Chorus Emissions: Mercury & Earth Share Magnetosphere Wave Properties

An international team from Kanazawa University (Japan), Tohoku University (Japan), LPP (France), and partners has demonstrated that chorus emissions, natural electromagnetic waves long studied in Earth's magnetosphere, also occur in Mercury's magnetosphere exhibiting similar chirping frequency changes. Using the Plasma Wave Investigation instrument aboard BepiColombo's Mercury orbiter Mio, six Mercury flybys between 2021 and 2025 detected plasma waves in the audible range. Comparison with decades of GEOTAIL data confirmed identical instantaneous frequency changes. This provides the first reliable evidence of intense electron activity at Mercury, advancing understanding of auroral processes across the solar system. The work is published in the journal Earth, Planets and Space. Chorus emissions and their significance Chorus emissions are electromagnetic waves generated when electrons resonate with plasma waves inside a magnetosphere. On Earth, they are known to play a key role in the formation and loss of radiation belts. These emissions exhibit rising and falling audible frequencies, often described as "birdsong" because of their interference with radio signals. Because the energy of affected electrons depends on wave frequency, understanding chorus characteristics is crucial for space weather forecasting and satellite radiation protection. Role of GEOTAIL and Mercury observations The GEOTAIL satellite, launched jointly by Japan and the United States in 1992, observed Earth's magnetotail for 30 years, providing invaluable knowledge of chorus generation, spatial distribution, and frequency properties. Mercury, with a magnetic field only about one‑hundredth that of Earth, had remained unexplored in this context. Observations by BepiColombo's Mercury orbiter Mio detected natural plasma waves in the audible range, suggesting the possible presence of chorus emissions and associated low‑energy electrons (cold electrons) near Mercury. This achievement was based on a deliberate strategy to apply decades of Earth magnetosphere research to Mercury. The Plasma Wave Investigation instrument aboard Mio was designed to test theoretical predictions of chorus emissions in Mercury's weak magnetosphere. Long‑term GEOTAIL data provided the essential benchmark for comparison. GEOTAIL's vantage point in the distant magnetotail, at about 10 Earth radii, offered conditions analogous to Mercury's much smaller magnetosphere. Mercury's plasma wave data quantitatively matched with GEOTAIL's chorus signatures, confirming: Frequency variation: rapid rising and falling tones, indicating nonlinear coupling between electrons and waves. Spatial distribution: concentration on the dawnside region where energetic electrons preferentially flow. Implications for planetary science and future research These findings demonstrate the universality of chorus generation mechanisms across planetary magnetospheres. They also provide supporting evidence for the presence of cold electrons around Mercury, a key prediction made in 2025, and highlight themes for Mio's orbital observations beginning in 2027. Previous studies have shown that Earth's hazardous radiation belt electrons are produced through chorus emissions. Extending this knowledge to Mercury advances space weather prediction and radiation protection for spacecraft. Although Mercury's weak magnetic field was thought to prevent radiation belt formation, the confirmed presence of chorus emissions with frequency variation indicates that efficient electron acceleration also occurs there. Mio will enter Mercury orbit in late 2026 and begin detailed observations, targeting spatial distribution, frequency dynamics, and the origin of cold electrons.