Vector Magnetic Field Diagnostics using Hanle Effect

Transcription

1 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

2 Non-magnetic (Rayleigh) scattering polarization in lines Scattering of anisotropic radiation on atoms and molecules produces linear polarization even in the absence of magnetic fields. Scattered beam: I r Unpol. incident beam I l = I r I l I = I + I r =2 I Q = l I I r = 0 l l I r Z Pol : Q/I = 100% U/I = 0 3 linear oscillators Y X 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, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

3 Non-magnetic (Rayleigh) scattering polarization in lines In the case of Sun, the incident radiation field is anisotropic due to limb darkening. Incoming radiation A < 0 Limb brightening Outgoing radiation A > 0 Limb darkening Anisotropic illumination of an atom in the solar atmosphere As the solar anisotropy (A) is small the degree of linear polarization Q/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). M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

4 Second Solar Spectrum Linearly Polarized Spectrum of the Sun observed near the limb is called the Second Solar Spectrum First Solar Spectrum Unpolarized Intensity Spectrum Second Solar Spectrum Linearly Polarized Spectrum It gives a wealth of information on the physics of light scattering on atoms and molecules. M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

5 Magnetic scattering polarization in lines (Hanle effect) The presence of a weak field modifies the Rayleigh scattered polarization and is termed as Hanle effect. Unpol. incident beam I = I l r I = I + I r =2 I l l Q = I l I r = 0 I l B U/I = 0 Z I 2 circular oscillators r X Scattered beam Hanle scattering (magnetic Rayleigh Scattering) Pol : Q/I < 100% Hanle effect causes : (1) a modification to the non-magnetic Q/I (basically depolarization), (2) rotation of polarization plane (fresh generation of U/I ). M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24 Y

6 Hanle effect a tool for highly sensitive field measurements Hanle sensitivity parameter Γ = Larmor Frequency Radiative 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 ( 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. M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

7 Hanle effect signatures in the polarized solar spectrum Sr I 4607 A, a photospheric line Ca I 4227A, a chromospheric line Hanle effect by photospheric turbulent magnetic field V/I signatures are due to longitudinal Zeeman effect Hanle effect by chromospheric oriented magnetic field. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

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

9 The limb observation of Ca i 4227 Å line Observations were taken in quiet regions on the Sun at IRSOL observatory, 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 M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

10 Modeling the limb observation of Ca i 4227 Å line The observed Q/I spectra of Ca i 4227 Å 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. n Obs LSA Obs n Q/I spectra has a triple peak structure a central core peak flanked 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. M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

11 Forward-scattering Hanle effect at disk center! Solar Atmosphere In the solar limb, Hanle effect produces depolarization : [(Q/I ) B 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 referred to as forward-scattering Hanle effect (discovered 10 years ago). Disk Center Observations B = 0: Q/I = 0, U/I = 0 B = 0: Q/I (say 2 %) U/I = 0 B = 0: Q/I = 0 (around 0.2 %) U/I = 0 Limb Observations B = 0: Q/I reduced (< 2 %) U/I = 0 It is a good diagnostic tool to determine weak, oriented magnetic fields. The first observational evidence was provided by Trujillo Bueno et al. (2002), who observed it in the He i Å line.. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

12 Forward-scattering Hanle observations of Ca i 4227 Å line Recently we performed near disk center spectro-polarimetric observations in Ca i 4227 Å 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 N Positive Q direction for theory Radius vector Slit α Positive Q direction for observations S M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

13 Modeling the forward-scattering Hanle effect in Ca i 4227 Å model profile observations I/I c (B, θ B, χ B ) represent the vector magnetic field parameters. Q/I (%) A realistic model atmosphere & a 20 level Ca i multi-level atom model are used. U/I (%) Observed (I, Q/I, U/I ) spectra were modeled using polarized radiative transfer. (B, θ B, χ B ) are free parameters. V/I (%) Unique (B, θ B, χ B ) values are determined by demanding a simultaneous fit of (I, Q/I, U/I ) using Hanle effect and V /I using Zeeman effect. (Anusha et al. 2011, ApJ, 737, 95) M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

14 Polarimetric limb observation of multi-line systems Scattering polarization in a multiplet can be used to determine the depth-dependence of turbulent magnetic fields. Scattering polarization in a multiplet arises due to quantum interference between the upper J-states. The theory required to model polarization in such a multiplet was formulated in Smitha et al. (ApJ 2011a, A&A 2011b). Cr i triplet at 5204, 5206, and 5208 Å provides a good example to study 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 allow simultaneous recording of all the three components of the Cr i triplet. M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

15 Modeling the Cr i triplet using J-state interference theory Figure shows a model fit to the limb observations in quiet regions. Micro-turbulent fields are again required to fit the line core polarization. Cross-over effect can be produced only by using quantum interference theory. The cross-over in Q/I in between the lines is the indicator of quantum interference between the line components. The triple-peak structure is an indicator of the partial frequency redistribution in scattering.. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

16 Enigmatic wing signatures in Ca i 4227 Å line Spectropolarimetry of Ca i 4227 Å line by Bianda et al. (2003, SPW3) in an active region, and by Sampoorna et al. (2009, ApJ) in quiet regions 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.. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

17 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 in terms of such a transfer of coherence (wing Hanle effect) failed. observed Q / I in the "magnetic region" theoretical ( Q / I ) mag for B = 0 theoretical (Q / I) non mag for B = 0 = ( Q / I ) mag ( Q / I ) non mag observed U/ I in the "magnetic region" theoretical U / I for B = 0 M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

18 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 has to 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 and polarization caused by spatial inhomogeneities. Such scattering polarization observations are completely novel. With NLST we can perform such high spatial resolution observations. M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

19 Origin of (Q/I, U/I ) wing signatures possibilities with NLST 45 cm Gregory-Coude telescope with ZIMPOL-3 polarimeter and adaptive optics, at IRSOL, Switzerland has following characteristics : Spatial resolution = 0.5 (with AO) Spectral resolution = 5 må at 5000Å Polarimetric accuracy = 10 5 Wavelength range = 3910 Å to 6995Å Our 2 m Gregorian on-axis NLST with Spectropolarimeter and adaptive optics : Spatial resolution = 0.1 (with AO) Spectral resolution = 10 må at 6500Å Polarimetric accuracy = 10 4 Wavelength range = 3800 Å to 2.5 microns M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

20 Proposal for Spectropolarimetry with NLST High spatial resolution Hanle effect observations in prominences : 1 Off-limb observations to determine the relation between vector magnetic 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 height variation of magnetic/ velocity fields (quantum interference effect). Measurement of Hanle signatures in strong resonance lines (like Ca i 4227 Å line) to explore the weak field structure near disk center (forward-scattering Hanle effect). M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

21 Summary NLST will deliver high-sensitivity imaging spectro-polarimetry in the wavelength range 3800 Å 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% in polarization within 10 seconds) for polarization measurements. Some of the unsolved problems in Hanle observations can be addressed using NLST. Scattering polarization observations at high spatial resolution which is possible with NLST may be used for exploring interesting physical effects. M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

22 IRSOL Instrumentation data Telescope: Gregory-Coude, evacuated, primary mirror has a diameter of 45 cm, total focal length: 25 m. Spectrograph: Czerny-Turner, focal length: 10 m, grating 180 mm x 360 mm, 316 lines / mm, blaze 63. Fabry-Perot filter system: Tunable narrow-band filter system based on two lithium-niobate Fabry-Perot etalons. It allows monochromatic imaging at any wavelength from 395 nm to 660 nm. Adaptive optic system: Tip-tilt and deformable mirror with 37 actuator Electronics: ZIMPOL-polarimeter, CCD-Wright Instruments, CCD-DTA-Pisa, FHSW-flare registration system, Guiding system (PIG). M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

23 Hanle Scattering (Quantum picture) J = 1 I 0 0 J = 0 ν L = Γ R U QI Γ R ν L Θ a scattering event M. Sampoorna (Indian Institute of Astrophysics, Vector Koramangala, Magnetic Field Bangalore) Diagnostics using Hanle Effect November 2, / 24

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