Arbitrary amplitude ion-acoustic soliton coexistence and polarity in a plasma with two ion species

Abstract

Using a Sagdeev pseudopotential formalism, large ion-acoustic waves are investigated in a multispecies plasma model consisting of two cold positive ion species in the presence of nonthermal Cairns electrons. Positive solitons are limited in amplitude by infinite compression of the lighter ion species. Negative solitons become possible at a sufficient degree of electron nonthermality, and their amplitudes are limited by the occurrence of double layers. What determines coexistence is the possibility of having parameter ranges for which there are solitons at the acoustic speed, which is a necessary and sufficient condition but unfortunately not an easy criterion to check analytically. For the first time a parameter range has been found where both negative and positive polarity solutions coexist, with the solitons at the acoustic speed always having a positive polarity, contrary to earlier results which seemed to indicate that polarity switches and coexistence ranges were linked. Finally, some useful proofs are presented in an Appendix, concerning the Sagdeev pseudopotential behavior at the acoustic speed, results of which are generic to a range of physical plasma models.