Slowing of the Solar Wind in the Outer Heliosphere
Abstract
We examine how the solar wind evolves with increasing distance from the Sun as it encounters an increasing amount of interstellar material. Here, we extend the solar wind proton data radial profiles for New Horizons (NH) out to nearly 45 au, (2) quantify the observed amount of slowing in the solar wind in the outer heliosphere by performing a detailed comparison between the speeds at NH (21-45 au) with speeds at 1 au, and (3) resolve discrepancies between the measured amount of slowing and estimates of the amount of slowing determined from the measured amount of interstellar pickup ions 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 previously found that the solar wind speed is reduced by 5%-7% between 30 and 43 au, and that the solar wind polytropic index (γsw) steeply decreases toward zero in the outer heliosphere from 21 to 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. Additionally, we compare and contrast with the Voyager 2 estimates of the slowing of the solar wind and the radial variation of the polytropic index.
- Publication:
-
43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.895E