Wu, S.Y., Whiter, D.K., Lamy, L., Wang, M.M., Zarka, P., Jackman, C.M., Ye, S.Y., Waters, J.E., Fogg, A.R., Mende, S.B., Kaweeyanun, N., Kasaba, Y., Kurita, Satoshi, Kojima, H., Paxton, L.J., Kasahara, Y., Miyoshi, Y., Shinbori, A. and Tsuchiya, F. (2025) Radio emissions reveal Alfvénic activity and electron acceleration prior to substorm onset. Nature Communications, 16, [10553]. (doi:10.1038/s41467-025-65580-8).
Abstract
Magnetospheric substorms are among the most dynamic phenomena in the Earth’s magnetosphere, yet their triggering mechanisms remain unclear. Ground-based observations have identified wave-like aurora as precursors to substorms. Here, we report a new precursor feature in space-based observations of auroral kilometric radiation (AKR), marked by the appearance of emissions with slowly frequency-drifting tones (<2 kHz/s) above 100 kHz. Simultaneous multi-instrument obser-vations and statistical analysis suggest that these AKR precursors occur concurrently with wave-like aurora, either manifesting as pseudo-breakup features (with about 5 minutes duration) or as pre-onset activity (about 2 minutes prior to substorm expansion), indicating a common driving mechanism. Analysis of the emissions with frequency-drifting tones suggests that they are linked to moving double-layers driven by dispersive Alfvén waves, consistent with an Alfvénic accelera-tion mechanism for pre-onset aurora. These findings highlight the importance of Alfvénic activity in substorms and suggest that Alfvénic acceleration is not only responsible for optical auroral fea-tures but also for radio emissions, potentially explaining also the ubiquitous frequency-drifting emission features observed at other magnetized planets like Saturn and Jupiter.
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