  Fluorescence emission lines are broadly applied in observation for diffuse
medium in the universe. They are normally observed around strong pumping
source, tracing the gas in circumstellar medium, reflection nebula, and H\,{\sc
ii} regions, etc. They reside in UV/optical and infrared bands and hence could
be directly observed with ground-base telescopes. In this letter, we
demonstrate the polarization of fluorescence lines as a magnetic field tracer
arising from ground state atomic alignment in diffuse medium, including our
solar system, supernova remnants (SNRs), as well as quasi-stellar object (QSO)
host galaxies. Two types of fluorescence emissions are considered: the primary
fluorescence from the excited states; and the secondary fluorescence from the
metastable state (forbidden lines). We find that the synergy of these lines
could measure three-dimensional magnetic direction: the polarizations of the
primary fluorescence lines could reveal the magnetic polar angle along the
line-of-sight, whereas the polarization of forbidden lines traces the
plane-of-sky magnetic direction. The expected degree of polarization is
$P>10\%$. Polarizations of both types of fluorescence emissions have shown
strong potential for observations, and are applicable to measure magnetic field
within and beyond our galaxy.

Astrophysics of Galaxies;Instrumentation and Methods for Astrophysics;Atomic Physics