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 technologicallyadvanced 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

ApertureDiffraction

Mirror roughness

Spiderdiffraction

Sky brightness

Corona at 1arcminsweet spot wherecorona is brighter than sky

The IR coronal spectrum: Discovery Science

Ion Wavelength Temperature ProspectsFe XIV 530.3nm 2MK okFeXIII 1075nm 1.7MK excellentSi X 1430nm 1.3MK okMg VIII 3027nm 0.8MK ?Si IX 3932nm 1.1MK goodMg VII 9031nm 0.6MK ?

3.9μ 1998 Mid-IR eclipse experiment

1994 Eclipse

Ideal V – IR, Blos 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 6 2004 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 & UStokes Q & U Scan:• RV = 0.25 R • From PAG 250° to 270°• Five 5° steps

FeXIII IR Coronal PolarimetryI 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

V

U

Q

I

V

U

Q

I

998422.0049915.00.00.0

049914.0998424.00.00.0

0.00.0999998.0004472.0

0.00.0004472.00.1

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 = 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

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-scaleswe can’t see a correlationbetween Blos and brightness.Does the “heating function” dependon 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: 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)

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)

CryoNIRSPCryoNIRSP’s IR personality

#Filter Name

Center Wavelength

(nm)1 Fe XIV 5302 Fe X 6373 H I 6564 Fe XI 789

5He I, Fe XIII

1080

6 S IX 12527 Si X 14308 Fe IX 22189 CO 232610 Si X 258011 Mg VIII 302812 Si IX 393513 CO 4651

14 TBD

15 Dark

16Empty slot

#/wheel

Filter NameCenter

Wavelength (nm)

CW tolerance (nm)

Effective Bandpass (nm)

ShapeComment

1-a GreenLine 530.27 0.1 0.2 3 cavity2-a Halpha 656.28 0.1 0.2 3 cavity3-a R 700 10 220 2 cavity Calibration/PSF

4-a FeXIII(1) 1074.7 0.2 1 3 cavity5-a HeI 1083.0 0.2 1 3 cavity

6-a Open8-b J 1250 5 20 2 cavity Calibration/PSF9-b K 2200 20 480 2 cavity Calibration/PSF

10-a SiIX 3923 5 20 2 cavity11-a M’ 3950 5 20 3 cavity SiIX/CO cont. ref.

12-a CO 4651 5 20 3 cavity

13-b ND 500-5000nm Density TBD

13-b Wiregrid 500-5000nm Q+

14-b Wiregrid 500-5000nm Q-

15-b Wiregrid 500-5000nm U+

16-b Wiregrid 500-5000nm U-

17-b Open 18-b Dark (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 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

1

1 1 1

( , ) ( ( , )sin ( , )cos )

(cos sin ) 0 ( sin ) ( cos )y z

y los z los

B y z B s B s

B B B B

s

Q

z

y

(Wikipedia)

The projection problem

The inversion

10 iterations over 12 projectionsspaced 15 degrees...

Another inversion

6 projections, 0-90 degrees...

Potential field...