The IBEX Ribbon and the Thickness of the Inner Heliosheath

Abstract

Abstract The dominant feature in global images of 1 keV heliospheric energetic neutral atom (ENA) emissions is the Interstellar Boundary Explorer Ribbon. Several mechanisms have been proposed for creating the Ribbon, including the so-called secondary ENA mechanism. Neutral solar wind that is generated inside the heliosphere through charge exchange with interstellar gas penetrates unimpeded into the outer heliosheath. Pickup ions are generated there, followed by a third charge exchange that forms 1 keV neutrals, which re-enter the heliosphere. The Ribbon is also observed at other energies, up to 6 keV and down to 0.2 keV, and thus far there has been little consideration of their sources. In the secondary ENA process, 0.2 keV ENAs originate from 0.2 keV ions in the inner heliosheath. Thus, the ratio of the Ribbon ENA flux at 0.2 and at 1 keV provides information on the sources of the original ion populations and ultimately on the relative sizes of the heliosphere and inner heliosheath. A simple one-dimensional model is constructed and a data-model comparison demonstrates how the different source regions of the Ribbon are exploited to estimate the thickness of the inner heliosheath in the direction of the Ribbon.