**1. Introduction**

Measurements conducted with various spacecraft across the Venus wake: the Mariner 5, the Venera, the Pioneer Venus Orbiter (PVO), and the Venus Express (VEX) spacecraft have provided evidence of plasma features that resemble vortex structures in fluid dynamic problems. It has been noted that the anti-solar directed flow of the plasma particles in the Venus wake is rotated to even become oriented back to Venus. The early measurement of that effect was reported by Pérez-de-Tejada et al. [1–3] from the PVO plasma data and that gave place to conditions that could be assessed as resulting from a vortex structure. Further information on the observation of vortex structures in the Venus wake was reported by Lundin et al. [4, 5] from measurements conducted with the ASPERA instrument onboard VEX. In this case it was possible to identify the scale size and position of vortex structures with information on the distribution of the velocity vectors of the solar wind and planetary ions that stream within and around those features.

A general view of the shape of a vortex structure is reproduced in the left panel of **Figure 1** showing in the plane transverse to the solar wind direction a vortex structure with a scale size comparable to that of the planet with a circulation sense that is counterclockwise when seen from the wake back to Venus. A circulation

**Figure 1.**

*(left panel) velocity vectors of H+* ≈ *1–300 eV ions measured with the VEX spacecraft in the Venus wake projected on the YZ plane transverse to the solar wind direction. Data are averaged in 1000 x 1000 km columns at X <* −*1.5 RV (adapted from Figure 4 of [5]). (right panel) average direction of solar wind ion velocity vectors across the Venus near wake collected from many VEX orbits and projected in cylindrical coordinates [15].*

pattern of the solar wind direction in cylindrical coordinates along the Venus wake is added in the right side panel of **Figure 1** with indications of a sunward directed flow return in the central wake. Comparable variations in the plasma velocity vector have also been recently reported from the VEX measurements [6]. Different from that motion there has also been information derived from the PVO and VEX measurements on an east–west displacement of planetary ion fluxes and that leads to the overall deflection of the trans-terminator ionospheric flow toward the dawn-hemisphere as it moves into the night-side ([7], see Fig. 15; [4], see Fig. 7b). A deflection in that direction can be accounted for in terms of the fluid dynamic Magnus force produced by the joint contribution of the unidirectional solar wind velocity and the rotation of the Venus atmosphere/ionosphere [8, 9].
