The nonlinear wave structures of large amplitude ion-acoustic waves are
studied in a plasma with positrons. We have presented the region of existence
of the ion-acoustic waves by analysing the structure of the pseudopotential.
The region of existence sensitively depends on the positron to electron
density ratio, the ion to electron mass ratio and the positron to electron
temperature ratio. It is shown that the maximum Mach number increases as the
positron temperature increases and the region of existence of the ion-acoustic
waves spreads as the positron temperature increases. The present theory is
applicable to analyse large amplitude ion-acoustic waves associated with
positrons which may occur in space plasmas.