Spectral interference events binned in 5° × 5° spatial windows. All burst-mode data from 2020 and 2021 are used for both Swarm Alpha and Bravo (spanning 290 days). 30-second time-windows for each day were used to compute power spectra, detrended over the band 70–110 Hz (A), and then in each detrended window we fit a quadratic to the 78–100 Hz range (B), from which we can also find the central frequency over all peaks (C). The labelled (in nT) contours are total field intensity on 01-Jan-2021 at 500 km altitude.