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

ATST is coming

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

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?

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

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

Ideal V – IR, B los measurement sensitivity 5 min observation, 10” pixel

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

April Observations 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. 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

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

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

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 direction and by measuring/calibrating the I-V cross-talk

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 lineshift 2x10 -3 pixels (1px = nm) and V/I peak amplitude of Stability requirements –D λ/ λ better than 3x10 -8 –D I/I better than Strategies – Measure I and V profiles simultaneously – Stabilize wavelength and photometry measurements – Minimize and calibrate telescope and polarimeter crosstalk B = 4.6G

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

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

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

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

ATST Polarimetry Requirements Polarization sensitivity: amount of fractional polarization that can be detected above a (spatially and/or spectrally) constant background, a relative measurement: 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)

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)

CryoNIRSP CryoNIRSP’s IR personality # Filter Name Center Wavelength (nm) 1Fe XIV530 2Fe X637 3H I656 4Fe XI789 5 He I, Fe XIII S IX1252 7Si X1430 8Fe IX2218 9CO Si X Mg VIII Si IX CO TBD 15Dark 16 Empty slot #/wheelFilter Name Center Wavelength (nm) CW tolerance (nm) Effective Bandpass (nm) Shape Comment 1-aGreenLine cavity 2-aHalpha cavity 3-aR cavityCalibration/PSF 4-aFeXIII(1) cavity 5-aHeI cavity 6-aOpen 8-bJ cavityCalibration/PSF 9-bK cavityCalibration/PSF 10-aSiIX cavity 11-aM’ cavitySiIX/CO cont. ref. 12-aCO cavity 13-bND nm Density TBD 13-bWiregrid nm Q+ 14-bWiregrid nm Q- 15-bWiregrid nm U+ 16-bWiregrid nm U- 17-bOpen 18-bDark (stop) Nominal net filter cost: $104K

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

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

CryoNIRSP 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

CryoNIRSP 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

“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”.

Scalar Algebraic Reconstruction Technique

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 s Q z y (Wikipedia)

The projection problem

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

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

Potential field...