  At times later than 200 days, Type Ia SN light curves are dominated by the
kinetic energy deposition from positrons created in the decay of $^{56}$Co. In
this paper, the transport of positrons after emission are simulated, for
deflagration and delayed detonation models, assuming various configurations of
the magnetic field. We find the light curve due to positron kinetic energy has
a different shape for a radially combed magnetic field than it does for a
tangled field that traps positrons. The radial field light curves fit the
observed light curves better than do the trapping field light curves for the
four SNe used in this study. The radial field permits a much larger fraction of
the positrons to escape, perhaps enough to explain a large percentage of the
511 keV annihilation radiation from the Galactic plane observed by OSSE.
