The inner heliosheath source for keV-ENAs observed with IBEX

Abstract

Context. The "IBEX ribbon" feature provides significant challenges to theoretical modeling attempts of the outer heliosphere. Classical ENA production by shock-processed nonthermal tails results in no relevant ENA signatures, mainly due to the downstream solar wind proton temperature observed by the Voyagers being one order of magnitude smaller than expected from the classical monofluid shock model. Aims. Here, we therefore study shock-processed pick-up ions (PUIs) in the inner heliosheath resulting from a multifluid shock model as a source of keV-energetic ENAs as have been detected by the IBEX mission. Unlike previous studies, we apply a semikinetic multifluid shock model, which did prove useful in explaining the apparent temperature discrepancy in the past. Methods. To convert upstream into downstream pick-up ions, we use both kinetic and multifluid theories describing the solar wind termination shock (TS) transition. This allows us to obtain the downstream PUI distribution function as a function of classical shock properties, such as the local magnetic field tilt angle and the compression ratio. In addition, this kinetic model also allows to derive a formulation of latitude-and longitude-dependent spectral intensities between 1 and 100 keV, a region which is not covered by missions such as the Voyagers and needs to be covered by theoretical considerations. Results. After converting shock-processed PUIs to ENAs by charge exchange with cold H-atoms, we find keV ENA fluxes of the same order of magnitude as those observed by IBEX. These fluxes also exhibit a pronounced ring-type feature, with about the correct ratio of ring-to-nose intensities, producing encouraging overall agreement with IBEX data, potential of further improvement.

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

“…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.…”

“…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.…”

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.…”

SPATIAL RETENTION OF IONS PRODUCING THEIBEXRIBBON

“…9. Siewert et al (2012) further studied the concept of shock accelerated particles using kinetic and multifluid theories describing the solar wind termination shock (TS) transition. This allowed derivation of the downstream pickup ion (PUI) distribution function as a function of shock properties, such as the local magnetic field tilt angle and the compression ratio.…”