Investigation of Venus plasma tail using the Solar Orbiter, Parker Solar Probe and BepiColombo flybys

(Solar Orbiter Nugget #45 by Niklas J.T. Edberg¹, David J. Andrews¹, J. Jordi Boldú¹,², Andrew P. Dimmock¹, Yuri V. Khotyaintsev¹,², Konstantin Kim¹², Moa Persson¹, Uli Auster³, Dragos Constantinescu³, Daniel Heyner³, Johannes Mieth³, Ingo Richter³, Shannon M. Curry⁴, Lina Z. Hadid⁵, David Pisa⁶, Luca Sorriso-Valvo¹,⁷,⁸, Mark Lester⁹, Beatriz Sánchez-Cano⁹, Katerina Stergiopoulou⁹, Norberto Romanelli¹⁰,¹¹, David Fischer¹², Daniel Schmid¹², Martin Volwerk¹²)

1. Introduction

Venus lacks a global magnetic field but generates an induced magnetosphere through its interaction with the solar wind. This interaction has been studied mainly close to the planet while the far tail has been less sampled. While the main objectives of Solar Orbiter are related to the Sun and the solar wind, the mission also provides a bonus for planetary scientists as it performs several close flybys of Venus. During these passes the spacecraft cuts through the induced magnetosphere of Venus and provides invaluable measurements of the planet’s far downstream tail. The Parker Solar Probe and Bepi Colombo missions have in recent years also used Venus for similar gravity assist maneuvers to steer the spacecraft into their correct trajectories. By combining most of these flybys to date in a recently published paper [1], we have advanced our understanding of the extent, structure and dynamics of Venus’ magnetotail arising from the solar wind-planet interaction.

The two boundaries focused on in this study are the bow shock (BS) and the induced magnetosphere boundary (IMB). The BS is the outermost boundary where the solar wind meets the planet obstacle such that the supersonic flow decelerates to subsonic levels and a shock forms. The IMB is a broader boundary located closer to the planet, and is the boundary where the plasma transitions from solar wind dominated composition to planetary dominated while at the same time the flow direction changes and the interplanetary magnetic field piles up to sharply increase in magnitude as it drapes around the planet. These definitions are valid on the dayside or close to the planet, while far downtail their appearance is less known.

Figure 1. (a) Overview of trajectories of Solar Orbiter (red), BepiColombo (green), and PSP (blue). An earlier bow shock model based on the work of [2], which was really only constrained to within 5 RV, is included (transparent grey color). (b) The trajectories, the bow shock model from [2] (dotted line), and induced magnetospheric boundary model from [3] (dashed line), are shown in cylindrical Venus Solar Orbital coordinates, where the x-axis is in the direction of the sun and the y-axis is anti-parallel to the planet’s orbital velocity vector.

2. Nine flybys combined

Figure 1 shows the flyby geometry for the flybys used in [1]. In total, 17 flybys will have been performed by SolO, PSP and BepiC by the end of the missions with the current planning, but not all are useful for studying the tail and so only nine are used in this study. The flybys included are all, more or less, passing through the far magnetotail, with the PSP trajectories generally closer to the terminator plane. Solar Orbiter and BepiColombo spent many hours in the tail and traversed several tens of Venus radii (1 RV = 6052 km) downstream while observing the gradually changing plasma and field parameters. The measured electron density and magnetic field throughout the Solar Orbiter flybys are shown in Figure 2. Remarkably, we could observe a structured plasma environment including plasma boundaries at least 60 RV downstream. In a similar way, PSP and BepiC magnetic field data were analyzed for identifying boundary crossings during their passes (see Figure 3 in [1]).

Figure 2. Time-series of Solar Orbiter/RPW and MAG data from the first three Venus flybys. The bow shock and induced magnetosphere boundary (IMB) crossings are indicated by red and black dashed lines, respectively. The dotted lines indicate an uncertainty interval for the cases when the crossings were not clear or abrupt. Solar Orbiter spent several hours and many tens of RV in the tail of Venus during these flybys.

3. New bow shock model

The results of the boundary crossing survey is shown in Figure 3, where the locations in space of all crossings are shown. It could be seen that while the IMB boundary crossings were all fairly well scattered around the pre-existing shape model, the bow shock (BS) crossings downtail of about 10 RV were found significantly closer to the central tail than the previous model suggested. We therefore compiled a new model of the BS surface, which fitted the far-tail better, assuming a standard conic section model of the form r = L/(1+ε cos(θ), where (r,θ) are the polar coordinates centred at X0 = 0.688 RV, L = 1.466 RV the semi-latus rectum and ε = 1.001 the eccentricity. These parameter values were obtained through least-squares fitting. In Figure 3, the new model (black line) can be compared with the previous one (dotted line). There is a significant difference with the new model being about 10 RV  narrower at 60 RV downstream.

4. Boundary variability

Not all boundary crossings were obvious to identify, as the further downstream the more gradual and variable they became. Eventually the planetary plasma environment presumably completely merges with the solar wind plasma, and the bow shock more and more transitions into a bow wave rather than a proper shock.
During Solar Orbiter’s third flyby, extreme solar wind speeds (~900 km/s) were recorded. This had a profound impact on the variability of the plasma and field throughout the tail, with significantly increased amplitude of the fluctuations (Figure 2e,f). Despite heightened solar wind pressure, the magnetotail was expanded instead of compressed at this time, which  likely was due to the increased solar extreme ultraviolet (EUV) flux as this was the first flyby occurring during the inclining phase of the solar cycle, as opposed to solar minimum as all previous flybys - see Figure 4. This highlights the ever changing balance between solar wind conditions and solar radiation in shaping Venus' plasma environment.

Figure 3. (a) Bow shock and IMB crossings along the trajectories of SolO, PSP and Bepi. The colored lines indicate the uncertainty interval (as in Fig 2). The dotted line is the previous BS model by [2], and the dashed lines the IMB model by [3]. The black solid line shows the new model from this study, which better fits the far-downtail BS. (b) A zoom in of panel a, (c) location of boundaries extrapolated to the terminator plane, and rotated into a frame where the vertical axis is aligned with the convective electric field direction. No clear asymmetry is found in this frame.

Figure 4. Time series of solar EUV flux (MgII EUV proxy) and the time of each Venus flyby indicated by the vertical lines. Note that most flybys have occurred during solar minimum while the VGAM3 of Solar Orbiter took place during the increasing phase of the solar cycle.

5. Conclusions

This study bridges a critical gap in Venusian plasma research. Earlier missions like Venus Express and Pioneer Venus Orbiter primarily explored the near-planet environment, with little coverage of the far tail. By utilizing gravity-assist flybys from Solar Orbiter, BepiColombo, and PSP, this research extends the empirical knowledge of Venus’ magnetotail, revealing its dynamic nature and dependence on solar activity. 

• The induced magnetosphere of Venus is studied down to 60 RV downstream of the planet, at which point the furthest BS crossing is recorded. The induced magnetospheric boundary is found to still exist at 20 RV downstream.
• Both plasma boundaries generally become less clear with increasing distance.
• A new BS model is presented which fits the far-tail region better. The pre-existing IMB model still seems valid 20 RV downstream.
• Boundary locations varied significantly among flybys, showcasing the magnetotail's dynamic response to solar wind conditions and EUV flux.

While several studies on the Venus plasma environment have been carried out from these flybys, there are more exciting measurements yet to come:  the upcoming planned passes of Solar Orbiter will take the spacecraft even closer to Venus such that hopefully the topside ionosphere can be studied. The next pass will take place already on 18 Feb 2025.

Affiliations

¹ Swedish Institute of Space Physics, Uppsala, Sweden
² Department of Physics and Astronomy, Uppsala University, Sweden
³ Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Braunschweig, Germany
⁴ Laboratory for Atmospheric and Space Plasmas, University of Colorado, Boulder, CO, USA
⁵ LPP, CNRS, Observatoire de Paris, PSL Research University, Sorbonne Université, École Polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France
⁶ Dept. of Space Physics, Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czechia
⁷ CNR/ISTP – Istituto per la Scienza e Tecnologia dei Plasmi, Via Amendola 122/D, 70126 Bari, Italy
⁸ Division of Space and Plasma Physics, KTH Royal Institute of Technology, Stockholm, Sweden
⁹ School of Physics and Astronomy, University of Leicester, Leicester, UK
¹⁰ NASA Goddard Space Flight Center, Greenbelt, MD, USA
¹¹ University of Maryland College Park, College Park, MD, USA
¹² Space Research Institute, Austrian Academy of Sciences, Graz, Austria
 

References

[1] Edberg et al. (2024). Extent of the magnetotail of Venus from the solar orbiter, Parker Solar Probe and BepiColombo flybys. Journal of Geophysical Research: Space Physics, 129, e2024JA032603. https://doi.org/10.1029/2024JA032603

[2] Signoles et al. (2023). Influence of solar wind variations on the shapes
of Venus plasma boundaries based on Venus express observations. The Astrophysical Journal, 954 (1), 95. doi: 10.3847/1538-4357/ace7b1

[3] Martinecz et al. (2008). Location of the bow shock and ion composition boundaries at Venus—initial determinations from Venus Express ASPERA-4. Planet.
Space. Sci, 56 , 780-784. doi: 10.1016/j.pss.2007.07.007