Very Local Interstellar Medium Revealed by 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.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 $(\lambda ,\beta )=(255.59^{\circ }\pm 0.23^{\circ },5.14^{\circ }\pm 0.08^{\circ })$, with speed $v_{\text{HP}}=25.86\pm 0.21$ km s${}^{-1}$ and temperature $T_{\text{HP}}=7450\pm 140$ 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 $v_{\text{VLISM}}=25.9$ 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.

arXiv comment31 pages, 6 figures, 8 tables, accepted for publication in ApJS

References in corpus (28)

• Atomic Data for Astrophysics. II. New Analytic Fits for Photoionization Cross Sections of Atoms and Ions
• Interstellar neutral helium in the heliosphere from IBEX observations. III. Mach number of the flow, velocity vector, and temperature from the first six years of measurements
• Warm Breeze from the starboard bow: a new population of neutral helium in the heliosphere
• Revisiting Ulysses Observations of Interstellar Helium
• Solar parameters for modeling interplanetary background
• Interstellar neutral helium in the heliosphere from IBEX observations. IV. Flow vector, Mach number, and abundance of the Warm Breeze
• Local Interstellar Neutral Hydrogen sampled in-situ by IBEX
• Neutral interstellar He parameters in front of the heliosphere 1994--2007
• Sun-Heliosphere Observation-based Ionization Rates Model
• Interstellar neutral helium in the heliosphere from IBEX observations. II. The Warsaw Test Particle Model (WTPM)
• Interstellar neutral helium in the heliosphere from IBEX observations. VI. The He${}^{+}$ density and the ionization state in the Very Local Interstellar Matter
• Interstellar Neutral Gas Species And Their Pickup Ions Inside The Heliospheric Termination Shock. Ionization Rates For H, O, Ne, And He
• On the Decades-Long Stability of the Interstellar Wind through the Solar System
• The helium Warm Breeze in IBEX observations as a result of charge exchange collisions in the outer heliosheath
• Interstellar Neutral Helium in the Heliosphere from IBEX Observations. I. Uncertainties and Backgrounds in the Data and Parameter Determination Method
• Interstellar neutral helium in the heliosphere from IBEX observations. V. Observations in IBEX-Lo ESA steps 1, 2, & 3
• Interstellar Neutral Gas Species And Their Pickup Ions Inside The Heliospheric Termination Shock. The Large-scale Structures
• Interstellar hydrogen fluxes measured by IBEX-Lo in 2009: numerical modeling and comparison with the data
• The Interstellar Neutral He haze in the heliosphere: what can we learn?
• The downwind hemisphere of the heliosphere: Eight years of IBEX-Lo observations
• Slowdown and Heating of Interstellar Neutral Helium by Elastic Collisions Beyond the Heliopause
• Precision Pointing of IBEX-Lo Observations
• Correcting the record on the analysis of IBEX and STEREO data regarding variations in the neutral interstellar wind
• Reconstruction of Solar Extreme Ultraviolet Flux 1740-2015
• Angular Scattering in Charge Exchange: Issues and Implications for Secondary Interstellar Hydrogen
• Evidence for Asymmetry in the Velocity Distribution of the Interstellar Neutral Helium Flow Observed by IBEX and Ulysses
• Science Opportunities from Observations of the Interstellar Neutral Gas with Adjustable Boresight Direction
• Synthesis of the distribution function of ISN He

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