tokens: ; Livadiotis McComas 2012 , 2013b ; Livadiotis 2016 ). Therefore, the IBEX maps may be consistent with the radial trend in γ sw detected with NH -SWAP. If we find that γ sw behaves in a systematic way using Voyager 2 observations, we may be able to monitor γ sw with NH and forecast when NH will reach the termination shock or at least determine if the location of the termination shock is moving dramatically inward or outward. Many theoretical and simulation studies have explored how the process of the solar wind picking up interstellar material can heat the solar wind (Matthaeus et al. 1999 ; Smith et al. 2001 ; Isenberg et al. 2003 ; Adhikari et al. 2017 ; Zank et al. 2018 ). Given that when we account for the radial variation of γ sw , we are then able to obtain excellent agreement between the inferred slowing from the amount of pickup and the amount of observed slowing, we conclude our work provides further observational evidence that the solar wind picking up interstellar pickup ions likely plays a role in heating the solar wind and altering the T – n relationship with increasing radial distance. However, additional work combining simulations and data should be performed to determine exactly why the solar wind T – n relationship systematically varies in a way that produces a linear decrease in γ sw with increasing distance from the Sun. Possibly, solar wind turbulence or wave particle interactions generated by the pickup process heat the solar wind altering the T – n relationship with increasing radial distance. In the inner heliosphere compressions, shocks, and shears are strong, but in the outer heliosphere all of those are much weaker. Therefore, those are not likely to impact the changes in the T – n relationship to the degree that waves and turbulence related to the waves could in the outer heliosphere. Our solar wind polytropic index radial profile γ sw ( r ) for the outer heliosphere, our initial analysis of the evolution of the total polytropic index ( γ ) from the inner to outer heliosphere, and the total polytropic index ( γ ) in the heliosheath from IBEX provide substantial metrics for outer heliospheric simulations. This work will become more relevant as NH gets closer to the termination shock heading in the direction of the ribbon and higher-resolution maps of the ENA ribbon from the Interstellar Mapping and Acceleration Probe ( IMAP ) mission become available (McComas et al. 2018 ). Below, we summarize our main findings and conclusions. 1. In the outer heliosphere (21–43 au), we found the density radial profile to decrease at either nearly or slightly less than the spherical expansion rate. 2. As NH moves farther away from the Sun, we can assess if picking up of interstellar material hinders the expansion of the solar wind altering the solar wind density profile. 3. In the outer heliosphere (21–43 au), the temperature profile is well above what is expected for an adiabatic profile. 4. The relationship between the solar wind
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