Particle simulation of the auroral zone showing parallel electric fields, waves, and plasma acceleration

This paper examines the self-consistent generation of the large-scale quasi-static, parallel electric fields that are formed in the auroral zone and how these fields affect local plasma distributions. A one-dimensional electrostatic particle-in-cell simulation is employed in this study with its axis aligned along a dipolar magnetic field line that includes the magnetic mirror force as well as a cold dense ionosphere at low altitudes Earthward drifting plasma from the magnetotail; is injected into the system at the high-altitude end of the simulation. Simulation results show that injection of magnetotail plasma leads to the mirroring of ions at lower altitudes than electrons, thereby creating a large-scale, quasi-static, parallel potential drop. In addition, the results show that the magnitude of the large-scale potential drop depends on the earthward directed drift speed; the drop can be as large as 2 kV over a distance of a few thousand kilometers for inflowing ion beam energies of 10 to 25 keV. The potential gradient corresponds to a parallel electric field that is directed away from the Earth. The field accelerates ionospheric ions away from the Earth, and the accelerated ions form upwelling beams with drift speeds that are in approximate agreement with observed high-altitude auroral ion beams. Electrons are accelerated earthward and appear as a precipitating high-energy tail in low-altitude ionospheric distribution functions. A broadband plasma wave spectrum is generated where the magnetotail and ionospheric plasma interact, and it plays an important role in auroral dynamics by modifying local plasma distributions. Not only do these modifications of the local distributions affect plasma flow in the region, they also decrease the magnitude of the large-scale potential drop by drawing off energy from the inflowing distributions that otherwise would support the potential drop.