Large scale current systems developed from substorm onset: Global MHD results

We investigate the large-scale substorm current systems developed from its onset in an idealized substorm event simulated by global magnetohydrodynamic (MHD) models. Mainly three current systems (loops) are revealed: (1) the classical substorm current wedge, which is composed by the disputed cross-tail current in the magnetotail, the nightside westward electrojet in the high-latitude ionosphere and a pair of region 1 field-aligned currents (FAC); (2) the partial-ring current system, which is braced by two region 2 FACs; and (3) the meridional current system, which is formed by an equatorial radial current (outward/inward in the morning/evening sector), and region 1 and region 2 FACs at its two ends. The region 2 FAC connects with region 1 FAC by a latitudinal horizontal current at each morning/evening ionosphere to complete Loops 2 and 3. A quantitative study shows the significant enhancement of these current systems during the substorm expansion phase, while Loop 1 dominates, which can reach a magnitude of ~1 MA. Empirical relations among the ionospheric currents and the related magnetotail currents are established based on the simulation results, implying that the substorm current systems are not evolved locally or separately, but must be viewed from a global perspective. This knowledge of large-scale substorm current system would deepen our understanding of the substorm development and could be validated by observations in the future.