The downward current region of the aurora is now recognized as an important region in the magnetospheric-ionospheric (MI) coupling process. In this region, energetic fluxes of electrons are accelerated upwards by electric fields parallel to the magnetic field. Despite the fact that the electric fields are directed downward, this region also has strong ion outflow. In fact, the ion outflow exceeds that of the upward current region. The main focus of this project is to make to understand the current-voltage relation and ion outflow in the downward current region. Three main questions will be addressed: (1) What is the relation between parallel electric fields and currents in the downward current region? (2) How do ionospheric or magnetospheric conditions affect the relation? (3) How is ion outflow modified by the combination of moving parallel electric fields and intense electrostatic turbulence? The ultimate goal of this study is to gain sufficient understanding of the downward current region to be able to incorporate its effects into global models. Direct observations of the electric fields in the downward current region indicate of that, at least in some cases, the parallel electric fields are localized and moving upwards. Observations also show that the localized parallel electric field sets up strongly unstable electron beams which, in turn, create a spatially-separated region of strong electrostatic wave turbulence with electron phase-space holes and ion cyclotron waves. The accelerated electron fluxes are substantially modified by the accompanying intense wave turbulence and electron phase-space holes. The wave turbulence also strongly heats the ions. The project is a collaborative effort carried out at the University of Colorado at Boulder by experimental space plasma physicists at the Laboratory for Atmospheric and Space Physics and theoreticians at the Center for Integrated Plasma Studies. A combination of methodologies will be used, including numerical simulations, test particle simulations and analytical modeling. Key to this study is the implementation of boundary conditions that are based on observations and use of large simulation domains to study the aggregate system of parallel electric fields and wave turbulence and evolution of electron and ion distributions. The small-scale physics will be investigated with existing 1-D and 2-D magnetized, open-boundary Vlasov codes. The medium scale physics will be investigated using analytical modeling and test-particle simulations with a newly developed code that incorporates moving double layers and realistic, time varying ion heating profiles. In addition to the direct relevance of the project to magnetosphere-ionosphere coupling, the processes being studied are also applicable to laboratory plasmas and astrophysical plasmas.
