The Impact of Kinetic Neutrals on the Heliotail

Abstract

The shape of the heliosphere is thought to resemble a long, comet tail, however, recently it has been suggested that the heliosphere is tailless with a two-lobe structure. The latter study was done with a three-dimensional (3D) magnetohydrodynamic code, which treats the ionized and neutral hydrogen atoms as fluids. Previous studies that described the neutrals kinetically claim that this removes the two-lobe structure of the heliosphere. In this work, we use the newly developed Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics (SHIELD) model. SHIELD is a self-consistent kinetic-MHD model of the outer heliosphere that couples the MHD solution for a single plasma fluid from the BATS-R-US MHD code to the kinetic solution for neutral hydrogen atoms solved by the Adaptive Mesh Particle Simulator, a 3D, direct simulation Monte Carlo model that solves the Boltzmann equation. We use the same boundary conditions as our previous simulations using multi-fluid neutrals to test whether the two-lobe structure of the heliotail is removed with a kinetic treatment of the neutrals. Our results show that despite the large difference in the neutral hydrogen solutions, the two-lobe structure remains. These results are contrary to previous kinetic-MHD models. One such model maintains a perfectly ideal heliopause and does not allow for communication between the solar wind and interstellar medium. This indicates that magnetic reconnection or instabilities downtail play a role for the formation of the two-lobe structure.

Signature of a heliotail organized by the solar magnetic field and the role of non-ideal processes in modeled IBEX ENA maps: a comparison of the BU and Moscow MHD models

“…The BU model uses the Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics (SHIELD) numerical model based on the works of Michael (2019); Michael et al (2021). This model extends the multi-fluid MHD model used in Opher et al (2015) (first implemented in the BU model in Opher et al (2009)) to include a kinetic treatment of neutral H atoms instead of a multi-fluid treatment of H atoms (Zank et al 1996).…”

“…The BU model uses the Space Weather Modeling Framework (SWMF) (Tóth et al 2005) and couples the Outer Heliosphere (OH) and Particle Tracker (PT) components. The PT component of SWMF is based on the Adaptive Mesh Particle Simulation (AMPS) (Tenishev et al 2021) to treat neutral atoms kinetically by solving the Boltzmann equation for neutral H atoms streaming through the domain and only incorporating effects due to charge exchange. Photoionization is not included, which is a negligible effect at 10 AU where the inner boundary is located.…”

“…Michael et al (2018) modeled the solar magnetic field both with a unipolar and bipolar configuration in a multi-fluid neutral treatment and found the split-tail persisted for the BU model. In a later work, Michael et al (2021) modeled the solar magnetic field with a unipolar configuration treating the neutrals kinetically and concluded a split tail was present in the BU model as well. Despite the inclusion of a bipolar solar magnetic field and kinetic neutrals treatment, the BU model has not been modeled with solar cycle conditions, so further investigation on whether the split tail remains is required.…”

The development of a split-tail heliosphere and the role of non-ideal processes: a comparison of the BU and Moscow models

“…The two numerical models of the heliosphere used in this work are the Moscow model (Izmodenov & Alexashov 2015;Izmodenov & Alexashov 2020) and the BU model (Opher et al 2015;Michael et al 2021). Both models are 3D kinetic-MHD models which treat the cold solar wind plasma and hot pick-up ion plasma as a single fluid, while treating neutral particles kinetically.…”

“…In this work, we compare the recently developed SHIELD model (Michael et al 2021) (herein referred to as the BU model), which extends the MHD model from Opher et al (2015) to include a kinetic treatment of neutrals, with the MHD model from Izmodenov & Alexashov (2015) and Izmodenov & Alexashov (2020). There are other 3D MHD models of the heliosphere (Florinski & Pogorelov 2008;Washimi et al 2011;Zank et al 2013;Pogorelov et al 2015;Heerikhuisen et al 2019;Guo et al 2019), but here we focus on the comparison of the BU and Moscow models to understand the effects of non-ideal MHD processes on the heliosphere.…”