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.