Slowing of the Solar Wind in the Outer Heliosphere

This study provides a deeper understanding of how the solar wind evolves with increasing distance from the Sun as it encounters an increasing amount of interstellar material. This work extends our prior work by (1) extending the solar wind proton data radial profiles for New Horizons (NH) out to nea...

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Bibliographic Details
Published in:	The Astrophysical journal Vol. 885; no. 2; pp. 156 - 169
Main Authors:	Elliott, Heather A., McComas, David J., Zirnstein, Eric J., Randol, Brent M., Delamere, Peter A., Livadiotis, George, Bagenal, Fran, Barnes, Nathan P., Stern, S. Alan, Young, Leslie A., Olkin, Catherine B., Spencer, John, Weaver, Harold A., Ennico, Kimberly, Gladstone, G. Randall, Smith, Charles W.
Format:	Journal Article
Language:	English
Published:	Philadelphia The American Astronomical Society 10-11-2019 IOP Publishing
Subjects:	Astrophysics Charged particles Density Heliosphere Interstellar matter ISM: atoms local interstellar matter Solar wind Solar wind density Solar wind velocity Sun: heliosphere Temperature profiles Wind observation Wind speed
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Summary:	This study provides a deeper understanding of how the solar wind evolves with increasing distance from the Sun as it encounters an increasing amount of interstellar material. This work extends our prior work by (1) extending the solar wind proton data radial profiles for New Horizons (NH) out to nearly 43 au, (2) quantifying the observed amount of slowing in the solar wind in the outer heliosphere by performing a detailed comparison between the speeds at NH (21-43 au) with speeds at 1 au, and (3) resolving discrepancies between the measured amount of slowing and estimates of the amount of slowing determined from the measured amount of interstellar pickup present in the solar wind. We find that the solar wind density radial profile may decrease at nearly or slightly less than a spherical expansion density profile. However, the temperature profile is well above what would be expected for an adiabatic profile. By comparing outer and inner heliospheric solar wind observations, we find the solar wind speed is reduced by 5%-7% between 30 and 43 au. We find the solar wind polytropic index (γsw) steeply decreases toward zero in the outer heliosphere (21-43 au) with a slope of ∼0.031 au−1. Using both this radial variation in γsw and the measured amount of interstellar pickup ions, we estimate the slowing in the solar wind and obtain excellent agreement with the observed slowing.
Bibliography:	The Sun and the Heliosphere AAS17767
ISSN:	0004-637X 1538-4357
DOI:	10.3847/1538-4357/ab3e49