BepiColombo probe links electron precipitation to stunning auroras on Mercury's surface

Mercury possesses an exosphere in place of a conventional atmosphere, and its formation is primarily attributed to the interaction between solar particles and the elements present on the planet's surface. This exosphere spans from space all the way to the surface of the planet.

The information was obtained by the Mercury Magnetospheric Orbiter (MMO), one of the spacecraft present onboard. The MMO collected data on various categories of charged particles located at an altitude of 124 miles (200 kilometers) above the surface of the planet.

By utilizing this comprehensive dataset, scientists successfully monitored the trajectory of electrons in the vicinity of Mercury.

Electron precipitation leads to aurora generation 

The data demonstrated that electrons emitted by the Sun accelerate rapidly in Mercury's magnetosphere.

Subsequently, the magnetosphere’s tail section fling these electrons toward Mercury. Due to the absence of an atmosphere on Mercury, these electrons do not collide with the air but rather descend upon the planet's surface, a phenomenon referred to as electron precipitation.

The fallen electrons then engage with the materials on Mercury's surface, triggering the emission of X-ray fluorescence. This process ultimately contributes to the formation of the planet's auroral glow.

“For the first time, we have witnessed how electrons are accelerated in Mercury’s magnetosphere and precipitated onto the planet's surface. While Mercury’s magnetosphere is much smaller than Earth’s and has a different structure and dynamics, we have confirmation that the mechanism that generates aurorae is the same throughout the Solar System,” explained Sae Aizawa, lead author of this study, at the University of Pisa, Italy, in an official release.

NASA’s MESSENGER mission previously spotted auroras on Mercury, although, the processes driving the X-ray fluorescence had not been properly understood due to the lack of direct evidence. 

BepiColombo is a joint mission of the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA), which was launched in October 2018 and is scheduled to reach Mercury's orbit in 2025.

The findings imply that aurorae have now been observed on every planet in the solar system, and the process of aurora formation might be “universal”.

The results of this new study have been reported in the journal Nature Communications.

Study Abstract:

Mercury’s magnetosphere is known to involve fundamental processes releasing particles and energy like at Earth due to the solar wind interaction. The resulting cycle is however much faster and involves acceleration, transport, loss, and recycling of plasma. Direct experimental evidence for the roles of electrons during this cycle is however missing. Here we show that in-situ plasma observations obtained during BepiColombo’s first Mercury flyby reveal a compressed magnetosphere hosts of quasi-periodic fluctuations, including the original observation of dynamic phenomena in the post-midnight, southern magnetosphere. The energy-time dispersed electron enhancements support the occurrence of substorm-related, multiple, impulsive injections of electrons that ultimately precipitate onto its surface and induce X-ray fluorescence. These observations reveal that electron injections and subsequent energy-dependent drift now observed throughout Solar System is a universal mechanism that generates aurorae despite the differences in structure and dynamics of the planetary magnetospheres.