  Energetic positrons produced in annihilation or decay of dark matter
particles in the Milky Way can serve as an important indirect signature of dark
matter. Computing the positron flux expected in a given dark matter model
involves solving transport equations, which account for interaction of
positrons with matter and galactic magnetic fields. Existing calculations solve
the equations inside the diffusion zone, where galactic magnetic fields confine
positrons, and assume vanishing positron density on the boundaries of this
zone. However, in many models, a substantial fraction of the dark matter halo
lies outside the diffusion zone. Positrons produced there can then enter the
diffusion zone and get trapped, potentially reaching the Earth and increasing
the expected flux. We calculate this enhancement for a variety of models. We
also evaluate the expected enhancement of the flux of energetic photons
produced by the inverse Compton scattering of the extra positrons on starlight
and cosmic microwave background. We find maximal flux enhancements of order 20%
in both cases.
