Energetic neutral atom flux from the inner heliosheath and its connection to termination shock properties
Date of Award
Doctor of Philosophy (PhD)
Gary P. Zank
Eric J. Zirnstein
Nikolai V. Pogorelov
Heliosphere (Astrophysics), Plasma (Ionized gases)
The energetic neutral atom (ENA) fluxes observed by the Interstellar Boundary Explorer (IBEX) spacecraft show power-law-like spectra in the energy range ~0.5 to 6 keV. However, the temperature of the thermal core solar wind (SW) protons in the inner heliosheath (IHS) measured by Voyager 2 spacecraft is too low to create keV ENAs with Earth-directed trajectories. On the other hand, the temperature of the pickup ions (PUIs) that are reflected and energized in the upstream motional electric field of the heliospheric termination shock (HTS) is too high to create a significant number of ENAs in the IBEX-Hi energy range. Instead, those PUIs that are transmitted directly across the HTS, i.e., without experiencing reflection, should be primarily responsible for the ENAs produced in that energy range. The energy gained by these PUIs upon crossing the HTS depends on a shock compression ratio that can vary with location. The work in this dissertation can be loosely divided into two connected parts. In part 1, we present statistical comparisons between ENA fluxes obtained using a global simulation of the heliosphere and data collected by the IBEX spacecraft. We use a 3D steady-state simulation of the heliosphere to simulate the ENA fluxes by post-processing the MHD plasma using a multi-Maxwellian distribution for protons in the IHS. The data for the comparison is from the IBEX-Hi instrument over the time period 2009-2015. The statistical comparison is performed by calculating the χ2 value between the simulated ENA fluxes and 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 determine the effect of solar wind (SW) properties on the IHS ENA fluxes. This study shows that our model matches the data well in the flanks and parts of the nose of the heliosphere, whereas, the match is less good 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 solar minimum (or maximum) conditions, and (ii) HTS parameters are likely different over the pole compared to equatorial latitudes. 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. In part 2, we further extend the statistical comparison between the observed and simulated IHS ENA fluxes to estimate the HTS compression ratio at multiple directions in the sky. For this purpose, the quantitative comparison is performed by calculating the fractional difference in the spectral slope between the observed and simulated ENA fluxes for a range of compression ratios, where the simulated ENA spectrum is varied as a function of downstream PUI temperature (as a function of compression ratio). The estimated compression ratio in a particular direction is determined by the minimum value of the fractional difference in spectral slope. Our study shows that the compression ratio estimated by this method is in close agreement with the large-scale compression ratio observed by Voyager 2 in its travel direction. Also, the compression ratio in other directions near the ecliptic plane is similar to the compression ratio in the Voyager 2 direction. The weakest shock compression is found to be on the port side of the heliosphere at the direction (27◦, 15◦). This is the first study to estimate globally the HTS compression ratio at multiple directions in the sky from IBEX data.
Shrestha, Bishwas L., "Energetic neutral atom flux from the inner heliosheath and its connection to termination shock properties" (2021). Dissertations. 250.