Colloquium

Galactic magnetic fields permeating through dust clouds lead to partial alignment of non-spherical dust grains along the field lines. Starlight passing through these dust clouds gets partially linearly polarized due to selective extinction. Measuring this polarization offers in principle, a way of tracing the magnetic fields as well as probing the nature of the dust grains. Dust induced polarization also creates a foreground smokescreen which makes it very difficult to measure underlying tiny signals such as that of B-mode polarization of CMBR.  Removing this foreground is not easy for many reasons. For example, along any given line of sight, there will be several dust clouds (two to three even along high Galactic latitude sight lines), each with possibly different grain compositions, magnetic field strengths and orientations. Due to vector addition effects, the net measured polarization may often not trace the polarization or magnetic fields in any of the individual dust clouds.  We plan to carry out a linear optical polarimetric survey (PASIPHAE: Polar-Areas Stellar Imaging in Polarization High-Accuracy Experiment) of specifically chosen high Galactic latitude areas with an unprecedented combination of depth (R<16.5), swath (> 1000 sq. degrees) and faith (error<0.1%). Two bespoke instruments (WALOP: Wide Area Linear Optical Polarimeter) will be used to carry out the survey simultaneously from a northern (Skinakas) and a southern (Sutherland) site. The high sample density (>100 stars per sq. degree) and large area coverage of PASIPHAE measurements, augmented with data on stellar distances from GAIA and dust cloud distributions from CO maps, will allow us to create a tomographic map of the magnetic field structure towards the survey regions. This talk presents the salient challenges of WALOP/PASIPHAE as well as the results from some path-finding experiments.