An Anomalous Cosmic-Ray Mediated Termination Shock: Implications for Energetic Neutral Atoms

Abstract

The Voyager 2 crossing of the termination shock indicated that most of the upstream energy from the thermal solar wind ions was transferred to pickup ions (PUIs) and other energetic particles downstream of the shock. We use hybrid simulations at the termination shock for the Voyager 2, flank, and tail directions to evaluate the distributions of different ion species downstream of the shock over the energy range of 0.52–55 keV. Here, we extend the work of Gkioulidou et al., which showed an energy-dependent discrepancy between modeled and energetic neutral atom (ENA) observations, and fit distributions to a hybrid model to show that a population of PUIs accelerated via diffusive shock acceleration (DSA) to become low-energy anomalous cosmic rays (ACRs) can bridge the gap between modeled and observed ENA fluxes. Our results with the inclusion of DSA via hybrid fitting give entirely new and novel evidence that DSA at the termination shock is likely to be an important physical process. These ACRs carry a significant fraction of the energy density at the termination shock (22%, 13%, and 19% in the Voyager 2, flank, and tail directions, respectively). Using these ACRs in global ENA modeling of the heliosphere from 0.52 to 55 keV, we find that scaling factors as large as 1.8–2.5 are no longer required to match ENA observations at energies of ∼1–4 keV. Large discrepancies between modeled and observed ENAs only remain over energies of 4–20 keV, indicating that there may be a further acceleration mechanism in the heliosheath at these energies.

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

“…For the MHD models, we use solutions from Kornbleuth et al (2021b), which we briefly describe in Section 2.1. Additionally, we use the ENA model from Kornbleuth et al (2023), which is based upon the model from Kornbleuth et al (2018Kornbleuth et al ( , 2020, and described briefly in Section 2.2.…”

“…The method for determining the parameters used for the ENA model and the parameters themselves are described in detail in Kornbleuth et al (2023). A fitting technique is used to match the total PUI distributions from the hybrid simulation of Giacalone et al (2021) for the Voyager 2, flank, and tail directions using the distributions described above.…”

“…A fitting technique is used to match the total PUI distributions from the hybrid simulation of Giacalone et al (2021) for the Voyager 2, flank, and tail directions using the distributions described above. The best-fit parameters, such as ion density and energy ratios relative to the plasma, are then extrapolated for all directions along the termination shock as a function of termination shock distance (see Equations (5)-( 13) and Table 2 in Kornbleuth et al 2023).…”

“…The long-tail solution is based on the Moscow model (Izmodenov & Alexashov 2015, where the confinement of the solar wind in the heliosheath resembles that seen in the short-heliotail solution of Opher et al (2015), except that the two heliospheric jets exist within a long tail. In this work, we extend the ENA model used in Kornbleuth et al (2021a), as described in Kornbleuth et al (2023), and use both a hybrid model of the termination shock (Giacalone et al 2021) that captures the heating and acceleration of PUIs to infer PUI distributions from the IBEX-Hi energies to the INCA energies and a model for diffusive shock acceleration of PUIs at the termination shock (Wang et al 2023). It is important to note that here we only calculate the acceleration of PUIs at the termination shock.…”

“…It is important to note that here we only calculate the acceleration of PUIs at the termination shock. There are indications (Gkioulidou et al 2022;Kornbleuth et al 2023) that there is an energy gap in the energy range of ∼4-20 keV between modeled ENA fluxes and observations, indicating that there is likely an additional PUI acceleration mechanism in the heliosheath that is not captured by presently available models. One advantage of using the Kornbleuth et al (2023) ENA model is the ability to compare modeled global ENA results with INCA observations, which to date has not been performed, and to investigate if heliotail differences can be discerned at these energies.…”