Small amplitude ion-acoustic double layers in a plasma with superthermal electrons and thermal positrons

Small-amplitude ion-acoustic double layers (IA-DLs) in a plasma with superthermal electrons and thermal positrons are investigated. Due to the electron superthermality and the relative fraction of positrons, our plasma model can admit compressive as well as rarefactive IA-DLs. As the electrons evolve toward their thermodynamic equilibrium, the negative IA-DL shrinks and, beyond a certain critical value of the electron spectral index κc, develops into a positive structure allowing therefore the existence of compressive IA-DLs. This critical value is lowered as the number of positrons becomes important. As the superthermal character of the plasma increases, the threshold pc above which only compressive IA-DLs are admitted is shifted toward higher values. Our investigation may help to understand the DL structures observed in the ionosphere and the auroral acceleration regions.