Structure and Dynamics in the Magnetotails of Unmagnetized and Weakly Magnetized Bodies

The plasma environment of our neighboring planets, Venus and Mars, differs significantly from Earth's. Although neither of them possesses a dominant intrinsic dipolar magnetic field, there are still induced magnetospheres forming around the two planets, due to the interaction of the solar wind and the interplanetary magnetic field (IMF) with their conductive ionospheres, exospheres, and the localized crustal magnetic fields in the case of Mars. Induced magnetospheres, their associated plasma environments, and the physical processes within them are particularly susceptible to the changing upstream conditions. The increasing number of successful and long-lived missions during the last few decades has been key for describing the fundamental structures and processes comprising the induced magnetospheres of the two planets. Nevertheless, their induced magnetotails have been more challenging to probe, due to the restrictions of the orbital geometry of planetary missions. Here, we present the latest discoveries and a comprehensive comparison between the Venusian and Martian induced magnetotails, and we highlight the need for further exploration of these regions. Atmospheric escape and energy transfer processes and paths are inextricably linked with the climate history and the disappearance of water at Mars, though there are many unknowns still in the case of Venus. Past and current missions utilizing particle and fields instruments have explored a great part of the plasma environments of Venus and Mars. Several plasma boundaries, shaped by both internal and external factors, divide the planetary environments and magnetospheres into different plasma regimes and have been described by observations and models. Simulations and observations have also been utilized to investigate the magnetotail structure of the two planets, which appears to be governed mainly by the IMF, the solar wind dynamic pressure, and the crustal magnetic fields in the case of Mars. At Mars, the presence of the crustal magnetic fields, which are regions of crustal magnetization on the surface of the planet clustered mostly in the southern hemisphere, further complicates the interaction of the solar wind and the IMF with the planet's plasma environment, thus justifying the term 'hybrid' - instead of induced - that is often used to describe the Martian magnetosphere. The existence of a magnetotail twist, as well as a first approach on mapping the structure of the current systems, has been reported at Mars, whereas different types of magnetotail current sheet flapping motion have been observed at both Mars and Venus. The magnetic topology, which describes the morphology of closed, open, and draped magnetic field lines over a planet, has also been inferred and explained for both planets. Escape processes, escape rates and their response to space weather have been reported, and we now have a better idea of the differences between the two planets. Escaping structures, contributing with a bulk removal of plasma, have also been observed in their magnetotails. Nevertheless, much still remains unknown, for example the specifics of how individual processes respond to solar drivers. Mars and Venus are not the only solar system bodies with no global intrinsic magnetic field. Induced magnetotails are formed around Saturn's moon Titan and comets too. A comparison between those bodies and Venus and Mars will provide a broader and general picture of induced and hybrid magnetotails, which could help future investigations of the plasma environments and tails of exoplanets. Lastly, in this review paper, we also summarize the questions that remain unanswered, emphasizing the need for future missions.