The terminator and nightside ionosphere of Venus as seen by Venus Express VeRa radio science

Fig. 1:. 4 VEX VeRa nightside electron density profiles. n_S are the smoothed electron density profiles. 6ˑσ_S indicates the conservative threshold for ionospheric signature detection. h_val is the lowest valid altitude of the observation. nu/hu and n_L/h_L are the identified upper and lower peak electron density/altitude, respectively.

Earth occultation radio science observations provide a powerful tool to probe the structure of planetary ionospheres. During its mission at Venus from 2006 and 2014, the Venus Express (VEX) spacecraft conducted more than 900 ingress and egress occultations with the Venus Radio Science experiment (VeRa), providing high resolution electron density profiles of the Venus dayside, terminator, and nightside ionosphere. In this study, 335 low noise level VeRa observations are analyzed to characterize the temporal and spatial variability of ionospheric structures at solar zenith angles (SZA) ranging from the terminator to the deep nightside.

On the planetary dayside, the altitude of the main ionospheric peak (V2) remains remarkably stable, with an average altitude of ~141 km and only minor variability. A modest increase in peak altitude is observed near the terminator, in good agreement with earlier results from the Pioneer Venus Orbiter radio science (ORO) experiment.

For SZAs > 90°, the ionosphere exhibits increased temporal and spatial variability. A valid ionospheric signature is detected in 97% of the lit nightside observations - where the ionosphere remains directly illuminated – and in 87% of observations on the deep nightside. Two distinct ionospheric regions are frequently identified on the planetary nightside: an upper peak and a lower peak separated by a significant reduction in electron density, indicating different formation mechanisms (Figure 1). Upper peaks occur on both the lit and deep nightside. In contrast, lower peaks are detected almost exclusively on the deep nightside, effectively ruling out direct solar EUV ionization as their primary source. Although lower peaks are observed under both low and high solar activity conditions, their occurrence rate increases with solar activity, suggesting an indirect solar-driven contribution to their formation.

Results show that the Venus nightside ionosphere is a persistent yet highly variable feature throughout the Venus Express mission time. A combined analysis of VEX VeRa electron density profiles and VEX ASPERA-4 plasma measurements will further constrain the origin and relative contributions of the processes responsible for the two distinct nightside ionospheric features.