  The expansion of a charge-neutral cloud of electrons and positrons with the
temperature 1 MeV into an unmagnetized ambient plasma is examined with a 2D
particle-in-cell (PIC) simulation. The pair outflow drives solitary waves in
the ambient protons. Their bipolar electric fields attract electrons of the
outflowing pair cloud and repel positrons. These fields can reflect some of the
protons thereby accelerating them to almost an MeV. Ion acoustic solitary waves
are thus an efficient means to couple energy from the pair cloud to protons.
The scattering of the electrons and positrons by the electric field slows down
their expansion to a nonrelativistic speed. Only a dilute pair outflow reaches
the expansion speed expected from the cloud's thermal speed. Its positrons are
more energetic than its electrons. In time an instability grows at the front of
the dense slow-moving part of the pair cloud, which magnetizes the plasma. The
instability is driven by the interaction of the outflowing positrons with the
protons. These results shed light on how magnetic fields are created and ions
are accelerated in pair-loaded astrophysical jets and winds.
