Center-to-limb Variation of Quiet Sun Intensity Contrasts. Alex Feller

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1 Center-to-limb Variation of Quiet Sun Intensity Contrasts Alex Feller

2 Introduction Analysis of quiet Sun rms intensity contrasts σ(i) / <I>, as a function of limb distance (µ value), and comparison with simulations Follow-up of earlier analysis of disk-center rms intensity contrasts (Hirzberger et al. 2010) Spectral windows used for the analysis SuFI: 300 nm, 312 nm (OH band), 388 nm (CN band), nm (Ca II H) IMaX: 525 nm (Fe I), continuum Major advantages of Sunrise data for this study UV wavelengths No significant time-varying atmospheric turbulence and straylight Other aspects of center-to-limb variation (e.g. granular intensity and velocity structuring) are currently studied by Fatima Rubio Solar Group Seminar 2

3 Sunrise 1m solar telescope on stratospheric balloon First science flight: June 8-13, 2009 Sunrise Filter Imager (SuFI) Imaging in 5 UV passbands: 214 nm and 300 nm continuum, 313 nm (OH), 388 nm (CN), nm (Ca II H) Imaging Magnetograph experiment (IMaX) Imaging spectropolarimetry in the Fe I nm line Sunrise launch from ESRANGE, Sweden Sunrise flight trajectory at an altitude of about 37 km Solar Group Seminar 3

4 Scientific context QS intensity contrasts are fundamental proxies for the thermal structure of the convective overshoot region High temperature sensitivity in the UV Good observational test for MHD simulations and spectral synthesis algorithms However: Observed intensity contrasts are very sensitive to optical aberrations and straylight! Normalized derivative of Planck function Solar Group Seminar 6

5 What is straylight? IMaX sample PSFs (1D cuts) Black: PSF resulting from lower order optical aberrations, as determined from phase diversity wavefront sensing Red : full PSF resulting from optical aberrations and straylight Solar Group Seminar 7

6 Why do we care about straylight? PD reconstructed IMaX continuum image (RMS contrast: 13%) Same image, after additional straylight correction (RMS contrast: 21%) Solar Group Seminar 8

7 How can straylight be measured? In the lab pinhole Observing a star Observing a celestial knife edge in front of the Sun, e.g. Mercury transit partial solar eclipse satellite transit? Mercury transit, observed by Hinode SOT/BFI and analyzed by Mathew et al None of these are available for the Sunrise data. Evaluate solar limb profiles! ISS transit (credits: P. Stetson) Solar Group Seminar 9

8 Limb images and 1D profiles 300 nm 397 nm Solar Group Seminar 12

9 Stray light model l: observed solar limb profile l t : true (intrinsic) solar limb profile PSF PD : PSF, as inferred from PD wavefront sensing g s,i : Gaussian straylight terms Solar Group Seminar 15

10 How to deal with the intrinsic limb profile? Theoretical Limb data for 550 nm, from different atmospheric models (Thuillier et al. 2011) Limb observed at total solar eclipse (Dunn et al. 1968) and erfc fit Solar Group Seminar 16

11 Straylight model l: observed solar limb profile l t : true (intrinsic) solar limb profile PSF PD : PSF, as inferred from PD wavefront sensing g s,i : Gaussian straylight terms Limb profile fitting function: Free parameters: w 1, w 2, σ 1, σ 2, σ Solar Group Seminar 17

12 Straylight parameters and disk-center contrasts [arcsec] Some disk-center rms contrast values from other sources Spectral window rms contrast Source 300 nm 0.31 MHD / RTE 312 nm (OH band) 0.28 MHD / RTE 388 nm (CN band) nm (CN band) 0.24 MHD / RTE 525 nm continuum 0.20 MHD / RTE Hinode SOT/BFI (Matthew et al. 2009) MHD / RTE Solar Group Seminar 23

13 Sunrise CLV dataset Solar Group Seminar 27

14 Sunrise CLV data basic reduction steps Image reconstruction incl. straylight Absolute pointing calibration (F. Rubio) based on limb analysis at different solar position angles valid from mission day 3 ( ), 07:47 UT offset (-71, 69) ± (10, 10) arcsec Additional fine-calibration for limb images Numerical rotation and selection of limbparallel slices corresponding to given range of µ values Sample 388 nm image Solar Group Seminar 28

15 CLV of rms contrasts - observed Solar Group Seminar 29

16 CLV of rms contrasts synthetic vs obs Solar Group Seminar 30

17 Spectral synthesis method Formal solution of the unpolarized radiative transfer eq.: 0 τ 0 I τ = 0 = exp k s ds k τ S τ dτ MHD cube model atmosphere providing physical parameters such as temp., velocity, pressure, Opacities provided by ATLAS9, opacity distr. functions (ODFs) SPINOR/STOPRO, incl. several 100 lines in the SuFI channels, and 20 lines in the IMaX channel Compute continuum optical depth τ along the red paths, up to a certain threshold Integrate the above RTE solution 6 Mm 1.4 Mm Radiative transfer through an MHD cube, for a given µ value (from V. Zakharov, 2006) Solar Group Seminar 32

18 What now? Is the problem in the data, in the MHD simulation, or in the spectral synthesis? Earlier results Schmidt et al. 1978, Durrant et al. 1983, white-light observations with Spektro-stratoskop, and from the ground during a partial solar eclipse: same qualitative behaviour as our resuls for 525 nm Zakharov, 2006: continuum around 678 nm: same qualitative behaviour as for 525 nm, except for very large mean magn. field of B=400G in the MHD cube Afram et al. 2011: synthetic vs observed contrasts (Hinode/BFI) 3 VIS continuum bands: compatible with our 525 nm results 388 nm CN band: suggest same qual. behaviour as our results Solar Group Seminar 33

19 Synthetic images at µ = 1 Greyscale: [0.5, 2] * I mean Solar Group Seminar 34

20 Synthetic images at µ = 1 Greyscale: [0.5, 2] * I mean Solar Group Seminar 35

21 Synth., ODF, 50G Observed Observed vs synthetic images 300 nm Solar Group Seminar 37

22 Synth., ODF, 50G Observed Observed vs synthetic power spectra 300 nm, µ= Solar Group Seminar 38

23 Effect of spatial filtering (300 nm) Solar Group Seminar 39

24 Synth., ODF, 50G, filtered Observed Observed vs synthetic images 300 nm Solar Group Seminar 40

25 A closer look at the simulations Approximation 1: S τ = B T τ = a + b τ plane-parallel atmosphere RTE solution for arbitrary μ: I x, z, τ = 0 = S x, z, τ = μ White lines: τ = 0.2, 0.4,, 1.0 at 300 nm Synthetic intensity contrast: 1 B B T ( T ) σ T Solar Group Seminar 41

26 A closer look at the simulations Approximation 2: S τ = B T τ = a + b τ RTE solution: I x, z, τ μ = 0 = S x, z, τ μ = 1 τ μ : Optical depth integrated along an oblique ray at angle θ = acos (μ) Synthetic intensity contrast: 1 B B T ( T ) σ T Solar Group Seminar 42

27 A closer look at the simulations Approximation 2 at 300 nm Approximation 2 at 525 nm Red: approx. 1 (plane parallel atmosph.) Black: approx. 2 Blue: Full RTE solution Solar Group Seminar 43

28 Summary The CLV of quiet Sun rms intensity contrasts of all Sunrise science channels (except 214 nm continuum and Ca II H) has been evaluated, and compared to synthetic data based on MHD simulations and LTE spectral synthesis. Significant discrepancy between observed and synthetic rms intensity contrasts, for all SuFI UV channels. The reason for this discrepancy is still unclear. Preliminary guesses: Amplitude of the temperature fluctuations vs height underestimated in the MHD simulations? Problem with UV continuum opacities? Inaccuracies in image reconstruction (incl. straylight) and image jitter estimates result in uncertainties of several % in the observed intensity contrasts. However, even within the range of these uncertainties, the qualitative behaviour of the synthetic intensity contrasts cannot be reproduced. Next steps: Spectral synthesis of Stagger MHD cube with current opacities Test different opacities in the spectral synthesis Test synthetic and observed limb darkening Solar Group Seminar 46