Vector Magnetic Field Diagnostics using Hanle Effect

Meeting on Science with Planned and Upcoming Solar Facilities in the Country, Bangalore,

India, November 2-3, 2011

Vector Magnetic Field Diagnostics using Hanle Effect

M. Sampoorna

Indian Institute of Astrophysics, Koramangala, Bangalore

November 2, 2011

Collaborators : K. N. Nagendra, J. O. Stenflo, L. S. Anusha, H. N. Smitha,

M. Bianda, H. Frisch, R. Holzreuter, R. Ramelli

Non-magnetic (Rayleigh) scattering polarization in lines

Scattering of anisotropic radiation on atoms and moleculesproduces linear polarization even in the absence of magnetic fields.

Ir

U/I = 0

Ir

Ir

Ir

Ir

X

Z3 linear oscillators

Y

Q/I = 100%Pol :

=

Scattered beam:

Unpol. incident beam

Il

=2 I

= 0

I = +I

−Q = l l

Il

Il

Resonance Scattering (non−magnetic Rayleigh Scattering)

Scattering polarization Q/I is the largest for 90◦ scattering.

Scattering polarization Q/I is zero for forward scattering.

M. Sampoorna (Indian Institute of Astrophysics, Koramangala, Bangalore)Vector Magnetic Field Diagnostics using Hanle Effect November 2, 2011 2 / 24

Non-magnetic (Rayleigh) scattering polarization in lines

In the case of Sun, the incident radiation field is anisotropic due tolimb darkening.

Limb darkening

Incoming radiation Limb brightening

Anisotropic illumination of an atom in the solar atmosphere

Outgoing radiation

A > 0

A < 0

As the solar anisotropy (A) is small the degree of linear polarizationQ/I is on the order of 0.1 to 4 %.

Q/I is largest in the limb observation (≈ 90◦ scattering).

Q/I is Zero in the disk center observation (≈ forward scattering).


Second Solar Spectrum

Linearly Polarized Spectrum of the Sun observed near the limb iscalled the Second Solar Spectrum

FirstUnpolarized Intensity Spectrum

Solar Spectrum

Second

Linearly Polarized Spectrum

Solar Spectrum

It gives a wealth of information on the physics of light scatteringon atoms and molecules.


Magnetic scattering polarization in lines (Hanle effect)

The presence of a weak field modifies the “Rayleigh scatteredpolarization” and is termed as Hanle effect.

Ir

U/I = 0

Ir

Ir

Ir

X

Z

Y

Q/I

2 circular oscillators

B

Scattered beam

Pol : < 100%

=Unpol. incident beam

I

=2I = +I I

= 0 −Q =

Il

l

Il

l

l

Hanle scattering (magnetic Rayleigh Scattering)

Hanle effect causes :(1) a modification to the non-magnetic Q/I (basically depolarization),(2) rotation of polarization plane (fresh generation of U/I ).


Hanle effect – a tool for highly sensitive field measurements

Hanle sensitivity parameter Γ = Larmor FrequencyRadiative width

Γ = 1 to 3 Hanle sensitivity regime

Γ� 1⇒ Zeeman sensitivity regime

Hanle effect is a good diagnostic to detect weak fields on the Sun(0.1-100 G).

It can be used for polarimetry both in the limb and in the disk center.

Zeeman effect is insensitive to weak turbulent magnetic fields.Hanle effect is very sensitive to weak turbulent magnetic fields.


Hanle effect signatures in the polarized solar spectrum

Sr I 4607 , a photospheric lineA

Hanle effect by photospheric turbulent magnetic field

I ,A4227 a chromospheric line

Hanle effect by chromospheric oriented magnetic field

V/I

longitudinal

signatures

Zeeman effect

are due to

Ca


Diagnostic potential of scattering polarization in lines

1 To test the classical and quantum theories of line scattering

2 To constrain the realistic solar atmospheric models

3 To determine the strength of the turbulent magnetic fields

4 To explore the spatial structuring and temporal variability of orientedand turbulent weak fields

5 To determine the vector magnetic fields in the chromosphere,transition region, and corona

6 To investigate depth-dependent magnetic fields and so on ....


The limb observation of Ca i 4227 A line

Observations were taken in quiet regions on the Sun at IRSOLobservatory, Switzerland, using the ZIMPOL-2 polarimeter.

The spectrograph slit was placed parallel to the north limb (about 6′′

inside, which corresponds to µ = cos θ = 0.1).

Zurich IMaging POLarimeter (ZIMPOL−2)Gregory−Coude telescope (diameter = 45 cm) at IRSOL


Modeling the limb observation of Ca i 4227 A line

The observed Q/I spectra of Ca i 4227 A line is modeled using :1 polarized radiative transfer (RT) approach (exact, but slow)2 last scattering approximation (LSA) (approximate, but fast)

(see Anusha et al. 2010, ApJ, 718, 988; for details).

Rad. Tran.

nObs Obs

n

LSA

Q/I spectra has a triple peak structure ⇒ a central core peakflanked by asymmetric peaks on both sides.These peaks are due to Partial Redistribution scattering mechanism.

A microturbulent field of 25 G is essential to fit the line core peak.


Forward-scattering Hanle effect at disk center!

Solar Atmosphere

= 0

Disk Center Observations

U/I

B = 0: Q/I = 0, U/I = 0 B = 0: Q/I (say 2 %)

U/I = 0

B =

Limb Observations

0: Q/I (around 0.2 %)

= 0

B = 0: Q/I reduced (< 2 %)

U/I = 0

In the solar limb, Hanle effect producesdepolarization : [(Q/I )B 6=0 < (Q/I )B=0],generation of non-zero U/I .

At the disk center :Rayleigh scattering produces no polarization,Hanle effect produces non-zero polarization.

Disk center Hanle observations are referredto as forward-scattering Hanle effect(discovered 10 years ago).

It is a good diagnostic tool to determineweak, oriented magnetic fields.

The first observational evidence wasprovided by Trujillo Bueno et al. (2002),who observed it in the He i 10830 A line.


Forward-scattering Hanle observations of Ca i 4227 A line

Recently we performed near disk center spectro-polarimetricobservations in Ca i 4227 A line.(Anusha et al. 2011, ApJ and Bianda et al. 2011, A&AL)

The data acquisition was done with the ZIMPOL-3 polarimeter.

Observations done near the active region NOAA 1112 (S19 W5).

These observations correspond to an average µ value of 0.93.

21 8

for observationsPositive Q direction

N

S

Slit

Positive Q directionfor theory

Radius vector α


Modeling the forward-scattering Hanle effect in Ca i 4227 A

model profile

observations

I/Ic

Q/I (%)

U/I (%)

V/I (%)

(B, θB , χB) represent the vector magneticfield parameters.

A realistic model atmosphere & a 20 levelCa i multi-level atom model are used.

Observed (I , Q/I , U/I ) spectra weremodeled using polarized radiative transfer.

(B, θB , χB) are free parameters.

Unique (B, θB , χB) values are determinedby demanding a simultaneous fit of (I ,Q/I , U/I ) using Hanle effect and V /Iusing Zeeman effect.(Anusha et al. 2011, ApJ, 737, 95)


Polarimetric limb observation of multi-line systems

Scattering polarization in a multiplet can be used to determine thedepth-dependence of turbulent magnetic fields.

Scattering polarization in a multiplet arises due to quantuminterference between the upper J-states.

The theory required to model polarization in such a multiplet wasformulated in Smitha et al. (ApJ 2011a, A&A 2011b).

Cr i triplet at 5204, 5206, and 5208 A provides a good example tostudy quantum interference phenomena.

ZIMPOL-3 was used to measure scattering polarization in Cr i triplet.

Micro-lensing, adaptive optics, and a large CCD in ZIMPOL-3 allowsimultaneous recording of all the three components of the Cr i triplet.


Modeling the Cr i triplet using J-state interference theory

Figure shows a model fit tothe limb observations in quietregions.

Micro-turbulent fields areagain required to fit the linecore polarization.

Cross-over effect can beproduced only by usingquantum interference theory.

The cross-over in Q/I in between the lines is the indicator ofquantum interference between the line components.

The triple-peak structure is an indicator ofthe partial frequency redistribution in scattering.


Enigmatic wing signatures in Ca i 4227 A line

Spectropolarimetry of Ca i 4227 A line by Bianda et al. (2003, SPW3)in an active region, and by Sampoorna et al. (2009, ApJ) in quietregions showed enigmatic scattering polarization in the far wings.

We refer to this enigmatic behavior in the wings of (Q/I U/I )spectra as “(Q/I , U/I ) wing signatures”.


What is Wing Hanle Effect??

It is well-known that Hanle effect ‘mainly affects the line core’.

Nagendra et al. (2002, A&A) and Sampoorna et al. (2007a,b, ApJ)predicted that elastic collisions “transfer” Hanle effect to line wings.

Attempts to explain the observed (Q/I , U/I ) wing signatures interms of such a “transfer” of coherence (wing Hanle effect) failed.

Q / I

Q / I )mag

Q / I )non−mag(

Q / I Q / I= )( )(mag non−mag

/ IU

/ IU

= 0for B

= 0for B

observed in the "magnetic region"

theoretical (

theoretical for B = 0

∆ −

observed in the "magnetic region"

theoretical


Origin of (Q/I , U/I ) wing signatures– possibilities with NLST

The failure to explain wing signatures points us to look for“non-magnetic interpretation” in terms of :

1 departures from plane-parallel stratification

2 inhomogeneities in the solar atmosphere (“hot spots”?).

For testing the viability of the “non-magnetic interpretation”, one hasto make high spatial resolution 2D mapping of the Stokes vector(instead of working with traditional single slit positions).

2D mapping helps to obtain the fluctuations in intensity andpolarization caused by spatial inhomogeneities.

Such scattering polarization observations are completely novel.

With NLST we can perform such high spatial resolution observations.


Origin of (Q/I , U/I ) wing signatures– possibilities with NLST

45 cm Gregory-Coude telescope with ZIMPOL-3 polarimeter andadaptive optics, at IRSOL, Switzerland has following characteristics :

Spatial resolution = 0.5′′ (with AO)

Spectral resolution = 5 mA at 5000A

Polarimetric accuracy = 10−5

Wavelength range = 3910 A to 6995A

Our 2 m Gregorian on-axis NLST with Spectropolarimeter andadaptive optics :

Spatial resolution = 0.1′′ (with AO)

Spectral resolution = 10 mA at 6500A

Polarimetric accuracy = 10−4

Wavelength range = 3800 A to 2.5 microns


Proposal for Spectropolarimetry with NLST

High spatial resolution Hanle effect observations in prominences :

1 Off-limb observations to determine the relation between vectormagnetic field and the observed vertical structure of prominence.

2 On-disk observations to exploit the diagnostic potential of Hanle effect,which is the only way to detect weak fields (0.1 to 100 Gauss).

High spatial resolution multi-line observations to study the heightvariation of magnetic/ velocity fields (quantum interference effect).

Measurement of Hanle signatures in strong resonance lines (like Ca i4227 A line) to explore the weak field structure near disk center(forward-scattering Hanle effect).


Summary

NLST will deliver high-sensitivity imaging spectro-polarimetry in thewavelength range 3800 A to 2.5 microns.

With the large aperture of NLST we may achieve good spatial (≈0.1′′ with adaptive optics) and temporal resolutions (0.1% inpolarization within 10 seconds) for polarization measurements.

Some of the unsolved problems in Hanle observations can beaddressed using NLST.

Scattering polarization observations at high spatial resolution which ispossible with NLST may be used for exploring interesting physicaleffects.


IRSOL Instrumentation data

Telescope: Gregory-Coude, evacuated, primary mirror has a diameterof 45 cm, total focal length: 25 m.

Spectrograph: Czerny-Turner, focal length: 10 m, grating 180 mm x360 mm, 316 lines / mm, blaze 63◦.

Fabry-Perot filter system: Tunable narrow-band filter system based ontwo lithium-niobate Fabry-Perot etalons. It allows monochromaticimaging at any wavelength from 395 nm to 660 nm.

Adaptive optic system: Tip-tilt and deformable mirror with 37actuator

Electronics: ZIMPOL-polarimeter, CCD-Wright Instruments,CCD-DTA-Pisa, FHSW-flare registration system, Guiding system(PIG).


Hanle Scattering (Quantum picture)

J = 1

J = 0

Θ

a scattering event

ΓR=Lν

UQII

00

ΓR

Lν


Fractal Magnetic fields


Vector Magnetic Field Diagnostics using Hanle Effect

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