Transition of Ambient Ions Motion Toward Field Alignment During Debris Plasma Expansion

Abstract A high‐altitude nuclear explosion (HANE) can transport plasma along geomagnetic field lines into distant regions, directly impacting the formation of artificial radiation belts and the regions of energy deposition. However, the early stage dynamics of ambient ion motion transition to field alignment remain poorly understood. In this paper, a two‐dimensional hybrid model in the field‐parallel plane is employed to analyze the dynamics of debris and ambient ions, with particular focus on the transition of ambient ions to field‐aligned motion. The simulations reveal that while debris ions expand nearly linearly, ambient ions are accelerated across the magnetic‐compression region perpendicularly within a single gyro‐period and then transition toward the geomagnetic field‐aligned motion. These transitions result from changes in the magnetic field direction at the boundary of the magnetic‐compression region. This process requires efficient energy transfer from debris to the ambient plasma. Through a series of simulations, we delineate the regime dependence of this transition using a dimensionless parameter and quantify the fraction of ambient ions streaming along geomagnetic field lines. In the simulations, under the condition in two‐dimensional simulations, more than 20% of ambient ions achieve field‐aligned velocities exceeding the initial debris expansion speed. This study clarifies the physical origin and parameter dependence of ambient‐ion field‐aligned transport, providing new insights into plasma distribution following HANE events.