Helicity in the Large-Scale Galactic Magnetic Field
  We search for observational signatures of magnetic helicity in data from
all-sky radio polarization surveys of the Milky Way Galaxy. Such a detection
would help confirm the dynamo origin of the field and may provide new
observational constraints for its shape. We compare our observational results
to simulated observations for both a simple helical field, and for a more
complex field that comes from a solution to the dynamo equation. Our simulated
observations show that the large-scale helicity of a magnetic field is
reflected in the large-scale structure of the fractional polarization derived
from the observed synchrotron radiation and Faraday depth of the diffuse
Galactic synchrotron emission. Comparing the models with the observations
provides evidence for the presence of a quadrupolar magnetic field with a
vertical component that is pointing away from the observer in both hemispheres
of the Milky Way Galaxy. Since there is no reason to believe that the Galactic
magnetic field is unusual when compared to other galaxies, this result provides
further support for the dynamo origin of large-scale magnetic fields in
galaxies.
