TY - JOUR
T1 - Constraining the Evolution of the Proton Distribution Function in the Heliotail
AU - Zirnstein, E. J.
AU - Kumar, R.
AU - Heerikhuisen, J.
AU - McComas, D. J.
AU - Galli, A.
N1 - Publisher Copyright:
© 2018. The American Astronomical Society. All rights reserved.
PY - 2018/10/1
Y1 - 2018/10/1
N2 - 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 km2 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 κ 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 km2 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.
AB - 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 km2 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 κ 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 km2 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.
KW - ISM: atoms
KW - Sun: heliosphere
KW - acceleration of particles
KW - magnetohydrodynamics (MHD)
KW - methods: numerical
KW - solar wind
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U2 - 10.3847/1538-4357/aadb98
DO - 10.3847/1538-4357/aadb98
M3 - Article
AN - SCOPUS:85054622161
SN - 0004-637X
VL - 865
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 2
M1 - 150
ER -