Energetic Neutral Atom Flux from the Inner Heliosheath and Its Connection to Termination Shock Properties

Abstract

We present statistical comparisons between energetic neutral atom (ENA) fluxes obtained using a global simulation of the heliosphere and data collected by the Interstellar Boundary Explorer (IBEX) spacecraft. The simulation of the inner heliosheath (IHS) ENA flux is based on a 3D steady-state heliosphere, while the data are from the IBEX-Hi instrument over the time period 2009–2015. The statistical comparison is performed by calculating the chi-square value between the simulated ENA fluxes and the data for each line of sight in the sky. A comparison with exposure-averaged data for solar minimum and solar maximum conditions is also performed to see the effect of solar wind (SW) properties on the IHS ENA fluxes. The model matches well with the data in the flanks and parts of the nose of the heliosphere, whereas the match is poor in the downwind tail, ribbon, and polar regions. We interpret these results to mean that (i) heliosheath plasma in the polar region consists of advected fast (or slow) SW during the solar minimum (or maximum) condition, and (ii) heliospheric termination shock parameters are likely different over the poles. A poor match at around 30° north and south of the downwind direction is likely due to the existence of a mixture of plasma that comes from fast and slow SW. While our results are consistent with a single heliotail, the shape of the heliosphere continues to be an area of active research, and more data and further modeling are needed to determine its true structure.

Tracking the Rapid Opening and Closing of Polar Coronal Holes through IBEX ENA Observations

“…We obtain the value of n p (r HTS ) by following the adiabatic expansion of the SW density at 1 au , where n p (r 1 au ) is the number density of SW at 1 au, and r comp = 3 is the HTS compression ratio, which is close to the average HTS compression ratio from the global MHD simulations of the heliosphere (Pogorelov et al 2009). We use an SW density of 5.74 and 1.82 cm −3 at 1 au depending on heliolatitude for slow and fast SW, respectively, using Equation (4) (Shrestha et al 2020). We note that this compression ratio is greater than the observed HTS compression ratio from V2 (Richardson et al 2008), and the HTS compression ratio inferred from the IBEX ENA observations (Shrestha et al 2021).…”

“…The density of SW plasma within the HTS is obtained by following the adiabatic expansion of SW density at 1 au. The SW density at 1 au is also varied based on the SW speed, i.e., 5.74 cm −3 for slow SW and 1.82 cm −3 for fast SW(Shrestha et al 2020) to keep a similar SW dynamic pressure.…”

The Structure of the Large-Scale Heliosphere as Seen by Current Models

“…For example, Shrestha et al. ( 2020 ) generated an all-sky map of the reduced values between the ENA flux observed by IBEX and the corresponding flux computed using a simulation of the SW–LISM interaction with multiple populations of PUIs. Figure 11 shows where the model agrees well with the data, and where it does not.…”

“…11 All-sky map of the reduced between model ENA flux and IBEX data (from Shrestha et al. 2020 ). For each direction we have summed over fluxes corresponding the IBEX-Hi ESA 2 to 6.…”

Heliospheric Energetic Neutral Atoms: Non-stationary Modeling and Comparison with IBEX-Hi data

“…The PUIs in the inner heliosheath can be divided into subpopulations, transmitted or reflected, depending on its interaction with TS (Zank et al 2010). In some works on the modeling of ENA fluxes the authors made attempts to model the quite distinct populations âĂŞ thermal and pickup protons âĂŞ using one kappa-distribution (e.g., Heerikhuisen et al 2008) or a superposition of Maxwell distributions (Zank et al 2010;Zirnstein et al 2017;Kornbleuth et al 2018;Shrestha et al 2020).…”