Very Local Interstellar Medium Revealed by a Complete Solar Cycle of Interstellar Neutral Helium Observations with IBEX

Abstract

The IBEX-Lo instrument on board the Interstellar Boundary Explorer (IBEX) mission samples interstellar neutral (ISN) helium atoms penetrating the heliosphere from the very local interstellar medium (VLISM). In this study, we analyze the IBEX-Lo ISN helium observations covering a complete solar cycle, from 2009 through 2020 using a comprehensive uncertainty analysis including statistical and systematic sources. We employ the Warsaw Test Particle Model to simulate ISN helium fluxes at IBEX, which are subsequently compared with the observed count rate in the three lowest energy steps of IBEX-Lo. The χ 2 analysis shows that the ISN helium flows from ecliptic λ , β = ( 255 .° 59 ± 0 .° 23 , 5 .° 14 ± 0 .° 08 ) , with speed v HP = 25.86 ± 0.21 km s−1 and temperature T HP = 7450 ±140 K at the heliopause. Accounting for gravitational attraction and elastic collisions, the ISN helium speed and temperature in the pristine VLISM far from the heliopause are v VLISM = 25.9 km s−1 and T VLISM = 6150 K, respectively. The time evolution of the ISN helium fluxes at 1 au over 12 yr suggests significant changes in the IBEX-Lo detection efficiency, higher ionization rates of ISN helium atoms in the heliosphere than assumed in the model, or an additional unaccounted for signal source in the analyzed observations. Nevertheless, we do not find any indication of the evolution of the derived parameters of ISN helium over the period analyzed. Finally, we argue that the continued operation of IBEX-Lo to overlap with the Interstellar Mapping and Acceleration Probe will be pivotal in tracking possible physical changes in the VLISM.

One Solar Cycle of Heliosphere Observations with the Interstellar Boundary Explorer: Energetic Neutral Hydrogen Atoms Observed with IBEX-Lo from 10 eV to 2 keV

“…One step absent from this list is the subtraction of the count rates caused by the ISN helium and hydrogen from the ENA maps in energy bins 1-4 around the ISN inflow direction. Such an approach would require a specific model of ISN He, which has to take into account all the physics of ISN He and H outside and inside the heliosphere (see, e.g., Kubiak et al 2014;Galli et al 2019;Swaczyna et al 2022). Because the ISN He inflow creates orders of magnitude more count rates than the ENA signal at low energies (see the wedge-shaped areas of high count rates in Figure 3), Poisson uncertainties in pixels dominated by this ISN signal are larger than the underlying ENA signal.…”

“…Because of limited statistics at low ENA energies, the finest spatial resolution of the maps is limited to 6°× 6°pixels. Due to a star tracker anomaly in May 2016, the pointing from orbit 326 onward is affected by an offset of 0°.6 (Swaczyna et al 2022). This is not an issue for ENA maps and derived products because this uncertainty is much smaller than the statistical scatter.…”

“…This despinning removes the drift to a large extent but the process introduces pointing inaccuracies on the order of 0°.1. For that reason, despun data are usually omitted from ISN-related studies (see, e.g., Swaczyna et al 2022). This inaccuracy is not an issue for heliospheric ENA studies because the finest spatial resolution is 6°× 6°.…”

The Heliosphere and Local Interstellar Medium from Neutral Atom Observations at Energies Below 10 keV

“…The 12 years of IBEX observations give the best-fit inflow direction in ecliptic coordinates, speed , and temperature (Swaczyna et al. 2022 ).…”

“…The pristine VLISM speed and temperature accounting for these collisions and gravitational attraction beyond the heliopause is , and temperature Swaczyna et al. ( 2022 ).…”