Coupling between the Magnetospheric Dipolarization Front and the Earth’s Ionosphere by Ultralow-frequency Waves

Magnetosphere–ionosphere coupling is representative of a class of astrophysical problems involving the interaction between very different plasma regimes that are connected by magnetic fields. It is a major issue of the planet's (e.g., Earth, Jupiter, Saturn) system how the magnetospheric plasma is coupled to the planet's ionosphere/atmosphere. Due to their roles on energy transport and conversion in the terrestrial magnetosphere, dipolarization fronts (DFs) in the magnetotail have been intensively studied. While the energy may also be transported toward the ionosphere during the magnetosphere–ionosphere coupling, the coupling processes during DF events remain unclear. Using high-quality MMS and Swarm measurements, we present new conjugated observations of ultralow-frequency waves associated with the DF in the magnetotail and multiscale field-aligned currents (FACs) in the ionosphere. The tail large-amplitude low-frequency magnetic disturbances and associated FACs were found connected with a pair of ionospheric FACs near the magnetic footprints of MMS. The earthward field-aligned Poynting flux of low-frequency waves reveals the energy transport from the DF toward the ionosphere. Meanwhile, small-scale and highly temporal ionospheric FACs suggest that the magnetotail shear Alfvén waves may evolve into kinetic Alfvén waves during the coupling processes. This finding implies that a very localized dynamic process (e.g., DFs in the terrestrial magnetotail) in the magnetosphere could couple the ionosphere with Alfvén waves.