Ponderomotive acceleration in the auroral region: A kinetic simulation

Alfvtn waves with frequencies of the order of several Hertz have been observed in the auroral region. These waves are thought to be responsible for the acceleration of !0nosphen'c particles out of the ionosphere and into the magnetosphere. As an Alfvtn wave propagates into an inhomogeneous ionosphere, the amplitude of the wave decreases resulting in a gradient in the electric field and a ponderomotive force. The nature of the ponderomotive force is to accelerate ions out of the ionosphere, while accelerating electrons into the ionosphere decreasing the ambpolar electric field. Ponderomotive acceleration of ions can penetrate to low enough altitudes affecting the production regions for O + and H + ions. To understand the influences of ponderomotive acceleration on the lower ionosphere, a one-dimensional hybrid particl e code is used. The simulation model allows for multiple species, atmospheric chemistry and phOtoionization, tilted dipolar coordinate system, corotation, and convection. The electrons are treated as a charge neutralizing fluid 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 simulations. The altitude dependence of an Alfvtn wave propagating from 6000 to 200 gan in an inhomogeneous ionosphere iS determined by solving Maxwell's equations. The parallel and perpendicular envelopes are determined and the resulting nonlinear, non-resonant ponderomotive acceleration of ionospheric ions is calculated. The ponderomotive force is included in the kinetic simulation in the same fashion as gravity or the self-consistent electric field. The acceleration event lasts for 10 min, by'which time the density and flow speed profiles have reached equ.ilibrium. The density and flow speed at 2500 km after 10 min of acceleration increase from 4.3 cm -3 and 1.6 km/s to 34 cm '3 and 6.0 km/s for 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 response of the ionosphere to the nonlinear ponderomotive force is an enhancement in ionospheric outflow of ions into the magnet0sphere.