Evolution of Entropy and Mediation of the Solar Wind by Turbulence
Abstract
We study the evolution of solar wind entropy based on a conservative formulation of solar wind and turbulence transport model equations, and compare the model results to Voyager 2 measurements. For a polytropic index of γ = 5/3 (>1), entropy increases with distance due to the dissipation of turbulence, being about 12.84% higher at 75 au than at 1 au. However, if the polytropic index satisfies γ < 1, entropy decreases. We show that not only the creation of pickup ions, but also stream-shear leads to a decrease of the solar wind speed. We show that the sum of the solar wind flow energy (kinetic plus enthalpy) and turbulent (magnetic) energy is constant, indicating that kinetic solar wind energy is transferred into turbulent energy via stream-shear and pickup ion isotropization, which then in turn heats the solar wind via the dissipation of turbulence. We compare the theoretical solutions of the solar wind entropy, the solar wind density, the thermal gas pressure, the solar wind proton temperature, and the fluctuating magnetic energy with those measured by Voyager 2. The results show that the theoretical results are in good agreement with the observed results.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- March 2020
- DOI:
- 10.3847/1538-4357/ab7010
- Bibcode:
- 2020ApJ...891...34A
- Keywords:
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- The Sun;
- Solar wind;
- Interplanetary turbulence;
- 1693;
- 1534;
- 830