Simulation of the scattering of continuously injected pickup ions in the outer heliosheath

We report on simulations of pickup ions (PUIs) that continuously enter the very local interstellar medium (VLISM) in the outer heliosheath. The PUIs are formed when neutral hydrogen atoms coming from the neutral solar wind charge exchange with plasma ions in the VLISM. The PUIs can charge exchange again becoming energetic neutral atoms (ENA) that return toward Earth to be observed by the Interstellar Boundary EXplorer (IBEX) (McComas 2014). This is the base of the secondary ENAs model, one of the possible explanations for the ENA ribbon observed by IBEX.

The simulations are performed using the 2D version of the hybrid model of Liu et al. (2012). The particle-in-cell (PIC) method is used to treat the kinetic motion of ions in a rectangular space with parameters representing a plasma parcel in the VLISM. The PUIs are continuously injected with a flowing Maxwellian velocity distribution. The evolution of the particle and field energies, the ion velocity distributions, and the magnetic and electric fields in the simulations are shown.

The system progresses from mirror mode waves to ion cyclotron waves as shown in the energy evolution and the wave dispersion relations. It is the ion cyclotron waves that are thought to quickly scatter the PUIs in the outer heliosheath, making it less likely that the PUIs have enough time to charge exchange again and become secondary ENAs observed by IBEX in the ribbon direction. Results for 3 different injection speeds for the PUIs, 200 km/s, 400 km/s and 600 km/s corresponding to variations in the solar wind speed show significant differences in thePUI pitch angle scattering time.

Reference

McComas, D. J., W. S. Lewis, and N. A. Schwadron (2014), IBEX's Enigmatic Ribbon in the sky and its many possible sources, Rev. Geophys., 52, 118-155, doi:10.1002/2013RG000438.

Liu, K., E. Möbius, S. P. Gary, and D. Winske (2012), Pickup proton instabilities and scattering in the distant solar wind and the outer heliosheath: Hybrid simulations, J. Geophys. Res. Space Physics, 117, A10102, doi:10.1029/2012JA017969.