Mirror mode-like structures around unmagnetised planets: 2. Venus as observed by the Venus Express spacecraft

Temperature anisotropy-driven instabilities such as the mirror mode and ion cyclotron instabilities are responsible for the generation of waves in the turbulent magnetosheath of planets. We present here a statistical study of mirror mode-like structures in the magnetosheath of Venus as observed by the Venus Express spacecraft. As in our Mars study (see poster by Simon Wedlund et al. in the same session), we use magnetic field-only measurements to constrain and identify these quasi-linear compressive structures and discuss ways to mitigate false positive detections based on one instrument only. After calculating the residence time of the spacecraft in the Venusian magnetoenvironment, we show two-dimensional statistical maps of mirror mode-like occurrence rates with respect to EUV solar flux levels, and type of bow shock (quasi-perpendicular vs quasi-parallel). We find detection probabilities of about 10% at most, with two main regions of occurrence, one behind the collisionless shock, the other close to the induced magnetospheric boundary, with the small modulation of the probability due to EUV solar flux conditions.