BEAM-DRIVEN ACOUSTIC SOLITARY WAVES IN THE AURORAL ACCELERATION REGION

The formation of ion acoustic solitary structures driven by electron and ion beams in the auroral acceleration region is studied using two-dimensional electrostatic particle simulations. The beams are consistently present in regions of moderate potential drop (less than or equal to 1 keV) where weak double layers have been observed on both the S3-3 and Viking spacecraft. The presence of more than one ion species introduces the ion two-stream instability besides the ion acoustic one into the system and modifies previous analysis and simulation results of solitary wave formation. Solitary structures Form as a result of the microinstability development. The numerical simulation results show that positively peaked (phi > 0) localized structures are formed in the system driven by a dense (n(ib) approximate to n(ic) approximate to n(e)/2) ion beam. The solitary waves move in the direction of the ion beam velocity. By contrast, negative potential solitary structures form when the ion beam density is reduced to 10 % (n(ib) approximate to 0.1n(e)) and electron drift relative to background ions is sustained by an applied electric field. In this case, solitary waves drift downward at subsonic speeds relative to the background ions, which may carry the localized pulses upward. Evolving solitary waves do not carry any significant net potential drop and therefore cannot contribute much to the auroral particle acceleration. They are found to be a consequence of the larger-scale V-shaped potential distribution in the auroral region.