The Role of Pickup Ion Dynamics Outside of the Heliopause in the Limit of Weak Pitch Angle Scattering: Implications for the Source of the IBEX Ribbon

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.

Turbulence in the Local Interstellar Medium and the IBEX Ribbon

The effects of turbulence in the very local interstellar medium (VLISM) have been proposed by Giacalone & Jokipii (2015) to be important in determining the structure of the Interstellar Boundary Explorer (IBEX) ribbon via particle trapping by magnetic mirroring. We further explore this effect by simulating the motion of charged particles in a turbulent magnetic field superposed on a large-scale mean field, which we consider to be either spatially-uniform or a draped field derived from a 3D MHD simulation. We find that the ribbon is not double-peaked, in contrast to Giacalone & Jokipii (2015). However, the magnetic mirror force still plays an important role in trapping particles. Furthermore, the ribbon's thickness is considerably larger if the large-scale mean field is draped around the heliosphere. Voyager 1 observations in the VLISM show a turbulent field component that is stronger than previously thought, which we test in our simulation. We find that the inclusion of turbulent fluctuations at scales ≳100 au and power consistent with Voyager 1 observations produces a ribbon whose large-scale structure is inconsistent with IBEX observations. However, restricting fluctuations to <100 au produces a smoother ribbon structure similar to IBEX observations. Different turbulence realizations produce different small-scale features (≲10°) in the ribbon, but its large-scale structure is robust if the maximum fluctuation size is ≲50 au. This suggests that the magnetic field structure at scales ≲50 au is determined by the heliosphere-VLISM interaction and cannot entirely be represented by pristine interstellar turbulence.

Parallax of the IBEX Ribbon Indicates a Spatially-Retained Source

In 2009, the Interstellar Boundary Explorer (IBEX) discovered the existence of a narrow "ribbon" of intense energetic neutral atom (ENA) emission projecting approximately a circle in the sky. It is believed that the ribbon originates from outside of the heliopause in radial directions ( r ) perpendicular to the local interstellar magnetic field (ISMF), B , i.e., B∙ r = 0. Swaczyna et al. (2016a) estimated the distance to the IBEX ribbon via the parallax method comparing the ribbon position observed from the opposite sides of the Sun. They found a parallax angle of 0.41° ± 0.15°, yielding a distance of140 - 38 + 84 au to a portion of the ribbon at high ecliptic latitudes. In this study, we demonstrate how the apparent shift of the ribbon in the sky, and thus the apparent distance to the ribbon's source found via the parallax, depends on the transport effects of energetic ions outside the heliopause. We find that the apparent shift of the ribbon based on the "spatial retention" model with ion enhancement near B∙ r = 0, as proposed by Schwadron & McComas (2013), agrees with the parallax of the source region. Parallax is also accurate for a homogeneously-distributed emission source. However, if there is weak pitch angle scattering and ions propagate freely along the ISMF, the apparent shift is significantly smaller than the expected parallax because of the highly anisotropic source. In light of the results from Swaczyna et al. (2016a), our results indicate that the IBEX ribbon source is spatially confined.

Strong Scattering of ~keV Pickup Ions in the Local Interstellar Magnetic Field Draped Around Our Heliosphere: Implications for the IBEX Ribbon's Source and IMAP

The leading hypothesis for the origin of the Interstellar Boundary Explorer (IBEX) "ribbon" of enhanced energetic neutral atoms (ENAs) from the outer heliosphere is the secondary ENA mechanism, whereby neutralized solar wind ions escape the heliosphere and, after several charge-exchange processes, may propagate back toward Earth primarily in directions perpendicular to the local interstellar magnetic field (ISMF). However, the physical processes governing the parent protons outside of the heliopause are still unconstrained. In this study, we compute the "spatial retention" model proposed by Schwadron & McComas (2013) in a 3D simulated heliosphere. In their model, pickup ions outside the heliopause that originate from the neutral solar wind are spatially-retained in a region of space via strong pitch angle scattering before becoming ENAs. We find that the ribbon's intensity and shape can vary greatly depending on the pitch angle scattering rate both inside and outside the spatial retention region, potentially contributing to the globally distributed flux. The draping of the ISMF around the heliopause creates an asymmetry in the average distance to the ribbon's source as well as an asymmetry in the ribbon's shape, i.e., radial cross section of ENA flux through the circular ribbon. The spatial retention model adds an additional asymmetry to the ribbon's shape due to the enhancement of ions in the retention region close to the heliopause. Finally, we demonstrate how the ribbon's structure observed at 1 au is affected by different instrument capabilities, and how the Interstellar Mapping and Acceleration Probe (IMAP) may observe the ribbon.

Strong Scattering of ~keV Pickup Ions in the Local Interstellar Magnetic Field Draped Around Our Heliosphere: Implications for the IBEX Ribbon's Source and IMAP

The leading hypothesis for the origin of the Interstellar Boundary Explorer (IBEX) "ribbon" of enhanced energetic neutral atoms (ENAs) from the outer heliosphere is the secondary ENA mechanism, whereby neutralized solar wind ions escape the heliosphere and, after several charge-exchange processes, may propagate back toward Earth primarily in directions perpendicular to the local interstellar magnetic field (ISMF). However, the physical processes governing the parent protons outside of the heliopause are still unconstrained. In this study, we compute the "spatial retention" model proposed by Schwadron & McComas (2013) in a 3D simulated heliosphere. In their model, pickup ions outside the heliopause that originate from the neutral solar wind are spatially-retained in a region of space via strong pitch angle scattering before becoming ENAs. We find that the ribbon's intensity and shape can vary greatly depending on the pitch angle scattering rate both inside and outside the spatial retention region, potentially contributing to the globally distributed flux. The draping of the ISMF around the heliopause creates an asymmetry in the average distance to the ribbon's source as well as an asymmetry in the ribbon's shape, i.e., radial cross section of ENA flux through the circular ribbon. The spatial retention model adds an additional asymmetry to the ribbon's shape due to the enhancement of ions in the retention region close to the heliopause. Finally, we demonstrate how the ribbon's structure observed at 1 au is affected by different instrument capabilities, and how the Interstellar Mapping and Acceleration Probe (IMAP) may observe the ribbon.