Breaking correlation of the interstellar neutral He flow parameters and determining the total ionization rate using the adjustable boresight of the IMAP-Lo detector

The temperature of interstellar neutral (ISN) helium and its flow vector relative to the Sun can be determined by direct sampling of the incoming atoms by a spaceborne sensor, like e.g., IBEX-Lo. However, because this sensor can only measure the flow direction of atoms at the detector but not their speed, and IBEX can observe the flow only for a short period each spring, a correlation between the flow direction, speed, and temperature of ISN He gas in the heliosphere appeared in the analysis. Here, we show that this parameter correlation can be removed when interstellar He is sampled over a sufficiently long portion of the detector's orbit around the Sun. This capability is obtained on the Interstellar Mapping and Acceleration Probe (IMAP) with an IMAP-Lo pivot platform that allows to adjust the boresight relative to the spacecraft spin axis in flight. This capability significantly extends the time when interstellar atoms are accessible for observations in comparison with IBEX-Lo, which can only look perpendicular to the spacecraft spin axis. Furthermore, while the ionization rate of neutral atoms can be assessed from observations of the solar EUV radiation and solar wind or via proxies, electron impact ionization is not accessible. We show that the total ionization rate of ISN gas can be deduced when atoms in both the direct and indirect trajectories are observed. This can be achieved using the adjustable boresight of IMAP-Lo. Since atoms in the indirect beam are observed after penetrating regions inside 1 au, this observation provides insight into electron-impact ionization closer to the Sun, and thus to the physical state of solar wind electrons in a region up to now hardly accessible to observations. We present observation requirements for a detector with IMAP-Lo capabilities to maximize the effectiveness in breaking the ISN gas flow parameter correlation and independent determination of the ionization rate for ISN gas via observations of indirect beam atoms