Estimating the Chromospheric Absorption of Transition Region Moss Emission

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 Å 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 Å, 195 Å and 186.88 Å, and compare the density determination from the 186/195 Å line ratio to that resulting from the 195/1242 Å line ratio. We find significant absorption of 195 Å 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.

24 pages, 7 figures, accepted for publication in ApJ

References in corpus (7)

• Dynamic Fibrils are driven by Magnetoacoustic Shocks
• High Resolution Observations and Modeling of Dynamic Fibrils
• High precision density measurements in the solar corona: I. Analysis methods and results for Fe XII and Fe XIII
• Magnetoacoustic shocks as driver of quiet Sun mottles
• Observation and Modeling of Coronal "Moss" With the EUV Imaging Spectrometer on Hinode
• Modeling X-ray Loops and EUV "Moss" in an Active Region Core
• Static and Impulsive Models of Solar Active Regions

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• Application of a magnetic-field-induced transition in Fe X to solar and stellar coronal magnetic field measurements
• Role of small-scale impulsive events in heating the X-ray bright points of the quiet Sun
• Where is the chromospheric response to conductive energy input from a hot pre-flare coronal loop?
• Undetected Minority-polarity Flux as the Missing Link in Coronal Heating
• A novel inversion method to determine the coronal magnetic field including the impact of bound-free absorption
• Undercover EUV Solar Jets Observed by the Interface Region Imaging Spectrograph
• The Transition Region of Solar Flare Loops
• Active region upflows in various coronal structures and their coupling to the lower atmosphere
• SolO/EUI Observations of Ubiquitous Fine-scale Bright Dots in an Emerging Flux Region: Comparison with a Bifrost MHD Simulation