filtergrams with spectrally resolved Ca II 854.2 nm images to understand observational effects on chromospheric magnetic structures, revealing that filtergrams may not accurately reflect high-frequency chromospheric behavior.

Contribution

The paper demonstrates that observational effects significantly influence Ca II K filtergrams, affecting the visibility of magnetic structures and high-frequency chromospheric dynamics, and compares these with Ca II 854.2 nm imaging.

Findings

Ca II K filtergrams lack apparent magnetic structures due to observational effects.

Fibrils are visible in Ca II 854.2 nm images but not in Ca II K filtergrams.

Ca II H filtergrams do not show significant chromospheric signatures in quiet Sun conditions.

Abstract

Filtergrams obtained in Ca II H, Ca II K and H-alpha are often employed as diagnostics of the solar chromosphere. However, the vastly disparate appearance between the typical filtergrams in these different lines calls into question the nature of what is actually being observed. We investigate the lack of obvious structures of magnetic origin such as fibrils and mottles in on-disk Ca II H and K images by directly comparing a temporal sequence of classical Ca II K filtergrams with a co-spatial and co-temporal sequence of spectrally resolved Ca II 854.2 images obtained with the Interferometric Bidimensional Spectrometer (IBIS), considering the effect of both the spectral and spatial smearing. We find that the lack of fine magnetic structuring in Ca II K filtergrams, even with the narrowest available filters, is due to observational effects. Signatures of fibrils remain however in the temporal evolution of the filtergrams, in particular with the evidence of magnetic shadows around the network elements. The Ca II K filtergrams do not appear, however, to properly reflect the high-frequency behavior of the chromosphere. Using the same analysis, we find no significant chromospheric signature in the Hinode/SOT Ca II H quiet-Sun filtergrams. The picture provided by H-alpha and Ca II 854.2, which show significant portions of the chromosphere dominated by magnetic structuring, appears to reflect the true and essential nature of the solar chromosphere. Data which do not resolve, spatially or spectrally, this aspect may misrepresent the behavior the chromosphere.