PRESSURE OF THE PROTON PLASMA IN THE INNER HELIOSHEATH

We develop a physical model to study the pressure of the source proton plasma in the inner heliosheath based on the IBEX Energetic Neutral Atom (ENA) spectra. A multiple linear regression technique is used to parameterize the proton distribution function, by connecting the observed ENA flux spectrum from IBEX-Hi with the power-law of the model proton distribution. We calculate the partial pressure over the measured energy range, using (1) a non-parametric method by integrating the observed ENA flux, and (2) a parametric method by integrating the modeled distribution of protons in the inner heliosheath. The two sky maps of the parametric and non-parametric partial pressures are nearly identical, owing to their power-law distributions at high energies; the kappa distribution is such a function that can be reduced to a power-law in the IBEX-Hi energy range. The slight differences between the two partial pressures may indicate protons that are not described by the kappa distribution, and instead, involve newer or "immature" (spherical shell) pick-up proton distributions. Ultimately, however, these become incorporated with the solar wind into one single proton population described by a kappa distribution. Moreover, we derive analytically (1) the ENA flux spectra, which suggests that this flux maximizes at similar to 30 eV, and (2) the differential pressure, which provides estimates of the partial pressures outside of those measured by IBEX-Hi. Under the assumptions of the modeled ENA spectra, the Ribbon emissions appear to be primarily limited to the energy ranges of IBEX-Lo and IBEX-Hi.