PONDEROMOTIVE ACCELERATION IN THE AURORAL REGION - A KINETIC SIMULATION

Alfven waves with frequencies of the order of several Hertz have been observed in the auroral region. These waves are thought to be responsi ble for the acceleration of ionospheric particles out of the ionospher e and into the magnetosphere. As an Alfven wave propagates into an inh omogeneous ionosphere, the amplitude of the wave decreases resulting i n a gradient in the electric field and a ponderomotive force. The natu re of the ponderomotive force is to accelerate ions out of the ionosph ere, while accelerating electrons into the ionosphere decreasing the a mbipolar electric field. Ponderomotive acceleration of ions can penetr ate to low enough altitudes affecting the production regions for O+ an d H+ ions. To understand the influences of ponderomotive acceleration on the lower ionosphere, a one-dimensional hybrid particle code is use d. The simulation model allows for multiple species, atmospheric chemi stry and photoionization, tilted dipolar coordinate system, corotation , and convection. The electrons are treated as a charge neutralizing f luid where the parallel and perpendicular temperatures and heat flows are given by the 16-moment transport equations. Coulomb, ion-neutral, and electron-neutral collisions are included in the hybrid simulation using newly developed collision techniques applicable to kinetic simul ations. The altitude dependence of an Alfven wave propagating from 600 0 to 200 km in an inhomogeneous ionosphere is determined by solving Ma xwell's equations. The parallel and perpendicular envelopes are determ ined and the resulting nonlinear, non-resonant ponderomotive accelerat ion of ionospheric ions is calculated. The ponderomotive force is incl uded in the kinetic simulation in the same fashion as gravity or the s elf-consistent electric field. The acceleration event lasts for 10 min , by which time the density and flow speed profiles have reached equil ibrium. The density and now speed at 2500 km after 10 min of accelerat ion increase from 4.3 cm(-3) and 1.6 km/s to 34 cm(-3) and 6.0 km/s fo r H+, while the same quantities for O+ increase from 1670 cm(-3) and - 0.0009 km/s to 4795 cm(-3) and 7.0 km/s. Collisional coupling between the H+ and O+ contributes to enhancing the outflow of H+. The net resp onse of the ionosphere to the nonlinear ponderomotive force is an enha ncement in ionospheric outflow of ions into the magnetosphere.