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Microstructure of the Heliospheric Termination Shock: Implications for Energetic Neutral Atom Observations

Microstructure of the Heliospheric Termination Shock: Implications for Energetic Neutral Atom Observations,10.1088/0004-637X/708/2/1092,Astrophysical

Microstructure of the Heliospheric Termination Shock: Implications for Energetic Neutral Atom Observations   (Citations: 8)
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The Voyager 2 plasma observations of the proton distribution function downstream of the quasi-perpendicular heliospheric termination shock (TS) showed that upstream thermal solar wind ions played little role in the shock dissipation mechanism, being essentially transmitted directly through the shock. Instead, the hot supra-thermal pickup ion (PUI) component is most likely responsible for the dissipation at the TS. Consequently, the downstream proton distribution function will be a complicated superposition of relatively cool thermal solar wind protons and hot PUIs that have experienced either direct transmission or reflection at the TS cross-shock potential. We develop a simple model for the TS microstructure that allows us to construct approximate proton distribution functions for the inner heliosheath. The distribution function models are compared to kappa-distributions, showing the correspondence between the two. Since the interpretation of energetic neutral atom (ENA) fluxes measured at 1 AU by IBEX will depend sensitively on the form of the underlying proton distribution function, we use a three-dimensional MHD-kinetic global model to model ENA spectra at 1 AU and ENA skymaps across the IBEX energy range. We consider both solar minimum and solar maximum-like global models, showing how ENA skymap structure can be related to global heliospheric structure. We suggest that the ENA spectra may allow us to probe the directly the microphysics of the TS, while the ENA skymaps reveal heliospheric structure and, at certain energies, are distinctly different during solar minimum and maximum.
Journal: Astrophysical Journal - ASTROPHYS J , vol. 708, no. 2, pp. 1092-1106, 2010
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    • ...[4] Substantial progress has been made in the modeling of the interaction of the solar wind plasma with the local interstellar medium through both fluid [Pauls et al., 1995; Zank et al., 2001; Pogorelov et al., 2006; Opher et al., 2009, and references therein] and kinetic [Baranov and Malama, 1993; Izmodenov et al., 2005; Malama et al., 2006; Heerikhuisen et al., 2006, 2008; Zank et al., 2010, and references therein] representations...
    • ...One mechanism [Zank et al., 1996, 2010] is the specular reflection of pickup ions at the narrow shock ramp, analogous to the reflection of solar wind ions...
    • ...For pickup ions, the velocities of interest here are v ∼ uu; this condition combined with l � rTS implies that the exponential factor is of order unity (as in equation (5) of Zank et al. [2010])...
    • ...Note that the latter two � values are considerably larger than the � = 1.5 value assumed in the model of Zank et al. [2010]...

    Pin Wuet al. Hybrid simulations of the termination shock: Suprathermal ion velocity...

    • ...A start was made at more carefully addressing the role of the TS in processing the solar wind and pickup ions [Zank et al., 2010], however much more theoretical work is needed in this area...

    D. J. McComaset al. Evolving outer heliosphere: Large-scale stability and time variations ...

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