Towards a unified theory of discrete auroras

Recent theoretical developments in auroral research are reviewed. In spite of the great complexity involved, a unified theory begins to emerge. The framework of this unified theory is provided by the imperfect magnetosphere-ionosphere coupling due to enhanced magnetospheric convection. The auroral potential structures in the imperfect coupling state result from the loss cone constriction on enhanced upward field-aligned currents following enhanced magnetospheric convections. Field-aligned potential drops for the inverted V precipitations are supported by a combination of the double layer process, the pitch-angle anisotropy process, the wave-particle interaction (anomalous resistivity) process, the electrostatic shock and the thermoelectric process in descending order of importance. Quasineutrality along inverted V field lines is maintained primarily by the trapped and back-scattered electrons produced respectively by wave-particle interactions in the magnetosphere and electron-neutral collisions in the ionosphere. Additional electron energization due to the ion cyclotron turbulence may lead to striations along the inverted V field lines to produce thin auroral arcs imbedded in the inverted V precipitation. The formation of discrete auroras can now be understood as a manifestation of the imperfect magnetosphere-ionosphere coupling due to enhanced magnetosphere-ionosphere interaction.