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 inthe cool plasmaimmediately adjacent tothe corona, unlesshigh energy photons/electrons in deeper layers can prevail.

In 1D models this meansthe uppermost scaleheight 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-knownlimb “He shell”

Unique “gap”, line forms ~ 1.5-2Mm

I I ISi 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

HAZELAsensio Ramos, Trujillo Bueno, et al. 2008

Particular observations:

FIRS 1083nm channel

22 Sep 2011, 20 minute sequence

NOAA 11302 at E limb

6 hours after X1.4 class flare.

Context

Boringspacecraft intensity data Boring

ground-basedintensity data

mildyusefulspace-basedmagneticdata

Boringintensity data -

becausethese are“effects”not“causes”

Stereo B (L=265 deg)

Usefulintensity data

Interesting dataHigh 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

opticalfringe

removal

Casini, Judge, Schad2012 ApJ

2D PCA

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

seeing-induced crosstalk at the limb

Spurious polarizationsignals 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”

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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 coronalbase 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….

2012

2014 - vector fields at the coronal base?

2013

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