Magnetospheric Dynamo Driving Large‐scale Birkeland Currents

Based upon recent global magnetohydrodynamic (MHD) simulations, we provide a theoretical foundation for magnetospheric dynamos driving large‐scale field‐aligned current (FAC) systems. In the framework of MHD, the dynamo is a process in which plasma thermal energy is converted to electromagnetic energy. Theoretical analysis using ideal MHD equations indicates the existence of two types of dynamos that allow this energy conversion. One is the expanding slow dynamo associated with flux tube expansion, and the other is the contracting slow dynamo associated with flux tube contraction. The expanding slow is a stationary dynamo without significant magnetic field curvature, with the field intensity gradient playing a central role. In contrast, the contracting slow is a dynamo with finite time variation and finite magnetic field curvature. A supplementary MHD simulation in this study, together with those in the past studies, indicates the existence of a general relation between the main voltage generator (i.e., dynamo) and the main FAC generator (i.e., divergence of FACs) that appears commonly for large‐scale FAC systems.. That is, the dynamo overlaps the FAC generator in a specific spatial configuration and that at the transition between compressible high beta plasma and incompressible low beta plasma. This overlap represents a coupling of slow mode and Alfvén mode disturbances. We performed linear analysis of that special configuration using ideal MHD equations and derived the general expression relating the voltage generator and the FAC generator.