Imaging the interaction of the heliosphere with the interstellar medium from Saturn with Cassini

Future Exploration of the Outer Heliosphere and Very Local Interstellar Medium by Interstellar Probe

A detailed overview of the knowledge gaps in our understanding of the heliospheric interaction with the largely unexplored Very Local Interstellar Medium (VLISM) are provided along with predictions of with the scientific discoveries that await. The new measurements required to make progress in this expanding frontier of space physics are discussed and include in-situ plasma and pick-up ion measurements throughout the heliosheath, direct sampling of the VLISM properties such as elemental and isotopic composition, densities, flows, and temperatures of neutral gas, dust and plasma, and remote energetic neutral atom (ENA) and Lyman-alpha (LYA) imaging from vantage points that can uniquely discern the heliospheric shape and bring new information on the interaction with interstellar hydrogen. The implementation of a pragmatic Interstellar Probe mission with a nominal design life to reach 375 Astronomical Units (au) with likely operation out to 550 au are reported as a result of a 4-year NASA funded mission study.

The Structure of the Large-Scale Heliosphere as Seen by Current Models

This review summarizes the current state of research aiming at a description of the global heliosphere using both analytical and numerical modeling efforts, particularly in view of the overall plasma/neutral flow and magnetic field structure, and its relation to energetic neutral atoms. Being part of a larger volume on current heliospheric research, it also lays out a number of key concepts and describes several classic, though still relevant early works on the topic. Regarding numerical simulations, emphasis is put on magnetohydrodynamic (MHD), multi-fluid, kinetic-MHD, and hybrid modeling frameworks. Finally, open issues relating to the physical relevance of so-called "croissant" models of the heliosphere, as well as the general (dis)agreement of model predictions with observations are highlighted and critically discussed.

The Early History of Heliospheric Science and the Spacecraft That Made It Possible

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.

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

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 PUIs 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 ∼ 47 au from the Sun or halfway to the heliospheric termination shock.

The Heliosphere and Local Interstellar Medium from Neutral Atom Observations at Energies Below 10 keV

As the heliosphere moves through the surrounding interstellar medium, a fraction of the interstellar neutral helium, hydrogen, and heavier species crossing the heliopause make it to the inner heliosphere as neutral atoms with energies ranging from few eV to several hundred eV. In addition, energetic neutral hydrogen atoms originating from solar wind protons and from pick-up ions are created through charge-exchange with interstellar atoms. This review summarizes all observations of heliospheric energetic neutral atoms and interstellar neutrals at energies below 10 keV. Most of these data were acquired with the Interstellar Boundary Explorer launched in 2008. Among many other IBEX breakthroughs, it provided the first ever all-sky maps of energetic neutral atoms from the heliosphere and enabled the science community to measure in-situ interstellar neutral hydrogen, oxygen, and neon for the first time. These observations have revolutionized and keep challenging our understanding of the heliosphere shaped by the combined forces of the local interstellar flow, the local interstellar magnetic field, and the time-dependent solar wind.

Cross-NASA divisional relevance of an Ice Giant mission

Robotic space exploration to the outer solar system is difficult and expensive and the space science community works inventively and collaboratively to maximize the scientific return of missions. A mission to either of our solar system Ice Giants, Uranus and Neptune, will provide numerous opportunities to address high-level science objectives relevant to multiple disciplines and deliberate cross-disciplinary mission planning should ideally be woven in from the start. In this review, we recount past successes as well as (NASA-focused) challenges in performing cross-disciplinary science from robotic space exploration missions and detail the opportunities for broad-reaching science objectives from potential future missions to the Ice Giants. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.

The Low-Energy Neutral Imager (LENI)

To achieve breakthroughs in the areas of heliospheric and magnetospheric energetic neutral atom (ENA) imaging, a new class of instruments is required. We present a high angular resolution ENA imager concept aimed at the suprathermal plasma populations with energies between 0.5 and 20 keV. This instrument is intended for understanding the spatial and temporal structure of the heliospheric boundary recently revealed by Interstellar Boundary Explorer instrumentation and the Cassini Ion and Neutral Camera. The instrument is also well suited to characterize magnetospheric ENA emissions from low-altitude ENA emissions produced by precipitation of magnetospheric ions into the terrestrial upper atmosphere, or from the magnetosheath where solar wind protons are neutralized by charge exchange, or from portions of the ring current region. We present a new technique utilizing ultrathin carbon foils, 2-D collimation, and a novel electron optical design to produce high angular resolution (≤2°) and high-sensitivity (≥10-3 cm2 sr/pixel) ENA imaging in the 0.5-20 keV energy range.

Global observations of the interstellar interaction from the Interstellar Boundary Explorer (IBEX)

The Sun moves through the local interstellar medium, continuously emitting ionized, supersonic solar wind plasma and carving out a cavity in interstellar space called the heliosphere. The recently launched Interstellar Boundary Explorer (IBEX) spacecraft has completed its first all-sky maps of the interstellar interaction at the edge of the heliosphere by imaging energetic neutral atoms (ENAs) emanating from this region. We found a bright ribbon of ENA emission, unpredicted by prior models or theories, that may be ordered by the local interstellar magnetic field interacting with the heliosphere. This ribbon is superposed on globally distributed flux variations ordered by both the solar wind structure and the direction of motion through the interstellar medium. Our results indicate that the external galactic environment strongly imprints the heliosphere.