Interstellar Conditions Deduced from Interstellar Neutral Helium Observed by IBEX and Global Heliosphere Modeling

Abstract

In situ observations of interstellar neutral (ISN) helium atoms by the IBEX-Lo instrument on board the Interstellar Boundary Explorer (IBEX) mission are used to determine the velocity and temperature of the pristine very local interstellar medium (VLISM). Most ISN helium atoms penetrating the heliosphere, known as the primary population, originate in the pristine VLISM. As the primary atoms travel through the outer heliosheath, they charge exchange with He+ ions in slowed and compressed plasma, creating the secondary population. With more than 2.4 million ISN helium atoms being sampled by IBEX during ISN seasons 2009–2020, we compare the observations with the predictions of a parameterized model of ISN helium transport in the heliosphere. We account for the filtration of ISN helium atoms at the heliospheric boundaries by charge-exchange and elastic collisions. We examine the sensitivity of the ISN helium fluxes to the interstellar conditions described by the pristine VLISM velocity, temperature, magnetic field, and composition. We show that comprehensive modeling of the filtration processes is critical for interpreting ISN helium observations, as the change in the derived VLISM conditions exceeds the statistical uncertainties when accounting for these effects. The pristine VLISM parameters found by this analysis are the flow speed (26.6 km s−1), inflow direction in ecliptic coordinates (255.°7, 5.°04), temperature (7350 K), and B − V plane inclination to the ecliptic plane (53.°7). The derived pristine VLISM He+ density is 9.7 × 10−3 cm−3. Additionally, we show a strong correlation between the interstellar plasma density and magnetic field strength deduced from these observations.

Influence of Heliolatitudinal Anisotropy of Solar FUV/EUV Emissions on Lyα Helioglow: SOHO/SWAN Observations and WawHelioGlow Modeling

“…The Sun is moving through the interstellar medium with a velocity of ∼26 km s −1 towards the ecliptic longitude of ∼256 • and the latitude of ∼5 • (Lallement et al 2004;Witte 2004;Bzowski et al 2008Bzowski et al , 2015Schwadron et al 2015;Swaczyna et al 2023;Bzowski et al 2023). The interaction of the solar wind and the local interstellar matter leads to the formation of the heliosphere, bounded by the heliopause, which is a separatrix for the he-liospheric and interstellar plasma flows (Baranov 1990;Ratkiewicz et al 1998;Izmodenov 2000;Pogorelov et al 2004).…”

Science Opportunities for IMAP-Lo Observations of Interstellar Neutral Hydrogen and Deuterium during a Maximum of Solar Activity

“…However, simulations of the flux of ISN He for the IBEX-Lo orbits for which ISN D is expected have never been performed before. The alternative model for ISN He is the so-called synthesis method, developed by Bzowski et al (2017) and Bzowski et al (2019), and subsequently used by Kubiak et al (2019) and Swaczyna et al (2023b). In this approach, it is assumed that the distribution function of ISN He is a Maxwell-Boltzmann function far ahead to the heliosphere (at ∼1000 au).…”

The Effect of Angular Scattering Imposed by Charge Exchange and Elastic Collisions on Interstellar Neutral Hydrogen Atoms

“…This appendix briefly describes the adopted boundary conditions of the global heliosphere model far from the Sun. A more detailed discussion can be found in Swaczyna et al (2023aSwaczyna et al ( , 2023c. The inner boundary conditions are selected based on the average calculated from the OMNI collection for four solar cycles before the maximum of solar cycle 24.…”