Characteristics of Broad‐Band Magnetic Signatures in the 10‐Hz Range Near the Plasmapause, as Observed by Swarm

The plasmapause (PP) marks the transition from a dense and cold plasma to a tenuous and hot plasma in the inner magnetosphere. Several electrodynamic processes are taking place at this steep electron density gradient. Here we make use of the high‐resolution magnetic field recordings by the Swarm A and C spacecraft for investigating broad‐band signal bursts in the 4–10 Hz frequency range. In more than 80% of the cases, when the Swarm passes the L‐shell bounding the plasmasphere on its low‐Earth orbit, an enhanced broad‐band signal is observed in the transverse field components. The actual location of the plasmapause is derived from in situ plasma measurements of the Arase satellite in the magnetosphere. The broad‐band signals around midnight are different from those near noon. During the dark hours in ∼90% of cases, a broad‐band signal is found right outside the PP continuing far into the magnetosphere. Its characteristic agrees well with the large‐amplitude kilometer‐scale FACs described recently by Zhou and Lühr (2025), https://doi.org/10.5194/angeo‐43‐667‐2025 . Different from that, around the noon sector broad‐band bursts appear rather confined in the L range (∼0.03 Re) and about 0.5 Re inward from the PP. The bursts observed on the dayside seem to represent resonances of preexisting waves, which suffer a chaotic breakdown after running into saturation. Opposed to the nightside, daytime broad‐band signal bursts are encountered only on about 80% of the passes. This implies that the resonance does not exist everywhere along the dayside PP. On the dawn and dusk sides, both types of broad‐band signatures can be found.