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Accueil > A propos du LPP > Communication > Actualités archivées > 2018 > First direct MHD numerical simulation of turbulent heating in the solar wind

First direct MHD numerical simulation of turbulent heating in the solar wind

The solar wind, as it escapes the Sun, cools slower than predicted by simple expansion, which implies that a heat source is embedded in the wind. A possibility, considered here, is that the source relies in waves transported by the wind, that would progressively dissipate into heat (or turbulent dissipation). To test this scenario, one has to measure the dissipation rate and compare it to how much temperature cools with distance. Two methods of measuring turbulent dissipation exist.

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Schéma du soleil et de la zone d’accélération du vent solaire (flèches rouges), guidé par les lignes magnétiques en vert et les ondes (serpentins rouge) servant de réservoir d’énergie pour le chauffage turbulent.

A first method consists in deducing the dissipation rate from the measured wave amplitude, via Politano & Pouquet (1998) relation, which is valid in the frame of magnetohydrodynamics or MHD. The measure has been made in the solar wind by several teams : italian, american and from LPP (Carbone et al 2009, Coburn et al 2012, Hadid et al 2017) ; it has shown that the dissipation of turbulent energy is large enough to explain the slow decrease of temperature, at least at Earth’s orbit.

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Quelques étapes de l’évolution du plasma (couleurs = intensité du champ magnétique) au cours du transport par le vent solaire (résolution numérique directe des équations de la MHD)

A second method, purely numerical, has been used for the first time by Montagud- Camps, Grappin (LPP) et Verdini (Florence). It consists in reproducing the turbulent evolution of a plasma volume transported by the wind by solving directly the MHD equations of fluid evolution. Until now, to do so, published works (Breech et al 2009) used model equations based on approximations difficult to assess. Integrating the primitive MHD equations has been possible thanks to a separation between the average radial wind and turbulent fluctuations, via the so-called "expansion box" method ; it has allowed to follow the plasma evolution transported by the wind between 0.2 astronomical units (AU) and the Earth’s orbit.
The turbulent temperature evolution simulated in this way was shown to correspond to that observed in the wind. This confirms the validity of the MHD description for wavelengths larger the the proton gyroradii. It also implies a direct, simple relation between temperature and heating, that remains to be explained.

More details in
Montagud-Camps, Grappin, Verdini Turbulent Heating between 0.2 and 1 au : A Numerical Study
The Astrophysical Journal 853:153, 2018


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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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Directeur de la publication : Pascal Chabert (Directeur)