Measuring Pickup Ion Distributions with the Solar Orbiter Heavy Ion Sensor

An inflow of interstellar neutral particles known as the interstellar wind is created as the Sun moves through the interstellar medium. This influences the composition of the Heliosphere, or the region of space affected by the Sun. After these neutrals enter the Heliosphere, they may become ionized by processes such as photoionization, electron impact ionization, and charge exchange with the solar wind. These “pickup” ions propagate radially outward with the solar wind. Studying distributions of these pickup ions is one of the most accessible means of studying Heliospheric ‒ interstellar interactions. One such feature that we measure to determine characteristics of this flow interaction is the helium focusing cone. Due to the high first ionization potential of helium, the focusing cone, a signature of enhanced helium which has been gravitationally focused on the downwind side of the Sun, leads to a measurable enhancement in the pickup ion population out to 1 AU. The Solar Orbiter Heavy Ion Sensor (SO-HIS) measures ions in the range H+ to Fe20+ with sufficient energy and angular ranges to measure characteristic signatures of pickup ions in the velocity distribution. To achieve our aim to study Heliospheric ‒ interstellar interactions using in situ pickup ion measurements, we present a characterization of the SO-HIS instrument required to enable identification of pickup ions in the data. We characterize the geometric factor and solid state energy detector efficiencies and develop validated pickup ion distribution measurements.