BEAM-DRIVEN ACOUSTIC SOLITARY WAVES IN THE AURORAL ACCELERATION REGION

The formation of ion acoustic solitary structures driven by electron a nd ion beams in the auroral acceleration region is studied using two-d imensional electrostatic particle simulations. The beams are consisten tly 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 intr oduces the ion two-stream instability besides the ion acoustic one int o the system and modifies previous analysis and simulation results of solitary wave formation. Solitary structures Form as a result of the m icroinstability development. The numerical simulation results show tha t positively peaked (phi > 0) localized structures are formed in the s ystem 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 bea m velocity. By contrast, negative potential solitary structures form w hen 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 a t subsonic speeds relative to the background ions, which may carry the localized pulses upward. Evolving solitary waves do not carry any sig nificant net potential drop and therefore cannot contribute much to th e auroral particle acceleration. They are found to be a consequence of the larger-scale V-shaped potential distribution in the auroral regio n.