  The large-scale magnetic field of the Milky Way is thought to be created by
an alpha-Omega dynamo, which implies that it should have opposite handedness
North and South of the Galactic midplane. Here we attempt to detect a variation
in handedness using polarization data from the Wilkinson Microwave Anisotropy
Probe. Previous analyzes of the parity-even and parity-odd parts of linear
polarization of the global dust and synchrotron emission have focused on
quadratic correlations in spectral space of, and between, these two components.
Here, by contrast, we analyze the parity-odd polarization itself and show that
it has, on average, opposite signs in Northern and Southern Galactic
hemispheres. Comparison with a Galactic mean-field dynamo model shows broad
qualitative agreement and reveals that the sign of the observed hemispheric
dependence of the azimuthally averaged parity-odd polarization is not
determined by the sign of alpha, but by the sense of differential rotation.
