Radio emissions reveal Alfvénic activity and electron acceleration prior to substorm onset

Magnetospheric substorms are among the most dynamic phenomena in Earth’s magnetosphere, yet their triggering mechanisms remain unclear. Ground-based observations have identified auroral beads as precursors to substorms. Here, we report a new precursor feature in space-based auroral kilometric radiation (AKR), marked by the appearance of emissions with slowly frequency-drifting tones (<2 kHz/s) above 100 kHz. Simultaneous observations and statistical analysis show that both AKR precursors and auroral beads occur simultaneously, ~10 minutes before substorm onset, indicating a shared physical process. Analysis of the emissions with frequency-drifting tones suggests they are linked to moving double-layers driven by dispersive Alfvén waves, consistent with the Alfvénic acceleration mechanism for auroral beads. These findings highlight the importance of Alfvénic activity in substorms and suggest that Alfvénic acceleration is not only responsible for optical auroral features but also for radio emissions, potentially explaining the ubiquitous frequency-drifting emission features observed at other magnetized planets like Saturn and Jupiter.