Return flows in Venus’ magnetotail and potential association with magnetic reconnection

Venus lacks an intrinsic magnetic field, and its interaction with the solar wind and interplanetary magnetic field (IMF) creates an induced magnetosphere [1]. The IMF drapes around the planet, forming the magnetotail on Venus’ nightside. The magnetotail is the main channel through which planetary ions escape [2]. However, ion escape in the magnetotail is significantly reduced by unexpected return flows (i.e., planetary ions flowing back towards Venus instead of downtail) [3]. The process responsible for reversing the velocity of magnetotail ions remains unexplained.

A possible mechanism causing the return flows is magnetic reconnection, a plasma process where oppositely directed magnetic field lines break apart and cross connect, releasing magnetic tension to the surrounding plasma, which is heated and accelerated. Magnetic reconnection could occur in Venus’ magnetotail at the current sheet, which separates magnetic field lines with opposite polarities. The process can potentially generate a plasmoid flowing toward Venus. Such magnetic reconnection events have been identified by magnetic and plasma data collected by the Venus Express (VEX) spacecraft [4], but have not been systematically correlated with return flows.

Here, we reassessed the VEX’s magnetometer (MAG) [5] data throughout the mission to identify typical Hall magnetic field signatures when the spacecraft crosses the plasma sheet, as evidence of the magnetic reconnection events [6]. We also used simultaneously measured ASPERA-4/IMA and ASPERA-4/ELS data [7] to detect ions and electrons heating as additional signatures of reconnection. We also systematically reassessed ion data (ASPERA-4/IMA) to identify return flow events when the ions are travelling in the Venusward direction.

In this presentation, we show case studies with simultaneous detections of the Hall magnetic field signature and ion return flows, with consistent velocities and directions of plasmoid ions. Additionally, we present a statistical analysis comparing the occurrence of return flows under quiet current sheet crossings and crossings exhibiting a Hall magnetic field structure. We will discuss the results of the statistical analysis and their implications regarding the correlation between magnetic reconnection and return flows in Venus’ magnetotail.

[1] Futaana, Y., Stenberg Wieser, G., Barabash, S., & Luhmann, G. J. 2017, SSR, 212, 1453, doi: 10.1007/s11214-017-0362-8

[2] Dubinin, E., Fränz, M., Zhang, T. L., et al. 2013, JGR, 118, 7624, doi: 10.1002/2013JA019164

[3] Persson, M., Futaana, Y., Fedorov, A., et al. 2018, GRL, 45, 10805, doi: 10.1029/2018GL079454

[4] Zhang, T.-L., Baumjohann, W., Lu, Q. M., et al. 2012, Science, 336, 567, doi: 10.1126/science.1217013

[5] Zhang, T.-L., Berghofer, G., Magnes, W., et al. 2007, ESA Special Publication SP 1295 (Paris: ESA)

[6] Yamada, M., Kulsrud, R., & Ji, H. 2010, RvMP, 82, 603, doi: 10.1103/RevModPhys.82.603

[7] Barabash, S., Sauvaud, J., Gunell, H., et al. 2007, PSS, 55, 1772, doi: 10.1016/j.pss.2007.01.014