A statistical study of ionopause perturbation and associated boundary wave formation at Venus

Previous missions to Venus have revealed that encounters with plasma irregularities of atmospheric origin outside the atmosphere are not uncommon. A number of mechanisms have been proposed to discuss their origins. One such mechanism involves an ionopause with a wavelike appearance. Extensive research on this characteristic of the ionopause is crucial in understanding the atmospheric evolution of Venus.

This thesis implemented an approach to identify potential boundary crossings resulting from ionospheric boundary waves. Coupled with Minimum Variance analysis, this approach was able to demonstrate whether or not the boundary crossings are `smooth' or `rippled'. By utilising the magnetic field and plasma data from Venus Express (VEX) over its entire mission from 2006 to 2014, this work presents the first observational statistical analysis of the ionospheric boundary waves at Venus.

The average estimated ionospheric boundary thickness is 57~$\pm$~4~km. This value which is estimated during solar minimum is roughly 1.5 times more than those estimated during solar maximum period. Further analysis shows that the boundary is thicker for weaker pressure and at higher altitudes. On the other hand, the boundary is thinner for stronger pressure at lower altitudes.

In the northern polar region of Venus, the normal directions of the rippled ionospheric boundary crossings lie mainly in the terminator plane with the largest component predominantly along the dawn-dusk ($Y_{VSO}$) direction. The average estimated wavelength of the boundary wave is 212~$\pm$~12~km and the average estimated velocity difference across the ionopause is 104~$\pm$~6~km~s$^{-1}$. The consistency shown between these results and the results from previous simulation studies of the Kelvin-Helmholtz Instability (KHI), suggests that the rippled boundary is a result of KHI. Furthermore, the magnetic field orientation in the barrier region is found to be quasi-perpendicular to the terminator plane, which is a favourable condition for the excitation of KHI along the dawn-dusk direction.

Analysis reveals a correlation between the normal directions and the locations of the boundary wave with respect to Venus. This indicates the draping of magnetic field lines appear to play a role in enhancing the plasma flow along the dawn-dusk direction, which could subsequently set up a velocity shear that favours the excitation of ionospheric boundary wave by the KHI along the dawn-dusk direction. Two other previously proposed boundary wave excitation mechanisms are also explored.

In addition, flux ropes are also identified using a similar approach. The average estimated flux ropes diameter is 90~$\pm$~6 km and they have axial orientations which lie mainly along the Venus-Sun ($X_{VSO}$) direction. The consistency in the sizes, locations and orientations shown between the identified flux ropes and boundary wave events suggest that these flux ropes are created as a result of the boundary waves reaching a turbulent stage.

In summary, this statistical study reveals that the ionopause of Venus does not always appear to be smooth, but often exhibits a wavelike appearance. Further analysis suggests that this wavelike appearance of the Venus ionopause is likely to be excited by the KHI, which arises as a result of the velocity shear across the ionopause driven by the draping pattern from the magnetic field. This study also shows that flux ropes can form and populate inside the ionosphere as a result of turbulent boundary wave. On the other hand, atmospheric bubbles can also form in similar manners and exist outside the ionosphere. Continuous scattering and the subsequent convection of atmospheric bubbles downstream and away from Venus over a prolonged period of time plays an important role in atmospheric loss from Venus.