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Solar Orbiter in Venus' Ionosphere: Observations From the 4th Flyby.

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Title: Solar Orbiter in Venus' Ionosphere: Observations From the 4th Flyby.
Authors: Edberg, Niklas J. T.1 (AUTHOR) ne@irfu.se, Boldu, Jordi1,2 (AUTHOR), Eriksson, Anders I.1 (AUTHOR), Kim, Konstantin1 (AUTHOR), Persson, Moa1 (AUTHOR), Andrews, David J.1 (AUTHOR), Khotyaintsev, Yuri V.1 (AUTHOR), Vecchio, Antonio3,4 (AUTHOR), Maksimovic, Milan4 (AUTHOR), Chust, Thomas5 (AUTHOR), Hadid, Lina Z.5 (AUTHOR), Horbury, Timothy S.6 (AUTHOR), Galand, Marina I. F.6 (AUTHOR), Matteini, Lorenzo6 (AUTHOR), Píša, David7 (AUTHOR), Souček, Jan7 (AUTHOR), Kretzschmar, Matthieu8 (AUTHOR), Owen, Chris J.9 (AUTHOR), Bale, Stuart D.10 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. May2026, Vol. 131 Issue 5, p1-17. 17p.
Subject Terms: Ionospheric plasma, Plasma boundary layers, Electron density, Venusian atmosphere, Solar wind
Abstract: Solar Orbiter entered the topside ionosphere of Venus for the first time when performing its 4th flyby of the planet, reaching an altitude of 378 km on 18 Feb 2025. High‐cadence electron density measurements showed previously unresolved fine‐structuring within plasma regions and boundaries, particularly at the ionopause. During the rapid flyby, a rare snapshot of the entire induced magnetosphere was captured during calm solar wind conditions around solar maximum. A well‐structured and a relatively steady plasma environment was observed. Assuming an electron temperature of 0.5 eV, pressure balance was found across the ionopause, while at the same time quasi‐periodic density and magnetic field variations suggest boundary oscillations. Near closest approach, non‐force free magnetic flux ropes were observed. Small‐scale perturbations in both magnetic field strength and density across them indicate them being dynamically evolving, rather than in a stationary state. Plain Language Summary: During its closest approach to Venus on 18 February 2025, the Solar Orbiter spacecraft flew through the planet's upper atmosphere—known as the ionosphere—at an altitude of only 378 km. This was the first time Solar Orbiter entered this region, allowing a study how Venus interacts with the solar wind, the stream of charged particles from the Sun. The measurements showed that the plasma (ionized gas) around Venus was structured but fairly stable. Detailed observations of electron density revealed fine‐scale variations, especially at the boundary between the ionosphere and the solar wind, called the ionopause. The data also indicated that pressure was balanced across this boundary, but small, regular fluctuations suggested that the boundary was oscillating. Near the point of closest approach, the spacecraft detected magnetic "flux ropes", twisted magnetic structures that appeared to be changing rapidly. These observations provide new insight into the complex and dynamic interaction between Venus and the solar wind. Key Points: Solar Orbiter passed by Venus at an altitude of 378 km, crossing the ionopause and reaching the topside ionosphere for the first timeA calm solar wind at solar maximum resulted in well‐structured plasma environment, a sharp ionopause and an un‐magnetized ionosphereFine‐scale structuring of both ionopause and ionospheric magnetic flux ropes were resolved with high‐time resolution data [ABSTRACT FROM AUTHOR]
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Abstract:Solar Orbiter entered the topside ionosphere of Venus for the first time when performing its 4th flyby of the planet, reaching an altitude of 378 km on 18 Feb 2025. High‐cadence electron density measurements showed previously unresolved fine‐structuring within plasma regions and boundaries, particularly at the ionopause. During the rapid flyby, a rare snapshot of the entire induced magnetosphere was captured during calm solar wind conditions around solar maximum. A well‐structured and a relatively steady plasma environment was observed. Assuming an electron temperature of 0.5 eV, pressure balance was found across the ionopause, while at the same time quasi‐periodic density and magnetic field variations suggest boundary oscillations. Near closest approach, non‐force free magnetic flux ropes were observed. Small‐scale perturbations in both magnetic field strength and density across them indicate them being dynamically evolving, rather than in a stationary state. Plain Language Summary: During its closest approach to Venus on 18 February 2025, the Solar Orbiter spacecraft flew through the planet's upper atmosphere—known as the ionosphere—at an altitude of only 378 km. This was the first time Solar Orbiter entered this region, allowing a study how Venus interacts with the solar wind, the stream of charged particles from the Sun. The measurements showed that the plasma (ionized gas) around Venus was structured but fairly stable. Detailed observations of electron density revealed fine‐scale variations, especially at the boundary between the ionosphere and the solar wind, called the ionopause. The data also indicated that pressure was balanced across this boundary, but small, regular fluctuations suggested that the boundary was oscillating. Near the point of closest approach, the spacecraft detected magnetic "flux ropes", twisted magnetic structures that appeared to be changing rapidly. These observations provide new insight into the complex and dynamic interaction between Venus and the solar wind. Key Points: Solar Orbiter passed by Venus at an altitude of 378 km, crossing the ionopause and reaching the topside ionosphere for the first timeA calm solar wind at solar maximum resulted in well‐structured plasma environment, a sharp ionopause and an un‐magnetized ionosphereFine‐scale structuring of both ionopause and ionospheric magnetic flux ropes were resolved with high‐time resolution data [ABSTRACT FROM AUTHOR]
ISSN:21699380
DOI:10.1029/2025JA034813