Vortex Dynamics in the Wake of Planetary Ionospheres

Abstract

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.

Mass Flux in Corkscrew Flow Vortices in the Venus Plasma Wake

“…As a whole its width in the near wake is comparable to the Venus radius and it is seen to exhibit a rotation in the counterclockwise direction when viewed from the wake [4]. In addition, the position of the vortex structure varies along the solar cycle and becomes more closely located to Venus under minimum solar cycle conditions [5,6].…”

Particle acceleration in the Venus plasma wake

“…An extended study of those results was conducted by considering VEX orbits that exhibit an indication of vortex structures throughout the operation time of that spacecraft (between 2006 and 2013). Even though the plasma conditions differed significantly among them, it was possible to collect a number of orbits for each year of operation and that led to their general distribution reproduced in Figure 4 [10]. In particular, they describe variations in their width as follows: For each orbit there is a segment bounded by the entry and exit of the spacecraft through a vortex with a number that marks the two last digits of the year when measurements were made (they include the four orbits for 2006 and for 2009 that were discussed in Figures 2 and 3).…”