The influence of atomic alignment on absorption and emission
  spectroscopy
  Spectroscopic observations play essential roles in astrophysics. They are
crucial for determining physical parameters in the universe, providing
information about the chemistry of various astronomical environments. The
proper execution of the spectroscopic analysis requires accounting for all the
physical effects that are compatible to the signal-to-noise ratio. We find in
this paper the influence on spectroscopy from the atomic/ground state alignment
owing to anisotropic radiation and modulated by interstellar magnetic field,
has significant impact on the study of interstellar gas. In different
observational scenarios, we comprehensively demonstrate how atomic alignment
influences the spectral analysis and provide the expressions for correcting the
effect. The variations are even more pronounced for multiplets and line ratios.
We show the variation of the deduced physical parameters caused by the atomic
alignment effect, including alpha-to-iron ratio ([X/Fe]) and ionisation
fraction. Synthetic observations are performed to illustrate the visibility of
such effect with current facilities. A study of PDRs in $\rho$ Ophiuchi cloud
is presented to demonstrate how to account for atomic alignment in practice.
Our work has shown that due to its potential impact, atomic alignment has to be
included in an accurate spectroscopic analysis of the interstellar gas with
current observational capability.
