Anisotropic Distribution of the Secondary Interstellar O Atoms in the Region outside the Heliopause

The IBEX-Lo neutral-atom camera on the Interstellar Boundary Explorer (IBEX) has observed the low-energy (< 50 eV/nuc in the spacecraft reference frame) neutral atoms flowing toward the Sun from the surrounding local interstellar medium (LISM). There are two sources of these low-energy neutral atoms: the unperturbed interstellar neutral atoms (i.e., the primary ISN atoms) and the neutralized interstellar ions (i.e., the secondary ISN atoms). The secondary ISN atoms are created by charge exchange between the disturbed interstellar ions and the unperturbed ISN atoms in the region outside the heliopause. The relative motion of the Sun with respect to the LISM results in entering of these neutral atoms into the inner heliosphere. Since charge-exchange reactions occur without momentum exchange between collision partners, the secondary ISNs inherit the kinematic properties of their parent ions and they can be a crucial tool to investigate the disturbed interstellar plasma ions in the region outside the heliopause. Initial analyses of these secondary ISN populations have typically used a superposition of two Maxwellian velocity distribution functions (VDF) at far away from the Sun. However, the reliability of the two-Maxwellian approximation is questionable because the Maxwellian VDF is likely a poor approximation for the secondary ISNs, which originate in the disturbed interstellar ions. In our previous study (Park et al. 2019, ApJ, 880, 4), we found the most likely flow parameters of the secondary ISN O via the analytical model of ISNs with the two-Maxwellian approximation. In this study, we are taking one step further. We assume that the secondary ISN O has a bi-Maxwellian VDF in the region beyond the heliopause and then reanalyze the IBEX-Lo O observations. We present the quantitative differences in the parameter estimation using the bi-Maxwellian VDF versus the two-Maxwellian approximation. Especially, we discuss anisotropic temperatures of the secondary ISN O atoms because the previous estimated temperature of T=10,000 K is much lower than the expected temperature of T~20,000 K for the region outside the heliopause, which is deduced by the Kinetic-MHD model of the global heliosphere.