  Tracing magnetic fields is crucial as magnetic fields play an important role
in many astrophysical processes. Earlier studies have demonstrated that Ground
State Alignment (GSA) is a unique way to detect weak magnetic fields (1G> B>
1exp(-15)G) in diffuse media, they consider the situation when the pumping
source is a point source, which applies when the star is very far away from the
diffuse media. In this paper, we explore the GSA in the presence of extended
radiation fields. For the radiation fields with a clear geometric structure, we
consider the alignment in circumstellar medium, binary systems, disc, and Local
Interstellar Medium (LISM). For the radiation fields with unidentified pumping
sources, we apply the method of multipole expansion and discuss the GSA induced
by each component. We demonstrate that for general radiation fields, it is
adequate to consider the contribution from dipole and quadrupole radiation
components. We find that in general polarization of absorption arizing from GSA
coincides with the projection of magnetic field in the 2D sky with 90 degree
degeneracy. We conclude that the GSA is a unique tool to detect the direction
of weak magnetic field, and it can be applied to diffuse media in any radiation
field.
