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Abstract

High-resolution magnetic field recordings by the Swarm A and C spacecraft have been used to investigate the properties of field-aligned currents (FACs) at auroral latitudes down to their smallest scales (<1 km). Particularly suitable for that purpose are the magnetic field recordings, taken at a rate of 50 Hz, during the 2 weeks around the quasi-coplanar orbit configuration around 1 October 2021. We have split the recorded signal caused by FACs of along-track scales from 0.2 to 20 km into 8 quasi-logarithmically spaced ranges. Our investigations revealed that the kilometer-scale FACs (0.2–5 km) show quite different characteristics from those of the small-scale FACs (5–20 km). The kilometer-scale FACs exhibit short-lived (<1 s) randomly appearing large current spikes. They are confined to certain latitude ranges, which depend on local time. Small-scale FAC structures last for longer times (>10 s) and are distributed over larger latitude ranges. Their largest amplitudes are achieved at latitudes that overlap with the kilometer-scale FACs. The small-scale FACs have earlier been identified as Alfvén waves that are partly reflected at the ionosphere, and they can oscillate within the ionospheric Alfvén resonator. When at the same time additional Alfvén waves are launched from the magnetosphere they will interfere with the reflected. We suggest that the interaction between oppositely travelling Alfvén waves, when continuing sufficiently long, is generating the large-amplitude and short-lived kilometer-scale FACs.