Electron temperatures in the ionosphere of Venus from Solar Orbiter/Radio and Plasma Waves instrument

¹ Radboud Radio Lab, Department of Astrophysics, Radboud University Nijmegen, The Netherlands
² LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
³ Department of Physics, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
⁴ Swedish Institute of Space Physics (IRF), Uppsala, Sweden
⁵ Department of Physics and Astronomy, Uppsala University, Uppsala 75121, Sweden
⁶ Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czech Republic
⁷ LPP, CNRS, Ecole Polytechnique, Sorbonne Université, Observatoire de Paris, Université Paris-Saclay, Palaiseau, Paris, France
⁸ LPC2E, UMR7328 CNRS, OSUC, University of Orléans, CNRS, CNES, Orléans, France
⁹ Space Sciences Laboratory, University of California, Berkeley, CA, USA
¹⁰ Physics Department, University of California, Berkeley, CA, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 October 2025
Accepted: 8 March 2026

Abstract

Context. On February 18, 2025, Solar Orbiter (SO) completed its fourth gravity assist maneuver of Venus (VGAM4) and reached an unprecedented proximity coming within 378 km of the planet. This flyby was necessary to steer the spacecraft into an orbit outside the plane of the ecliptic. Near the closest approach, only the Radio and Plasma Wave (RPW) and Magnetometer (MAG) instruments were operational; this enabled high-cadence measurements to be taken to investigate the plasma properties of the Venusian ionosphere.

Aims. The main goal of this study is to derive the electron density and temperature in the ionosphere of Venus using electric potential measurements from RPW, and to characterize them.

Methods. During approximately five minutes around the closest approach, the High Frequency Receiver of RPW detected radio emissions of a type naturally generated by planetary ionospheres whose frequency can be related to the electron density. Using quasithermal noise spectroscopy, we inferred the electron temperature at discrete altitudes and solar zenith angles.

Results. Solar Orbiter measured an average density and electron temperature in the ionosphere of Venus of 12 385 ± 148 cm⁻³ and 0.43 ± 0.05 eV, respectively. These values are in agreement with in-situ measurements by Pioneer Venus Orbiter (PVO) obtained at the solar maximum. Binned magnetic fields and temperatures are anticorrelated, which suggests that the magnetic flux ropes, observed in the Venus ionosphere, are more likely non-force-free structures.

Conclusions. The findings presented in this paper, together with the measurement from the Parker Solar Probe (PSP) during the third gravity assist, support the conclusion that the plasma density in the Venusian ionosphere above 350 km varies with solar activity, whereas the electron temperature shows a much weaker dependence. Notably, the electron temperature remains consistent across the three missions (SO, PSP, and PVO), despite varying levels of solar activity. This suggests that, over the altitude and solar zenith regions probed, the thermal structure of the Venusian ionosphere is not driven by solar extreme ultraviolet (EUV) heating alone, but is also shaped by external heat sources near the ionopause. Processes such as the damping of whistler mode waves, solar wind ion heating, and thermal conduction from the hot ionosheath appear to play a major role.