The first attempt to produce the trace-back flux maps of the secondary ISNs on the outer boundary

The secondary interstellar neutral (ISN) atoms are produced by charge exchange reactions between the perturbed interstellar plasma and the ambient neutral atoms in the outer heliosheath. Due to minimized momentum exchange in charge exchange reactions, the secondary ISNs inherit the kinematic properties of their parent ions. Since the plasma conditions in the outer heliosheath are strongly anisotropic with relatively strong spatial gradients of the plasma velocity, density, and temperature, the local production rate of the secondary ISNs should be anisotropic. Hence, the distribution function of the secondary ISNs is expected to be anisotropic and inhomogeneous. However, the predicted distribution function of secondary ISNs at the outer boundary has not been verified by the observations. In this study, we discuss the trace-back flux maps of the secondary ISNs penetrating into the heliosphere through the outer boundary, which is a sphere of heliocentric radius R. The fluxes on the boundary sphere are computed from the IBEX-Lo ISN observations at 1 au with a Keplerian equation of motion in the solar gravity field and a total ionization rate along an atoms trajectory. The trace-back flux maps could provide the distribution function of the secondary ISNs flowing toward the Sun at the outer boundary and the directional distributed features of the secondary ISNs.