Authors: Sarah A. Spitzer (University of Michigan), Susan T. Lepri (University of Michigan), Jim M. Raines (University of Michigan), Frederic Allegrini (Southwest Research Institute), Stefano Livi (Southwest Research Institute, University of Michigan), Jason A. Gilbert (University of Michigan), Connor P. Raines (University of Michigan), Austin N. Glass (University of Michigan), Keiichi Ogasawara (Southwest Research Institute), Mark Phillips (Southwest Research Institute)

Solar Orbiter was launched on 9 February, 2020 as a collaborative effort between ESA and NASA, and is an instrumental mission utilizing both remote sensing and in situ measurements to study how the Heliosphere is created and shaped by the Sun. Solar Orbiter provides a unique opportunity to observe the Sun away from its equator, reaching orbits as close to the Sun as ~2.8 AU at up to 33 degrees out of the ecliptic in the extended mission. The Solar Orbiter Heavy Ion Sensor (SO-HIS) is an in situ triple-coincidence ion mass spectrometer intended to measure ion and elemental composition as well as 3D velocity distribution functions of ions in the range of He ‒ Fe with charge-states from He+ to Fe20+ in the full energy-per-charge (E/q) range 0.5 ‒ 80 keV/e. SO-HIS measures the bulk solar wind in the range 0.5 ‒ 18 keV/e and the suprathermal solar wind components of He, C, O, and Fe up to at least 60 keV/e in addition to pickup ions. SO-HIS uses 64 E/q steps with an energy resolution of 6 ‒ 10% and 16 elevation steps in the range +/-17 degrees with an angular resolution of <3.5 degrees and has a continuous azimuthal acceptance in the range -30 ‒ +66 degrees. We create a high resolution ion optical model (IOM) of the SO-HIS instrument from its CAD model using the SIMION software. Using the SIMION IOM, we determine the instrument’s geometric factor (GF) as well as characterize the angular acceptances at each elevation bin. We validate SIMION outputs against laboratory measurements and present a function of the instrument’s GF under standard solar wind conditions using the spaceflight voltages. We additionally include higher energy ions as well as angles outside the radial solar wind direction. Using laboratory measurements, we additionally characterize the instrument’s detector efficiencies.