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Abstract

In this study we make use of the closely spaced Swarm A and C spacecraft to determine the main wave parameters of micro-pulsation in the Pc1 range. Generally, the interpretation of wave events in satellite recordings suffers from the spatial/temporal ambiguity of such data. To resolve that problem, we use for the first time in the 0.2–5 Hz apparent frequency range high-resolution magnetic field data from the Swarm A/C spacecraft pair. Particularly suitable for this analysis is the period of Counter Rotating Orbit Phase in 2021. During that period the along-track distance between the spacecraft was varied from 2 to 40 s. Surprisingly many of the wavy signals at middle latitudes were identified as small-scale (order of 10s km) quasi-static field-aligned current (FAC) structures. Some of the events turned out to be medium-scale (100–200 km) waves oscillating at 1 Hz or less. A number of published electromagnetic ion cyclotron (EMIC) waves were checked on their consistency. One characteristic is their long wavelength (∼1,000 km). About half of the cases passed the check, the other did not satisfy the spatial homogeneity. The small-scale FAC structures we believe to be driven by atmospheric gravity waves. While the medium-scale waves we prefer to relate to the effects of the ionospheric Alfvén resonator.