1. Dust-polarization maps and interstellar turbulence
Marc Kamionkowski (Johns Hopkins University)
(work with Chris Hirata and Robert Caldwell, in preparation)

2. Planck map (353 GHz) of dust polarization

3. From which (since dust polarization perpendicular to local magnetic field) is inferred

4. E/B (grad/curl) decomposition
Polarization map can be decomposed geometrically into E modes and B modes
Kamionkowski, Kosowsky, Stebbins 1996; Seljak, Zaldarriaga 1996;
Recent review article: “The Quest for B modes From Inflationary Gravitational Waves,” Kamionkowski & Kovetz, arXiv: (ARAA, in press)

6. Power Spectra

7. Pre-Planck expectation:
E.g., from random polarization field
Or if dust is randomly distributed in uniform magnetic field

8. What Planck finds! (Also seen, at lower l, but with less significance,
in WMAP polarized-synchrotron maps)

9. WTF?!?!?

10. Another fun fact
TE (temperature-polarization) cross-correlation is measured by Planck and found to be positive

11. Magnetized-fluid perturbations
ISM is magnetized plasma
Magnetic-field lines pinned to plasma
Perturbations to plasma/field configuration decomposed into slow, fast, and Alfven waves (one plasma-density degree of freedom and two magnetic-field degrees of freedom given )
In weak-field limit, fastsound, and slow/Alfventransverse-field perturbations
But more generally (for arbitrary ), slow/fast waves are combinations of plasma-density and transverse-field perturbations.
Alfven waves are always transverse-field perturbations and involve no plasma-density perturbation

12. Calculation
Polarization from region of dust density n and magnetic field H is where the plane of the sky is the x-y plane and z is the line of sight. A is a constant (and A<0 so dust polarization is perpendicular to H field)
Consider background magnetic field in x-z plane at some angle wrt the line of site (z)
Consider H/n perturbations from one Fourier mode of slow, fast, or Alfven wave with wavevector in x-y plane.
Calculate E/B/T amplitudes for that mode
Average over all background magnetic-field orientations

13. Parameters
Parameter for power anisotropy:

14. Results

15. Implication/Interpretation
EE/BB~2 and TE>0 does not fit with expectations from MHD turbulence for power in Alfven/slow/fast waves
Neither does spectral index (as opposed to expected for MHD turbulence)
Interpretation: ~ pc scales probed by Planck dust polarization overlap “outer scale” of turbulence; i.e., observations may have more to do with large-scale physics stirring the ISM rather than the turbulent cascade to smaller scales

16. Lots of future measurements/analyses to do
We’ve figured out how to “read” dust-polarization maps---tons of detailed information about ISM that cannot be accessed in any other way! A powerful new probe of the ISM!
E.g., there should be variations in EE/BB and TE (and local departures from statistical isotropy) in small patches of sky that reflect the background magnetic-field orientation there
Outer-scale hypothesis implies departures from scale-invariance in E/B/T dust power spectra at larger and smaller scales
Analysis tools can be applied to high-resolution polarization maps (radio/submm/IR) of individual molecular clouds

17. Tons of cool stuff to do with forthcoming projects!!
Deep integrations of small regions: perfect for BlastPol/BFORE/PILOT (cf. Kovetz-MK, )
Implications for large-angle measurements (CLASS/LiteBird)
Whole array of new science targets for CMB-S4 (and Simons/BICEP- Keck/SPTPol/ACTPol/you-name-itPol)
Powerful new analysis tool for radio/submm/IR polarization maps of molecular clouds, star-forming regions,
…..improved understanding will also improve algorithms for foreground separation for CMB-polarization studies (cf., MK-Kovetz, )