Influence of planetary space weather on the shapes of Venus plasma boundaries

As Venus does not have an intrinsic magnetic field, the solar wind interacts directly with the Venusian atmosphere, altering its structure and composition, for example through atmospheric ion escape to space. In particular, the interaction will result in the formation of plasma boundaries, which separate regions of different plasma populations around Venus. Knowing how space weather influences the shape of these boundaries is one of the key pieces to understanding the current state of the Venusian atmosphere.

During its eight years mission, including more than 3000 orbits around Venus, Venus Express made measurements of the plasma environment, covering a wide range of upstream conditions. Using conjoint plasma and magnetic field measurements from the ASPERA-4 (Analyser of Space Plasma and Energetic Atoms) and the magnetometer instruments, we identified the locations where the spacecraft crossed the bow shock and the ion composition boundary for each orbit. Using the derived dataset, we then determined the boundary shapes with a two-parameter fit. The boundary shape fittings were done with respect to one or multiple upstream conditions.

Here we report that both boundaries are highly dependent on solar wind extreme ultraviolet (EUV) flux, expanding further from the planet at solar maximum. A likely explanation is that at solar maximum, combined heating of the exosphere ions and a higher photoionization rate lead to a higher planetary ion production. These additional ions increase the internal thermal pressure, pushing the boundaries outward.

Additionally, at solar minimum, solar wind parameters like dynamic pressure and energy flux were found to not affect the shape of the bow shock, which is consistent with previous studies. The influence of the strength and orientation of the interplanetary magnetic field, the Mach number, and potential correlations between multiple upstream parameters, are also discussed in this talk.