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

Abstract

<p>Venus’ lack of an intrinsic magnetic field allows the solar wind to closely interact with its atmosphere [1], making it a<br>prime target for investigating how unmagnetized atmospheric bodies in our Solar System [2] or elsewhere [3] interact<br>with magnetized plasma flows. This close interaction means that solar-activity correlations exhibited by the solar wind and<br>other heliospheric parameters [4, 5] cause solar-cycle variations in Venus’ plasma environment and plasma phenomena. We<br>investigate these variations by characterizing the proton population around Venus during periods of solar minimum (2006–2009)<br>and maximum (2010–2014). We use data from the Ion Mass Analyser (IMA) instrument, a particle mass-energy spectrometer<br>which was onboard the Venus Express (VEX) mission. We apply a previously developed methodology which fits Maxwellian<br>models to measurements of the protons’ velocity distribution functions [6] to produce statistical distributions of bulk speeds and<br>temperatures in various regions of Venus’ plasma environment. We also present spatial maps and probability-density histograms<br>comparing the proton parameters between the two time periods.<br>We find that the temperatures perpendicular (T<sub>⊥</sub>) and parallel (T<sub></sub>) to the background magnetic field are 20–35% lower<br>in the magnetosheath during solar maximum. This suggests that the heating of particles as they cross the bow shock varies<br>between the two time periods. We also find that the regions in the magnetosheath with highest temperature ratio T<sub>⊥</sub>/T<sub></sub> are<br>farther downstream from the bow shock during solar maximum than minimum. This is consistent with previous observations of<br>how mirror-mode structures presumably generated at the bow shock strictly decay as they are convected into the magnetosheath<br>during solar minimum, whereas during solar maximum they first grow and then decay [7]. We also present ongoing work to<br>further characterize the plasma environment as a function of upstream solar-wind parameters (such as Mach number or cone<br>angle) and bow shock geometry. We discuss preliminary results concerning energy conversion processes at Venus’ bow shock.</p><p><br>REFERENCES<br>[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.<br>[2] C. Bertucci, F. Duru et al., <em>The induced magnetospheres of mars, venus, and titan</em>, 2011, vol. 162, no. 1-4.<br>[3] C. Dong, M. Jin et al., “Atmospheric escape from the TRAPPIST-1 planets and implications for habitability,” Proceedings of the National Academy of<br>Sciences of the United States of America, vol. 115, no. 2, 2017.<br>[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.<br>[5] P. R. Gazis, “Solar cycle variation in the heliosphere,” Reviews of Geophysics, vol. 34, no. 3,  1996.<br>[6] A. Bader, G. Stenberg Wieser et al., “Proton Temperature Anisotropies in the Plasma Environment of Venus,” Journal of Geophysical Research: Space<br>Physics, vol. 124, no. 5, 2019.<br>[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.</p>