Atomic Alignment: New tool for studies of Cosmic Magnetic fields

1. Atomic Alignment: New tool for studies of Cosmic Magnetic fields Huirong Yan (CITA, Canada) with: A. Lazarian (UW-Madison, USA)

2. Atomic alignment is sensitive to weak magnetic fields Level interferences occur v(cm/s) G    L–Larmor frequency A-Einstein coefficient J– EJ /h Lower level Hanle effect Hanle effect Atomic realignment Zeeman effect J R-1      L (s-1) Weak field (<1G) are sufficient. History: Laboratory studies on masers (experimental) (Brossel et al. 1952; Hawkins 1955; Kastler 1957)

3. atoms Angular momentum radiation Atoms are aligned by anisotropic radiation Toy model: Atomic alignment is differential occupation of the sublevels ofthe ground (or metastable) state. Atomic alignment is induced by anisotropic radiation. - + 0

4. Requirement for alignment: Incoming light transmitted light medium • anisotropic radiation (unpolarized light is sufficient) • fine or hyperfine structure on the ground level are required.

5. B J(F) M=2 z M=1 r M=0 M=-1 M=-2 Magnetic field induced precession and realigns atoms B   0

6. Alignment depends on the angle between the B and light r r Alignment is determined by r. The polarization degree depends on  and r .For the degenerate case, r and  coincide.

7. Using QED calculations we obtain Stokes parameters Absorption coefficients, emissivities Transitional Equilibrium eqs Density tensor Stokes parameters IQU I, Q, U kq I,Q,U Main object: density tensor: kq 2: dipole moment 4: quardripole moment

8. Na I N V Al III H I Cr I Cr II 5892- 5898 1239- 1243 1855- 1863 912- 1216 3580- 3606 2046- 2066 O I S II Ti II Ti III C II C I 5513-7254 1250-1260 3058-3385 1282-1299 1036-1336 1115-1657 S I S III Fe II Fe III Si II Si I 1205-1826 1012-1202 2344-2600 1123-1132 990-1533 1695-2529 Our results: many observed absorption lines have appreciable polarization

9. Our results: polarization from resonance scattering

10. Polarization of absorption lines Yan & Lazarian (2006) Polarization changes direction at Van Vleck angle r =54.7o,180o- 54.7o.

11. Information from absorption lines 2. 3D information: from degree of polarization P(20(r),). Two lines are enough  -1.82 -11% “Easy” information: Polarization is || or B in the planeof sky. Available with one line.

12. Strength of B can be obtainedwhen G = nL / tR-1 ~1 Polarization of SII absorption G  polarization depends on r, ,  as well as G

13. An example: synthetic observation of a comet wake Simulated magnetic field lines P: -8%~14% for Na D2 (a) (b) (a) MHD simulations of comet’s wake; (b) Resonance scattering of solar light by sodium tail from comet;

14. Possible to study magnetic field in the epoch of Reionization OI 63.2 m transmission From Cantalupo et al. (2008) • UV excitation rate/ CMB excitation rate (Yan & Lazarian 2008)

15. Polarization from Poloidal Disk LOS z   r  x

16. Polarization from Toroidal Disk LOS z    x

17. circumstellar medium ISM Interplanetary medium QSOs Interglactic medium Where is atomic alignment applicable?

18. Summary • Aligned atoms and ions widely exist and provide unique information about 3D magnetic field direction. • Polarization of optical and UV absorption and emission • lines can be used for the studies. • This tool is applicable when other tools fail, e.g. from comet’s wake to early universe. • It also can detect time variation of B, thus allow an cost effective way of studying turbulence.

19. Related works • Diffuse medium (but very idealized situations) (Varshalovich 1971; Landolfi & Landi Degl’Innocenti 1986) • AGN polarization (but problems with formalism) (Lee, Blandford & Western 1994) • Solar magnetism (but different regime) (See papers by Landi Degl’Innocenti, Trujillo Bueno, Stenflo, etc)