1. Magnetic field measurements in and above a limb active region Philip Judge HAO, NCAR and Montana State University Roberto Casini Thomas Schad Lucia Kleint Sarah Jaeggli Rebeca Centeno coronal structure is imprinted in the chromosphere

2. The Quest: measure the magnetic free energy over active regions

3. Method: use He I lines formed at the coronal base what? surely you are joking?

4. A quirk of nature. Sometimes we need a bit of luck. 4 He triplets can be populated only in the cool plasma immediately adjacent to the corona, unless high energy photons/ electrons in deeper layers can prevail. In 1D models this means the uppermost scale height of the chromo- sphere. Low beta

5. Yes, use He I lines formed at the coronal base But 1D models? Surely you are STILL joking? Ah… not so fast…

6. 6 here is the well-known limb “He shell” Unique “gap”, line forms ~ 1.5-2Mm I I I Si He gap = absence of emission (opt thin vs Eddington Barbier)

7. ..and a lot of hard work..and a lot of hard work: Lin, Jaeggli (PhD 2011, U. Hawaii)..

8. ..and a lot more hard work: inversion codes for He 1083 HAZEL Asensio Ramos, Trujillo Bueno, et al. 2008

9. Particular observations: FIRS 1083nm channel 22 Sep 2011, 20 minute sequence NOAA 11302 at E limb 6 hours after X1.4 class flare.

10. Context Boring spacecraft intensity data Boring ground- based intensity data mildy useful space- based magnetic data

11. Boring intensity data - because these are “effects” not “causes”

12. Stereo B (L=265 deg) Useful intensity data

13. Interesting data High sensitivity He I polarimetry FIRS data and processing 13 High sensitivity demands special care in both data acquisition and processing. - use AO for residual seeing reduction - flat fields - deep integrations - polarization calibration - use 1 of 4 slits (spectral purity) - use PCA to remove residual fringes - diverse PCA training set of spectra needed

14. optical fringe removal Casini, Judge, Schad 2012 ApJ 2D PCA

15. FIRS monochromatic images from 20min scan (75 steps, slit || limb)

16. seeing-induced crosstalk at the limb Spurious polarization signals at several % level at/above limb Not completely understood rows at/ above limb are crosstalk

17. Seeing (DST G band) Frames 17:00-17:09, rms (limb) 0.3”

18. Seeing (DST Ca II camera) Frames 17:00-17:09, rms (limb) 0.3”

19. 19 1083 nm region IQUV spectra (x=λ, y=position along slit) Redshifted v>>0 post-flare loop data within coronal plasma: REAL He I Polarization at coronal base in active region, v~0 residual fringes Polarized spectra t (t)

20. Disk data: Milne- Eddington inversions

21. on disk inversions of “plumes” ME: SLAB: (binned 2x2 pixels)

22. off-limb inversions: optically thin slabs

23. Observed and inverted profiles along a plume

24. Inversion maps Caveat: seeing- induced noise, crosstalk and other systematic errors seriously affect limb/off- limb data.

25. results: surface fields

26. results: plumes

27. FIRS Si vs He inversions: curious

28. Conclusions: He I 1083 high s/n spectropolarimetry challenging (small B, polarization, non-ideal setup) (our limb data do not address free energy problem) in spite of difficulties: we achieved 0.03%* sensitivity with FIRS demonstrated credible inversions though not an ideal dataset inversions “make sense” (ME vs slab vs thin) first measurement of B above limb in post flare loops He I should be a prime line for Solar C Imaging IR spectropolarimeter (coronagraph) needed Judge, Heliophysics decadal survey BBSO NST ATST? *residual noise, rms, sensitivity, not accuracy

29. You failed!

30. Failed indeed… but this is not easy… and remember…. 2012 2014 - vector fields at the coronal base? 2013

31. Maybe! T. Schad, Ph. D. Thesis, U. Arizona: The Right Stuff