Tracing Magnetic Fields with Ground State Alignment
  Observational studies of magnetic fields are vital as magnetic fields play a
crucial role in various astrophysical processes, including star formation,
accretion of matter, transport processes (e.g., transport of heat), and cosmic
rays. We identified a process "ground state alignment" as a new way to
determine the magnetic field direction in diffuse medium. The alignment is due
to anisotropic radiation impinging on the atom/ion, while the magnetic field
induces precession and realign the atom/ion and therefore the polarization of
the emitted or absorbed radiation reflects the direction of the magnetic field.
The atoms get aligned at their low levels and, as the life-time of the
atoms/ions we deal with is long, the alignment induced by anisotropic radiation
is susceptible to weak magnetic fields ($1{\rm G}\gtrsim B\gtrsim 10^{-15}$G).
Compared to the upper level Hanle effect, atomic realignment is most suitable
for the studies of magnetic field in the diffuse medium, where magnetic field
is relatively weak. In fact, the effects of atomic/ionic alignment, including
the realignment in magnetic field, were studied in the laboratory decades ago,
mostly in relation to the maser research. Recently, the atomic effect has been
already detected in observations from circumstellar medium and this is a
harbinger of future extensive magnetic field studies. A unique feature of the
atomic realignment is that they can reveal the 3D orientation of magnetic
field. In this article, we shall review the basic physical processes involved
in atomic realignment and its applications to interplanetary, circumstellar and
interstellar magnetic fields. In addition, our research reveals that the
polarization of the radiation arising from the transitions between fine and
hyperfine states of the ground level can provide a unique diagnostics of
magnetic fields, including those in the Early Universe.
