Turbulence in the Local Interstellar Medium and the IBEX Ribbon

Abstract

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 have considered to be either spatiallyuniform or a mean 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 the fluctuations to ~10 au or smaller 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 ~10 au. This suggests that the magnetic field structure at scales ≲10 au is determined by the heliosphere's interaction with the VLISM and cannot entirely be represented by homogeneous interstellar turbulence.

On the Energization of Pickup Ions Downstream of the Heliospheric Termination Shock by Comparing 0.52–55 keV Observed Energetic Neutral Atom Spectra to Ones Inferred from Proton Hybrid Simulations

“…The heated PUIs that populate the HS charge-exchange with the interstellar neutrals and are measured remotely by the Interstellar Boundary Explorer (IBEX; 0.01-6 keV) and Cassini/Ion and Neutral Camera (INCA; 5.2-55 keV), providing energetic neutral atom (ENA) full-sky maps (McComas et al 2009;Krimigis et al 2009). These images showed the existence of a bright and narrow ribbon (Schwadron et al 2009) of ENA emissions that is measured by IBEX-Hi and is thought to lie beyond the HP, formed through a secondary ENA process (e.g., Heerikhuisen et al 2010;McComas et al 2017) and the globally distributed flux (GDF; Schwadron et al 2011), a "background" ENA flux, with the IBEX ribbon, which is largely produced in the HS, removed (Dayeh et al 2011;Zank et al 2010;McComas et al 2020;Zirnstein et al 2020). Spectral agreements between Cassini/ INCA ENA measurements and the Voyager ion measurements suggest that the source of ENA emissions at energies above 5.2 keV are most likely produced by charge-exchange interactions inside the HS (Dialynas et al 2013(Dialynas et al , 2017.…”

Random Walk and Trapping of Interplanetary Magnetic Field Lines: Global Simulation, Magnetic Connectivity, and Implications for Solar Energetic Particles

“…Although this approach may be adapted rather directly to different applications, we should recall that this theoretical development is approximate, given simplifications such as the neglect of correlations between the deflections due to large-scale expansion and the deflections due to the modeled small-scale turbulence. It may be of particular interest to extend this approach to the outer heliosphere and the very local interstellar medium, locations where field line connectivity may be relevant to explaining observations by Voyager, IBEX, and the upcoming IMAP missions (McComas et al 2018;Zank et al 2019;Zirnstein et al 2020;Fraternale et al 2020). To obtain a relationship between the 2D bendover scale λ 2 and the 2D correlation scale λ c2 (see Section 2.3), we begin with the following form for the 2D modal power spectrum (Matthaeus et al 2007): where C 2 is a dimensionless constant, k ⊥ is a 2D wavenumber, and p is a power-law index that determines the longwavelength behavior of the spectrum.…”

Nearly a Decade of Cosmic Ray Observations in the Very Local Interstellar Medium

“…However, as the authors noted, turbulence could instead participate in the formation of the anisotropies in protons or possibly contribute to the scattering of electrons in some other way. For example, an isotropic turbulence model by [58] demonstrated that pre-existing VLISM turbulence could effectively trap suprathermal protons (of a few keV in energy) with near-90° pitch angles through magnetic mirroring, and their results were generally consistent with the enhanced energetic neutral atom signature of the IBEX ribbon [59]. Although the Voyager…”