Coordinated observations on the spatiotemporal development of global-scale dipolarizations and their coupling to meso-scale dipolarizations

Global- and meso-scale dipolarizations are well-known features of Earth’s magnetosphere, but their coupling remains poorly understood. Here, using a new approach that combines 2D ionospheric field-aligned current (FAC) maps with coordinated observations from a network of magnetospheric satellites, we directly show that individual global-scale dipolarizations can expand from the nightside to, or even into, the dayside. These expansions are enduring (20–30 minutes), slow (2–4 deg/min), and global in extent (up to 12 hours in local time), consistent with previous statistical inferences but now explicitly observed. The expanding FACs form a two-sheet current system as described by the Boström II model. In contrast, meso-scale dipolarizations are bursty (a few minutes), fast (10s deg/min, 10s = several ten), and localized (several hours in local time), as evidenced by auroral expansions and satellite data. They are associated with the line-current system as described by the Boström I model, i.e., the substorm current wedge. Notably, meso-scale dipolarizations often emerge near the expanding edge of a global-scale dipolarization, suggesting a dynamic coupling between the two scales. These observations provide a complementary scenario to the simulation-based interpretation that global-scale dipolarizations result from the accumulation of meso-scale dipolarizations. Here, meso-scale dipolarizations appear far less frequently than in simulations and occur around the edge of global-scale dipolarizations. This implies that meso-scale dipolarizations may be sporadically triggered during the azimuthal expansion of global-scale dipolarizations.