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

TL;DR

This study analyzes 12 years of IBEX-Lo data to characterize the local interstellar medium's helium flow, revealing stable parameters and emphasizing the importance of continued observations for understanding VLISM dynamics.

Contribution

First comprehensive analysis of a complete solar cycle of IBEX-Lo interstellar helium data, providing refined flow parameters and insights into VLISM conditions.

Findings

ISN helium flow from ecliptic coordinates (255.59°, 5.14°)

Flow speed of 25.86 km/s and temperature of 7450 K at the heliopause

No significant evolution of ISN helium parameters over 12 years

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.W e 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 $\chi^2$ analysis shows that the ISN helium flows from ecliptic $(\lambda,\beta)=(255.59^{\circ}\pm0.23^{\circ}, 5.14^{\circ}\pm0.08^{\circ})$, with speed $v_\text{HP}=25.86\pm0.21$ km s$^{-1}$ and temperature $T_\text{HP}=7450\pm140$ 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 $v_\text{VLISM}=25.9$ km s$^{-1}$ and $T_\text{VLISM}=6150$ K, respectively. The time evolution of the ISN helium fluxes at 1 au over 12 years 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 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 (IMAP) will be pivotal in tracking possible physical changes in the VLISM.