Rickard Lundin

By H. Pérez-de-Tejada and R. Lundin

Measurements conducted with the Venus Express (VEX) spacecraft at its entry and exit through vortex structures in the Venus wake reveal that their position varies with the solar cycle. Both crossings are consistently measured closer to Venus during minimum solar cycle conditions and are gradually encountered at larger distances downstream from the planet along the solar cycle. At the same time their width along the VEX trajectory on the plane transverse to the solar wind direction is larger during minimum solar cycle conditions and show a gradual decrease along the solar cycle. As a result the vortex structures are envisioned as features that gradually become thinner as they extend along the Venus wake and agree with the geometry of a vortex flow in fluid dynamics whose thickness decreases with the downstream distance from an obstacle. Similar conditions should also be applicable to Mars and other bodies within the solar system and also possibly to exo-planets in external stellar systems.

Part of the book: Solar System Planets and Exoplanets

By Hector Pérez-de-Tejada and Rickard Lundin

Measurements conducted with spacecraft around Venus and Mars have shown the presence of vortex structures in their plasma wake. Such features extend across distances of the order of a planetary radius and travel along their wake with a few minutes rotation period. At Venus, they are oriented in the counterclockwise sense when viewed from the wake. Vortex structures have also been reported from measurements conducted by the solar wind-Mars ionospheric boundary. Their position in the Venus wake varies during the solar cycle and becomes located closer to Venus with narrower width values during minimum solar cycle conditions. As a whole there is a tendency for the thickness of the vortex structures to become smaller with the downstream distance from Venus in a configuration similar to that of a corkscrew flow in fluid dynamics and that gradually becomes smaller with increasing distance downstream from an obstacle. It is argued that such process derives from the transport of momentum from vortex structures to motion directed along the Venus wake and that it is driven by the thermal expansion of the solar wind. The implications of that momentum transport are examined to stress an enhancement in the kinetic energy of particles that move along the wake after reducing the rotational kinetic energy of particles streaming in a vortex flow. As a result, the kinetic energy of plasma articles along the Venus wake becomes enhanced by the momentum of the vortex flow, which decreases its size in that direction. Particle fluxes with such properties should be measured with increasing distance downstream from Venus. Similar conditions should also be expected in vortex flows subject to pressure forces that drive them behind an obstacle.

Part of the book: Vortex Dynamics