Proton Temperature Anisotropies in the Venus Plasma Environment during Solar Minimum and Maximum

Venus’ lack of an intrinsic magnetic field allows the solar wind to closely interact with its atmosphere [1], making it a
prime target for investigating how unmagnetized atmospheric bodies in our Solar System [2] or elsewhere [3] interact
with magnetized plasma flows. This close interaction means that solar-activity correlations exhibited by the solar wind and
other heliospheric parameters [4, 5] cause solar-cycle variations in Venus’ plasma environment and plasma phenomena. We
investigate these variations by characterizing the proton population around Venus during periods of solar minimum (2006–2009)
and maximum (2010–2014). We use data from the Ion Mass Analyser (IMA) instrument, a particle mass-energy spectrometer
which was onboard the Venus Express (VEX) mission. We apply a previously developed methodology which fits Maxwellian
models to measurements of the protons’ velocity distribution functions [6] to produce statistical distributions of bulk speeds and
temperatures in various regions of Venus’ plasma environment. We also present spatial maps and probability-density histograms
comparing the proton parameters between the two time periods.
We find that the temperatures perpendicular (T_⊥) and parallel (T) to the background magnetic field are 20–35% lower
in the magnetosheath during solar maximum. This suggests that the heating of particles as they cross the bow shock varies
between the two time periods. We also find that the regions in the magnetosheath with highest temperature ratio T_⊥/T are
farther downstream from the bow shock during solar maximum than minimum. This is consistent with previous observations of
how mirror-mode structures presumably generated at the bow shock strictly decay as they are convected into the magnetosheath
during solar minimum, whereas during solar maximum they first grow and then decay [7]. We also present ongoing work to
further characterize the plasma environment as a function of upstream solar-wind parameters (such as Mach number or cone
angle) and bow shock geometry. We discuss preliminary results concerning energy conversion processes at Venus’ bow shock.

REFERENCES
[1] Y. Futaana, G. Stenberg Wieser et al., “Solar Wind Interaction and Impact on the Venus Atmosphere,” Space Science Reviews, vol. 212, no. 3-4, 2017.
[2] C. Bertucci, F. Duru et al., The induced magnetospheres of mars, venus, and titan, 2011, vol. 162, no. 1-4.
[3] C. Dong, M. Jin et al., “Atmospheric escape from the TRAPPIST-1 planets and implications for habitability,” Proceedings of the National Academy of
Sciences of the United States of America, vol. 115, no. 2, 2017.
[4] C. T. Russell, E. Chou et al., “Solar and interplanetary control of the location of the Venus bow shock,” Journal of Geophysical Research, vol. 93, no. A6, 1988.
[5] P. R. Gazis, “Solar cycle variation in the heliosphere,” Reviews of Geophysics, vol. 34, no. 3,  1996.
[6] A. Bader, G. Stenberg Wieser et al., “Proton Temperature Anisotropies in the Plasma Environment of Venus,” Journal of Geophysical Research: Space
Physics, vol. 124, no. 5, 2019.
[7] M. Volwerk, D. Schmid et al., “Mirror mode waves in Venus’s magnetosheath: Solar minimum vs. solar maximum,” Annales Geophysicae, vol. 34, no. 11, 2016.