Proton Temperature Anisotropies in the Venus Plasma Environment during Solar Minimum and Maximum
Authors: Rojas Mata Sebastián , Stenberg Wieser Gabriella , Futaana Yoshifumi , Bader Alexander , Persson Moa , Fedorov Andrey , Zhang Tielong 
Published: March 2022

&lt;p&gt;Venus&amp;#8217; lack of an intrinsic magnetic field allows the solar wind to closely interact with its atmosphere [1], making it a&lt;br&gt;prime target for investigating how unmagnetized atmospheric bodies in our Solar System [2] or elsewhere [3] interact&lt;br&gt;with magnetized plasma flows. This close interaction means that solar-activity correlations exhibited by the solar wind and&lt;br&gt;other heliospheric parameters [4, 5] cause solar-cycle variations in Venus&amp;#8217; plasma environment and plasma phenomena. We&lt;br&gt;investigate these variations by characterizing the proton population around Venus during periods of solar minimum (2006&amp;#8211;2009)&lt;br&gt;and maximum (2010&amp;#8211;2014). We use data from the Ion Mass Analyser (IMA) instrument, a particle mass-energy spectrometer&lt;br&gt;which was onboard the Venus Express (VEX) mission. We apply a previously developed methodology which fits Maxwellian&lt;br&gt;models to measurements of the protons&amp;#8217; velocity distribution functions [6] to produce statistical distributions of bulk speeds and&lt;br&gt;temperatures in various regions of Venus&amp;#8217; plasma environment. We also present spatial maps and probability-density histograms&lt;br&gt;comparing the proton parameters between the two time periods.&lt;br&gt;We find that the temperatures perpendicular (T&lt;sub&gt;&amp;#8869;&lt;/sub&gt;) and parallel (T&lt;sub&gt;&lt;/sub&gt;) to the background magnetic field are 20&amp;#8211;35% lower&lt;br&gt;in the magnetosheath during solar maximum. This suggests that the heating of particles as they cross the bow shock varies&lt;br&gt;between the two time periods. We also find that the regions in the magnetosheath with highest temperature ratio T&lt;sub&gt;&amp;#8869;&lt;/sub&gt;/T&lt;sub&gt;&lt;/sub&gt; are&lt;br&gt;farther downstream from the bow shock during solar maximum than minimum. This is consistent with previous observations of&lt;br&gt;how mirror-mode structures presumably generated at the bow shock strictly decay as they are convected into the magnetosheath&lt;br&gt;during solar minimum, whereas during solar maximum they first grow and then decay [7]. We also present ongoing work to&lt;br&gt;further characterize the plasma environment as a function of upstream solar-wind parameters (such as Mach number or cone&lt;br&gt;angle) and bow shock geometry. We discuss preliminary results concerning energy conversion processes at Venus&amp;#8217; bow shock.&lt;/p&gt;&lt;p&gt;&lt;br&gt;REFERENCES&lt;br&gt;[1] Y. Futaana, G. Stenberg Wieser et al., &amp;#8220;Solar Wind Interaction and Impact on the Venus Atmosphere,&amp;#8221; Space Science Reviews, vol. 212, no. 3-4,&amp;#160;2017.&lt;br&gt;[2] C. Bertucci, F. Duru et al., &lt;em&gt;The induced magnetospheres of mars, venus, and titan&lt;/em&gt;, 2011, vol. 162, no. 1-4.&lt;br&gt;[3] C. Dong, M. Jin et al., &amp;#8220;Atmospheric escape from the TRAPPIST-1 planets and implications for habitability,&amp;#8221; Proceedings of the National Academy of&lt;br&gt;Sciences of the United States of America, vol. 115, no. 2, 2017.&lt;br&gt;[4] C. T. Russell, E. Chou et al., &amp;#8220;Solar and interplanetary control of the location of the Venus bow shock,&amp;#8221; Journal of Geophysical Research, vol. 93, no. A6, 1988.&lt;br&gt;[5] P. R. Gazis, &amp;#8220;Solar cycle variation in the heliosphere,&amp;#8221; Reviews of Geophysics, vol. 34, no. 3,&amp;#160; 1996.&lt;br&gt;[6] A. Bader, G. Stenberg Wieser et al., &amp;#8220;Proton Temperature Anisotropies in the Plasma Environment of Venus,&amp;#8221; Journal of Geophysical Research: Space&lt;br&gt;Physics, vol. 124, no. 5, 2019.&lt;br&gt;[7] M. Volwerk, D. Schmid et al., &amp;#8220;Mirror mode waves in Venus&amp;#8217;s magnetosheath: Solar minimum vs. solar maximum,&amp;#8221; Annales Geophysicae, vol. 34, no. 11, 2016.&lt;/p&gt;