First sky map of the inner heliosheath temperature using IBEX spectra


Analysis of the IBEX-Hi Energetic Neutral Atom spectra reveals, for the first time, the sky map of the source ion temperatures. The solar wind exists in non-equilibrium stationary states and can be described by kappa distributions. The high-energy asymptotic behavior of kappa distributions leads to a power law of the flux versus energy spectrum, while its specific formulation derives the temperature and kappa index that govern these distributions. We find that the observed temperature in most directions is about a million degrees, in agreement with most heliospheric models for the inner heliosheath. Thus, the termination shock is stronger in most places than revealed by the Voyagers' observations at their two unique crossing points. The global sky maps indicate low-temperature regions in various directions, including toward Voyager 2 where the temperature is an order of magnitude lower, consistent with in situ Voyager observations. Interestingly, the vast majority of measured kappa indices are between similar to 1.5 and similar to 2.5, consistent with the far-equilibrium "cavity" of minimum entropy discovered by Livadiotis & McComas. The sky maps suggest that the Ribbon is a string of localized regions, while its thermodynamical behavior differentiates it from the global distributed flux. A simple model is developed to derive the density, heliosheath thickness, and thermal pressure. We find that the Ribbon thermal pressure is similar to 3.5 pDyn cm(-2), roughly equal to the mechanical pressure exerted by the Local Interstellar Medium.

Publication Date


Journal Title

Astrophysical Journal



Digital Object Identifier (DOI)


Document Type



© 2011. The American Astronomical Society. All rights reserved.