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

The Quest: measure the magnetic free energy over active regions

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

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

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

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)

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

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

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

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

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

Stereo B (L=265 deg) Useful intensity data

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

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

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

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

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

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

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)

Disk data: Milne- Eddington inversions

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

off-limb inversions: optically thin slabs

Observed and inverted profiles along a plume

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

results: surface fields

results: plumes

FIRS Si vs He inversions: curious

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

You failed!

Failed indeed… but this is not easy… and remember… vector fields at the coronal base? 2013

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