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Title: Constraining the Evolution of the Proton Distribution Function in the Heliotail

Abstract

We use Interstellar Boundary Explorer (IBEX) measurements of energetic neutral atoms (ENAs) to constrain the proton (mostly pickup ion, PUI) distribution in the heliotail. In our previous study, we solved the Parker transport equation and found that the velocity diffusion coefficient $D(v)$ for PUIs is approximately $D(v)$ ~ 1.1 × 10 -8 km 2 s -3 ($$v/v_0$$) 1.3, assuming the initial proton distribution processed by the termination shock (TS), $$f_{p,0}$$, is a kappa distribution with kappa index $$ₖ_{p,0}$$ = 1.63. In this study, we test different forms for $$f_{p,0}$$. We find that if $$f_{p,0}$$ is kappa-distributed and $D(v)$ = $$D_0(v/v_0)^{1.3}$$, any kappa index in the range 1.5 < $$κ_{p,0}$$ < 10 is consistent with IBEX data if $$D_0 ~$$ 0.8–1.3 × 10 -8 km 2 s -3. While the case where $D(v)$ ∝ $$v^{1.3}$$ yields ENA fluxes that appear to best reproduce IBEX data for any $$κ_{p,0}$$, it is possible for $D(v)$ to scale close to $$~v^{2/3}$$ or $~v^2$ within our uncertainties by changing $$D_0$$. We also show that an upstream PUI filled-shell distribution that is heated by a quasi-stationary TS, generating a downstream filled-shell with large cutoff speed, yields an excess of ENAs > 2 keV compared to IBEX. However, using a fully kinetic particle-in-cell simulation to process a PUI filled-shell across the TS yields ENA spectra consistent with IBEX, reinforcing the significance of self-consistent, preferential PUI heating and diffusion at the TS. Interestingly, an upstream PUI distribution inferred from the particle-in-cell simulation to reproduce Voyager 2 observations of the nose-ward TS is inconsistent with IBEX observations from the heliotail, suggesting differences in the upstream PUI distribution or TS properties.

Authors:
ORCiD logo [1]; ORCiD logo [1];  [2]; ORCiD logo [1]; ORCiD logo [3]
  1. Princeton Univ., NJ (United States). Dept. of Astrophysical Sciences
  2. University of Alabama, Huntsville, AL (Unites States). Dept. of Space Science
  3. University of Bern (Switzerland). Physics Inst.
Publication Date:
Research Org.:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC)
Sponsoring Org.:
USDOE
OSTI Identifier:
1544067
Resource Type:
Journal Article
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Volume: 865; Journal Issue: 2; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; Astronomy & Astrophysics

Citation Formats

Zirnstein, E. J., Kumar, R., Heerikhuisen, J., McComas, D. J., and Galli, A. Constraining the Evolution of the Proton Distribution Function in the Heliotail. United States: N. p., 2018. Web. doi:10.3847/1538-4357/aadb98.
Zirnstein, E. J., Kumar, R., Heerikhuisen, J., McComas, D. J., & Galli, A. Constraining the Evolution of the Proton Distribution Function in the Heliotail. United States. doi:10.3847/1538-4357/aadb98.
Zirnstein, E. J., Kumar, R., Heerikhuisen, J., McComas, D. J., and Galli, A. Mon . "Constraining the Evolution of the Proton Distribution Function in the Heliotail". United States. doi:10.3847/1538-4357/aadb98.
@article{osti_1544067,
title = {Constraining the Evolution of the Proton Distribution Function in the Heliotail},
author = {Zirnstein, E. J. and Kumar, R. and Heerikhuisen, J. and McComas, D. J. and Galli, A.},
abstractNote = {We use Interstellar Boundary Explorer (IBEX) measurements of energetic neutral atoms (ENAs) to constrain the proton (mostly pickup ion, PUI) distribution in the heliotail. In our previous study, we solved the Parker transport equation and found that the velocity diffusion coefficient $D(v)$ for PUIs is approximately $D(v)$ ~ 1.1 × 10-8 km2s-3 ($v/v_0$)1.3, assuming the initial proton distribution processed by the termination shock (TS), $f_{p,0}$, is a kappa distribution with kappa index $ₖ_{p,0}$ = 1.63. In this study, we test different forms for $f_{p,0}$. We find that if $f_{p,0}$ is kappa-distributed and $D(v)$ = $D_0(v/v_0)^{1.3}$, any kappa index in the range 1.5 < $κ_{p,0}$ < 10 is consistent with IBEX data if $D_0 ~$ 0.8–1.3 × 10-8 km2s-3. While the case where $D(v)$ ∝ $v^{1.3}$ yields ENA fluxes that appear to best reproduce IBEX data for any $κ_{p,0}$, it is possible for $D(v)$ to scale close to $~v^{2/3}$ or $~v^2$ within our uncertainties by changing $D_0$. We also show that an upstream PUI filled-shell distribution that is heated by a quasi-stationary TS, generating a downstream filled-shell with large cutoff speed, yields an excess of ENAs > 2 keV compared to IBEX. However, using a fully kinetic particle-in-cell simulation to process a PUI filled-shell across the TS yields ENA spectra consistent with IBEX, reinforcing the significance of self-consistent, preferential PUI heating and diffusion at the TS. Interestingly, an upstream PUI distribution inferred from the particle-in-cell simulation to reproduce Voyager 2 observations of the nose-ward TS is inconsistent with IBEX observations from the heliotail, suggesting differences in the upstream PUI distribution or TS properties.},
doi = {10.3847/1538-4357/aadb98},
journal = {The Astrophysical Journal (Online)},
issn = {1538-4357},
number = 2,
volume = 865,
place = {United States},
year = {2018},
month = {10}
}