As the closest planet to the Sun, has the most direct exposure to the solar winds of any planet in the Solar System. The planet has a weak magnetic field, with a magnetosphere strongly linked to the surface and exosphere (a zone around its solid surface within which individual atoms and molecules can be found moving freely, but seldom, if ever, interacting) by a range of processes. These are largely driven by the exchange of energy with the solar winds, and loss of material from the planetary surface. The magnetosphere contains a mixture of ions derived from the solar winds, primarily hydrogen⁺ and helium²⁺ ions, and others derived from the planet's surface via ionization of material in the exosphere.
The Mariner 10 spacecraft made three flybys of Mercury in 1974 and 1975, detecting heavy ions thought to be derived from the planet's exosphere. Subsequent observations of Mercury by Earth-based telescopes were able to identify ions such as sodium⁺, potassium⁺, and calcium⁺. Between 2011 and 2015 the MESSENGER spacecraft orbited Mercury, during which time its Fast Imaging Particle Spectrometer was able to detect a variety of ions in the planet's magnetosphere, including hydrogen⁺ and helium²⁺ derived from the solar winds, as well as heavier ions such as helium⁺, oxygen⁺, water group ions (hydroxide⁺, water⁺, and hydrogen peroxide⁺), sodium⁺, magnesium⁺, aluminium⁺, and silicon⁺, derived from the planet. The helium⁺ ions showed a relatively even distribution around the planet, but the other planetary-derived ions were concentrated around the planetary cusp (i.e. directly facing the Sun) and in the equatorial band of the nightside of the planet, although it was not possible to further define the distribution of individual types of ion.
The BepiColombo spacecraft is a joint project by the European Space Agency and the Japan Aerospace Exploration Agency which was launched in 2018, on aa trajectory which would lead it to make close flybys of Mercury in October 2021, June 2022, June 2023, September 2024, December 2024, and January 2025, before finally entering the planet's orbit in November 2026.
In a paper published in the journal Communications Physics on 3 October 2024, a team of scientists led by Lina Hadid of the Observatoire de Paris, Sorbonne Université, Université Paris Saclay, École polytechnique, and Institut Polytechnique de Paris, present the result of a study of ion plasma observations made by BepiColombo's Mercury Plasma Particle Experiment instruments, during the flyby of Mercury made on 19 June 2023.
The 19 June 2023 flyby took BepiColombo to about 235 km above the surface of Mercury, enabling sampling of ions within the magnetosphere plasma at low altitudes along the spacecraft's trajectory. During the flyby BepiColombo approached Mercury from its dusk-nightside, passing through the post-midnight magnetosphere close to the equator of the planet, and moving away towards dawn-dayside.
BepiColombo crossed the bow-shock of the planet inbound at 6.44 pm GMT, and outbound at 7.52 pm. Outside of this bowshock region, both before and after the crossing, ions with energies of about 10 and about 20 electronvolts were constantly observed, with mass-per-charge ratios of 1 and 16. This is consistent with hydrogen⁺ and oxygen⁺ ions, derived from water molecules outgassed from the planet.
Projections of BepiColombo’s third Mercury flyby trajectory in the aberrated Mercury–Sun magnetospheric (aMSM) coordinate system. (a) X'–Z' and (b) X'–Y' planes, all expressed in Mercury radii (The radius of Mercury is 2440 km). Note the displacement in (a) of the magnetopause relative to the planetary centre because of the northward offset of the magnetic dipole by approximately 0.2 of the radius of Mercury. In traditional MSM coordinates, the X-axis and Z-axis point to the sun and north pole, respectively, and the Y-axis completes a right-hand system. In the aberrated coordinates, Mercury’s orbital velocity is considered. The X-axis is anti-parallel to the solar wind direction in the rest of the reference frame of Mercury. The aberration angle varies between about 5.5° and about 8.4° assuming a solar wind speed of 400 km per second. The black arrows indicate the viewing direction of instrument during this flyby. The magenta and cyan crosses represent the observed inbound (and outbound) bow shock and magnetopause crossings, respectively. The red dot highlights the closest approach of BepiColombo to Mercury. The black solid and dashed lines represent the modelled dayside bow shock and magnetopause that are obtained from the statistical distribution of observed crossing points, respectively. Hadid et al. (2024).
As BepiColombo entered the dusk magnetosphere of Mercury, it encountered ions with energies of around 20 000 electronvolts, but following this the energy of the ions fell to a few tens of electronvolts. Hadid et al. interpret this area as the low latitude boundary layer, the area along the magnetospheric side of a planet's low-latitude magnetopause where plasmas form the magnetosheath and magnetosphere are mixed. This kind of energy dispersion within the plasma mantle is typically seen at high latitudes; its presence close to the equator of Mercury suggests a relationship between the plasma mantle and the low latitude boundary layer, with convection carrying ions deep into the magnetosphere.
Hydrogen⁺ ions were detected in the low latitude boundary layer region which Hadid et al. interpret as having derived from the duskside magnetosphere (i.e. the part of the magnetosphere where the Sun is setting). The low latitude boundary layer region is also likely to contain heavy ions derived from the dayside exosphere of the planet and transported over the polar caps, although Hadid et al. are careful to emphasise that determining the origin of the low latitude boundary layer is beyond the scope of the current study.
Model of the hydrogen⁺ ion trajectories. (a) Shows various particle trajectory projections in the equatorial plane traced backward in time. (b) Shows the particle kinetic energy versus time. The ions are launched from different locations (closed circles) along BepiColombo’s orbit, and their trajectories are traced backward in time. The colour code depicts the different magnetospheric regions, viz., the Low- Latitude Boundary Layer in green, the umbra in blue, the Plasma Sheet Horns in yellow, and ring current in red. The test hydrogen⁺ ion trajectories were computed using a modified Luhmann–Friesen model for the magnetic field combined with a two-cell convection pattern for the electric field. The full equation of motion was integrated backward in time using a fourth-order Runge–Kutta technique. Hadid et al. (2024).
As BepiColombia enetered the umbra (shadow) of Mercury and inner part of the low latitude boundary layer at 7.24 pm it encountered 'cold' ions with energies as low as 30-100 electronvolts. Hadid et al. suggest that this might be because negatively charged, causing low-energy ions from the exosphere to become attracted towards it. The ions encountered around the low latitude boundary layer include oxygen⁺ and calcium⁺ and/or potassium⁺ ions thought to have originated from the dayside of the planet and lighter hydrogen⁺ and helium²⁺ ions, probably of solar origin.
At 7.28 pm, shortly after leaving the low latitude boundary layer, BepiColombo began to encounter 'hot' ions with energies in the kiloelectronvolt range, in an area corresponding to the 'plasma sheet horns' detected by the MESSENGER spacecraft a decade previously. These ions are thought to originate from the tail of the magnetosphere, and to be accelerated towards the planet by convection currents.
After passing through this region at 7.32pm, BepiColombo encountered a region with intense ion fluxes, with ion energies in the 5-40 kiloelectronvolt range. Because this layer is present at low altitudes in the equatorial region, Hadid et al. interpret this as a tenuous ring current, in which charged particles could become trapped in orbits of the planet at altitudes of 1.3-1.5 times its radius. At this altitude a hydrogen⁺ could orbit Mercury in about four minutes, bouncing back and forth on either side of the equatorial plane throughout its motion around the planet. The presence of such a ring current had been suggested from the MESSENGER data, but the data was rather limited, with particles with energies of no more than 13 kiloelectronvolts being detected. The greater energy range detected in the BepiColombo data provides much better support for the presence of such a ring current, although again it is not sufficient to state definitively that this is what is being detected.
The high energy particles within this band appear to be hydrogen⁺ and helium²⁺ ions, but BepiColombo also encountered larger particles. The most common of these have energies of around 2 kiloelectronvolts, and are interpreted as oxygen⁺ ions, while more energetic particles, with energies of around 10 kiloelectronvolts, and are interpreted as being predominantly calcium⁺ and potassium⁺ ions, with some sodium⁺ ions also present. BepiColombo also encountered cold ions, with energies of about 15 electronvolts. These cold ions are presumed to have originated from the surface of Mercury, and peaked at the closest to the planet, an altitude of 332 km.
After passing through the post-midnight magnetosphere of Mercury, BepiColombo re-entered the planet's magnetosheath and then moved back into the solar wind. This solar wind comprises a compressed and heated stream of hydrogen⁺ and helium²⁺ ions, although outgasses water group ions could again be detected in this region.
Mercury’s magnetosphere during BepiColombo’s third flyby. European Space Agency.
The 19 June 2023 flyby of BepiColombo has provided us with our first reasonably detailed view of the structure of Mercury's magnetosphere, demonstrating that it is not greatly different from that of the Earth. Both low and high energy ions were observed in the planet's magnetosphere, including the deepest parts encountered, suggesting that ion sputtering (the dislodging of low energy ions from the surface of the planet by the impact of high energy ions from the Sun) plays a significant role in the system. The evidence collected by the spacecraft supports the presence of a ring current encircling Mercury, and for the first time demonstrate the presence of a low latitude boundary layer.
The magnetosphere of Mercury will remain a subject of study for the remainder of the BepiColombo mission, including the planned further flybys and orbital stage, which should serve to greatly enhance our understanding of the planet's magnetic environment.
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