Ring current formation influenced by solar wind substorm conditions

Abstract

Single-particle tracking with time-dependent global magnetic and electric fields is used to investigate the generation of the ring current from ionospheric outflows during an internally driven substorm. We show that the energization of the ions is not correlated with the time that the ions leave the ionosphere; instead energization is correlated with the formation of an injection front driven by an earthward moving flux rope at onset. Because of the large gyroradius of the O + ions, they experience strong dawn-dusk acceleration in the vicinity of the injection front. The acceleration is strongly influenced by small-scale structures including the Hall electric field and the development of kinks across the tail. H + is mainly energized by betatron acceleration as it is injected into the inner magnetosphere with less average energy than the O + ions. In this paper we investigate the conditions that lead to the formation of the injection front and small-scale structures (∼1 R E ) in the current sheet, such as tail kinking and flux ropes, that are correlated with particle convection and energization at substorm onset. High-resolution capabilities allow us to resolve these small-scale processes within a thin (<1000 km) current sheet, and we show that simulations with coarse grid resolution underestimate the energization of ring current particles. The role of the interplanetary magnetic field (IMF) B z on dayside particle loss is also examined. It is found that northerly turning IMF at or shortly after onset is important in producing a symmetric ring current, but the degree of turning is not as critical.