A theoretical approach to the morphology and the dynamics of diffuse auroral zones

Abstract

Direct observations of the high‐latitude ionosphere have established the continuous presence of large‐scale emissions, referred to as diffuse auroras. Neglecting localized structures, such as discrete arcs, we focus upon a quantitative description of the effects of large‐scale convection on diffuse precipitation. The transport of 100 eV‐10 keV electrons from the geomagnetic tail earthward by the convection electric field, and their pitch angle diffusion into the loss cone by wave‐particle interactions are believed to be the main cause of diffuse auroral electron precipitation. A self‐consistent treatment balancing wave generation and particle diffusion appears to be still beyond the present state of the art. For the main purpose of diffuse precipitation modeling, we propose a simplified approach to this problem and test its validity against direct observations of the location and dynamics of diffuse auroral emissions. Using basically the strong pitch angle diffusion limit in the way proposed by Kennel [1969], we derive a set of fluid equations describing the three‐dimensional transport of plasma‐sheet electrons. Their integration provides the latitude and local time distribution of precipitation fluxes and characteristic energies at the top of the ionosphere as a function of the large‐scale dawn to dusk electrostatic potential drop. The calculated expansion of the auroral oval with magnetic activity, deduced from our model, approaches the experimental results. So this simplified theoretical study permits us to reproduce and explain the main characteristics of the diffuse auroral zone. However, for high values of magnetic activity, the theoretical results are found to be shifted poleward relative to the observations. This suggests that the assumption of strong pitch angle diffusion overestimates the efficiency of wave‐particle interactions.