Swarm absolute scalar magnetometer burst mode: observed ELF signals and their origins

The Absolute Scalar Magnetometers (ASM) onboard each European Space Agency Swarm satellites nominally provide 1 Hz magnetic scalar data. An additional experimental mode is available on the ASM consisting of a 250 Hz burst-mode which is periodically switched on allowing magnetic signals with frequencies up to 125 Hz to be investigated. By analysing burst-mode data from 2014 to 2023 in the frequency-time domain, we find a range of signals both man-made and geophysical, which we present here. Known features include lightning whistlers, auroral hiss and plasma bubbles that produce broadband incoherent signatures, and powerline harmonic radiation observed as stable 50 or 60 Hz lines. Ground based extremely low frequency (ELF) communication systems are also detected in the data. However, many as yet unexplained signals are observed. Rising tone 70–125 Hz high-intensity bursts, lasting ∼3 min, are found over the Antarctic, which we suggest are associated with local He + ion gyrofrequencies and thus are possibly narrow-banded ionospheric hiss. These signals appear to be associated with the South Atlantic Anomaly due to their clustering on its southern edge, with the low field strength producing He + gyrofrequencies less than 125 Hz. Additionally, extremely weak linear and quadratic chirps lasting tens of seconds to tens of minutes occur in the data without an obvious temporal or spatial trend which we tentatively attribute to onboard electronic or instrumental noise. Their origin requires further investigation to avoid artefacts in future magnetic missions. We also find long lasting, high frequency narrow-band features around 80–125 Hz which persist for 60–300 s, occurring orbit-on-orbit at fixed magnetic local times (MLT) around 0900 and 1500, mainly in equatorial latitudes, which are encountered during all burst sessions spanning these MLTs. The Swarm ASM burst-mode data suggest there are new geophysical magnetic phenomena yet to be fully understood, potentially offering new insights into magnetospheric and ionospheric wave-plasma interactions.