Mercury’s magnetic landscape mapped in 30 minutes

As BepiColombo passed Mercury during its June 2023 flyby, it encountered various features in the small planet’s magnetic field. These measurements provide a tantalizing taste of the mysteries the mission will investigate when it arrives in orbit around the solar system’s innermost planet.

Like Earth, Mercury has a magnetic field, albeit 100 times weaker on the planet’s surface. Nevertheless, this magnetic field creates a bubble in space, called the magnetosphere, which acts as a buffer to the continuous flow of particles blown by the sun in the form of solar wind.

Because Mercury orbits so close to the sun, the solar wind’s interaction with the magnetosphere and even the planet’s surface is much more intense than on Earth. Exploring the dynamics of this bubble and the properties of the particles it contains is one of the main objectives of the BepiColombo mission.

BepiColombo is expected to arrive at Mercury in 2026 by flying past Earth, Venus and Mercury itself to adjust its speed and trajectory to allow it to be captured in orbit around the planet. The currently “stacked” spacecraft will separate and deploy two science orbiters – the ESA-led Mercury Planetary Orbiter (MPO) and the JAXA-led Mercury Magnetospheric Orbiter (MMO, or Mio) – into complementary orbits to enable measurements essential dual spacecraft. necessary to provide a complete picture of Mercury’s dynamic environment.

As the spacecraft passes Mercury during flybys, many of its science instruments are capable of providing a glimpse of the exciting science to come. Additionally, flybys provide unique information about regions of the planet that will not be directly accessible from orbit.

Lina Hadid, a former ESA researcher currently at the Paris Observatory’s Plasma Physics Laboratory, used the Mercury Plasma Particle Experiment (MPPE) suite of instruments active on Mio during the June 19, 2023 flyby, the third of Mercury’s six gravitational assists from BepiColombo, build an impressive image of the planet’s magnetic landscape in a very short period of time.

Hadid is the co-principal investigator of MPPE and responsible for one of its instruments, the mass spectrum analyzer. She worked on the article published in Physics of communications who presented the results with former instrument leader Dominique Delcourt.

“These flybys are fast: we crossed Mercury’s magnetosphere in about 30 minutes, from dusk to dawn and at our closest, just 235 km above the planet’s surface,” she explains. “We sampled the type of particles, their temperature and their movement, which allowed us to clearly trace the magnetic landscape during this brief period.”

Combining BepiColombo’s measurements with computer modeling to determine the origin of detected particles based on their motion allowed Hadid and colleagues to sketch the different features found in the magnetosphere.

“We saw expected structures like the ‘shock’ boundary between the free-flowing solar wind and the magnetosphere, and we also passed through the ‘horns’ flanking the plasma layer, a region of hotter and more electrically charged gas dense which flows like a tail in the direction opposite to the sun. But we also had some surprises.

Delcourt says: “We detected a so-called low-latitude boundary layer defined by a region of turbulent plasma at the edge of the magnetosphere, and here we observed particles with a much wider range of energies than we have ever seen previously at Mercury. , largely due to the sensitivity of the mass spectrum analyzer designed specifically for Mercury’s complex environment.”

“BepiColombo will be able to determine the ionic composition of Mercury’s magnetosphere in more detail than ever before.”

“We have also observed energetic hot ions near the equatorial plane and at low latitudes trapped in the magnetosphere, and we think the only way to explain this is by a ring current, whether partial or complete, but It’s an area that’s very debated,” adds Hadid.

A ring current is an electric current carried by charged particles trapped in the magnetosphere. Earth has a well-understood ring current located tens of thousands of kilometers from its surface. On Mercury, it is less clear how particles can become trapped a few hundred kilometers from the planet, especially since the magnetosphere is crushed against the planet’s surface. This debate will likely be settled once MPO and Mio are collecting data full time.

Hadid and her colleagues also observed the spacecraft’s direct interaction with the surrounding space plasma. When the spacecraft is heated by the sun, it cannot detect the colder, heavier ions because the spacecraft itself becomes electrically charged and repels them.

But as the spacecraft moves through the planet’s night shadow, the charge is different and suddenly a sea of ​​cold plasma ions becomes visible. For example, the spacecraft detected ions of oxygen, sodium and potassium, which were likely sent from the planet’s surface by micrometeorite strikes or interactions with the solar wind.

“It’s as if we suddenly see the surface composition ‘exploding’ in 3D through the planet’s very thin atmosphere, known as the exosphere,” notes Delcourt. “It’s really exciting to start to see the connection between the planet’s surface and the plasma environment.”

“During this rare dusk-to-dawn scan through the large-scale structure of Mercury’s magnetosphere, we tasted the promise of future discoveries,” said Go Murakami, JAXA’s BepiColombo project scientist.

“The observations highlight the need for the two orbiters and their complementary instruments to tell us the whole story and provide a complete picture of how the magnetic and plasma environment is changing over time and space,” adds Geraint Jones, ESA BepiColombo project scientist.

“We look forward to seeing what impact BepiColombo will have on our broader understanding of planetary magnetospheres.”

Meanwhile, scientists are already studying data obtained during Mercury’s fourth close flyby last month, while flight controllers prepare for the final two consecutive flybys scheduled for December 1, 2024 and January 8, 2025, respectively.