Ongoing project
Evolution of the outer heliosphere seen in neutral atom fluxes
- Grant No. 2023/51/D/ST9/0126
- Funder: National Science Centre, Poland (NCN)
- Program: SONATA 19
- Grant value: 878 421 PLN
- Host institution: Space Research Centre of the Polish Academy of Sciences
- Implementation period: October 2024 – September 2027
- Project team:
- Dr. Paweł Swaczyna, Principal Investigator
- Dr. Małgorzata Antonik, Post-doc
General public abstract
The space between planets and stars is filled with a low-density, partially ionized plasma. Close to Earth, the main source of this interplanetary medium is the Sun, which continuously emits fully ionized plasma called the solar wind. The solar wind expands from the Sun with a very high speed of ~400-800 km/s, blowing away the interstellar plasma from the solar system, creating the heliosphere. The heliosphere extends over the entire solar system, reaching more than 100 times further away from the Sun than Earth’s orbit. The Sun and the solar wind change over the solar cycle, resulting in dynamic heliosphere changes. The studies of the heliosphere are complex because only a few spacecraft have left the solar system and can detect the particles in the solar wind plasma far away from the Sun. One such spacecraft is New Horizons, which continues its mission through the heliosphere after passing Pluto. Voyagers had measured the heliosphere plasma, but they crossed the heliopause – the heliosphere outer boundary – and continued their journey in the interstellar medium. While in situ measurements from New Horizons and Voyagers are critical for studying the processes operating in the outer heliosphere plasma, the heliosphere is also imaged globally using energetic neutral atoms.
Energetic neutral atoms are created in all regions in the heliosphere from energetic ions that capture electrons from ambient interstellar neutral atoms present in all regions of the heliosphere. As all charged particles, ions are subject to electromagnetic forces, and thus, their dynamic is governed by the plasma’s global flow. On the other hand, neutral atoms are only subject to gravitational forces and can travel through the heliosphere on ballistic trajectories. Consequently, energetic neutral atoms detected close to the Sun may inform about the state of the plasma from which they originate. These atoms are detected currently by instruments on NASA’s Interstellar Boundary Explorer (IBEX) mission. This small satellite scans through the sky, producing images of the heliosphere every year. The first results of the mission showed that in addition to the predicted broad structure from the heliosphere, there is another narrow circular feature in the sky called the IBEX ribbon. This feature is created outside the heliopause in the local interstellar medium and shows us the orientation of the interstellar magnetic field.
In the project, we plan to analyze the observations of energetic neutral atoms to understand how the heliosphere changes in response to the solar wind evolution caused by the solar cycle and long-term changes. We will try to find the heliosphere’s shape from observations of energetic neutral helium atoms, which should reveal the structure of the heliospheric tail. Such observations are not currently available from IBEX but should be possible with the next-generation instruments on the Interstellar Mapping and Acceleration Probe (IMAP) mission. We will carefully investigate those observations with newly developed tools. Moreover, we plan to understand the source of energization of ions in the solar wind as observed by the New Horizons mission.
The outer heliosphere and its interstellar surrounding revealed in neutral atom and pickup ion observations
- Grant No. BPN/PPO/2022/1/00017 (NAWA) & 2023/02/1/ST9/00004 (NCN)
- Funder: Polish National Agency for Academic Exchange (NAWA) & NCN (research component)
- Program: Polish Returns 2022
- Grant value: 2 376 000 PLN (NAWA) & 199 996 PLN (NCN)
- Host institution: Space Research Centre of the Polish Academy of Sciences
- Implementation period: May 2023 – April 2027
- Project team:
- Dr. Paweł Swaczyna, Principal Investigator
- Dr. Małgorzata Antonik, Post-doc
- Dr. Marcin Bury, Physicist
- Find related publications in SAO/NASA ADS
Abstract
The heliosphere inflated by the solar wind in the very local interstellar medium (VLISM) protects the solar system from interstellar charged particles, while interstellar neutral (ISN) atoms propagate through the heliosphere. Inside the heliosphere, some ISN atoms are ionized, creating a population of pickup ions (PUIs) propagating through the heliosheath. Most PUIs are eventually neutralized and become energetic neutral atoms (ENAs). ISN atoms and ENAs that survive travel to 1 au are detected by dedicated instruments on space missions such as IBEX or IMAP. In the proposed project, we will analyze observations of ISN atoms, PUIs, and ENAs to learn about the structure of the outer heliosphere and the VLISM. We will study the temporal evolution of PUIs and ENAs over the solar cycle. We will find how the ISN atoms are modified in the heliospheric boundaries and estimate the physical conditions of the VLISM, which we will interpret in the context of astrophysical observations.
Completed projects
Angular Scattering of Neutral Atoms: Observations and Interpretation from IBEX and Consequences for IMAP
Any opinions, findings, conclusions or recommendations expressed in this website are those of the author and do not necessarily reflect the views of NASA.
- Grant No. 80NSSC20K0781
- Funder: National Aeronautics and Space Administration (NASA)
- Program: ROSES-2019 Outer Heliosphere Guest Investigators Program
- Grant value: 416 921 USD
- Host institution: Princeton University
- Implementation period: May 2020 – April 2023
- Project team:
- Dr. Paweł Swaczyna, Princeton University, Principal Investigator
- Dr. Eric J. Zirnstein, Princeton University, Co-Investigator
- Dr. Fatemeh Rahmanifard, University of New Hampshire, Co-Investigator
- Dr. Jacob Heerikhuisen, University of Waikato, New Zealand, Collaborator
- Find related publications in SAO/NASA ADS
Proposal summary
Science goals and objectives:
Interstellar neutral (ISN) atoms penetrating the heliosphere provide a remote diagnostic of the very local interstellar medium (VLISM). Fluxes of ISN atoms are modified in the outer heliosheath by charge exchange (CX) collisions, which result in the production of secondary ISN atoms. A large part of our understanding of the VLISM and the outer heliosphere comes from the attenuation of the ISN signal as observed at 1 au to the source that lies outside the heliosphere. For this reason, details of the CX process are crucial for interpretating the plasma properties in the heliosphere and its vicinity. So far, analyses of the ISN atom observations from IBEX-Lo and Ulysses/GAS have neglected elastic scattering and assumed that particles in CX collisions conserve momenta of parent particles, i.e., that there is no angular scattering in these collisions. Recent theoretical calculations of the differential CX cross section between protons and hydrogen atoms show that this assumption is not valid for collision speeds of the order of tens of kilometers per second, as expected in the outer heliosheath, and hence this process impacts the distribution function of secondary atoms. We will quantitatively verify the importance of angular scattering for the populations of ISN atoms observed by IBEX-Lo and in the future by IMAP-Lo. We will answer the following scientific questions:
- How does angular scattering in CX and elastic collisions change distribution functions of ISN atoms at 1 au? How is this change reflected in IBEX-Lo observations?
- Are physical parameters of the VLISM obtained from analyses of the ISN helium observations from IBEX-Lo affected by angular scattering in the outer heliosheath?
- Do IBEX-Lo observations support an out-of-equilibrium distribution of the pristine ISN atoms ahead of the heliosphere?
We will address whether discrepancies between models and IBEX-Lo observations manifested by statistically too high chi-square estimators are explained by angular scattering (Q1). We will further check if the lack of these physical mechanisms in the prior analyses resulted in biased parameters of interstellar flow deducted from ISN atom observations (Q2). Finally, we will determine (Q3) whether claims of non-Maxwellian distributions of the ISN helium population are supported in the light of angular scattering in the outer heliosheath. One reason for departure from equilibrium can be a close distance from the edge of the local interstellar cloud, as indicated by the UV observations of absorption lines of nearby stars.
Data:
We will use the ISN data (IBEX-Lo observations in the energy steps 1–3) published in the IBEX data releases 3, 6, 9, and 11. We also plan to make predictions for IMAP-Lo observations based on anticipated specifications.
Methodology:
We will utilize the existing forward model of ISN atom transport in the heliosphere and detection by the IBEX-Lo sensor (Schwadron et al. 2015, ApJS 220:25). Model ISN atom fluxes will be evolved due to CX and elastic collisions as they move through the outer heliosheath. We will use the plasma properties in the outer heliosheath resulting from the global simulations of the heliosphere (Zirnstein et al. 2016, ApJL 818:18). Model results and IBEX-Lo data will be analyzed using methods that account for multiple sources of IBEX-Lo uncertainties (Swaczyna et al. 2015, ApJS 220:26).
Relevance:
We will analyze ISN atom observations using methods beyond the IBEX mission to determine the physical state of the interstellar medium and find the role of angular scattering in the interaction of ISN atoms from the VLISM with the heliosphere. Therefore, this proposal directly addresses the interaction of the Sun with the interstellar medium included in Decadal Survey Goal 3: “Determine the interaction of the Sun with the solar system and the interstellar medium”, and as such is relevant for the OH-GI program.
Separation and Time-evolution of the Ribbon ENA Source Observed by IBEX
Any opinions, findings, conclusions or recommendations expressed in this website are those of the author and do not necessarily reflect the views of NASA.
- Grant No. 80NSSC21K0582
- Funder: National Aeronautics and Space Administration (NASA)
- Program: ROSES-2020 Heliophysics Guest Investigator “Open” Program
- Grant value: 303 485 USD
- Host institution: Princeton University
- Implementation period: April 2021 – March 2023
- Project team:
- Dr. Paweł Swaczyna, Princeton University, Principal Investigator
- Dr. Eric J. Zirnstein, Princeton University, Co-Investigator
- Dr. Maher A. Dayeh, Southwest Research Institute, Co-Investigator
- Dr. Jacob Heerikhuisen, University of Waikato, New Zealand, Collaborator
- Find related publications in SAO/NASA ADS
Proposal summary
Science goals and objectives
The Interstellar Boundary Explorer (IBEX) provides all-sky maps of energetic neutral atom (ENA) fluxes, which represent integrations of ENA production over lines of sight. The maps show an arc-like bright structure extending over the sky, dubbed the “ribbon” on top of the globally distributed flux (GDF) seen from all directions. The fluxes from these two sources are comparable. Therefore, analyses of the time-evolution of the ribbon are complicated. In this project, we propose a novel method of ribbon separation to answer the following questions:
- What is the angular size of the spatial structures observed in the globally distributed flux? Are these structures different in angular scales when compared with structures associated with the ribbon?
- How do the separated ribbon intensity and position change with the solar cycle? How does the accuracy of these observables increase due to the separation?
If the angular scales of the structures are different in the ribbon and the GDF, the proposed method will allow for a separation of the ribbon signal. The existence of small-scale structures in the GDF may affect the separation. Still, we will be able to quantify statistical uncertainties that may emerge from small structures behind the ribbon that cannot be separated due to similar angular scales. However, we note that the time-cumulative maps from IBEX do not show indications of small structures in the GDF.
The second question aims to verify how the ribbon flux relates to the variation of the solar wind. The separated flux gives the possibility to correlate the temporal variation seen in the ribbon with the observed variation in the solar wind. This will allow us to differentiate between several proposed scenarios for the ribbon origin, and physical processes occurring in the ribbon source region.
Mission data
We will use the IBEX-Hi ENA flux maps published in the IBEX Data Releases. We also plan to use the in situ solar wind conditions at 1 au as collected in the OMNI database.
Data analysis methodology, models, and simulations
We will separate the IBEX using spherical harmonic expansions of the GDF based on the signal observed outside the region covered by the ribbon. The flux predicted by the expansion in the regions covered by the ribbon will be subtracted from the observations, and the remaining part will be classified as the ribbon flux. For comparisons with the solar wind conditions, we will employ existing models of the solar wind latitudinal structure, and the OMNI database of the solar wind conditions. We will also use existing ribbon models to check the correlation of the time-variation of the ribbon with the solar wind conditions.
Relevance
The IBEX ribbon is created as a result of the interaction between the solar wind and the interstellar medium; therefore, this study directly addresses the IBEX mission science objective, which is “discovering the nature of the interaction between the solar wind and the interstellar medium at the edge of our solar system.”