Selected topics on Cosmic Magnetic fields

Huirong YanKIAA-PKU

http://kiaa.pku.edu.cn/~hryan

Astrophysical magnetic fields are ubiquitous

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Cluster of galaxies

Accretion disk

galaxy QSO

Solar wind

Also in Molecular clouds, outflows, supernovae remnants, planetary nebulae…

There is no universal magnetic diagnostics in diffuse medium

Zeeman splitting (BZeeman splitting (B ||||):):

Dense and cold clouds, with molecules or even denser with masersDense and cold clouds, with molecules or even denser with masers

Faraday rotation (BFaraday rotation (B ||||):):

Uncertainty with the electron density distribution along losUncertainty with the electron density distribution along los

Light polarization by dust (BLight polarization by dust (B⊥⊥):):

••Star light polarizationStar light polarization

••Grain IR emissionGrain IR emission

Most common ways of magnetic field studies:

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Zeeman Effect

-

(Δml=-1 )

π(Δml=0 )

+

(Δml=+1 )

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Zeeman Effect

5

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Faraday Rotation

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Magnetic realignment

Goldreich - Kylafis effect (upper state)polarization in radio and FIR emission due to anisotropic optical depth

applicable to molecular clouds

Ground state realignment

unique mechanism to induce polarization in absorption lines

due to anisotropic radiation

ranging from submilimeter, IR, optical to UV

general diffuse medium7

Toy model:s

Atomic alignment is differential occupation of the sublevels of the ground (or metastable) state.

Atomic species on the ground state can be aligned by anisotropic radiation

radiation

Classical AnalogyClassical Analogy

−−

Thermal systemRadiatively pumped system

Induced 1 transition followed by isotropic emission.

• anisotropic radiation (unpolarized light is sufficient)

• there are at least 3 sublevels on the ground state.

Alignment by unpolarized light requires >2 sublevels

Incoming light transmitted lightmedium

History: Laboratory studies on masers (experimental) (Brossel et al. 1952; Hawkins 1955; Kastler 1957)

Suggested by Varshalovich (1971) for B studies in diffuse medium

Magnetic field induces precession and realigns atoms

M=2M=2

M=1M=1

M=0M=0

M=-1M=-1

M=-2M=-2

zz

BB J(F)J(F)

θθrr

• Long lived ground state means

sensitivity to weak B.

• Alignment is either parallel or

perpendicular to B due to fast

magnetic precession.

BB

Magnetic field induces precession and realigns atoms

M=2M=2

M=1M=1

M=0M=0

M=-1M=-1

M=-2M=-2

zz

BB J(F)J(F)

θθrr

• Long lived ground state means sensitivity to weak B.

• Alignment is either parallel or perpendicular to B due

to fast magnetic precession.

BB

Atomic realignment is sensitive to weak magnetic fields

νL–Larmor frequency

A-Einstein coefficient, νΔJ– EΔJ /h

ννL L (s(s-1-1))

ΔΔv(cm/s)v(cm/s) ΒΒGG

HanleHanleeffecteffect

LowerLowerlevellevelHanleHanleeffecteffect

Level interferences Level interferences occuroccur

AtomicAtomicrealignmerealignmentnt Zeeman Zeeman

effecteffect

ννΔΔJJ⋅⋅

ΑΑ⋅⋅

⋅⋅

ττRR-1-1

For a wide range of field (>~10For a wide range of field (>~10-15-15G, ~<1G) in G, ~<1G) in diffuse medium, atomic alignment happens.diffuse medium, atomic alignment happens.

ντπτννΒμπτνπ

Realignment depends on the angle between the B and light

Alignment is determined by θr. The polarization degree depends on θ and θr . For the degenerate case, θr and θ coincide.

r

r

How to observe it? - I. Absorption

νττνμνττθττμμντθττμνν

Δντπτπν

πττντνττμττττπττν≡ ∆ （τ（

Polarization of absorption is either || or ⊥ to the magnetic field

Polarization changes direction at Van Vleck angle θr =54.7o,180o- 54.7o.

Yan & Lazarian (2006)Yan & Lazarian (2006)

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How to observe it? - II. Emission

ννννντ

πττντνττμττττπττντ （

ντννπτνπτ

Our results: many observed absorption lines have appreciable polarization

Ion C I C II Si I Si II O I S I

Wavl(A)

1329-1561

1336 1695-2529

1265 1302 1807

Pmax 18% 15% 20% 7% 29% 22%

Ion S II Ti II Cr I Cr II NI S III

Wavl(A)

1250 3385 4254-4290

2741-2767

1200 1012-1202

Pmax 12% 7% 5% 21% 5.5% 24.5%

Calculated Examples:Calculated Examples:

Fe I, Fe II, Fe III, Fe III, Mn II, Ti III, C lI, N II, Cr I … Many more lines:Many more lines:

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Magnetic field changes optical depth and line intensity!

πτπτμννντντ

One has to take into account the effect of magnetic field when analyzing spectrum.

Effect of nuclear spin

2. Reduce alignment for atoms with fine structures.

Enables alignment for Alkalii atoms.

Similar structureSimilar structure

θθ θθ

θθ rr θθ rr

polarization of NI absorptionpolarization of NI absorption

Yan & Lazarian (2007)Yan & Lazarian (2007)

polarization of SII absorptionpolarization of SII absorption

NI and SII have same electron configurations (4SNI and SII have same electron configurations (4S3/23/2 4P 4P3/2,3/23/2,3/211). The ). The

difference in their polarizations shows the effect of hyperfine structure.difference in their polarizations shows the effect of hyperfine structure.

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Is there any circular polarization?

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Our results: polarization of resonant lines scattered from aligned atoms

ion HI(Bal)

Na I K I N V O I P V S II Al II Ti II

Wavl 3646-6365

5892

7667

1243 5555-7254

1118

1254-1259

8843

3073

Pmax 25% 21% 20% 22% 2.3% 27% 31% 20% 7.3%

Many more lines:Many more lines:

N I, N II, N III, P III, Al I, Al III, Fe I, Fe II, Fe III, Fe III, Mn II, Ti III, C lI, N II, Cr I …

Calculated Examples:Calculated Examples:

Using several lines, it’s possible to get 3D B

3D information: from degree of polarization 3D information: from degree of polarization PP((22

00((θθrr),),θθ). ). Two lines are enough （（

P/P/ττ

θθrrθθ

ττ τ

τ

SII

SII

-1.82-1.82

-11%-11%

Optical depth

20(θr) ---Normalized

density matrix coefficient from atomic physics

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3rd possibilty: Forbidden (submm, IR) transitions within the aligned ground state

Schematics of UV pumpingof OI 63.2μm emissionSchematics of UV pumpingof OI 63.2μm emissionSchematics of UV pumpingof OI 63.2μm emission

Schematics of UVpumpingof CI 610μm emission μττντ 3Po

610μm3P1

3P0

3P2

3P2

[CI] Emission

θττμμντθττμννΔντπτπν

Forbidden (submm, IR) transitions within the aligned ground state are

polarizedCalculated Examples:Calculated Examples:

Many more lines:Many more lines:

Ex. I: alignment allows studies of magnetic field in a comet wake

Resonance scattering of solar light by sodium tail from comet.

MHD simulations of comet’s wake.

P: -8%~14% for Na D2P: -8%~14% for Na D2

Time variation of turbulent magnetic field can be studied.

Ex. II: Alignment allows tracing heliosphere B with comet

Satellite based model of Heliospheric B (Opher et al. 2008)

Earth

Comet orbit

Cost effective way of study of B in heliosphere!

Sun

synthetic measurements of comet B

∗

Ex. III: Polarization from circumstellar envelope

O I ( ) emissionQuickTime™ and aTIFF (Uncompressed) decompressor

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Polarization vectors are spherically symmetric without magnetic realignment and theclassical expression applies Spherical symmetry is broken

With alignment

ΒΒ

LOSLOS

zz

xx

Polarization from Toroidal Disk

Ex. V: Atomic alignment influences the FIR line intensity

ouudistortion to CMB

This influences Distortion of CMB and our estimates for early universe metalicity.

ΔT/T

cmbτ

Schematics of UV pumpingof OI 63.2μm emission

3S1

63.2μm

3P0

3P13P2

1306A 1302A

1304A

Ex. VI: alignment allows study of magnetic field in the epoch of Reionization

OI 63.2 μm transmission

UV excitation rate/ CMB excitation rate (Yan & Lazarian 2008)

From Cantalupo et al. (2008)

Schematics of reionization

63.2μm

3S1

3P0

3P13P2

1306A 1302A

1304A

Schematics of UV pumpingof OI 63.2μm emission

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CSO

Steward Observatory

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Sounding rocket flight is funded.(PI Nordsieck)

Observations for IBEX. (w. Paul Smith)

Studies of [C I] polarization are intended.(w.Houde)

Ongoing projects open wide avenues for the technique

Observations for SNRs. (w. F. Snick)

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Grain alignment

Renewed interest to grain alignment also arise from CMB polarization studies

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Paramagnetic alignment and its modifications fail observational testing

• Grains get aligned with rotation axis parallel to B (grain 1)

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B

Davis-Greenstein mechanism 1

2

This is textbook solution

Rao et al. 98

τνΔτμτπτνττττ

Δμντνμντντν

ττντνττν

ννμντππντν⊥τΒττμπν

ντννμντμνΒντττντννΔν

τνντνμντπππντΒννντνττνμνττΑν

πτνΑνμντνΒνττντπντμ

ττθνν

Δντπννννττντντππτν“ττν”νππττθνμντ

ΔνντνντνΔν

μτ

τ

ΒΔνντττθνΔΔΑ

τντνπτν

“τ”

ν

Draine & Weingrartner 97

L. Spitzer: Alignment torques are not universal. How can a universal alignment exist?

Angle between B and light direction

πμτπττπνντ

Αντντθ

a1

k

e2

e3

e1Θ

Lazarian & Hoang 07

νπτνΘνΘμττΘνπνν

a1 is grain max inertia axis

k is light direction

μ

ΑντμΑπθντττντντττθΑνμντ

μπντπνΘνΘτθ

Θ

Θ

Θ

Lazarian & Hoang 07

ντνμτθνμτΑ

τΘμΘ

μνμντν

μπνΘ

ΑντμΑπνττΑνντν

ΔτνμΑν

J//a1

B

τμ

ντ

πτντΒπτνμνττθν

ν

νμνττθτπνπτθτ

τπντΒ

’ντττπντπνπτθ

ττττττ

ττνπντ

is angle between magnetic field and radiation

is angle between a and magnetic field

k

B

a

Βμτ

No wobbling: “wrong alignment”

“wrong” alignment for a narrow range of angles

μππν“ν”νμντ

Suppression of “wrong alignment” by wobbling

νν

ντπττνπμτ

Ανμντπντ

νμντ

νΒν

ττντ

ννμνττνπνντντννΒ

Δπνθπντττττνμ

νν

ττντττττπνττν

ττνννμ

RATs increase with aeff

Αν

τντντντπννμπμν

νν

ΔΔΑτννντττντπτττμνμν

τνπντθνττττνννμντ

Αττττπνπτθντθτ

μτννμ

• RAT alignment explains the existing data.• RAT testing constrains grain composition.• RAT alignment is present beyond ISM

νμμνμνττνττττντπντνΑνμντπττ

B

Davis-Greenstein mechanism

1

2

Ανμντ

• Atomic alignment is sensitive to smaller scale fluctuations of magnetic field.

• Combining the information from both, we

can get 3D information of magnetic field.

• Provide independent test to grain alignment

theory.

Comparison of atomic and grain alignment:

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Get magnetic strength from Chandrasekar-Fermi (CF) method

• Assuming equipartition between kinetic energy and perturbed magnetic energy

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μμττντ

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How good is the assumption of equipartition?

sub-Alfvenic turbulence

super-Alfvenic turbulence, turbulence dynamo

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Summary of interstellar medium polarimetry

Magnetic field in ubiquitous in astrophysics and need to be diagnosed from multiple scales with the synergy of various polarimetry techniques.

Ground state alignment is a unique mechanism to induce

polarization in absorption lines and detect ISM magnetic field.

It also influence emission polarimetry and forbidden line

polarimetry

Multiple wavebands from submillimeter, IR, optical to UV can be

used to detect 3D small scale magnetic field.

Combined with grain alignment, CF techniques, it can be used to

study both spatial and temporal variations of magnetic field.

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