Strength of the Termination Shock Inferred from the Inner Heliosheath ENA Flux.

Abstract:

The energetic neutral atom (ENA) fluxes measured by the IBEX spacecraft show a power law-like dependence on energy in the range ∼0.5 to 6 keV. However, the temperature of the thermal core of the inner heliosheath (IHS) plasma measured by Voyager 2 is ∼180,000 K, which is too small to create ENAs with Earth-directed trajectories. ENAs with energies higher than the thermal core must have come from a suprathermal population of protons, known as pick up ions (PUIs). Some of these PUIs are significantly energized at the heliospheric termination shock (HTS) and are then advected through the IHS along with other PUIs and core solar wind (SW) protons. The energy gained by PUIs upon their crossing at the HTS depends on the strength of the cross-shock potential, which in turn depends on a shock compression ratio that can vary with location. In this work, we estimate the HTS compression ratio at different directions in the sky from a quantitative comparison between the simulated ENA fluxes from the IHS and data collected by the IBEX spacecraft. The simulation of the IHS ENA flux is based on a 3D steady-state simulation of the heliosphere with a multi-Maxwellian description for protons in the IHS, while the data is from the IBEX-Hi instrument over the first three years of the mission. The quantitative comparison is performed by calculating the fractional difference in spectral slope between the simulated ENA flux and data.