Auroral Signatures of Ballooning Instability and Plasmoid Formation Processes in the Near‐Earth Magnetotail

Abstract The nonlinear development of ballooning instability and the subsequently induced plasmoid formation in the near‐Earth magnetotail demonstrated in MHD simulations has been proposed as a potential trigger mechanism for substorm onset over the past decade, and their connections to the in situ satellite and ground all‐sky auroral optical observations have been a subject of continued research. In this work, a set of THEMIS substorm onset events with good conjunction of auroral observations has been selected for comparative simulation study, whose pre‐onset magnetotail configuration and conditions are inferred from in situ data and compared with the onset conditions of ballooning instability obtained in our MHD simulations. The evolution of the near‐Earth magnetotail is followed, where the signatures of ballooning instability and the plasmoid formation are extracted from simulations and compared with the magnetic fields and flow patterns within the magnetotail region from observation data. The field‐aligned current (FAC) density is evaluated at the Earth side boundary of the magnetotail domain of simulation, which is further mapped along magnetic field lines to the auroral ionosphere as input to the newly developed TREx‐ATM model for auroral simulation. The comparison between the simulation and the THEMIS‐ASI observation data on the auroral bead and arc has enabled the identification of their magnetotail origins. In particular, a scenario for the subsequent emergence of new poleward auroral arc above the substorm onset auroral beads has been proposed, which brings new understanding on how the ballooning induced plasmoid formation process may contribute to substorm onset and expansion.