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Illuminating the Outer Heliosphere: ENA imaging from IBEX to IMAP

The Interstellar Boundary Explorer (IBEX) Mission, launched in 2008, in concert with in situ measurements by the Voyager spacecraft have initiated a remarkable scientific quest to discover the global heliosphere and its interaction with the local galactic environment through which our Sun and solar system move. The global boundaries that surround our solar system and the IBEX ribbon are created through a myriad of complex physical processes that mediate the interactions between the solar wind, the local interstellar flow, and the local interstellar magnetic field. At this point in time, more than a solar cycle of IBEX data has been accrued, revealing not only the global properties of our heliosphere, but also our first views of their variations in time. The rich science of the global heliosphere, our growing understanding of suprathermal particle populations that influence interstellar interactions, and expanded research into the properties of the local interstellar medium have helped to usher in the next steps of exploration to be taken by the upcoming Interstellar Mapping and Acceleration Probe (IMAP), due to launch in 2025. This session is devoted to the science that is advancing our quest to discover the complex physics of our global heliosphere and its interaction with the local interstellar medium.

Convener: Nathan Schwadron | Co-conveners: Reka WinslowECSECS, Jamie RankinECSECS
| Thu, 26 May, 15:10–16:23 (CEST)
Room 1.14

Thu, 26 May, 15:10–16:40

Chairpersons: Nathan Schwadron, Reka Winslow, Jamie Rankin

On-site presentation
Bertalan Zieger et al.

The Voyager spacecraft are the first man-made objects to cross the termination shock (TS), where the solar wind becomes sub-fast magnetosonic due to the interaction with the local interstellar medium. Voyager 2 observations revealed that classical single-fluid magnetohydrodynamic (MHD) or multispecies single-fluid MHD models are not sufficient to describe the microstructure of the TS and the observed nonlinear waves downstream the TS. Consequently, more sophisticated physical models, like multifluid, hybrid or fully kinetic solar wind models, are needed to capture nonlinear waves, dispersive shock waves, and ion-ion instabilities, where each ion species (and electrons) can move independently with their own bulk velocities, and the fluctuating parts of the ion velocities are often comparable to the mean velocity of the collective plasma fluid. The multifluid simulation of the TS by Zieger et al. [2015] shows a remarkable agreement with high-resolution Voyager 2 observations, reproducing not only the microstructure of the third TS crossing (TS3) but also the energy partitioning among thermal ions, pickup ions (PUI), and electrons across the shock. It was demonstrated that TS3 is a subcritical dispersive shock wave with low fast magnetosonic Mach number and high plasma ß. Here we present multifluid, hybrid, and particle-in-cell (PIC) simulations of the second TS crossing (TS2) by Voyager 2, which was somewhat stronger than TS3, with an observed compression ratio of 2.2. All three types of simulations confirm the dispersive nature of the TS in agreement with Voyager 2 observations. We conclude that TS2, just as TS3, is a subcritical dispersive shock wave with a soliton (overshoot) at the leading edge of the shock and a quasi-stationary nonlinear wave train downstream of the shock front. We compared the cross-shock electric field in the multifluid, hybrid, and PIC simulations and found a reasonable agreement. We show that the Hall electric field is dominating over the convective and ambipolar electric fields, which indicates that electrons play an important role in the shock transition. Finally, we demonstrate that the microstructure of the termination shock is controlled by dispersion rather than ion reflection, and only slightly affected by reflected solar wind ions in the hybrid and PIC simulations, which validates the multifluid model on fluid scale. The dispersive nature of the termination shock has important implications for the transition and acceleration of PUIs across the termination shock, which is revealed in the PUI distributions in our hybrid [Giacalone et al., 2021] and PIC simulations.

How to cite: Zieger, B., Giacalone, J., Swisdak, M., Zank, G., and Opher, M.: The Dispersive Nature of the Heliospheric Termination Shock, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3325, https://doi.org/10.5194/egusphere-egu22-3325, 2022.

Virtual presentation
Eric Zirnstein

The heliosphere surrounding our solar system is formed by the interaction between the solar wind and the local interstellar medium as the Sun moves through interstellar space. With dimensions on the order of hundreds to potentially thousands of au, it is extremely difficult to pinpoint the 3D structure of the heliosphere and its boundaries, and the properties of the plasma within it. However, we can remotely measure the properties of the heliosphere with energetic neutral atoms (ENAs) which are created as a product of charge exchange between interstellar neutrals and ions within the solar wind plasma. ENAs can propagate hundreds of au before ionizing, allowing us to remotely view the distant boundaries of the heliosphere.

The Interstellar Boundary Explorer (IBEX) mission, a NASA smaller explorer mission which has been measuring ENA fluxes at ~0.5-6 keV for more than a solar cycle, has revealed at least two separate sources of ENAs: the “ribbon” of enhanced ENA fluxes forming a narrow, circular band across the sky, and the “globally distributed flux” (GDF) that forms lower intensity, broad features near the nose and tail of the heliosphere. While it is believed that the ribbon is formed from secondary ENAs from outside the heliopause, and ENAs from the inner heliosheath inside the heliopause are a major contributor to the GDF, it is not clear exactly how much of the GDF may originate outside the heliopause, and how much of the flux observed in the Ribbon is from the GDF itself. To help solve this issue, we present recent developments in our understanding of the ribbon vs. GDF sources from both IBEX data analysis and modeling perspectives. Moreover, with the upcoming IMAP mission set to launch in 2025, we provide insight into how IMAP’s enhanced capabilities may improve our current understanding of the heliosphere.

How to cite: Zirnstein, E.: Differentiating the Ribbon and Globally Distributed ENA Flux in IBEX Observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13186, https://doi.org/10.5194/egusphere-egu22-13186, 2022.

Virtual presentation
George Livadiotis

Space plasmas reside in non-equilibrium stationary states described by kappa distributions. The high-energy asymptotic behavior of kappa distributions leads to a power-law relationship of the energy-flux spectra; this relationship, when observed, can be analyzed for determining the thermodynamic parameters of the plasma. In the presented analysis, we use Energetic Neutral Atom (ENA) observations from the IBEX-Hi sensor, converted to the corresponding proton plasma spectra of the inner heliosheath, and timestamped with the ENA creation time. We, then, model the proton spectra with kappa distributions and derive the sky maps of the (radially averaged) values of temperature, density, kappa, and other thermodynamic parameters of the proton plasma in the inner heliosheath. We examine the variations of the determined thermodynamics and whether a correlation exists with solar activity during the 24th solar cycle.

How to cite: Livadiotis, G.: Thermodynamics of the proton plasma in the inner heliosheath during the 24th solar cycle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13574, https://doi.org/10.5194/egusphere-egu22-13574, 2022.

On-site presentation
Konstantinos Dialynas et al.

The Voyager 1 and Voyager 2 (V1 & V2) crossings of the termination shock (TS; ~94 and ~84 AU, respectively), led to the first measurements of ions and electrons that constitute the heliosheath (HS). Their crossings of the heliopause (HP; ~122 AU and ~119 AU), pinpointed the extent of the upwind heliosphere's expansion into the Very Local Interstellar medium (VLISM). The Cassini/INCA >5.2 keV ENA images of the celestial sphere, have placed the local V1&2/LECP measurements in a global context and have led to the discovery of a high intensity and wide ENA region that encircles the celestial sphere, called “Belt” and corresponds to a “reservoir” of particles that exist within the HS. The heliosphere forms a time-dependent, roughly symmetric obstacle to the inward interstellar flow, responding within ~2-3 yrs, in both the nose and anti-nose directions to the outward propagating solar wind changes through the solar cycle. The shape of the ion energy spectra plays a critical role in determining the pressure balance and acceleration mechanisms inside the HS. Energy spectra from ~10 eV to 344 MeV show that the PUIs dominate the total pressure distribution inside the HS, but suprathermal ions provide a significant contribution that cannot be neglected, revealing that >5.2 keV ENAs serve as important indicators of the acceleration processes that the parent H+ population undergoes inside the HS, thus imposing a key constraint on any future interpretation concerning the HS dynamics. The combination of ENAs and ions in the HS show that the plasma beta is >>1, the magnetic field upstream at the heliopause required to balance the pressure from the HS is >0.5 nT (V1 direction) and ~0.67 nT (V2 direction) and that the neutral Hydrogen density is ~0.12/cm3. These inferred values are consistent with measurements from both V1 and V2 spacecraft. Energetic ion measurements from V1/LECP in and beyond the HP show an average radial inflow of 40-139 keV ions for ~10 AU inside the HS and an average radial outflow over a spatial range of ~28 AU past the HP. These particles correspond to an ion population leaking from the HS into interstellar space, most likely due to the flux tube interchange instability at the boundary and provide a direct observation of the communication between the HS and the VLISM. They may also provide an important constraint for future models that aim to explain the <6 keV ENA ribbon fluxes (measured from the IBEX mission), which are likely formed from the neutralization of energetic pickup ions gyrating in the IS magnetic field outside the HP, reflecting (in part) those ion distributions that are responsible for the formation of this unexpected structure.

How to cite: Dialynas, K., Krimigis, S., Decker, R., and Hill, M.: Highlights of the combination of “Ground truth” >28 keV in-situ ions from the Voyagers and >5.2 keV ENAs from Cassini in the study of the global heliosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4316, https://doi.org/10.5194/egusphere-egu22-4316, 2022.

Kaijun Liu et al.

Scattering of pickup ion ring-beam distributions in the outer heliosheath is a fundamental element in the spatial retention scenario of the energetic neutral atom (ENA) ribbon observed by the Interstellar Boundary EXplorer (IBEX). According to our earlier linear instability analysis, pickup ion ring-beam distributions trigger magnetic field-aligned, right-hand polarized unstable waves in two separate frequency ranges which are near and far above the proton cyclotron frequency, respectively. We have performed hybrid simulations to study the unstable waves near the proton cyclotron frequency. However, the high-frequency waves well above the proton cyclotron frequency are beyond the reach of hybrid simulations. In the present study, particle-in-cell simulations are carried out to investigate the parallel- and anti-parallel-propagating high-frequency waves excited by the outer heliosheath pickup ions at different pickup angles as well as the scattering of the pickup ions by the waves excited. In the early stages of the simulations, the results confirm the excitation of the parallel-propagating, right-hand polarized high-frequency waves as predicted by the earlier linear analysis. Later in the simulations, enhanced anti-parallel-propagating modes also emerge. Furthermore, the evolution of the pickup ion ring-beam distributions of the selected pickup angles reveals that the high-frequency waves do not significantly contribute to the pickup ion scattering. These results are favorable regarding the plausibility of the spatial retention scenario of the IBEX ENA ribbon.

How to cite: Liu, K., Mousavi, A., and Sadeghzadeh, S.: High-frequency waves driven by pickup ion ring-beam distributions in the outer heliosheath, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1608, https://doi.org/10.5194/egusphere-egu22-1608, 2022.

Jonathan Gasser et al.

The IMAP mission by NASA is dedicated to extending the physical understanding of our heliosphere and its interaction with the interstellar medium by enhancing and refining the results obtained from IBEX. The neutral atom analysis instrument IMAP-Lo will observe and map fluxes of low-energy heliospheric neutral atoms (ENAs) and interstellar neutral (ISN) H, D, He, O and Ne with energies as low as 10 eV up to 1000 eV.

The instrument testing and calibration with a neutral atom beam is foreseen in the MEFISTO test facility for ion and neutral particle instruments at the University of Bern. MEFISTO is equipped with an electron-cyclotron resonance ion source that provides ion beams at a beam energy 3keV/q up to 100 keV/q. The beam fed into the test chamber is decelerated to 10 eV/q – 3 keV/q and effectively neutralized in a removable neutralization stage via surface reflection on a highly polished single crystal tungsten surface. The relative neutral beam intensity is permanently monitored via the neutralizing surface current. The neutralization process induces a considerable reduction of particle kinetic energy and conical widening of the neutral beam.

Thus, one key improvement for the calibration of a neutral atom instrument such as IMAP-Lo is to be able to measure the absolute neutral particle flux and beam energy into the instrument in the test chamber. To achieve this goal, the Absolute Beam Monitor (ABM) was developed recently.

The ABM is a dedicated laboratory device for absolute neutral particle flux measurements below 3 keV and coarse kinetic energy determination. Neutrals entering the ABM aperture strike a single crystal W conversion surface at grazing angle and are reflected into a channeltron to generate a stop pulse. The simultaneous monitoring of secondary electrons released at the W surface as start signal, the stop signal and the coincidence event rate allows inferring the rate of neutral atoms into the ABM aperture. The average neutral beam energy is obtained from the start-stop time-of-flight spectrum. The ABM is the first and so far the only device to measure the absolute neutral atoms flux in this low energy range below a few 100 eV. It serves as a primary standard for gauging the MEFISTO neutral beam source.

We report on recent calibration results of neutral H, He, O, Ne beams in the 10 eV – 1 keV energy range with the ABM in MEFISTO.

How to cite: Gasser, J., Galli, A., and Wurz, P.: Recent results from neutral beam source calibration by means of the novel Absolute Beam Monitor, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11016, https://doi.org/10.5194/egusphere-egu22-11016, 2022.

Maciej Bzowski

As the Sun moves through its local galactic neighborhood, it disturbs the ionized component of interstellar matter, forming a complex bow-wave structure of a slowed and heated, magnetized plasma, flowing past the heliosphere. This region is called the outer heliosheath. The neutral component of interstellar matter is in equilibrium with the ionized component far ahead of the heliosphere, but within the outer heliosheath it decouples kinematically from the perturbed plasma. A complex interaction begins, mostly by charge exchange, between the ions and the neutral atoms, and as a result, a new population of neutral atoms is created, with kinematic parameters similar to that of the surrounding plasma. In addition, elastic collisions operate, additionally modifying both the original primary and the secondary population of neutral. Both the primary and the secondary populations penetrate inside the heliosphere and can be directly sampled at 1 au. We will review results of observations of these populations obtained from IBEX-Lo and results of modeling of the secondary populations of interstellar hydrogen, helium, and oxygen in the outer heliosheath. We will illustrate how these populations can be better observed owing to enhanced capabilities of the planned IMAP-Lo instrument and demonstrate how they can be leveraged to resolve the primary and secondary populations, and investigate the properties of local interstellar matter in the immediate neighborhood of the Sun.

How to cite: Bzowski, M.: The primary and secondary populations of interstellar neutral gas as seen now by IBEX and in the future by IMAP, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13535, https://doi.org/10.5194/egusphere-egu22-13535, 2022.

On-site presentation
Nathan Schwadron et al.

The Sun’s motion relative to the surrounding interstellar medium leads to an interstellar neutral (ISN) wind through the heliosphere. For several species, including He, this wind is moderately depleted by ionization and can be analyzed in-situ with pickup ions and direct neutral atom imaging.  Since 2009, observations of the wind at 1 AU with the Interstellar Boundary Explorer (IBEX) have returned a precise 4-dimensional parameter tube for the flow vector (speed VISN, longitude λISN, and latitude βISN) and temperature TISN of interstellar He in the local cloud, which organizes VISN, λISN, and TISN as a function of λISN, and the local flow Mach number (Vth−ISN/VISN).  We refer to this functional dependence as the 4D IBEX parameter tube. On IBEX, the limitation of measuring the ISN flow observations to nearly perpendicular to the Earth-Sun line limits the range of observations in ecliptic longitude to ∼ 30º.  This limitation results in large uncertainties along the IBEX parameter tube and relatively small uncertainties across the parameter tube.   Over the past three years, IBEX operations were modified to let the spin axis pointing of IBEX drift to the maximum offset (7º) west of the Sun, which is the limit for the IBEX spacecraft. This expansion of the IBEX viewing helps break the degeneracy of the ISN parameters along the 4D IBEX parameter tube. It complements the full χ-square-minimization to obtain the ISNs parameters through comparison with detailed models of the ISN flow. The next generation IBEX-Lo sensor on IMAP will be mounted on a pivot platform, enabling IMAP-Lo to follow the ISN flow over almost the entire spacecraft orbit around the Sun.  A near-continuous set of 4D parameter tubes on IMAP will be observed for He, and for O, Ne, and H that cross at varying angles in the full ISN parameter space. This analysis substantially reduces the flow parameter uncertainties for these species and mitigating systematic uncertainties, such as those from ionization effects and the presence of secondary components. We discuss implications of these measurements for understanding our environment and its relationship to the structure of the local interstellar medium. Thus, we discuss how IMAP will probe the interstellar neutral gas flow in detail to derive the precise parameters of the interstellar flow and relate these conditions to understand our place within the interstellar medium. 

How to cite: Schwadron, N., Moebius, E., McComas, D., Bower, J., Bzowski, M., Fuselier, S., Heirtzler, D., Kubiak, M., Lee, M., Rahmanifard, F., Sokol, J., Swaczyna, P., and Winslow, R.: Interstellar Neutral He Parameters from Crossing Parameter Tubes with the Interstellar Mapping and Acceleration Probe (IMAP) informed by 10 Years of Interstellar Boundary Explorer (IBEX) Observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6402, https://doi.org/10.5194/egusphere-egu22-6402, 2022.

On-site presentation
Paweł Swaczyna et al.

The pristine very local interstellar medium (VLISM) is not available for in situ observations even with the Voyager spacecraft because the influence of the heliosphere on the VLISM plasma extends up to several hundred au from the Sun. Therefore, observations of interstellar neutral (ISN) helium, the species least modified at the heliospheric boundaries, are used to determine the pristine VLISM flow speed, direction, and temperature. For more than one solar cycle, the Interstellar Boundary Explorer (IBEX) has sampled ISN helium atoms at 1 au, significantly reducing the statistical uncertainties of the ISN helium flow parameters. Launching in 2025, the Interstellar Mapping and Acceleration Probe (IMAP) will further lower these uncertainties thanks to the utilization of a pivot platform, which provides a range of viewing orientations and reduces the parameter degeneracy seen from IBEX data. Even though interstellar helium is the least modified ISN species, recent studies show that the ISN helium flux is affected by charge exchange and elastic collisions beyond the heliopause. Charge exchange collisions outside the heliopause filter the primary ISN helium and produce a secondary population from perturbed He+ ions in the interstellar plasma. The secondary helium population, originally called the Warm Breeze, was discovered from IBEX observations. Moreover, the distribution function of the primary ISN helium population is modified by elastic collisions with slowed down and heated plasma ahead of the heliopause. Consequently, the combined primary and secondary ISN helium populations at 1 au are complex and cannot be separated. Furthermore, the modifications of the population properties are larger than the statistical uncertainty of IBEX observations. We use global heliosphere models to estimate the magnitude of the filtration and scattering caused by charge exchange and elastic collisions. Together with other sources of information about the VLISM, these estimates allow us to assess the pristine VLISM conditions outside of heliosphere influences. 

How to cite: Swaczyna, P., Bzowski, M., Fuselier, S., Galli, A., Heerikhuisen, J., Kubiak, M., McComas, D., Möbius, E., Rahmanifard, F., Schwadron, N., and Zirnstein, E.: Filtration and Scattering of Interstellar Neutral Helium beyond the Heliopause, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6502, https://doi.org/10.5194/egusphere-egu22-6502, 2022.

Eberhard Möbius and Jeffey Linsky

The pressures exerted by the solar wind from the inside and the interstellar medium from the outside control the size and shape of the heliosphere. Magnetic fields and cosmic rays play important roles to varying extents on both sides. We will assess the pressure balance by assembling the relevant component pressures in the (inner) heliosheath inside the heliopause, in the Very Local Interstellar Medium (VLISM, interstellar medium affected by the presence of the heliosphere or outer heliosheath), and in the Local Interstellar Cloud (LIC) unaffected by the heliosphere. We take the cosmic ray pressure from Voyager observations, which don’t show substantial gradients beyond the heliopause. The remaining pressure on the heliopause from inside is due to thermal and suprathermal ions in the subsonic solar wind, obtained from IBEX and INCA ENA observations and Voyager in situ measurements at the higher energies.

Besides cosmic rays, the pressure in the undisturbed LIC is composed of magnetic field pressure taken from IBEX Ribbon observations and related modeling. Thermal and turbulent pressures are based on H and He neutral and ion densities from pickup ion and interstellar gas flow observations, combined with the temperature and turbulent speed from absorption-line observations. The total LIC pressure in its rest frame is almost 40% lower than the pressure inside the heliopause, whereas adding the full ram pressure based on the LIC velocity relative to the Sun exceeds that pressure substantially. We estimate the likely effective pressure on the heliopause by combining the compressed interstellar magnetic field, as measured by Voyager, and the compressed and heated interstellar plasma, resorting to results from global heliospheric modeling. An interesting result of these pressure comparisons is that the effective ram pressure on the heliopause is somewhat larger than the combined magnetic field, thermal, and turbulent pressure in the LIC, which points to the importance of the LIC ram pressure for the shape of the heliosphere. We also compare the LIC pressure with the gravitational pressure on the galactic disk at the location of the Sun.

How to cite: Möbius, E. and Linsky, J.: Pressure Balance at the Heliosphere Boundary and in the Local Interstellar Cloud, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13568, https://doi.org/10.5194/egusphere-egu22-13568, 2022.