IR Coronal Tools: Jeff Kuhn Institute for Astronomy, University of Hawaii

1. IR Coronal Tools: Jeff Kuhn Institute for Astronomy, University of Hawaii • Current progress and harmonic convergences • IR is good • Stokes V is better • What’s needed

2. ATST is coming

4. The most technologically advanced optical and IR “polarimeter” ever built

5. Renaissance opportunities for ground-based coronal science… • ATST’s non-incremental features: • Aperture (by an order of magnitude) • Wavelength opportunity -- IR • Polarimetric sensitivity (including calibration) • Complex problems are “solved” using forward modeling with new observational and computational tools…like we’ve been talking about doing this week. What’s needed to realize all of this?

7. The IR coronal advantage from Haleakala with ATST at thermal wavelengths Sky brightness sweet spot where corona is brighter than sky Corona at 1arcmin Spider diffraction Mirror roughness Aperture Diffraction 0.5m 4.0m

8. The IR coronal spectrum: Discovery Science 1994 Eclipse 3.9μ 1998 Mid-IR eclipse experiment

9. Ideal V – IR, Blosmeasurement sensitivity 5 min observation, 10” pixel

10. Echelle Grating Camera Lens Collimator NICMOS3 IR camera Fiber Bundle SOLARC Imaging Spectropolarimeter Secondary mirror Prime focus inverse occulter/field stop Re-imaging lens LCVR Polarimeter Input array of fiber optics bundle “OFIS” spectrograph Primary mirror

11. April 6 2004 Observations • Full Stokes vector observations were obtained on April 6, 2004 on active region NOAA 0581 during its west limb transit. • Stokes I, Q, U, & V Observation: • 20arcsec/pixel resolution • 70 minutes integration on V • 15 minutes integration on Q & U • Stokes Q & U Scan: • RV = 0.25 R • From PAG 250° to 270° • Five 5° steps Fe X 171Å image of the solar corona at approximately the time of SOLARC/OFIS observation from EIT/SOHO. The rectangle marks the target region of the coronal magnetic field (Stokes V) observation.

12. FeXIII IR Coronal Polarimetry Q I B=4.6G V U

13. Crosstalk: Gregorian Focus • Aluminum coating at 400 nm • Polarization effects depend on wavelength, field of view, coating properties and age • Instrumental polarization fixed with respect to telescope

14. Measuring Stokes V for Coronal Fields • Unlike photospheric Zeeman observations, in the corona there is a strong linear polarization signal, and only a weak intrinsic Stokes V signal. Even small U-V cross-talk dominates measured Stokes V • In weak-field approximation, V = c·B·dI/d, the observed circular polarization can be written as • V’ () =  ·I () + c·B·dI() /d =  ·I (+ c·B/), • Thus, B can be directly obtained by comparison with the shift of V with respect to I in the spectral directionand by measuring/calibrating the I-V cross-talk

15. SOLARC Magnetometry, Useful Magnitudes • FeXIII Q/I or U/I is of order 10% • Magnetic V/I amplitude sensitivity should be of order 5x10-5 for B ~ 3 G • Peak magnetic flux density of 6G corresponds to I-V lineshift2x10-3 pixels (1px = 0.017 nm) and V/I peak amplitude of 0.0001 • Stability requirements • Dλ/ λ better than 3x10-8 • DI/I better than 10-4 • Strategies • Measure I and V profiles simultaneously • Stabilize wavelength and photometry measurements • Minimize and calibrate telescope and polarimeter crosstalk B = 4.6G

16. Results: Coronal Magnetograms Contours B=4,2,0,-2 G

17. Coronal model for B and observations Abbett, Ledvina, Fisher,… SOLARC observations

18. What light’s up the loops? NB: At least on small-scales we can’t see a correlation between Blos and brightness. Does the “heating function” depend on spatial scale…? Kuhn, IAS, 2008

19. What’s needed? • Observational tools – more than ATST, sensitive polarimetry from the telescope and instruments

20. ATST PolarimetryRequirements • Polarization sensitivity: amount of fractional polarization that can be detected above a (spatially and/or spectrally) constant background, a relative measurement: 10-5 • Polarization accuracy: absolute error in measured fractional polarization, an absolute measurement: 5·10-4 • Derived telescope polarization requirements: • < 1% instrumentally induced polarization at all wavelengths before polarization modulation (to keep second-order effects small enough to achieve required polarization sensitivity) • Instrumental polarization calibration error: < 5·10-4 (to achieve polarization accuracy requirement) • Instrumental polarization stability: < 5·10-4 within 15 min (to achieve polarization accuracy requirement)

21. What’s needed? • Observational tools – more than ATST, sensitive polarimetry from the telescope and instruments • Instruments designed for sensitive coronal polarimetry (Stokes V and IR)

22. CryoNIRSP’s IR personality Nominal net filter cost: $104K

23. Cryogenic photon backgrounds CryoNIRSP must use cooled optics and baffling Disk Disk Warm Optics Corona Corona 2% 0.5% Imager Spectrograph

24. CryoNIRSP T=130K T=200K Mass: 2500kg

25. What’s needed? • Observational tools – more than just ATST, need sensitive polarimetry from the telescope and instruments • Instruments designed for sensitive coronal polarimetry (Stokes V and IR) • People, and a growing interested community

26. How to rebuild community • Let’s do better advertising our progress on the long-standing problems, e.g. coronal magnetometry • Let’s connect with a broader astronomical community, e.g. “night-time solar physics” is a real discipline, more radio, extrasolar planets, stellar magnetism… • Let’s make the hard quantitative interpretation of 3-d polarimetric tools more accessible, such a tool provides a natural venue for linking broader communities

27. “forward” and CryoNIRSP • … a CN “instrument personality” module that accepts CN instrument configuration parameters and generates simulated “observables” suitable for developing CN-ATST coronal experiments • …starting to look for long-term support for tools like “forward”.

29. Scalar Algebraic Reconstruction Technique

30. ART and Vector Inversions • FF and potential model from Low (1993) • External potential field+FF at r<R + dipole • Radon transform using Algebraic Reconstruction Technique (Wikipedia) z y Q s

31. The projection problem

32. The inversion 10 iterations over 12 projections spaced 15 degrees...

33. Another inversion 6 projections, 0-90 degrees...

34. Potential field...