Nonlinear spatiotemporal signatures of whistler-mode wave activity around Mercury during six flybys of BepiColombo mission.

Cyclotron resonance between electromagnetic waves and plasmas may be a universal acceleration phenomenon of charged particles in magnetized planets. Coherent fine structures of whistler-mode waves serve as a signature of nonlinear resonance. However, the fine wave structures at Mercury have remained unknown due to limited spacecraft observations. Here we show that plasma wave observations by the third BepiColombo mission Mercury flyby (2023) have identified discrete whistler-mode emission waves similar to those observed in Earth's magnetosphere. The frequency sweep rates of Mercury's wave chirping tones correspond to those at Earth, based on the scaling law for planetary magnetospheric size. Furthermore, although the spatial coverage on the dayside and dusk sectors is insufficient, the spatial characteristics of Mercury's whistler-mode waves during all the Mercury flybys (2021 to 2025) reveal an asymmetric dawn-to-night sector, which suggests nonlinear growth characterized by the distorted magnetospheric shape. These spatiotemporal features strongly indicate that electron precipitation events occur primarily in the active wave (dawn-side) region through nonlinear resonant mechanisms similar to those in Earth's magnetosphere. This study highlights the potential significance of nonlinear resonant processes in shaping Mercury's unique plasma environment within its small magnetosphere.