An Analytical Model to Predict Directional Distribution of the Secondary ISN O Atoms at the Heliopause Derived From IBEX-Lo Observations

Due to the Sun's motion relative to the local interstellar medium, a fraction of interstellar neutral (ISN) atoms cross the heliospheric interface and enters the heliosphere. The penetration of the ISN oxygen has been one of the interesting topics in the heliospheric interface due to its strongly coupling with protons through charge exchange reaction. The two-shock heliospheric model proposed that the charge exchange reaction between hydrogen atoms and oxygen ions in the outer heliosheath causes an enhancement in the number density of neutral oxygen atoms upstream of the heliopause, which is the so-called oxygen wall. The neutral oxygen atoms created by charge exchange reactions in the outer heliosheath are called the secondary ISN oxygen atoms, and a fraction of them penetrates into the inner heliosphere. Therefore, the oxygen atoms provide observational tests of processes at these heliospheric boundaries. The IBEX-Lo neutral-atom detector on the Interstellar Boundary Explorer (IBEX) observed not only the primary ISN oxygen atoms but also the secondary ISN oxygen atoms. Based on the IBEX-Lo oxygen observations from 2009 to 2011, we created an analytic model to derive the directional distribution of the secondary ISN oxygen atoms at the heliopause. This distribution provides the physical parameters of the oxygen wall in the outer heliosheath. Also, this study could be a useful guide for the future Interstellar Probe mission.