Chorus emissions are plasma waves generated when magnetospheric electrons interact resonantly with electromagnetic waves, producing characteristic rising and falling tones in the audible frequency range often described as birdsong. On Earth, these emissions play a key role in forming and depleting the Van Allen radiation belts and can interfere with radio communications, making their detailed properties important for space weather forecasting and satellite radiation protection strategies.

GEOTAIL, launched jointly by Japan and the United States in 1992, spent more than 30 years observing electromagnetic waves and plasma in Earths magnetotail, building a long-term reference data set on chorus generation, spatial distribution, and frequency behavior. Until recently, similar processes at Mercury were largely unconstrained because the planet has a magnetic field only about one hundredth as strong as Earths and an extremely tenuous atmosphere.

The Plasma Wave Investigation instrument aboard BepiColombos Mio spacecraft has now recorded natural plasma waves in the audible range during multiple close Mercury encounters. These measurements suggested that chorus-like emissions and associated low-energy, or cold, electrons could exist in Mercurys magnetosphere, contrary to earlier expectations that such electrons would be absent around an almost airless planet.

Researchers deliberately applied decades of knowledge from Earths magnetosphere to interpret the new Mercury data. GEOTAILs vantage point in the distant magnetotail, roughly ten Earth radii from the planet, provides conditions that resemble the compact magnetosphere of Mercury, making it an effective benchmark for comparative analysis of plasma wave phenomena.

Quantitative comparison showed that Mercurys plasma wave signatures match key properties of chorus emissions observed with GEOTAIL at Earth. The team identified rapid rising and falling frequency sweeps, indicating nonlinear coupling between electrons and waves, as well as a similar spatial concentration of emissions in the dawnside sector where energetic electrons preferentially stream through the magnetosphere.

These results demonstrate that the mechanisms responsible for generating chorus emissions can operate in very different planetary environments and magnetic field strengths, revealing a universal plasma process across magnetospheres. The findings also support earlier theoretical work from 2025 that predicted the presence of cold electrons around Mercury based on wave analyses, and they highlight new priorities for Mio once it begins orbital operations in 2027.

Previous studies have established that at Earth, hazardous radiation belt electrons can be efficiently accelerated by chorus waves, posing risks to spacecraft and satellites. Extending this understanding to Mercury enhances the broader framework for space weather prediction and radiation protection in the inner solar system and for missions that operate in strong particle environments.

Although Mercurys weak magnetic field was thought to preclude the formation of stable radiation belts, the confirmed presence of chorus emissions with clear frequency variation indicates that efficient electron acceleration processes are active there as well. Mio is scheduled to enter Mercury orbit in late 2026, enabling detailed investigations of how these emissions vary with location, how they interact with cold electron populations, and how they shape the local space environment.

The new observations open a path to systematic comparative studies of auroral and radiation processes at multiple planets, including Mars, Jupiter, and Saturn. By examining how chorus emissions, cold electrons, and magnetospheric structure interplay across different planetary systems, scientists aim to build a unified picture of universal plasma wave phenomena in planetary space environments.

Credits for the visual materials used in this research include Mercury imagery from NASA, the Johns Hopkins University Applied Physics Laboratory, and the Carnegie Institution of Washington, BepiColombo spacecraft imagery from ESA, and Earth imagery from NASA.

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

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