Estimating the Chromospheric Absorption of Transition Region Moss Emission

TL;DR

This study investigates how chromospheric absorption affects EUV emission measurements in the transition region moss, revealing significant absorption factors and improving the understanding of coronal heating models.

Contribution

It demonstrates that chromospheric absorption significantly impacts EUV emission observations in transition region moss, providing a new correction factor for coronal heating models.

Findings

Absorption of EUV emission in moss is about a factor of 2.

Observed absorption is well reproduced by 3D radiative MHD models.

Increased line-of-sight angles lead to more absorption.

Abstract

Many models for coronal loops have difficulty explaining the observed EUV brightness of the transition region, which is often significantly less than theoretical models predict. This discrepancy has been addressed by a variety of approaches including filling factors and time-dependent heating. Here we focus on an effect that has been ignored so far: the absorption of EUV light with wavelengths below 912 {\AA} by the resonance continua of neutral hydrogen and helium. Such absorption is expected to occur in the low-lying transition region of hot, active region loops, that is co-located with cool chromospheric features and called ``moss'' as a result of the reticulated appearance resulting from the absorption. We use co-temporal and co-spatial spectroheliograms obtained with SOHO/SUMER and Hinode/EIS of Fe XII 1242 {\AA}, 195 {\AA} and 186.88 {\AA}, and compare the density determination from the 186/195 {\AA} line ratio to that resulting from the 195/1242 {\AA} line ratio. We find significant absorption of 195 {\AA} emission caused by the chromospheric inclusions in the moss. We find that the amount of absorption is generally of order a factor of 2. We compare to numerical models and show that the observed effect is well reproduced by 3D radiative MHD models of the transition region and corona. We use STEREO A/B data of the same active region and find that increased angles between line-of-sight and local vertical cause additional absorption. Our determination of the amount of chromospheric absorption of TR emission can be used to better constrain coronal heating models.