  Recently there have been significant interests in the spin hydrodynamic
generation phenomenon from multiple disciplines of physics. Such phenomenon
arises from global polarization effect of microscopic spin by macroscopic fluid
rotation and is expected to occur in the hot quark-gluon fluid (the ``subatomic
swirl'') created in relativistic nuclear collisions. This was indeed discovered
in experiments which however revealed an intriguing puzzle: a polarization
difference between particles and anti-particles. We suggest a novel application
of a general connection between rotation and magnetic field: a magnetic field
naturally arises along the fluid vorticity in the charged subatomic swirl. We
establish this mechanism as a new way for generating long-lived in-medium
magnetic field in heavy ion collisions. Due to its novel feature, this new
magnetic field provides a nontrivial explanation to the puzzling observation of
a difference in spin hydrodynamic generation for particles and anti-particles
in heavy ion collisions.
