Transit-time aspects of ENA production models for the inner heliosheath

Abstract

It is still a major challenge to understand the data of energetic neutral atoms (ENAs) that arrive from the outer heliosphere. These data are provided by the IBEX mission. With the most recent data covering three years of dynamical evolution, it is now possible to study temporal correlations with the solar cycle. We present in this paper for the first time an extended study dedicated to the transit-time delay between the plasma that is ejected from the Sun towards the outer heliosphere and the ENA fluxes that reach the detector after the solar wind protons undergo charge exchange processes in their distant production regions in the inner heliosheath. We derive transit-time delays caused by both general and non-trivial model-dependent contributions for arbitrary points in the inner heliosheath. In addition, we also calculate the average transit-time delay along the line-of-sight in selected directions, allowing to estimate how strongly an extended emission region can reduce the resolution that is possible with transit-time studies. We are able to demonstrate that ENAs generated beyond a certain characteristic streamline distance are mostly removed by charge exchange, and thus do not contribute significantly to the keV-ENA fluxes observed by IBEX. For this reason, transit-time delays do provide a viable tool for studying the impact of the solar cycle on ENA fluxes.

Geometry and Characteristics of the Heliosheath Revealed in the First Five Years of Interstellar Boundary Explorer Observations

“…Since our analysis does not assume radial flow streamlines with constant speed, the cooling depth is a reasonable proxy for the radial LOS integration distance for the majority of ENA flux observed at 1 au. Siewert et al (2014) also supported the importance of this variable when analyzing the IHS plasma properties and resulting ENA fluxes.…”

“…In the flow model, which is derived in Appendix A, the solar wind is represented as flow from a point source, the LISM flow past the Sun is represented as a uniform flow from infinity, and a flow divergence term is included due to the presence of charge exchange (and therefore the flow is slightly compressible, but still irrotational). Similar flow models have been used in recent heliospheric studies by, e.g., Siewert et al (2014), Isenberg et al (2015), Röken et al (2015), Sylla & Fichtner (2015), and Zirnstein & McComas (2015).…”

“…Several early models of ENA flux from the heliosphere focused on sources from the IHS (e.g., Gruntman 1992; Hsieh et al 1992;Czechowski et al 2001;Gruntman et al 2001). The IHS has remained an important subject for ENA modeling in recent years (e.g., Opher et al 2013;Desai et al 2014;Siewert et al 2014;Zirnstein et al , 2015bGloeckler & Fisk 2015;Heerikhuisen et al 2015;Zirnstein & McComas 2015).…”

Tracking the Solar Cycle Through Ibex Observations of Energetic Neutral Atom Flux Variations at the Heliospheric Poles

“…Recent models (e.g., Izmodenov et al 2008;Washimi et al 2011;Pogorelov et al 2013) are now fully three-dimensional and use as constraints data provided by the Voyager spacecraft as they journey through the heliosheath. Particularly relevant to our work, recent efforts have gone into modeling thetime variation in the 1 auENA flux based on realistic solar cycle parameters (e.g., Fahr & Scherer 2004;Sternal et al 2008;Siewert et al 2014;Zirnstein et al 2015). See Zirnstein et al (2015) for a comprehensive summary of the theoretical body of research on the influence of the solar cycle on the heliosphere.…”

On the evolution of the κ distribution of protons in the inner heliosheath

“…The transport of isotropically distributed suprathermal protons in the IHS is governed by five physical processes, namely, convection with the solar wind velocity , magnetic cooling [ Fahr , ; Fahr and Siewert , , , Fahr and Fichtner , ]), diffusion in velocity space, and injection as well as loss due to charge exchange. In a very good approximation one can consider the plasma in the IHS to be incompressible, i.e., for the solar wind velocity field [ Czechowski et al , ], which is a consequence of its rather low Mach number M ≤0.1 [e.g., Siewert et al , ; Fahr and Siewert , ]. This implies that the adiabatic cooling vanishes.…”