Characterizing Heliospheric and Interstellar Interactions Using Pickup Ion Measurements

The Heliosphere, orbiting the galactic center of the Milky Way, is a plasma bubble carved out of the Local Interstellar Medium (LISM) by the Sun's dynamic and magnetic influence. Its relative motion creates a steady inflow of neutral particles and dust from the LISM through the Heliosphere, influencing the space environment throughout and impacting the entire Solar System, including the Earth. This thesis addresses the question: How does the Heliosphere - Local Interstellar Medium interaction evolve over time? These Heliosphere-interstellar interactions can be studied in situ using measurements of pickup ions (PUIs), ions originating from neutrals, such as interstellar neutrals, ionized through photoionization, electron impact ionization, or charge exchange with the solar wind. This thesis presents measurements of the longitudinal inflow direction using He+ PUI measurements in the downwind focusing cone. This new method is validated, measuring a flow direction comparable to recent literature studies of 75.37 deg +/-0.43 deg over the ACE/SWICS data set including focusing cone crossings in the years 1998 through 2010. This method is applied to 3-orbit boxcar averages of combined count distributions with center years 1999 through 2009 to measure the flow direction trend over an 11-year solar cycle. A trend of 0.00 deg +/-0.51 deg is measured, indicating that no evidence is found of a longitudinal flow direction variation over this 11-year period. This thesis additionally discusses ongoing efforts to further characterize Heliosphere-interstellar interactions by measuring a more precise yearly flow direction using upwind He+ PUI measurements, as well as a characterization of new spaceflight hardware and challenges associated with current analysis and measurement techniques. Future studies to characterize these interactions will be greatly enhanced with improved measurements implementing new instrument designs and techniques, such as by the Solar Orbiter Heavy Ion Sensor (SO-HIS) launched as a joint ESA/NASA mission on 9 February, 2020. SO-HIS is an in situ time-of-flight triple-coincidence ion mass spectrometer with elevation selection measuring elemental composition and 3D velocity distribution functions. Its higher cadence, specifically at the 30 s and 4 s resolutions, along with the ability to resolve angular measurements, stepping through 16 elevation steps and having an increased continuous azimuthal acceptance of -30 deg to +66 deg to include more of the PUI population off the Sun–spacecraft line in the ecliptic, will greatly improve the future available PUI data set for use in studying Heliosphere-interstellar interactions. This thesis presents the cross-calibration of SO-HIS with its SIMION ion optical model to characterize the geometric factor, a measure of the instrument's useful particle intake geometry, under standard solar wind conditions. Validation of the model with laboratory measurements is presented, along with a function of the instrument's geometric factor for elevations from -15 deg to +18 deg, +/-3 deg, for standard solar wind energies and azimuths. This characterization will allow the measurements taken by the SO-HIS instrument to be converted to higher level data products useful for studying the in situ plasma, including PUIs. Solar Orbiter, with the SO-HIS instrument, having a trajectory up to ~30 deg out of the ecliptic in the extended mission, as well as future missions and advancements, will enable further studies of Heliosphere–interstellar interactions, including measuring precise yearly interstellar flow directions and mapping and tracking the 3D structures of the of the interstellar He+ pickup ion distribution in the inner Heliosphere.PhDClimate and Space Sciences and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/172694/1/saraylet_1.pd