Spectral properties of keV-energetic ion populations inside the heliopause reflected by IBEX-relevant energetic neutral atoms

Abstract

For more than three years now, the IBEX mission monitors fluxes of energetic neutral atoms (ENAs) in the range between 0.1 to 6 keV from all directions of the sky. During this extended observation period, it could be confirmed that an outstanding ENA emission feature, called the "ribbon", has been continuously found over the past years. In addition, spectral flux features also have become the target of refined observations. In this study, we thus aim at delivering theoretically calculated results on spectral ENA flux properties. The ENAs we are considering in this paper are due to two types of transcharged pick-up ions present in the heliosheath region: primary pick-up ions that have been energetically processed at their passage over the termination shock, and ions induced to the pick-up ion regime from adiabatically cooled anomalous cosmic ray ions. Both components have different spectra, so we need to consider ENA fluxes generated by charge-exchange processes from the superposed distributions of these two populations. We derive the spectral features of ENA fluxes along a line-of-sight as a function of the celestial direction and compare them with IBEX observations. The power-law characteristics of the resulting spectra exhibit a good match with the average power indices observed by IBEX in large sections of the heliosphere, with the notable exception of the heliotail, for which our adopted heliospheric model does not provide an accurate enough representation.

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

“…This allowed us to solve a complication with ENA modeling that was identified by, for example, Siewert et al (2013), namely the upper limit for the line-of-sight integral in the heliotail, where the heliopause distance grows very large, reaching distances where ENA removal by reionization becomes important. In the earlier study, we had introduced a cutoff distance of 300 AU, which is quite close to the cutoff obtained here.…”

“…We are currently working on a new study by updating our earlier model calculations Siewert et al 2012Siewert et al , 2013 to include these additional corrections; for this, we carefully investigate how these new effects would quantitatively modify ENA flux spectra and related properties. This also includes a modified heliosheath pressure distribution and hence as a reaction of that, the heliosheath streamline configuration.…”

“…thermal solar wind, pick-up ions) of the plasma flow in this region (see e.g. Zank et al 2010;Livadiotis et al 2011;Siewert et al 2013). In contrast to this, identifying a possible modulation of more distant sources beyond the heliopause is less trivial because of the specific configurations and assumptions used by models devoted to these sources (see e.g.…”

“…In this study, we first summarize the equations and formulae required to calculate transit-time delays in sufficiently general situations, which serve as "building blocks" that can then be used to study explicit models. Following this, we use this formalism to study our own model for ENA production in the inner heliosheath Siewert et al 2012Siewert et al , 2013 that is based on established analytic models for relevant properties of the inner heliosheath, such as the termination shock (TS) geometry, the plasma flow profile in the inner heliosheath (iHS), and the heliopause (HP). As we demonstrate, the finite line-ofsight extent of the ENA production region reduces the amount of information that can be gained from transit-time and solar wind correlation studies, but nevertheless, it delivers a selection criterion between competing models, especially those with transheliopause ENA production sites.…”

On the electron temperature downstream of the solar wind termination shock

“…Especially the so-called pick-up ions definitely play an important role (see, e.g. Fahr and Chalov, 2008;Zank et al, 2010;Siewert et al, 2013). They collectively as a multifluid plasma define the consistent compression ratio as shown in the implicit multifluid compression equation s = S(s), as derived by Fahr et al (2012).…”