BepiColombo mission confirms stagnation region of Venus and reveals its large extent

The study investigates the solar wind interaction with Venus, a non-magnetized planet whose ionosphere forms an induced magnetosphere that acts as an obstacle to the solar wind. Gas-dynamic models predict a subsolar magnetosheath stagnation region where solar wind flow decelerates to very low bulk speeds and heats significantly, as well as a sonic line where the flow transitions between subsonic and supersonic regimes. Prior missions (Pioneer Venus Orbiter and Venus Express) provided important insights into the Venusian magnetosheath but could not sample the subsolar magnetosheath in situ during solar minimum, leaving key predictions unconfirmed. The BepiColombo second Venus flyby (August 10, 2021), coincident with upstream measurements by Solar Orbiter under stable solar wind conditions, offered a unique opportunity to confirm the existence, properties, and spatial extent of the stagnation region and to assess energy transfer from the solar wind to the Venusian ionosphere near solar minimum.

Multiple gas-dynamic and MHD models have described the Venusian magnetosheath and predicted a subsolar stagnation region and a sonic line (e.g., Spreiter & Stahara; Tanaka; Biernat et al.). Pioneer Venus Orbiter (1978–1992) provided extensive magnetic field observations but limited plasma measurements and did not sample the subsolar magnetosheath during solar minimum due to periapsis raise. Venus Express (2006–2014) improved understanding of the magnetosheath but, due to its polar, highly elliptical orbit, did not perform in situ measurements in the subsolar magnetosheath. Earlier studies established the presence of a magnetic pile-up boundary (MPB) acting as an inner boundary that largely excludes solar wind protons, especially during solar maximum and on the flanks during solar minimum, but the subsolar MPB conditions at solar minimum remained poorly constrained. Consequently, the predicted stagnation region and its extent near solar minimum had not been experimentally confirmed by in situ plasma measurements before this study.

Observational campaign: BepiColombo conducted a ~2-hour Venus flyby on 2021-08-10, traversing the magnetosheath from the nightside to the subsolar region and along the flank, approximately following plasma streamlines in Venus-Solar-Orbital (VSO) coordinates. The stacked spacecraft configuration included Mio (MMO) and MPO, along with MTM and MOSIF. Instruments used: on Mio, MPPE suite comprising MEA (electrons, 0.003–26 keV), MIA (ions, 0.015–29 keV), MSA (mass-resolved ions, 0.001–38 keV; switched off at 13:49 UT due to high flux), and ENA (0.01–3.3 keV energetic neutral atoms; in this configuration some pixels monitor neutralized protons). On MPO, SERENA/MIPA (ions, 0.015–15 keV) and MPO-MAG dual fluxgate magnetometers. Complementary upstream solar wind and IMF were provided by Solar Orbiter SWA/PAS (ions, 0.20–20 keV) and MAG; Solar Orbiter was ~200 Venus radii upstream along the same Parker spiral and measured stable conditions throughout the interval. Data analysis: Time–energy spectrograms for electrons and ions; magnetic field magnitude and vector; identification of plasma regions (electron foreshock, bow shock ramp and over/undershoot, subsolar magnetosheath subregions including stagnation region and sonic line, closest approach near MPB, flank magnetosheath, and return to solar wind). Angular distributions of proton counts within MIA and MIPA fields-of-view were compared with model-predicted bulk flow directions. Temperature estimation: Electron and ion temperatures in the stagnation region were derived by fitting drifting Maxwellian distributions to measured spectra (MEA for electrons, MIA for ions), using 3D angular scans available every 10 minutes (one within the stagnation region). Modeling: A LatHyS global hybrid simulation (ions as particles, electrons as massless fluid) for Venus was run and constrained by Solar Orbiter upstream conditions; model outputs (proton bulk and thermal speeds, temperature, magnetic field orientation and magnitude) were compared with in situ observations to interpret plasma regimes and validate region identifications. Pressure balance and gyro-kinematics: Using upstream dynamic pressure and measured/estimated magnetic fields, the team inferred MPB magnetic field strength at subsolar point and estimated proton gyroradii to evaluate solar wind penetration potential.

• BepiColombo provided the first in situ confirmation of the subsolar stagnation region at Venus during near-solar-minimum conditions. The stagnation region extends to at least ~1900 km altitude near the subsolar point. - Multi-spacecraft configuration with Solar Orbiter upstream established very stable solar wind and IMF, enabling separation of spatial from temporal variability. - Bow shock crossing was quasi-perpendicular near subsolar point with θ_Bn ≈ 82°. - In the stagnation region, protons are significantly slowed and heated: the proton thermal speed exceeds the bulk speed (model and measurements). - Measured temperatures in the stagnation region: electrons ~34 eV (from MEA Maxwellian fit at 13:57 UT) and ions ~280 eV (from MIA fits over 13:57–13:58 UT). Electron temperature is lower than earlier expectations (~90–140 eV), implying reduced electron-impact ionization efficiency. - Sonic line region identified (13:53–13:55 UT) with changes in energies, reduced magnetic fluctuations, and flow direction aligned with the obstacle boundary, consistent with transition toward supersonic flow further downstream. - Closest approach (~13:52 UT; ~550 km altitude) coincided with thermal electron depletion and a peak in |B|, indicating entry into the magnetic pile-up boundary (MPB) region; likely skimming of the induced magnetosphere boundary without full crossing. - Flank magnetosheath showed accelerated protons approaching solar wind energies by ~13:42 UT, before exit through the bow shock to pristine solar wind. - Upstream dynamic pressure measured by Solar Orbiter during the flyby was ~1.4 nPa (used for pressure balance), giving an estimated subsolar MPB magnetic field strength ~55 nT. Using measured Tp and B, average proton gyroradius is ~30 km (or ~100 km assuming Tp ~3 keV), much smaller than MPB thickness (~300 km) and altitude (~600 km), implying negligible direct penetration of gyrating solar wind protons to the ionosphere. - LatHyS hybrid simulations (constrained by upstream conditions; example caption shows dp ~2.8 nPa for a specific interval) matched MPO-MAG magnetic field orientation and magnitude and reproduced the low bulk speed relative to thermal speed in the stagnation region, supporting interpretation of observations. - Overall, observations indicate limited entry of and energy transfer from the solar wind to the Venusian ionosphere at the subsolar point under low dynamic pressure and large IMF-flow angle conditions.

The results directly confirm a long-predicted, gas-dynamics-dominated stagnation region in the subsolar Venusian magnetosheath and quantify its large vertical extent. The combination of strong heating and deceleration (bulk speed below thermal speed) and low electron temperature supports a scenario of reduced ion production and mass loading in the subsolar magnetosheath near solar minimum. The pressure balance analysis, together with small proton gyroradii relative to MPB scales, indicates that the MPB efficiently excludes solar wind ions at the subsolar point in these conditions, preventing direct Coulomb energy transfer to the ionosphere. These findings align with earlier flank and solar-maximum observations that suggested a strong MPB but extend them to the crucial subsolar region during solar minimum. The implications are significant for understanding atmospheric escape and the role of intrinsic magnetic fields: even without an intrinsic or crustal field, Venus’s ionosphere can withstand solar wind forcing under low dynamic pressure, limiting energy transfer and potentially reducing solar-wind-driven atmospheric erosion. The multi-point configuration with Solar Orbiter was critical to ensure that spatial structure, not temporal variability, dominated the signatures observed.

This study provides the first in situ confirmation and characterization of the subsolar stagnation region at Venus, demonstrating that it extends to at least ~1900 km altitude during stable, low dynamic pressure conditions near solar minimum. Measured low electron temperature (~34 eV) and significant proton heating/slowdown, corroborated by hybrid simulations, indicate limited mass loading and restricted solar wind energy transfer to the ionosphere. Pressure balance and gyro-kinematic estimates show the MPB remains an effective barrier at the subsolar point under these conditions. These results illuminate how non-magnetized planets can resist solar wind erosion, informing comparative planetology and habitability. Future work should: (i) sample the subsolar magnetosheath under a broader range of solar wind pressures, Mach numbers, and EUV levels (including solar maximum); (ii) perform additional flybys and, ideally, dedicated orbital passes through the subsolar region to resolve boundary structures and dynamics; (iii) obtain composition-resolved measurements of planetary pickup ions to quantify mass loading; and (iv) integrate multi-point upstream monitors to isolate spatial-temporal variability in future campaigns.

• Single flyby snapshot near solar minimum limits generalizability across solar conditions and solar wind parameters. - Stacked spacecraft configuration (MOSIF shielding) reduced instrument fields-of-view; only certain pixels observed the plasma, complicating full angular coverage. - MSA was automatically turned off at 13:49 UT due to high flux, limiting composition-resolved ion data in later intervals. - Only one 3D electron angular scan was available inside the stagnation region (scans every 10 min), constraining temperature determination cadence. - Evidence suggests skimming but not a complete crossing of the induced magnetosphere boundary; thus, interior boundary properties were inferred rather than fully observed. - Upstream conditions were measured by Solar Orbiter ~200 Venus radii away; although conditions were stable, small spatial/temporal gradients cannot be entirely excluded. - Low-energy electron measurements (<10 eV) were affected by spacecraft photoelectrons and excluded from fits. - Some parameters (e.g., MPB altitude and thickness) rely on prior mission statistics and model assumptions; uncertainties remain for the specific state during the flyby.