A. Lagg – He 10830 lecture NAOJ, Aug 2008 1 He 10830 lecture He 10830 lecture : some aspects as...

A. Lagg – He 10830 lecture NAOJ, Aug 2008A. Lagg – He 10830 lecture NAOJ, Aug 200811

He 10830 lectureHe 10830 lecture:some aspects as seen from an observer‘s viewpoint

Andreas LaggAndreas Lagg

National Astronomical Observatory of JapanNational Astronomical Observatory of Japanandand

Max-Planck-Institut für Sonnensystemforschung,Max-Planck-Institut für Sonnensystemforschung,Katlenburg-Lindau, GermanyKatlenburg-Lindau, Germany

no quantum theoryno quantum theory

no derivation of formulaeno derivation of formulae

no in depth explanation of no in depth explanation of Hanle theoryHanle theory

no solar physicsno solar physics

phenomenological explanation phenomenological explanation of effects (Hanle, PB, atomic of effects (Hanle, PB, atomic polarization)polarization)

application of formulae to application of formulae to demonstrate influence of CI, demonstrate influence of CI, geometry, PB, Hanle on Stokes geometry, PB, Hanle on Stokes IQUVIQUV


He 10830 - History

first solar obs. in He 10830:D‘Azambuja (1938), Zirin (1956), Mohler & Goldberg (1956), Namba (1963), Fisher (1964), Milkey et al. (1973)

Harvey & Hall (1971)

Giovanelli & Hall (1977)

Lites et al. (1985): report on steady flows (9 km/s, hours to days)

Avrett (1994): formation of He 10830

He 10830 spectropolarimetry:Lin (1995), Lin et al. 1996, 1998

Trujillo-Bueno (2002): atomic polarization in He 10830 solved

Giovanelli & Hall (1977)

Harvey & Sheeley (1977)Harvey & Sheeley (1977)


Para / Ortho Helium

Centeno et al., 2008Centeno et al., 2008


Ionization / Recombination Scheme



The He 10830 Triplet

Transition 23S1 – 23P2,1,0

absorption depends on:

density and extend of upper chromosphere

coronal radiation in the λ<504 Å continuum

2s 3S level populated by recombination of He II orcollisional excitation from 11S

Tr1: 10829.0911 Å, f=0.1111, geff=2.00

Tr2: 10830.2501 Å, f=0.3333, geff=1.75

Tr3: 10830.3397 Å, f=0.5556, geff=1.25


The He D3 line

Transition 23P2,1,0 - 33D3,2,1

formation mechanism similar to He 10830 (CI required)

difference to 10830:optical thickness of the observed solar plasma structures is weaker on the solar disk it is much easier to see structures in 10830 than in 5876both lines are clearly seen in emission when observing offlimb structures such as prominences and spicules.

He 10830 preferable because:forward scattering creates measurable linear polarization signals in the lines of the He I 10830 when the magnetic field is inclined (Trujillo Bueno et al. 2002)nearby presence of Si I line coupling science

Asensio Ramos et al., 2008Asensio Ramos et al., 2008


He 10830 – Formation Height

equil.hydrostat.

Tr1

Tr2+3

He

de

ns

ity

3S

1

WL

z

Avrett et al. (1994)

model atmospheres: T-profile pressure models A (cell-center), C (average),

F (bright network), P (plage) CH/CL hi/lo coronal irradiance


Influence of Height above Limb


He 10830 He D3 5876

FAL-C, nominal CI

highest

lowest


Influence of Coronal Illumination (CI)


change of ratio!(additional diagnostic tool)

He 10830 He D3 5876


Zeeman Effect

reliable magnetic field information for B >200 Gsimultaneous observation of photosphere (Si) and chromosphere (He)three (blended) HeI lines("blue" line + 2 "red" lines)

The HeI 10830 diagnostics: Zeeman effectThe HeI 10830 diagnostics: Zeeman effect

Line WL [Å] Transition geff rOS

Si I 10827.088 4s 4s 33PP22 - 4p - 4p 33PP22 1.50

He Ia 10829.091 2s 2s 33SS11 - 2p - 2p 33PP00 2.00 0.11

He Ib 10830.250 2s 2s 33SS11 - 2p - 2p 33PP11 1.75 0.33

He Ic 10830.340 2s 2s 33SS11 - 2p - 2p 33PP22 1.25 0.56

Atomic Parameters: [Lagg et al., 2007]Atomic Parameters: [Lagg et al., 2007]


The HeI 10830 diagnostics: Paschen Back effectThe HeI 10830 diagnostics: Paschen Back effect

Paschen-Back Effect

B LS HH Weak B

BLSC HHHH

The Hamiltonian of an electron in an atom in an external uniform magnetic field:

Hamiltonian of the electron affected by the

Coulomb interaction

Coupling between S and LThe interaction betweenthe external B and the

magnetic moment of the eBLS HH Strong B

Zeeman effectRegime

Paschen-Back effect Regime

B LS HH IPBS Regime


The HeI 10830 diagnostics: Paschen Back effectThe HeI 10830 diagnostics: Paschen Back effect

Paschen-Back Effect

Socas-Navarro et al. (2004)Socas-Navarro et al. (2004)

LZS IPBS

Positions and strengths of the Zeeman components as a function of the magnetic

fieldTr 1 Tr 1

Tr 2 Tr 2

Tr 3 Tr 3

Δλ

(Å)

Δλ

(Å)

Δλ

(Å)

rela

tive

str

engt

hre

lati

ve s

tren

gth

rela

tive

str

engt

h


Paschen Back Effect: influence on Q, U, V

Sasso et al. (2006)Sasso et al. (2006)

dashed = w/o PBdotted = with PB


Paschen-Back effect: Error on parameters

Sasso et al. (2006)Sasso et al. (2006)


Hanle Effect (Trujillo-Bueno, 2002, Landi Degl'Innocenti, 1982)(Trujillo-Bueno, 2002, Landi Degl'Innocenti, 1982)

non magnetic case:• anisotropic illumination of

atoms (3 independent, damped oscillators in x,y,z) with unpolarized light

• no polarization in forward scattering

• complete linear polarization in 90° scattering

Hanle effect:modification of (atomic) polarization caused by the action of a magnetic field

The HeI 10830 diagnostics: Hanle effectThe HeI 10830 diagnostics: Hanle effect


magnetic case:now the 3 oscillators are not

independent:1 osc. along B (ω0)

2 osc. around B(ω0-ωL ; ω0+ωL )

damped oscillation precesses around B→ rosette like pattern→ damping time tlife = 1/γ

lifeJ tgB /1106

ωL >> 1/tlife

LP in forward scattering: max. polarization along ±y90° scattering: no polarization

ωL ≈ 1/tlife

LP in forward scattering: weaker, but still ±y90° scattering: lin.pol. in Q, U, smaller than in non-magnetic case

The HeI 10830 diagnostics: Hanle + BThe HeI 10830 diagnostics: Hanle + B

Hanle Effect (Trujillo-Bueno, 2002, Landi Degl'Innocenti, 1982)(Trujillo-Bueno, 2002, Landi Degl'Innocenti, 1982)


Atomic Polarization: the quantum picture

'normal‘ (scattering) case:upper level atomic polarization polarization only in emission (90° scattering) no polarization in absorption (forward scattering)

Transition: JL = 0 → JU = 1


Hanle Effect, the He 10830 case

He Blue Line (JHe Blue Line (JLL=1, J=1, JUU=0):=0):

degenerate lower leveldegenerate lower levelupper level cannot carry atomic

polarization→ emitted beam to (1) unpolarized→ transmitted beam (2) has excess of linear polarization ┴ to B (=dichroism)

Trujillo-Bueno, 2001Trujillo-Bueno, 2001

'normal‘ (scattering) case:upper level atomic polarization


The HeI 10830 diagnostics: Atomic PolarizationThe HeI 10830 diagnostics: Atomic Polarization


Hanle Effect, the He 10830 case Trujillo-Bueno, 2001Trujillo-Bueno, 2001

'normal‘ (scattering) case:upper level atomic polarization


The HeI 10830 diagnostics: Atomic PolarizationThe HeI 10830 diagnostics: Atomic Polarization

He Red Lines (JHe Red Lines (JLL=1, J=1, JUU=1 or 2):=1 or 2):

degenerate upper & lower leveldegenerate upper & lower levelboth levels carry atomic polarization

→ emitted beam to (1) polarized→ transmitted beam (2) has excess of linear polarization ┴ to B


90° scattering: linear polarization only in red line


The prominence case


forward scattering: linear polarization in red & blue line


The filament case


Hanle effect saturation

Hanle effect sensitive

linear polarization signal depends on

1) magnetic field strength

2) magnetic field direction

(around B = 10−2 G, the density matrix elements start to be affected by the magnetic field caused by a feedback effect that the alteration of the lower-level polarization has on the upper levels)

Hanle saturation regime

linear polarization signal depends on

1) magnetic field direction

(coherences are negligible and the atomic alignment values of the lower and upper levels are insensitive to the strength of the magnetic field)

Application:disk center, horizontal field:tan(2*AZI) = Q/U

↑ <

8 G

auss

↑↓

> 8

Gau

ss ↓

0 – 8 Gauss 8 - 100 Gauss


Ambiguities of Hanle effect

solid lines: INC=const, AZI=(-90,90)dashed lines: AZI=±90, INC=(0,-90)

B=25 Gauss, off-limb, red comp. polarization diagram:same QU diagramfor:

INC 180-INCandAZI -AZI

andAZI 180-AZI(but: different V)

(traditional ambiguities)

Merenda et al., 2006Merenda et al., 2006

Van Vleck ambiguityVan Vleck ambiguitysaturated regime


Ambiguities: van Vleck ambiguity + traditional ambiguity

INC=80°, AZI=-46°, B=22G or INC=40°, AZI=19°, B=25G

plus traditional 180° ambiguity:INC=100°, AZI=46°, B=22G or INC=140°, AZI=-19°, B=25G

Merenda et al., 2006Merenda et al., 2006

The Van Vleck ambiguity occurs only for some combinations of the inclinations and azimuths. Moreover, it occurs mainly in the saturation regime of the Hanle effect.


Dependence of LP on optical thickness of He slab


no change in ratio!


Dependence of Hanle signal on inclination and observing angle


μ=0.1

μ=0.1

μ=1

μ=1

cos2(ΘVV)=1/3

B=10G, h=3”

red

com

p.

blue

com

p.

U/IU/I

Q/I

Q/I


Dependence of Stokes Q on magnetic field strength

Trujillo Bueno and Asensio Ramos, 2007Trujillo Bueno and Asensio Ramos, 2007




atomic polarization must not be neglected even for strong fields!


Some pitfalls for Zeeman-used scientists

Zeeman:total linear polarization is proportional to transversal field

disk centerB=500G

blue: INC=54° (more horizontal)green: INC=44°

red: INC=34° (more vertical)



Zeeman:total linear polarization is proportional to transversal field

Hanle:not at all!(van Vleck angle)

disk centerB=50G





Zeeman:ratio between linear and circular polarization is proportional to inlination

Hanle:not at all!(van Vleck angle)(same example)

disk centerB=50G





Zeeman:strength of polarization signal is a measure of strength of magnetic field

Hanle:not for very weak fields!(Hanle depolarizes)

saturation regime (10-100G):strength of linear polarization does not depend on B

disk centerINC=60°

blue: B=100G (strongest)green: B=25G

red: B=1G (weakest)


Conclusions

Strong fields (active region, plage fields):reliable measurements for B > 200 G (100 G for special geometries)Paschen-Back effect important for correct determination of |B|atomic polarization important for B < 1.5 kG10-3 polarization signal sufficient

Weak fields:10 – 100 G: saturated Hanle regime: LP determined by direction of B<10 G: Hanle sensitive regime: LP depends on direction and on strength of Baveraging: weak fields do not cancel out!good: 4x10-4 polarization signal, ideal: 1x10-4

Hanle: additional complications in analysis of data

Ambiguities: 180° Hanle ambiguity Van Vleck ambiguity

Computation: x10-100 as compared to

Zeeman only

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