Exploring the Structure of the Local Interstellar Medium with the Interstellar Boundary Explorer (IBEX)
Final Report Abstract
The goal of the research project was a quantitative assessment of a model to explain the so-called ribbon in the all-sky maps of differential fluxes of energetic neutral atoms (ENAs). These ENAs originate in the interaction region between the solar wind and the local interstellar medium (ISM), and the most prominent feature in the all-sky ENA flux maps is the ENA ribbon, i.e. a band of enhanced differential fluxes detected with the Interstellar Boundary Explorer (IBEX) spacecraft. According to the model tested in this project, the ribbon is caused by an inhomogeneity of the ISM, namely by a so-called H-wave. The study contributed in two ways to the contemporary research regarding the outer heliosphere and the local ISM: First, the study helped to clarify the nature of the IBEX ribbon: While the extended modelling of the effect of an H-wave on the production of ENAs in the inner heliosheath (IHS) confirmed the possibilty of the existence of such structures in the ISM, the apparently absent motion of the ribbon over nearly a solar activity cycle appears to indicate that an H-wave may not be its cause. Second, a detailed kinetic modelling of the evolution of the pickup proton distribution and their interaction with interstellar neutral hydrogen atoms in the IHS was performed and improved our understanding of the interaction of the solar wind with its interstellar environment. The results also indicate that the analytical large-scale Parker model of the heliosphere is a useful approximation that can significantly simplify various studies. The investigation of H-waves had to remain limited to a linear dispersion study of waves in the local ISM was carried out in the framework of a Bachelor thesis on the basis of a two-fluid approximation for the plasma and neutral gas. What concerns the tasks assigned to the funded doctoral researcher, the project work developed according to the work plan defined in the proposal. We analysed the evolution of the velocity distribution of suprathermal pick-up protons, which represent a significant ENA source population, along the solar wind streamlines in the IHS and studied their influence on the form of the ENA energy spectra. This evolution was modelled both semi-analytically using socalled kappa-distributions and fully numerically by solving the pick-up ion transport equation. The simulations yielded the proton distributions as a function of location and particle speed and, after computing the production rates and subsequent line-of-sight integrations, the corresponding differential ENA fluxes. This way we were also able to determine the relative contribution of different regions of the IHS to these fluxes. The model results, described in detail in A. Sylla’s Ph.D. thesis, are not only useful in relation to meaurements made with the IBEX spacecraft but, by considering higher energies, also as predictions for the forthcoming follow-up mission IMAP, i.e. the Interstellar Mapping and Acceleration Probe. This work was mainly carried out in collaboration with scientists from Germany (Ruhr-Universität Bochum, Universität Bonn, and TU Berlin), also from the USA and resulted in a number of related joint publications.
Publications
- On the evolution of the κ-distribution of protons in the inner heliosheath, Journal of Geophysical Research 121, 8203, 2016
Fahr, H.J., Sylla, A., Fichtner, H., Scherer, K.
(See online at https://doi.org/10.1002/2016JA022561) - Testing the H-wave Hypothesis for the Origin of the IBEX Ribbon, Journal of Physics: Conference Series 767, article id. 012007, 2016
Fichtner, H., Sylla, A., Scherer, K., Fahr, H.J., Zönnchen, J.
(See online at https://doi.org/10.1088/1742-6596/767/1/012007) - An Improved Analytical Model of the Local Interstellar Magnetic Field: The Extension to Compressibility, The Astrophysical Journal 838, article id. 75, 2017
Kleimann, J., Röken, C., Fichtner, H.
(See online at https://doi.org/10.3847/1538-4357/aa5f15) - Uncertainties in the heliosheath ion temperatures, Annales Geophysicae 36, 37-46, 2018
Scherer, K., Fahr, H.J., Fichtner, H., Sylla, A., Richardson, J.D., Lazar, M.
(See online at https://doi.org/10.5194/angeo-36-37-2018) - Erforschung von Strukturen des lokalen interstellaren Mediums mit dem Interstellar Boundary Explorer (IBEX) / Exploration of structures of the local interstellar medium with the Interstellar Boundary Explorer (IBEX), Ph.D. thesis, Ruhr-Universität Bochum, 2019
Sylla, A.
