  Magnetic fields have important or dominant effects in many areas of
astrophysics, but have been very difficult to quantify. Spectropolarimetry from
Ground State Alignment (GSA) has been suggested as a direct tracer of magnetic
field in interstellar diffuse medium. Owing to the long life of the atoms on
ground states the Larmor precession in an external magnetic field imprints the
direction of the field onto the polarization of absorbing species. This
provides a unique tool for studies of sub-gauss magnetic fields using
polarimetry of UV, optical and radio lines. Many spectral lines with strong
signals from GSA are in the UV band. By discerning magnetic fields in gas with
different dynamical properties, high spectral resolution measurement of
spectral polarization will allow the study of 3D magnetic field distribution
and interstellar turbulence. GSA provides also a unique chance to map 3D
direction of magnetic field on small scales, e.g., disks, where grain alignment
is unreliable. The range of objects suitable for studies is extremely wide and
includes magnetic fields in the interplanetary medium, in the interstellar
medium, and in circumstellar regions as well as diffuse media in extragalactic
objects.
