Energetic Neutral Atom Maps from a Kinetic-MHD Description of the "Croissant-like" Heliosphere

Opher et al. (2015) suggested that due to the collimation of the solar wind plasma by the solar magnetic field, two high latitude lobes would emerge, resulting in a shortened heliotail with a "croissant-like" shape. Other works hypothesized that using a kinetic treatment of neutrals in modeling the heliosphere would lead to the disappearance of this "croissant-like" shape. Recently, Michael et al. (2019) showed that using the Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics (SHIELD) model, which is a 3D MHD model coupled with a kinetic description of neutrals, the "croissant-like" structure of the heliosphere persists. The Interstellar Boundary Explorer (IBEX) is probing the heliosphere by using energetic neutral atoms (ENAs). McComas et al. (2013) and Schwadron et al. (2014) showed two high latitude lobes of increased ENA flux at the highest IBEX energies, with a deficit of ENA flux in the low latitude tail. This observed structure was suggested to be the result of the latitudinal variation of the solar wind. Zirnstein et al. (2017) showed that using a time dependent model of the heliosphere, the ENA structure observed by IBEX could be reasonably replicated. Kornbleuth et al. (2018) showed that the collimation of the solar wind plasma seen by Opher et al. (2015) could also lead to the emergence of high latitude lobes of increased ENA flux in the absence of a varying solar wind structure. In this work, we use the SHIELD model of the heliosphere to investigate the underlying effect of solar wind collimation on ENA maps. We present maps from a case where no collimation is present (by neglecting solar magnetic field) and compare with a case where collimation is present. In both cases we include the solar wind latitudinal variations as in solar minimum in 2008 using a model developed by Sokol et al (2015), which is based on the interplanetary scintillation observations of the solar wind structure (Tokumaru et al 2012). We find that while a latitudinally-varying solar wind structure can replicate IBEX observations in the absence of solar wind collimation, the inclusion of collimation causes an enhancement of the high latitude lobes at the highest IBEX energies. As the solar magnetic field strengthens or weakens over the course of a solar cycle, the varying strength of the collimation should be observable in IBEX ENA observations.