Mercury’s magnetosphere during BepiColombo’s third flyby (annotated)
The ESA/JAXA BepiColombo spacecraft sped through the magnetosphere of Mercury on 19 June 2023 during its third of six flybys at the planet. Using data collected by the JAXA-led Mercury Magnetospheric Obiter’s Mass Spectrum Analyser (MSA), Mercury Ion Analyser (MIA), and Mercury Electron Analyser (MEA), and combining these measurements with a computer model to determine the origin of detected ions based on their motion, various features encountered along the trajectory of the spacecraft were revealed.
BepiColombo’s trajectory is shown as the yellow line in this graphic, with the various features encountered along the way labelled accordingly. A detailed explanation is provided below:
The first feature BepiColombo encountered, at 18:44:22 Universal Time (UT), was the bow shock. This is the boundary between the free-flowing solar wind and the planet’s magnetosphere.
Next, it crossed the magnetopause, which separates the shocked solar wind in the magnetosheath from the rest of Mercury’s magnetosphere, at 19:14:00 UT.
Measurements made between 19:10:30 and 19:27:34 UT revealed a region of turbulent plasma at the edge of the magnetosphere, hosting particles with a wide range of energies. This region is called the low-latitude boundary layer (LLBL).
At 19:24:25 UT, just as BepiColombo entered the shadow of Mercury, the MSA and MIA instruments detected an intense cold ion signature while MEA observed a strong reduction in the number of electrons. The spacecraft remained in the planet’s shadow until about 19:39, during which time it also detected heavy ions reflecting the chemistry of the planet’s surface.
Immediately after the LLBL, at 19:28:41 UT, BepiColombo encountered thermalised hot ions and, for the first time, an electron population in the plasma sheet extending to higher latitudes. The plasma sheet is a region of hotter, denser electrically charged gas that streams out like a tail in the direction away from the Sun. What BepiColombo detected is characteristic of plasma sheet horns resulting from electrons being accelerated from the distant plasma tail toward the planet.
From about 19:32:00 until 19:44:04 UT the spacecraft travelled through a region with high-energy ions and electrons. The presence of these energetic ions both near the equator and at low altitudes – BepiColombo passed just 235 km above the planet’s surface – strongly suggest that the spacecraft travelled through a tenuous ring current around Mercury. This is an electric current carried by charged particles trapped in the magnetosphere.
The spacecraft remained inside the magnetosphere until 19:45:00, crossing the outbound bow shock at 19:52:00 and there-on into the ‘normal’ dawn-side solar wind made up mostly of hydrogen and helium ions.
A clearer picture of the dynamic processes in Mercury’s magnetosphere will be built up over the course of BepiColombo’s main science mission.
Click here for an unannotated version of this graphic.
[Image description: A textured sphere representing Mercury is shown with magnetic field lines compressed on the sunward side and streaming out into a tail on the nightside. The BepiColombo spacecraft’s trajectory is drawn passing through the magnetosphere from dawn to dusk, close to the planet’s surface. Various features in the magnetosphere are depicted and labelled with text. Following the order in which they were detected by the spacecraft, this includes the bow shock, magnetopause, low-latitude boundary layer, cold ion cloud, plasma sheet horn and ring current.]
