A radio oval above Earth's auroral oval

Auroral kilometric radiation (AKR), Earth's strongest radio emission, has long been associated with discrete auroras and electrons near a few kilo-electron volt (keV) range. However, auroras also occur in diffuse forms with broader electron energies, raising the question of why AKR has not been observed above diffuse auroras or linked to electrons outside the kilo-electron volt population. Comprehensive AKR source distributions have remained elusive because of observational limitations, and their local-time coverage remains largely unknown. Using spacecraft measurements, we identify a "radio oval" above the optical auroral oval, spanning the full local-time range, where AKR is emitted over both discrete and diffuse auroras. The AKR source electrons display diverse precipitation features, including monoenergetic (peak flux at 3.82 kilo-electron volts), broadband (1.34 kilo-electron volts), low-energy (0.47 kilo-electron volts), and diffuse types (>1 kilo-electron volt). These results reveal that the cyclotron maser instability-the mechanism driving AKR-can arise in diverse plasma environments, broadening our understanding of both AKR generation and auroral complexity.