Universal plasma wave phenomena revealed: Mercury’s BepiColombo Mio and Earth’s GEOTAIL show shared wave frequency properties across planetary magnetospheres

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.

Background:

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 interfere 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. The GEOTAIL satellite*¹⁾, 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*²⁾ detected natural plasma waves in the audible range, suggesting the possible presence of chorus emissions and associated low‑energy electrons (cold electrons*³⁾) near Mercury.

Results:

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 ten 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.

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.

Future Prospects:

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. This breakthrough opens the way to comparative studies across multiple planets, including Mars, Jupiter, and Saturn. Understanding how auroral phenomena manifest not only at Earth but also at Mercury and other planets will significantly expand our knowledge of planetary space environments and their universal plasma processes.

Glossary

*1) GEOTAIL satellite

Launched in 1992, GEOTAIL observed Earth’s magnetotail for more than 30 years, measuring electromagnetic waves and plasma. It provided globally important long‑term data on chorus emissions, contributing significantly to magnetospheric science.

*2) Mercury Magnetospheric Orbiter “Mio”

Launched in 2018 as part of the international BepiColombo mission by JAXA and ESA, together with the Mercury Planetary Orbiter (MPO). Mio is scheduled to enter Mercury’s orbit in 2026 and carries a wide range of scientific instruments to study magnetic fields, plasma, and electromagnetic waves.

*3) Cold electrons

Electrons in space plasma with relatively low energy (temperature). Because Mercury has almost no atmosphere, cold electrons were previously thought not to exist there. However, analyses of electromagnetic waves observed by Mio suggest the possible presence of cold electrons near Mercury.

Reference: https://doi.org/10.1186/s40623-025-02305-7