Interstellar neutral atoms propagating into the heliosphere experience charge exchange with the supersonic solar wind (SW) plasma, generating ions that are picked up by the SW. These pickup ions (PUIs) constitute ∼25% of the proton number density by the time they reach the heliospheric termination shock (HTS). Preferential acceleration of PUIs at the HTS leads to a suprathermal, kappa-like PUI distribution in the heliosheath, which may be further heated in the heliosheath by traveling shocks or pressure waves. In this study, we utilize a dynamic, 3D magnetohydrodynamic model of the heliosphere to show that dynamic heating of PUIs at the HTS and in the inner heliosheath (IHS), as well as a background source of energetic neutral atoms (ENAs) from outside the heliopause, can explain the heliospheric ENA signal observed by the Interstellar Boundary Explorer (IBEX) in the Voyager 2 direction. We show that the PUI heating process at the HTS is characterized by a polytropic index larger than 5/3, likely ranging between
The shape of the heliosphere is currently under active debate. Energetic neutral atoms (ENAs) offer the best method for investigating the global structure of the heliosphere. To date, the Interstellar Boundary Explorer (IBEX) and the Ion and Neutral Camera (INCA) that was on board Cassini provide the only global ENA observations of the heliosphere. While extensive modeling has been done at IBEX-Hi energies (0.52–6 keV), no global ENA modeling has been conducted for INCA energies (5.2–55 keV). Here, we use an ENA model of the heliosphere based on hybrid results that capture the heating and acceleration of pickup ions (PUIs) at the termination shock to compare modeled global ENA results with IBEX-Hi and INCA observations using both a long- and short-tail model of the heliosphere. We find that the modeled ENA results for the two heliotail configurations produce similar results from the IBEX-Hi through the INCA energies. We conclude from our modeled ENAs, which only include PUI acceleration at the termination shock, that ENA observations in currently available energy ranges are insufficient for probing the shape and length of the heliotail. However, as a prediction for the future IMAP-Ultra mission (3–300 keV) we present modeled ENA maps at 80 keV, where the cooling length (∼600 au) is greater than the distance where the long- and short-heliotail models differ (∼400 au), and find that IMAP-Ultra should be able to identify the shape of the heliotail, predicting differences in the north lobe to downwind flux ratio between the models at 48%.
more » « less- Award ID(s):
- 2148653
- NSF-PAR ID:
- 10400517
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 945
- Issue:
- 1
- ISSN:
- 2041-8205
- Format(s):
- Medium: X Size: Article No. L15
- Size(s):
- ["Article No. L15"]
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract γ ∼ 2.3 and 2.7, depending on the time in solar cycle 24 and SW conditions. The ENA fluxes at energies >1.5 keV show large-scale behavior in time with the solar cycle and SW dynamic pressure, whereas ENAs < 1.5 keV primarily exhibit random-like fluctuations associated with SW transients affecting the IHS. We find that ≲20% of the ENAs observed at ∼0.5–6 keV come from other sources, likely from outside the heliopause as secondary ENAs. This study offers the first model replication of the intensity and evolution of IBEX-Hi ENA observations from the outer heliosphere. -
Abstract The Voyager 2 crossing of the termination shock indicated that most of the upstream energy from the thermal solar wind ions was transferred to pickup ions (PUIs) and other energetic particles downstream of the shock. We use hybrid simulations at the termination shock for the Voyager 2, flank, and tail directions to evaluate the distributions of different ion species downstream of the shock over the energy range of 0.52–55 keV. Here, we extend the work of Gkioulidou et al., which showed an energy-dependent discrepancy between modeled and energetic neutral atom (ENA) observations, and fit distributions to a hybrid model to show that a population of PUIs accelerated via diffusive shock acceleration (DSA) to become low-energy anomalous cosmic rays (ACRs) can bridge the gap between modeled and observed ENA fluxes. Our results with the inclusion of DSA via hybrid fitting give entirely new and novel evidence that DSA at the termination shock is likely to be an important physical process. These ACRs carry a significant fraction of the energy density at the termination shock (22%, 13%, and 19% in the Voyager 2, flank, and tail directions, respectively). Using these ACRs in global ENA modeling of the heliosphere from 0.52 to 55 keV, we find that scaling factors as large as 1.8–2.5 are no longer required to match ENA observations at energies of ∼1–4 keV. Large discrepancies between modeled and observed ENAs only remain over energies of 4–20 keV, indicating that there may be a further acceleration mechanism in the heliosheath at these energies.more » « less
-
Abstract Heliospheric energetic neutral atoms (ENAs) originate from energetic ions that are neutralized by charge exchange with neutral atoms in the heliosheath and very local interstellar medium (VLISM). Since neutral atoms are unaffected by electromagnetic fields, they propagate ballistically with the same speeds as parent particles. Consequently, measurements of ENA distributions allow one to remotely image the energetic ion distributions in the heliosheath and VLISM. The origin of the energetic ions that spawn ENAs is still debated, particularly at energies higher than ∼keV. In this work, we summarize five possible sources of energetic ions in the heliosheath that cover the ENA energy from a few keV to hundreds of keV. Three sources of the energetic ions are related to pickup ions (PUIs): those PUIs transmitted across the heliospheric termination shock (HTS), those reflected once or multiple times at the HTS, i.e., reflected PUIs, and those PUIs multiply reflected and further accelerated by the HTS. Two other kinds of ions that can be considered are ions transmitted from the suprathermal tail of the PUI distribution and other particles accelerated at the HTS. By way of illustration, we use these energetic particle distributions, taking account of their evolution in the heliosheath, to calculate the ENA intensities and to analyze the characteristics of ENA spectra observed at 1 au.more » « less
-
Abstract Interstellar neutrals (ISNs), pick-up ions (PUIs), and energetic neutral atoms (ENAs) are fundamental constituents of the heliosphere and its interaction with the neighboring interstellar medium. Here, we focus on selected aspects of present-day theory and modeling of these particles. In the last decades, progress in the understanding of the role of PUIs and ENAs for the global heliosphere and its interaction with very local interstellar medium is impressive and still growing. The increasing number of measurements allows for verification and continuing development of the theories and model attempts. We present an overview of various model descriptions of the heliosphere and the processes throughout it including the kinetic, fluid, and hybrid solutions. We also discuss topics in which interplay between theory, models, and interpretation of measurements reveals the complexity of the heliosphere and its understanding. They include model-based interpretation of the ISN, PUI, and ENA measurements conducted from the Earth’s vicinity. In addition, we describe selected processes beyond the Earth’s orbit up to the heliosphere boundary regions, where PUIs significantly contribute to the complex system of the global heliosphere and its interaction with the VLISM.more » « less
-
more » « less
Abstract The LEXI and SMILE missions will provide soft X‐ray images of the Earth's magnetosheath and cusps after their anticipated launch in 2023 and 2024, respectively. The IBEX mission showed the potential of an Energetic Neutral Atom (ENA) instrument to image dayside magnetosheath and cusps, albeit over the long hours required to raster an image with a single pixel imager. Thus, it is timely to discuss the two imaging techniques and relevant science topics. We simulate soft X‐ray and low‐ENA images that might be observed by a virtual spacecraft during two interesting solar wind scenarios: a southward turning of the interplanetary magnetic field and a sudden enhancement of the solar wind dynamic pressure. We employ the OpenGGCM global magnetohydrodynamics model and a simple exospheric neutral density model for these calculations. Both the magnetosheath and the cusps generate strong soft X‐rays and ENA signals that can be used to extract the locations and motions of the bow shock and magnetopause. Magnetopause erosion corresponds closely to the enhancement of dayside reconnection rate obtained from the OpenGGCM model, indicating that images can be used to understand global‐scale magnetopause reconnection. When dayside imagers are installed with high‐ENA inner‐magnetosphere and FUV/UV aurora imagers, we can trace the solar wind energy flow from the bow shock to the magnetosphere and then to the ionosphere in a self‐standing manner without relying upon other observatories. Soft X‐ray and/or ENA imagers can also unveil the dayside exosphere density structure and its response to space weather.
