Three-Dimensional Simulations of Magnetospheric Substorm Onset by Kinetic Cross-Tail Instabilities

Abstract:

Satellite observations show that magnetospheric substorms are preceded by a slow growth phase of magnetic flux loading in the tail lobes and a thinning of the tail current sheet down to kinetic scales. This can eventually lead to explosive disruptions associated with transient features including bursty flows of plasma with enhanced magnetic flux, energetic particle acceleration, and auroral brightening. Theoretical and observational considerations have led to the development of substorm models involving magnetic reconnection and three-dimensional cross-tail instabilities such as ballooning and drift-kink, however the exact mechanism that triggers the onset and the potential interplay between these various instabilities are not yet well understood. Here we model substorm dynamics using two- and three-dimensional kinetic particle-in-cell simulations with a domain that captures both the distant tail and the critical near-Earth region which features a transition to a dipolar field. Our novel simulations are initialized using an exact kinetic equilibrium, which eliminates the artificial transients associated with non-rigorous initialization procedures, and comparing two- and three-dimensional simulations allows us to isolate reconnection from competing instabilities. We find that three-dimensional cross-tail instabilities play a critical role that significantly modifies the two-dimensional reconnection dynamics by explosively disrupting dipolarization fronts traveling into the near-Earth region, leading to the production of field-aligned currents and nonthermal particle acceleration. We study the contributions of kinetic ballooning and drift kink modes to the disruption, and examine the particle acceleration mechanisms and energy partitioning. The signatures from these simulations are compared with recent observations from multi-spacecraft missions.