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:
€
1
λ= RZ 2 1
n12 −
1
n22
⎛
⎝ ⎜ ⎞
⎠ ⎟
• Lyman continuum: n1 = 1, n2 = ∞
H: < 912 ÅHeI: < 504 ÅHeII: < 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:
1) Temperature dependence of 195/1242 ratio
2) Contaminant lines in spectroheliograms
3) 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 fromsimulation
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.