Energy Conversion Associated With Field‐Aligned Currents at Very High Altitudes During Substorm: MMS Observations

Abstract

Field‐aligned currents (FACs) play a central role in magnetosphere–ionosphere coupling by mediating energy transport and current closure along magnetic field lines. Using Magnetospheric Multiscale (MMS) observations, we report a clear detection of a Region 1 FAC at an altitude of about 6.8 Re on the dawnside during a substorm, accompanied by pronounced injections of H ⁺ and He ²⁺ ions. Magnetic field line tracing maps the FAC footprint to the plasma sheet boundary layer (PSBL) or the dawnside near‐flank magnetopause, placing the event between the canonical equatorial and ionospheric FAC source regions. Energy conversion diagnostics reveal a generator‐dominated system characterized by predominantly reversible perpendicular energy exchange, while localized and persistent positive parallel EJ within the FAC core indicates effective dissipation mediated by parallel electric fields. Complementary observations of charge density, ion composition, and parallel Poynting flux support a coherent picture in which positive charge accumulation, energetic ion injection, and field‐aligned energy transport coexist along the Region 1 FAC. These results highlight the critical role of FACs as key conduits for energy transport and current closure during substorm recovery.