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

The interaction between a Solar System object and the solar wind is significantly influenced by the presence of a planetary intrinsic magnetic field and/or an ionosphere. This interaction dictates the nature of the interaction and its effects on atmospheric evolution. Venus, despite being an Earth-like planet in size, lacks both a global magnetic field like Earth's and crustal magnetic fields like Mars'. Consequently, only its ionosphere interacts with the solar wind. This interaction induces currents within the conductive ionosphere, forming an induced magnetosphere that acts as an obstacle to the solar wind. As the solar wind encounters this obstacle, it's decelerated at a detached bow shock, forming a magnetosheath around Venus. At the subsolar point of the magnetosheath, where the solar wind dynamic pressure and ionospheric thermal pressures are maximal, the solar wind flow stagnates – reaching a very low bulk speed and high temperature. The Venusian magnetosheath presents a unique opportunity to study a purely gas-dynamic interaction between the supersonic solar wind and a conductive ionosphere, uninfluenced by complexities associated with intrinsic planetary magnetic fields. It's a valuable natural laboratory for investigating energy transfer from the solar wind to the ionosphere of non-magnetized bodies and the characteristics of stagnated flow near the subsolar point. Previous missions like Pioneer Venus Orbiter (PVO) and Venus Express (VEx) have provided valuable data, but their limitations – including insufficient time resolution and energy range for PVO's plasma instruments and VEx's inability to directly sample the subsolar magnetosheath due to its orbital characteristics – prevented a complete experimental confirmation of theoretically predicted gas-dynamic models. The BepiColombo mission, with its trajectory and synergistic observations with Solar Orbiter, addresses these limitations, offering the first comprehensive in-situ measurements of the subsolar magnetosheath.

Several gas-dynamic models have been developed to describe the gas-dynamics-dominated magnetosheath of Venus. The Pioneer Venus Orbiter (PVO) mission, orbiting Venus from 1978-1992, provided a detailed view of magnetic fields but lacked the high time resolution and energy range needed for comprehensive plasma characteristics. Its periapsis also increased after the initial years, limiting subsolar magnetosheath sampling during solar minimum. The Venus Express (VEx) mission (2006-2014) offered more detailed observations, but its highly elliptical polar orbit prevented in-situ measurements in the crucial subsolar region. No previous mission possessed a flyby trajectory capable of sampling the subsolar magnetosheath, the critical region defining the solar wind-planet interaction. Consequently, in-situ plasma measurements had not experimentally confirmed the theoretical predictions of this textbook example of a gas-dynamic interaction.

The BepiColombo mission's second Venus flyby on August 10, 2021, provided the unique opportunity to experimentally verify the subsolar magnetosheath subregions predicted by models. The dual-spacecraft BepiColombo (consisting of the Mercury Magnetosphere Orbiter (Mio) and the Mercury Planetary Orbiter (MPO)), passed through the Venusian magnetosheath, following plasma streamlines from the nightside to the subsolar region. Simultaneous measurements were acquired from various onboard instruments including the Mercury Plasma Particle Experiment (MPPE) on Mio (comprising the Mercury Electron Analyzer (MEA), the Mercury Ion Analyzer (MIA), the Mass Spectrum Analyzer (MSA), and the Energetic Neutral Atom analyzer (ENA)), the Search for Exospheric Refilling and Emitted Natural Abundances (SERENA) instrument suite on MPO (including the Mercury Ion and Proton Analyzer (MIPA)), and the magnetometer (MPO-MAG) on MPO. These measurements were complemented by data from the Solar Orbiter mission, which had performed a Venus flyby the previous day, providing upstream solar wind and magnetic field measurements. The multi-spacecraft configuration, coupled with stable solar wind conditions observed by Solar Orbiter, allowed for investigation of spatial magnetosheath variability without the interference of temporal solar wind fluctuations. The data analysis involved identifying different magnetosheath subregions based on changes in plasma parameters and comparing the observations with results from the LatHyS global hybrid model, a well-constrained model validated through comparisons with Solar Orbiter data. Electron and ion temperatures were determined by fitting Maxwellian distributions to the measured energy spectra.

The BepiColombo flyby revealed a complete picture of the Venusian magnetosheath, including its previously unexplored subsolar region. The spacecraft crossed a quasi-perpendicular bow shock near the subsolar point. The observations confirmed the existence of a stagnation region extending to altitudes of at least 1900 km, indicating a large region of stagnated flow even during low solar wind pressure. The spacecraft also traversed the sonic line, a transition region where the flow changes from subsonic to supersonic speeds. Near the closest approach, a depletion in thermal electron flux was observed, possibly indicating an encounter with the magnetic pile-up boundary. After closest approach, the spacecraft entered the flank magnetosheath, where protons were accelerated to near solar wind speeds. The comparison with the LatHyS model confirmed these interpretations, showing the lower proton bulk speed compared to the thermal speed in the stagnation region. In-situ electron temperature measurements in this region revealed a value of 34 eV, lower than previously expected. The lower electron temperature limits the efficiency of electron impact ionization and, combined with lower oxygen corona density near solar minimum, suggests a lower pickup ion density in the subsolar magnetosheath. Calculations based on Solar Orbiter measurements indicated a magnetic field strength of approximately 55 nT at the magnetic pile-up boundary. The small proton gyroradius, much smaller than the magnetic pile-up boundary thickness and its distance to the ionosphere, indicated that the solar wind does not directly interact with ionospheric particles via Coulomb collisions. This signifies that the inner boundary of the Venusian magnetosheath is effective at excluding the solar wind, even under solar minimum conditions and low dynamic pressure.

The BepiColombo findings address the research question concerning the nature of the solar wind interaction with Venus' ionosphere in the absence of a global magnetic field. The large extent of the stagnation region, the low electron temperature, and the small proton gyroradius collectively indicate that the Venusian ionosphere is robust even at the subsolar point during solar minimum, limiting the solar wind's energy transfer to the planet's atmosphere. This supports previous theoretical predictions and refines our understanding of gas-dynamic interactions in planetary environments lacking intrinsic magnetic fields. The results are highly relevant to the broader field of comparative planetology, informing our knowledge of atmospheric escape mechanisms driven by solar wind erosion and their implications for planetary habitability. The mission successfully demonstrated the significant contributions single flybys can make to our understanding of solar system objects.

The BepiColombo mission's second Venus flyby, in conjunction with Solar Orbiter observations, provided crucial in-situ measurements confirming the existence and extent of Venus' subsolar stagnation region. This confirms the limited energy transfer from the solar wind to the ionosphere under solar minimum conditions. Future research could focus on extending these observations to different solar wind conditions and exploring the detailed processes governing energy transfer at the magnetic pile-up boundary.

The study primarily focuses on conditions observed during a single flyby at near solar minimum with stable solar wind conditions. Further measurements under diverse solar wind conditions (e.g., high solar wind dynamic pressure, different Interplanetary Magnetic Field orientations) are needed to fully characterize the variability of the Venusian interaction with the solar wind. The analysis relies on modeling, and while the LatHyS model is well-constrained and validated, discrepancies between the model and observations may still exist.