Measurements of Venus' plasma environment during the 4th Solar Orbiter flyby

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Abstract
During its fourth Venus flyby on 18 February 2025, Solar Orbiter reached an altitude of 378 km, significantly deeper than during previous encounters, allowing the spacecraft to enter the Venusian ionosphere for the first time. The Magnetometer (MAG) and Radio and Plasma Wave (RPW) instruments operated in burst mode during most of the flyby, providing high-time-resolution measurements of the entire induced magnetosphere. The peak electron density reached approximately 2x104 cm-3, derived from a spacecraft potential of about –45 V and calibrated using the plasma frequency line.Solar Orbiter approached Venus from the tail region and entered the plasma environment without detecting a clear inbound bow shock. The upstream solar wind was steady and calm, as observed a few hours before and after the flyby and inferred from stable magnetosheath conditions, resulting in a structured and relatively steady plasma environment. High-cadence electron density measurements resolved fine-scale structures within plasma regions and boundaries, particularly at the ionopause, on spatial scales of 1–10 km, comparable to the local H+ and O+ ion length scales (2 and 8 km, respectively). Assuming an electron temperature of 0.5 eV, pressure balance was found across the ionopause, while quasi-periodic density and magnetic field variations suggest boundary oscillations on ion length scales during the pass. Near closest approach, magnetic flux ropes were observed. These features were generally not in full pressure balance with the surrounding plasma and exhibited small-scale perturbations in both magnetic field strength and density, consistent with a dynamically evolving rather than stationary state.