The inner heliospheric source for keV-energetic IBEX ENAs

Abstract

Context. Up to now, three succesive full-sky maps of heliospheric fluxes of KeV-energetic neutral H-atoms have been registered with the satellite IBEX. The most outstanding feature persisting in these maps is the so-called ribbon which appears as an arc around the upwind direction with unexpectedly high fluxes. This radiation feature was not predicted by models, but at present is tentatively explained as due to energetic plasma sites outside the heliosphere in regions beyond the heliopause. Aims. Since all these proposed explanations, however, need to rely on unproven energizing ion processes, we shall investigate in this article alternative ion sources of keV-ENAs that in contrast appear much closer to the sun, but are based on a clear concept of ion energization to KeV energies, namely suprathermal pick-up ions at a few AU inside and outside of the solar wind termination shock. Methods. Using well-established models for the solar wind magnetic field and the heliospheric TS surface, we derive skymaps of the magnetic field tilt angle, the resulting injection efficiency and the resulting ENA flows. The results are normalised to the ion flux observations made by Voyager-1. We also discuss basic modifications of the TS geometry within the framework of the model. Results. We calculate ENA fluxes of these pick-up ions and show that they, due to their sensitivities to magnetic tilt angles at the termination shock surface, reveal a radiative sickle feature around the upwind direction which in many of its properties resembles the ENA ribbon feature seen by IBEX, although the highest IBEX intensities do not seem to be quantitatively representable by our calculations.

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

“…In two recent publications (Fahr et al 2011;Siewert et al 2012), we have studied two different sources of ENAs at 1 keV. The resulting ENA fluxes as a function of celestial position were in good agreement, both in intensity and spatial shape, with those flux intensities observed by IBEX.…”

“…This ribbon, which consists of a special discrete region in the sky observed in essentially all energy bands measured by both IBEX ENA detectors from ≈100 eV−6 keV, is motivating the development of new ideas and models aiming to explain this unexpected feature (see e.g. McComas et al 2009aMcComas et al , 2010Schwadron et al 2009;Heerikhuisen et al 2010;Grzedzielski et al 2010;Chalov et al 2010;Fahr et al 2011;Siewert et al 2012). One of the most challenging properties of the ribbon is that, while generally exhibiting a power-law characteristic, the power index fitting to the flux data does not result turn out to be a special feature in the ribbon , which suggests that the physical processes generating the ribbon -besides of their absolute rates -are qualitatively and physically similar to those operating in the remaining regions of the sky, thus resulting in the same qualitative spectral shape, and that the only difference between both regions might be the spectral intensity of ENA sources.…”

The inner heliosheath source for keV-ENAs observed with IBEX

“…3.4.1) for the models applied by us). Keeping in mind that the PUI abundance at the shock position essentially turns out to be independent of the direction, given by ξ(α, δ) ξ s (0, 0) 0.15 (see Fahr & Ruciński 1999;Fahr et al 2011), we finally obtain the above expression in the following form…”

Low Energy Neutral Atoms From the Heliosheath

“…Some of these pickup ions charge exchange in the inner heliosheath, creating a population of ENAs with energy that is higher than energies of populations created in processes 1 and 2 in Figure 1 (Malama et al 2006). Finally, the highest energy ENAs (process 4 in Figure 1) are produced by solar wind (and pickup ions) that are accelerated at the termination shock and by turbulence in the inner heliosheath before charge exchanging in the inner heliosheath (Chalov & Fahr 2000;Chalov et al 2003;Fahr et al 2011;Siewert et al 2012;Kucharek et al 2013). The energy of these ENAs extends well beyond the 6 keV upper limit of the IBEX ENA cameras.…”