Date of Award

Winter 2024

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Nathan A Schwadron

Second Advisor

Eberhard Moebius

Third Advisor

Shawna Hollen

Abstract

The Interstellar Boundary Explorer (IBEX) mission is playing a pivotal role in advancing our understanding of the interaction between the very local interstellar medium (VLISM) and the heliosphere. By sampling interstellar neutral (ISN) atoms and energetic neutral atoms (ENA), IBEX has provided over 15 years of continuous data over a range of energy from 15 eV to 6 keV, offering an invaluable opportunity to study these interactions across various phases of solar activity. The IBEX-Lo instrument, which detects lower-energy ISNs, has been particularly useful in exploring the properties of hydrogen and helium in the interstellar medium, as well as their interactions with the boundaries of the heliosphere.

IBEX-Lo has produced annual count rates and exposure time maps for ISN hydrogen and helium at energy steps 2 (29 eV) and 3 (55 eV) throughout solar cycle 24 (2009–2020). These maps, now publicly available, qualitatively align with model predictions and highlight key patterns in the ISN data. For instance, they show a remarkable decrease in the visibility of secondary helium during solar maximum. Similarly ISN hydrogen, which was observed during the rising and falling phases of solar activity, disappearing during solar maximum and reappearing as activity diminished towards solar cycle 25. These results further contribute to the broader understanding of how interstellar material interacts with the solar wind and how solar activity influences this interaction.

In addition to the primary ISN helium dataset, IBEX-Lo has observed secondary interstellar helium. Systematic analysis of these observations has revealed a significant discrepancy between the simulated and the observed secondary helium, particularly during solar cycle 24. In this study, we show that this discrepancy is naturally explained by the elastic scattering of primary helium atoms by the solar wind protons, which redistributes helium atoms from the core of the flux to form a halo in the wings of the distribution. Correcting for this elastic scattering requires an increase by approximately 12\% of the density of primary helium in the interstellar medium when detected directly at 1 au.

By addressing these key aspects of the interstellar medium's interaction with the heliosphere, IBEX has provided critical data that enhances our understanding of the VLISM and its role in shaping the solar system’s boundaries. The findings from IBEX-Lo, combined with simulations that incorporate elastic scattering effects, offer a refined estimate of neutral interstellar helium density, advancing both heliophysics and astrophysics. These insights will also serve as a foundation for future missions like the Interstellar Mapping and Acceleration Probe (IMAP), which will further investigate ISN atoms and their dynamics at the edge of the solar system.

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