BepiColombo’s First Mercury Flyby Unmasks Electron Rain As Trigger for X-Ray Auroras

by Santiago Fernandez
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X-Ray Auroras

BepiColombo’s First Mercury Flyby Unveils Electron Rain as a Trigger for X-Ray Auroras

A significant breakthrough has been achieved by the joint European-Japanese mission, BepiColombo, shedding light on the formation of high-energy auroras on Mercury. According to research findings, electrons, accelerated within Mercury’s magnetosphere and descending onto its surface, interact with the surface material, leading to the emission of X-rays and the creation of mesmerizing auroras. This discovery highlights the commonality of auroral mechanisms across the entire Solar System.

BepiColombo, a collaborative effort between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA), embarked on its journey towards the innermost planet of the Solar System back in 2018. On October 1, 2021, the mission executed its first flyby of Mercury, during which data from three of BepiColombo’s instruments were collected and later analyzed by an international team of scientists. Their findings, published on July 18 in Nature Communications, provided groundbreaking insights into Mercury’s auroras.

Unlike Earth, where auroras are the result of the interaction between the solar wind and the ionosphere, Mercury’s thin exosphere causes its auroras to form directly from the solar wind’s interaction with the planet’s surface.

BepiColombo comprises two spacecraft: the Mercury Planetary Orbiter (MPO) led by ESA and the Mercury Magnetospheric Orbiter (MMO or Mio post-launch) overseen by JAXA. These spacecraft have been in a docked configuration for their seven-year journey to reach their final orbit. During the first Mercury flyby, BepiColombo approached within 200 kilometers of the planet’s surface, with Mio’s plasma instruments providing simultaneous observations of various charged particles from the solar wind in Mercury’s vicinity.

Lead author Sae Aizawa, affiliated with the Institut de Recherche en Astrophysique et Planétologie (IRAP) and currently associated with JAXA’s Institute of Space and Astronautical Science (ISAS) and the University of Pisa, Italy, expressed enthusiasm over the discovery, stating that they witnessed the acceleration of electrons within Mercury’s magnetosphere and their eventual precipitation onto the planet’s surface. This phenomenon, occurring despite Mercury’s magnetosphere being smaller and having a distinct structure and dynamics compared to Earth’s, confirms the universality of auroral generation mechanisms throughout the Solar System.

During the flyby, BepiColombo approached Mercury from its night side in the northern hemisphere and made its closest approach near the morning side in the southern hemisphere. The mission observed the magnetosphere on the southern hemisphere’s daytime side before reentering the solar wind. Notably, the data revealed an unusually compressed magnetosphere, likely due to high-pressure conditions in the solar wind.

The process of electron acceleration takes place on the dawn side of Mercury’s magnetosphere, where high-energy electrons are transported from the tail region towards the planet’s surface. With no atmosphere to impede their progress, these electrons interact with the surface material, leading to the emission of X-rays and the manifestation of an awe-inspiring auroral glow. Prior to this discovery, NASA’s MESSENGER mission had observed auroras on Mercury, but the exact processes triggering X-ray fluorescence from the surface had remained elusive until now.

The groundbreaking study was conducted by a collaborative research team involving institutions such as the French Institut de Recherche en Astrophysique et Planétologie (IRAP), Kyoto University, ISAS, the Laboratoire de Physique des Plasmas (France), the Max Planck Institute for Solar System Research (Germany), the Swedish Institute of Space Physics, Osaka University, Kanazawa University, and Tokai University. The research received partial support from the European Commission under grant agreement No 871149, as part of the Europlanet 2024 Research Infrastructure funding.

Frequently Asked Questions (FAQs) about X-Ray Auroras

What did the joint European-Japanese mission, BepiColombo, discover during its first Mercury flyby?

BepiColombo’s first Mercury flyby revealed how high-energy auroras on Mercury are formed. The research findings indicate that electrons, accelerated in Mercury’s magnetosphere and precipitating onto the planet’s surface, interact with the surface material to emit X-rays and produce the auroras.

What is the significance of the discovery?

The discovery highlights the commonality of auroral mechanisms throughout the Solar System. Despite the differences in Mercury’s magnetosphere’s size, structure, and dynamics compared to Earth’s, the mechanism that generates auroras is similar, indicating a universal phenomenon across celestial bodies in the Solar System.

How does Mercury’s thin atmosphere contribute to the formation of auroras?

Unlike Earth, which has a thicker atmosphere (ionosphere) where auroras are produced by the interaction with the solar wind, Mercury’s thin exosphere causes its auroras to be formed directly from the solar wind’s interaction with the planet’s surface.

What instruments were used during BepiColombo’s flyby to gather data?

BepiColombo consists of two spacecraft: the Mercury Planetary Orbiter (MPO) led by ESA and the Mercury Magnetospheric Orbiter (MMO or Mio post-launch) overseen by JAXA. The flyby utilized Mio’s onboard plasma instruments to facilitate the first simultaneous observations of different types of charged particles from the solar wind near Mercury.

How was the research conducted, and where can I find more details?

The research was carried out by an international team of scientists affiliated with various institutions, including the French Institut de Recherche en Astrophysique et Planétologie (IRAP), Kyoto University, ISAS, the Laboratoire de Physique des Plasmas (France), the Max Planck Institute for Solar System Research (Germany), the Swedish Institute of Space Physics, Osaka University, Kanazawa University, and Tokai University. For more detailed information, the findings were published in the scientific journal, Nature Communications, on July 18, 2023, under the reference “Direct evidence of substorm-related impulsive injections of electrons at Mercury” by Sae Aizawa et al.

How did BepiColombo’s flyby trajectory and observations contribute to the research?

During the flyby, BepiColombo approached Mercury from the night side of the northern hemisphere and made its closest approach near the morning side of the southern hemisphere. The mission observed the magnetosphere on the southern hemisphere’s daytime side before exiting back into the solar wind. The data collected during this trajectory provided insights into Mercury’s magnetosphere’s structure and boundaries, indicating an unusually compressed magnetosphere due to high-pressure conditions in the solar wind.

What causes the emission of X-rays and the creation of auroral glow on Mercury’s surface?

Electron acceleration occurs in Mercury’s magnetosphere, particularly on the dawn side, where high-energy electrons are transported from the tail region towards the planet. These electrons then precipitate onto Mercury’s surface and interact with the material, causing X-rays to be emitted and resulting in the beautiful auroral glow. This phenomenon had not been well understood or directly observed until the recent findings from BepiColombo’s flyby.

What organizations were involved in the BepiColombo mission?

The BepiColombo mission is a joint effort between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA). The two spacecraft involved are the Mercury Planetary Orbiter (MPO) led by ESA and the Mercury Magnetospheric Orbiter (MMO or Mio post-launch) overseen by JAXA.

How was the research funded?

The work was partially supported through Europlanet 2024 Research Infrastructure funding from the European Commission under grant agreement No 871149.

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