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1. Probing the Length of the Heliospheric Tail with Energetic Neutral Atoms (ENAs) from 0.52 to 80 keV

   https://doi.org/10.3847/2041-8213/acbc73  

   Kornbleuth, M. ; Opher, M. ; Dialynas, K. ; Zank, G. P. ; Wang, B. B. ; Baliukin, I. ; Gkioulidou, M. ; Giacalone, J. ; Izmodenov, V. ; Sokół, J. M. ; et al ( March 2023 , The Astrophysical Journal Letters)

   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 80more »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%.

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2. Explanation of Heliospheric Energetic Neutral Atom Fluxes Observed by the Interstellar Boundary Explorer

   https://doi.org/10.3847/2041-8213/ac92e2  

   Zirnstein, E. J. ; Kim, T. K. ; Dayeh, M. A. ; Rankin, J. S. ; McComas, D. J. ; Swaczyna, P. ( September 2022 , The Astrophysical Journal Letters)

   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 betweenγ∼ 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 <more »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.

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3. In Situ Observations of Interstellar Pickup Ions from 1 au to the Outer Heliosphere

   https://doi.org/10.1007/s11214-022-00895-2  

   Zirnstein, E. J. ; Möbius, E. ; Zhang, M. ; Bower, J. ; Elliott, H. A. ; McComas, D. J. ; Pogorelov, N. V. ; Swaczyna, P. ( June 2022 , Space Science Reviews)

   Abstract Interstellar pickup ions are an ubiquitous and thermodynamically important component of the solar wind plasma in the heliosphere. These PUIs are born from the ionization of the interstellar neutral gas, consisting of hydrogen, helium, and trace amounts of heavier elements, in the solar wind as the heliosphere moves through the local interstellar medium. As cold interstellar neutral atoms become ionized, they form an energetic ring beam distribution comoving with the solar wind. Subsequent scattering in pitch angle by intrinsic and self-generated turbulence and their advection with the radially expanding solar wind leads to the formation of a filled-shell PUI distribution, whose density and pressure relative to the thermal solar wind ions grows with distance from the Sun. This paper reviews the history of in situ measurements of interstellar PUIs in the heliosphere. Starting with the first detection in the 1980s, interstellar PUIs were identified by their highly nonthermal distribution with a cutoff at twice the solar wind speed. Measurements of the PUI distribution shell cutoff and the He focusing cone, a downwind region of increased density formed by the solar gravity, have helped characterize the properties of the interstellar gas from near-Earth vantage points. The preferential heating of interstellar PUIsmore »compared to the core solar wind has become evident in the existence of suprathermal PUI tails, the nonadiabatic cooling index of the PUI distribution, and PUIs’ mediation of interplanetary shocks. Unlike the Voyager and Pioneer spacecraft, New Horizon’s Solar Wind Around Pluto (SWAP) instrument is taking the only direct measurements of interstellar PUIs in the outer heliosphere, currently out to $\sim47~\text{au}$ ∼ 47 au from the Sun or halfway to the heliospheric termination shock.« less
4. The Early History of Heliospheric Science and the Spacecraft That Made It Possible

   https://doi.org/10.1007/s11214-022-00900-8  

   Zank, G. P. ; Sterken, V. ; Giacalone, J. ; Möbius, E. ; von Steiger, R. ; Stone, E. S. ; Krimigis, S. M. ; Richardson, J. D. ; Linsky, J. ; Izmodenov, V. ; et al ( June 2022 , Space Science Reviews)

   Abstract Our understanding of the interaction of the large-scale heliosphere with the local interstellar medium (LISM) has undergone a profound change since the very earliest analyses of the problem. In part, the revisions have been a consequence of ever-improving and widening observational results, especially those that identified the entrance of interstellar material and gas into the heliosphere. Accompanying these observations was the identification of the basic underlying physics of how neutral interstellar gas and interstellar charged particles of different energies, up to and including interstellar dust grains, interacted with the temporal flows and electromagnetic fields of the heliosphere. The incorporation of these various basic effects into global models of the interaction, whether focused on neutral interstellar gas and pickup ions, energetic particles such as anomalous and galactic cosmic rays, or magnetic fields and large-scale flows, has profoundly changed our view of how the heliosphere and LISM interact. This article presents a brief history of the conceptual and observation evolution of our understanding of the interaction of the heliosphere with the local interstellar medium, up until approximately 1996.
5. Exploring turbulence from the Sun to the local interstellar medium: Current challenges and perspectives for future space missions

   https://doi.org/10.3389/fspas.2022.1064098  

   Fraternale, Federico ; Zhao, Lingling ; Pogorelov, Nikolai V. ; Sorriso-Valvo, Luca ; Redfield, Seth ; Zhang, Ming ; Ghanbari, Keyvan ; Florinski, Vladimir ; Chen, Thomas Y. ( November 2022 , Frontiers in Astronomy and Space Sciences)

   Turbulence is ubiquitous in space plasmas. It is one of the most important subjects in heliospheric physics, as it plays a fundamental role in the solar wind—local interstellar medium interaction and in controlling energetic particle transport and acceleration processes. Understanding the properties of turbulence in various regions of the heliosphere with vastly different conditions can lead to answers to many unsolved questions opened up by observations of the magnetic field, plasma, pickup ions, energetic particles, radio and UV emissions, and so on. Several space missions have helped us gain preliminary knowledge on turbulence in the outer heliosphere and the very local interstellar medium. Among the past few missions, the Voyagers have paved the way for such investigations. This paper summarizes the open challenges and voices our support for the development of future missions dedicated to the study of turbulence throughout the heliosphere and beyond.