2022/01/14 by P. Swaczyna, M. A. Kubiak, M. Bzowski +9 · 36 citations
Physics and Astronomy · #Astrobiology #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Atomic physics #Helium #Interstellar medium #Physics #Solar and Space Plasma Dynamics #Stellar, planetary, and galactic studies #astro-ph.EP #astro-ph.GA #astro-ph.SR #physics.space-ph
paper · pdf · doi:10.3847/1538-4365/ac4bde
published in The Astrophysical Journal Supplement Series 259(2), 42 (Institute of Physics) · 31 pages, 6 figures, 8 tables, accepted for publication in ApJS
arxiv created 2022/01/14 · openalex publication_date 2022/03/23 · arxiv updated 2022/03/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close=")" open="("> <mml:mrow> <mml:mi>λ</mml:mi> <mml:mo>,</mml:mo> <mml:mi>β</mml:mi> </mml:mrow> </mml:mfenced> <mml:mo>=</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>255</mml:mn> <mml:mo>.°</mml:mo> <mml:mn>59</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0</mml:mn> <mml:mo>.°</mml:mo> <mml:mn>23</mml:mn> <mml:mo>,</mml:mo> <mml:mn>5</mml:mn> <mml:mo>.°</mml:mo> <mml:mn>14</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0</mml:mn> <mml:mo>.°</mml:mo> <mml:mn>08</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:math> , 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.