Hybrid Simulations of Interstellar Pickup Protons Accelerated at the Solar-wind Termination Shock at Multiple Locations

Abstract

We estimate the intensity of interstellar pickup protons accelerated to ∼50 keV at various locations along the solar-wind termination shock, using two-dimensional hybrid simulations. Parameters for the solar wind, interstellar pickup ions (PUIs), and magnetic field just upstream of the termination shock at one flank of the heliosphere, and at the location in the downwind (or tail-ward) direction are based on a solar-wind/pickup-ion/turbulence model. The parameters upstream of the shock where Voyager 2 crossed are based on observations. The simulation is limited in size, and therefore cannot accurately model the distribution to energies much beyond ∼50 keV. This is sufficient to study the origin of the high-energy tail of the distribution, which is the low-energy portion of the anomalous cosmic-ray spectrum. We also extrapolate our results to other locations along the termination shock, such as the other flank, and the poles of the heliosphere. We find that the intensity of ∼10–50 keV accelerated pickup protons is remarkably similar at all three locations we simulated, suggesting that particles in this energy range are relatively uniformly distributed along the termination shock, and are likely quite uniform throughout the entire heliosheath. In addition, we find significant differences in the distribution in the 0.5–1 keV energy range for energetic neutral atoms coming from the tail region of the heliosphere compared to that at the nose or flank look directions. This is because the peak in the PUI distribution is at a higher energy there.

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

“…We use the hybrid model by Giacalone et al (2021) to produce interstellar pickup protons downstream of the TS, which have been reflected and some further accelerated at the TS (Zank et al 1996). The hybrid simulation is twodimensional, however, vectors, such as average proton velocity, magnetic field, electric field, etc., are not confined to the simulation plane, and they point in three directions.…”

Anomalous Cosmic Rays and Heliospheric Energetic Particles

“…This was supported by numerical simulation of pickup ion acceleration at the termination shock at multiple locations (Giacalone et al. 2021 ).…”

“…This figure is taken from Giacalone et al. ( 2021 ). The black squares in the right figure are Voyager 2 LECP observations taken just after the crossing of the termination shock (see also Giacalone and Decker 2010 , for more details) …”

“…Hybrid simulations of the interaction of pickup ions with the termination shock, showing efficient acceleration, were performed initially by Giacalone and Decker ( 2010 ), and more recently by Giacalone et al. ( 2021 ). The simulations treat all the ions (solar wind and pickup) kinetically and effectively solve for the kinetic shock physics on ion scales.…”

“…Recently, Giacalone et al. ( 2021 ) extended the study of Giacalone and Decker ( 2010 ) to study the acceleration of pickup ions at other locations along the termination shock in addition to that where V2 crossed. The right panel of Fig.…”