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Estimating the Chromospheric Absorption of Transition Region Moss Emission. Bart De Pontieu, Viggo H. Hansteen, Scott W. McIntosh, Spiros Patsourakos. What is Moss?. Reticulated EUV emission seen in TRACE 171 Å
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Estimating the Chromospheric Absorption of Transition Region Moss Emission Bart De Pontieu, Viggo H. Hansteen, Scott W. McIntosh, Spiros Patsourakos
What is Moss? • Reticulated EUV emission seen in TRACE 171 Å • TR footpoints of hot (3-10 MK) coronal loops (Berger et. al. 1999b, Martens et. al. 2000) From Martens, Kankelborg, & Berger 2000
What’s the problem? • Moss is observed to have similar EUV brightness as loops • Coronal loop models predict moss emission should be much greater than loop emission (e.g., Schrijver et. al. 2004) • Some folks (Winebarger et. al. 2008) have used filling factors to explain this discrepancy, but…
Absorption in the TR • TR is known (Berger et. al. 1999b) to contain mixture of hot EUV emitting plasma, and cool chromospheric plasma (H, HeI, HeII) • Could faint moss EUV emission be explained by absorption due to cool neutral material?
Absorption in the TR • Hydrogen-like Rydberg Equation: • Lymancontinuum: n1 = 1, n2 = ∞ H: < 912 Å HeI: < 504 Å HeII: < 228 Å
Absorption in the TR • Known EUV absorption in TR must be accounted for to constrain loop models (e.g., constrain filling factors) • Measure emission at > 912 Å (no absorption), and at < 228 Å (most absorption), compare with radiative transfer model (e.g., CHIANTI)
Details • We don’t know how much stuff is emitting, but we can get densities with line ratios. • With known density, we can use CHIANTI to predict ratio of long wavelength to short wavelength emission, in absence of absorption. • We need observations of 3 spectral lines from one ion - one long wavelength, and a density sensitive line pair which experience similar absorption.
Observations • Weak active region observed November 14, 2007 • Hinode/EIS raster containing Fe XII 186.88 Å and 195.1 Å spectral lines • SOHO/SUMER raster containing 1242 Å spectral lines • AR also observed by Hinode/XRT, TRACE and STEREO A and B
Hinode/EIS Observations • W-E raster, 256 positions, 1” spatial cadence, taken from 16:44 UTC to 17:50 UTC • Data reduced with eis_prep.pro, and continuum subtracted • Processed data is summed over spectral line (with exclusion of contaminants, e.g., Ni Xi 186.98 Å) to obtain spectroheliograms in Fe XII 186.88 Å and 195.1 Å
SOHO/SUMER Observations • E-W raster, 1.125” spatial cadence, taken from 17:12 UTC to 20:35 UTC • Data reduction given by McIntosh et. al. 2007 • Spectroheliogram created as with EIS data?
Alignment issues • Observations are not co-temporal • Appreciable changes occur in AR over long SUMER raster • Authors feel best co-alignment is achieved in the eastern half of the rasters.
Analysis • Fe XII density derived from CHIANTI and density sensitive EIS line pair • Derived density used to calculate expected 195/1242 ratio, using CHIANTI and Keenan et. al. 1990
Analysis • Derived 195/1242 ratio compared to observation • Authors feel CHIANTI gives better results • Quantify results by making histograms of observed and calculated 195/1242 ratio in moss MA and loops A and F…
Results • Moss shows 195/1242 ratio systematically reduced by ~ 2. This is taken to be absorption of 195 Å emission in TR • Loop ratio shows good agreement with CHIANTI prediction. Long tail in observed ratio is explained by changing AR structure in western FOV. Observed (solid) and predicted (dashed, CHIANTI) 195/1242 line ratio in loops (A & F) and moss (MA).
Other Considerations • Other factors might contribute to the mismatch between observed and calculated 195/1242 ratio in moss: • Temperature dependence of 195/1242 ratio • Contaminant lines in spectroheliograms • Image noise
Temperature dependence of 195/1242 ratio • Is is possible that the moss is cooler than expected, and therefore the calculated 195/1242 ratio smaller than previously estimated? • If this were true, the overlying loops would also be cooler, and would be expected to be very bright in the EIS spectra. This is not observed.
Contaminant lines in spectroheliograms • Care was taken when extracting the 186.88 Å line from the EIS spectra to not include close contaminants such as Ni XI 186.98 Å. Likewise for close contaminants of the 195 Å lines. Other weak contaminants are felt by the authors to be too small to impat their result.
Image noise • The authors investigated noise in the EIS spectra, and found it to be dominated by photon shot noise. Such pixel-to-pixel uncorrelated noise cannot account for the systematic shift seen in the moss, and not in the loops.
STEREO Observations • Center-to-limb variation of moss EUV emission measured with STEREO A (disk center) and B (LOS 40° from local vertical) • Observed between 16 and 18 UTC on November 14, 2007 in Fe XII 195 Å • Data reduced with STEREO ssw and corrected for distance from the sun.
STEREO analysis • STEREO results quantified with histograms of EUV intensities of two patches of moss (A and B) and one loop. • Loop shows same average intensity in both A and B spacecraft. • Moss intensity is reduced when viewed near the limb (reduction ~ cos?) Moss A Intensity: 340±130(A), 240±93(B) Moss B Intensity: 310±100(A), 260±60(B)
MHD Simulation • 3D MHD model spanning volume from convection zone to corona. • Includes convection, nongray, non-LTE radiative losses, and conduction along the magnetic field. • Evolution of an initially potential field, stressed by convective motion, results in a simulated atmospheric structure. • The model TR is very rough, as in observations, and shows a mix of plasma at temperatures of 5000 K to 1 MK.
MHD Simulation • Contribution function dI = Aelne2g(T)e-dy is integrated along y • Optical depth is due to cool plasma: = HINHI+ HeINHeI+ HeIINHeII • 195 Å emission shows significant extinction at model TR heights. Y-integrated Fe XII 1242 Å (top) and 195 Å (bottom) emission from simulation
Summary • Authors have shown absorption by cool chromospheric material as a plausible mechanism for reducing observed moss EUV intensity. • Center-to-limb variation was measured with STEREO. Observed dimming of moss near the limb is due to increased absorption by viewing a greater cross section of chromospheric material. • Expected extinction of EUV in the TR is seen in 3D MHD simulations. • Taking TR absorption into account does not fully resolve the observed discrepancy between moss and loop EUV emission, but it is a significant effect, and quantifying this absorption will help to constrain coronal loop models.