Magnetotail plasma dynamics and return flow at Venus

Venus’ induced magnetosphere forms through the interaction between the solar wind and the planet’s highly conductive ionosphere. The solar wind, which is frozen-in to the interplanetary magnetic field (IMF), slows down upstream of the obstacle, while it flows largely unaffected away from it. The interaction causes the IMF to drape around Venus, forming the induced magnetotail on the nightside. The magnetotail acts as the main channel for atmospheric escape, as planetary plasma is accelerated downstream by the highly bent magnetic field lines [1]. However, plasma in the magnetotail is often observed flowing Venusward, reducing the net escape to space [2]. These Venusward flows, also known as return flows, have been quantified by Venus Express plasma measurements [3]. However, the process responsible for reversing the plasma velocity and creating the return flows is yet unknown, although magnetic reconnection was proposed as a possible triggering mechanism [4].

Here, we analyse magnetic field (MAG) [5], ion (ASPERA-4/IMA), and electron (ASPERA-4/ELS) [6] data throughout the Venus Express (VEX) mission, aiming to understand the physics of return flows. We perform a statistical analysis correlating return flows with magnetotail plasma processes, including Hall magnetic field configurations, electron bursts, multiple current sheet crossings, and flux ropes. Hall magnetic fields are a consequence of ions and electrons decoupling from the magnetic field around a reconnecting X line, and are widely interpreted as a signature of collisionless reconnection [7]. Energetic and periodic electron bursts [8], multiple current sheet crossings associated with magnetic field fine structures or current sheet flapping [9], and closed magnetic loops such as flux ropes [10] were also observed in the Venus magnetotail and suggested to be a consequence of reconnection.

We find that ion return flows are observed more frequently when the Hall magnetic field configuration is measured, compared to orbits with quiet magnetotail intervals. This indicates that magnetic reconnection can drive the return flows. However, return flows are still observed in cases without Hall magnetic field structures. Correlations with the other potential magnetic reconnection signatures were also studied, but our results show that return flows occur less frequently with these plasma processes than with Hall magnetic field events. 

In the presentation, we will discuss the different magnetotail plasma processes, their effect on magnetotail dynamics, and their role as possible return flow drivers.

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