Multi-Height Full Stokes Polarimetry of a B-Class Flare Tom Schad – in collaboration with – Ali...

Multi-Height Full Stokes Polarimetry of a B-Class Flare

Tom Schad

– in collaboration with –Ali Tritschler & Matt Penn

CANFIELD-FEST Aug 8 – 11, 2010

Image courtesy A.O. Benz, “Flare Observations” Living Rev. Solar Phys. 5

Introduction

9 August 2010 2Tom Schad - Canfield-Fest

How do flares acquire and release free magnetic energy into thermal and nonthermal energy?

The Problem:

Constraining the Problem: Quantifying the transfer of

energy…usual methods: multi-spectral diagnostics energetic particle spectrum

measurement (@ 1 AU) Evaluating magnetic field

topology from photospheric field.

This talk A not-as-usual method chromospheric polarimetry of flaring regions

A few previous studies…

Hénoux & Semel, 1981 Penn & Kuhn 1995 Metcalf, Leka, & Mickey 2005 Xu et al. 2006, 2007 Sasso et al. 2007 Firstova et al. 2008

9 August 2010 Tom Schad - Canfield-Fest 3

Achieving polarimetric signals from the chromosphere is observationally and as well as theoretically challenging.

Why Multi-Height Flare Polarimetry?


Assumed Non-Linear Force-Free Photospheric Field Extrapolation

‘Actual?’ force free boundary for extrapolation

Field Reconstruction

Field Topology Advantage

Chromospheric Zeeman Polarimetry

More accurate topology for examining of flaring region skeleton

Direct measure of total field energy

Probe of not-well classified horizontal fine structure in chromosphere..perhaps flaring role

Accelerated Particle Dist. MeasureMeasuring spectra of accelerated particles (1 AU)

Impact Linear Polarization

Measure of accelerated particle anisotropy in solar atmosphere.


Line FormationComplex NLTE formationWeak polarizationLarge range of contribution heights -Photospheric contamination

Necessary ObservationsInteresting fields largely horizontalFine scale structureLarge scale features [+large Grad(VLOS)]Short scale dynamics

Technical NeedsMulti-wavelength spectropolarimetry, large field of view, high cadence, diffraction-limited, high spectral resolution, high polarization accuracy,

CHAL

LEN

GES


He I Triplet @10830- promising diagnostic of vector magnetic fields in upper chromosphere

Solanki et al., 2003, 2004

Observational Magnetic Field Reconstructions NLFF Extrapolation

Coupled Zeeman, Hanle, Atomic Polarization, PB effect inversions possible

HELIX+ and HAZEL codes available

Observations@ the Dunn Solar Telescope, May 2010

FIRS Fe I 6302, Si 10827, He I 10830 – Full

Stokes Polarimetry

IBIS Ca II 8542 – Full Stokes Whitelight Imaging

+ G-Band Imaging

+ DST High Order Adaptive Optics

+ Supporting Observations @ SOLIS (10830 spectroheliograms & 8542 L)and McMath/Pierce (10830 – Full Stokes)


FIRSThe Facility Infrared Spectropolarimeter at the Dunn Solar Telescope

Multi-Spectral Diff.Limited Slit-Spectropolarimeter Simultaneous IR & Vis Full Stokes Polarimetry

Multi-Slit (Fast scanning of solar image across slit unit) Large Field of View (up to ~ 170” x 70”) Currently operates in 630, 1565, and 1083 nm spectral

windows

8

Raw VIS Frame – 630 nm

Raw IR Frame – 1083 nm

Two

Beam

s

Slit 1 Slit 2 Slit 3 Slit 4

Sunspot in Slit 2

Fe I 630.1 nm

Fe I 630.2 nm

Si I 1082.7 nm

He I 1083 nm Triplet

Telluric

Two

Beam

s

Jaeggli et al. (2008)

IBISThe Interferometric Bidimensional Spectro(polari)meter

Raw IBIS Frame with Dual-Beam PolarimetryFOV ~ 40” X 80”

Builds 3(or 4) dim data cubes [X, Y, LAMBDA, (STOKES STATE)] using tunable scanning of a given spectral window or combinations of spectral windows

Usable Range: 5800 Å - 8600 Å High Spectral ( R≥ 200 000), spatial ( ≃

0.2″), and temporal resolution (several frames per second)


Cavallini (2006); Reardon & Cavallini (2008)

May 25, 2010 Operations

Ca II λ8542 Å Full Stokes Dual-Beam Polarimetry 20 spectral points (Stokes definition

modulation sequence at each wavelength + simultaneous whitelight continuum frames)

36 second period for all 120 [i.e. 20-λ x 6-states] measurements

Near continuous scanning between 13:00 and 18:00 UT

Unbinned Polarimetry RMS noise ~ 1x10-2 IC


FIRSIBIS Visible Arm: Fe λ 6301 Å and λ 6302 Å

(1 pm spectral disp) Infrared Arm: Si λ 10827 Å and He I λ

10830 Å (3.8 pm spectral disp) Full Stokes Dual-Beam Polarimetry 4-slit unit used FOV ~168” x 70” Deeper integration scan (1 hour for full

FOV scan…FOV can be scan ~ 20 min) Angular resolution: 0.29” RMS noise @ 029” res ~ 2x10-3 IC

NOAA AR 11072May 25, 2010

μ ≈ 0.6S15 °, W35°

FIRS slit aligned perpendicular to Earth’s Horizon (reason for FOV rotation between scans)

IBIS FOV

FIRS FOV


OBSERVING TARGET

A B6.5 Flare on May 25, 2010


H-alpha movieFrom BBSO Full Disk Images

The Observed Flare - Context

• Put in continuum int map, hewid equivalent width map, IBIS frame (c.int + lcen), & BBSO flare map


First, a photometric/spectroscopic look:


GOES X-ray Flux

IBIS 8542 Line Center Intensity Movie

H alpha and Ca II 8542 intensity follows closely the X-ray flare flux.

He I 10830 Flare Response


Photoionization-Recombination (PRM) – due to coronal EUV

Collisional excitation

Collision ionization (CM)

In large flares, 10830 emission common.

Emission seen in small flares, but spatial extent is often limited.

Formation of 10830 flare emission not well known [You et al. 2001]

Most likely enhanced coronal EUV deepens some spectra during event.

13:05:39 UT

14:15:24 UT

15:20:29 UT

16:25:59 UT

Primary formation processes [Avrett et al. 1994; Ding et al. 2005]:

Hard to distinguish.

FIRS Observations

He I 10830 – Red Line Intensity

He I 10830 – Blue Line Intensity

Continuum Intensity Near Si I 10827



A Possible Nonthermal Effect to He I 10830 Line Formation

Ding et al. (2005) suggest a nonthermal electron beam may deepen absorption prior to flare heating excited 10830 emission

Slit #3 Flare Light Curve

Quick note on velocity diagnostics


Deepened 10830 absoption in contrast to 8542, 6563, etc., allows for better velocity determination.


Magnetic Energy Injection at Flare Location

Hagyard et al. 1990

Flare location association with large magnetic shear along neutral line [Hagyard et al. 1990]

Injection of energy into corona derived from displacement of magnetic features (emergence/cancellation, MMFs, sunspot rotation [Regnier & Canfield 2006])

Injected energy can be estimated from transverse field motions [Kusano et al. 2002]:

Flare Polarization Observations….

13:05:39 UT

14:15:24 UT

15:20:29 UT

16:25:59 UT

FIRS Scan Start Times


TEMPORAL EVOLUTION OF PHOTOSPHERIC FIELD VECTOR

Significant Transverse Field

Horizontal Motion of Magnetic Field

FLARE START: 15:46 UT

Photosphere/Chromosphere Field Comparison


13:05:39 UT

14:15:24 UT

15:20:29 UT

16:25:59 UT

Chromospheric Vertical Field More Diffuse

Field more ‘pinched’ in chromosphere?

Chromospheric Magnetic and Velocity Structure


13:05:39 UT

14:15:24 UT

15:20:29 UT

16:25:59 UT


Chromospheric Magnetic and Velocity Structure


13:05:39 UT

14:15:24 UT

15:20:29 UT

16:25:59 UT

BSE?

Finally, a look at the full Stokes Spectra…Flare 10830 Emission Stokes Spectra


Only a preliminary look, as reduction needs work.

Flare

10830 in emission

As seen in Stokes V map, flare location near polarity reversalHint of Stokes U

structure

Curious Stokes V signal in flare. Red component < noise Blue comp. shows Zeeman profile.Perhaps optical thickness difference?

IBIS 8542 Stokes Vector Movies


Complete polarization calibration (unfortunately poor MPEG quality)

8542 PreFlare vs . Flare Emission Stokes Spectra


Flare fibrils show distinct 8542 emission Stokes I U crosstalk, but clear but complext changes in Q, U

8542 Stokes Vector Time Series

Flare Start 15:46

Flare Peak 15:51

Flare End

15:55


Anti-symmetric Stokes V profile turned anomalous by flare?

Stokes U amplitude seems to increase during flare?

What could explain it?


A few possible causes….

[Firstova et al. 2008]

Impact Polarization

Formation Height Change?

Field Topology Shift?

Not real? Instrumental, seeing cross-talk? Effect of limited spectral resolution? Temporal effect?

Concluding Remarks

FIRS and IBIS join observations offer beneficial new diagnostics and instrumental opportunities for flare studies.

He I 10830 triplet could be an important tool for field topology and evolution assessment.

Polarization signatures within flare for 10830 and 8542 shows interesting complex behavior.

Stay tuned…


Thank You

Multi-Height Full Stokes Polarimetry of a B-Class Flare Tom Schad – in collaboration with – Ali...

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