Slowing of the Solar Wind in the Outer Heliosphere

Heather A. Elliott, David J. Mccomas, Eric J. Zirnstein, Brent M. Randol, Peter A. Delamere, George Livadiotis, Fran Bagenal, Nathan P. Barnes, S. Alan Stern, Leslie A. Young, Catherine B. Olkin, John Spencer, Harold A. Weaver, Kimberly Ennico, G. Randall Gladstone, Charles W. Smith

Research output: Contribution to journalArticlepeer-review

47 Scopus citations


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 (γ sw) 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 γ sw and the measured amount of interstellar pickup ions, we estimate the slowing in the solar wind and obtain excellent agreement with the observed slowing.

Original languageEnglish (US)
Article number156
JournalAstrophysical Journal
Issue number2
StatePublished - Nov 10 2019

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • ISM: Atoms
  • Sun: Heliosphere
  • local interstellar matter
  • solar wind


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