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Accueil > A propos du LPP > Communication > Actualités archivées > 2018 > Plasma turbulence in the Earth magnetosheath : the heating rate amplified by density fluctuations

Plasma turbulence in the Earth magnetosheath : the heating rate amplified by density fluctuations

The solar wind is a supersonic plasma (charged particles mainly consisting of protons and electrons) which is continuously flowing out from the Sun. It spreads out in the interplanetary medium and interacts with the planets of the solar system. For the planets that have their intrinsic magnetic field, a magnetosphere is formed around them and acts as an obstacle opposing the flow of the solar wind. The solar wind-magnetosphere interaction generates then a bow shock, followed by a very turbulent region called the magnetosheath in which the solar wind slows down, becomes denser and hotter (Figure 1). Plasma turbulence in the magnetosheath is considered a key ingredient for understanding energy transfers and penetration of the solar wind particles into the magnetosphere, processes that are responsible for several dynamical phenomena such as the polar auroras.

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Figure 1
Left (©SOHO/LASCO/EIT NASA, ESA) : illustration showing the solar wind interaction with the Earth magnetosphere. Right (©James Burch) : scheme showing the key regions induced by this interaction, in particular the magnetosheath (main topic of this work).

Despite many decades of research on plasma turbulence in the magnetosheath, several important features remain unknown. One of these features is the rate at which the energy is dissipated in the medium. In a turbulent fluid (air, water in a river), large vortices of similar size collide with each other, and then decompose into smaller and smaller ones until they reach the very small scales where the kinetic energy of the vortices is converted into heat (dissipation). The rate by which this dissipation is done is the same as the one with which the energy of the big vortex is transferred to the smaller ones (Figure 2). In the magnetosheath, this cascade can cover scales ranging from 100,000 km up to 1km. Unlike neutral fluids, in plasma the energy involved is the one of the electric and magnetic fields and its dissipation results in heating or acceleration of the plasma particles. These processes occur in
many astrophysical plasmas (e.g. heating the solar corona, acceleration of cosmic rays, ...).

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Figure 2
simplified sketch of the process of the energy turbulent cascade from the large scales (injection) to the smaller ones (dissipation) with a constant energy flux ε.

In the article, Hadid et al., PRL, 2018, the authors have made significant progress in understanding turbulence in the Earth’s magnetosheath by obtaining the first estimate of the energy dissipation rate. Due to the complex nature of the magnetosheath turbulence and the importance of density fluctuations (i.e., large plasma compressibility), this estimation could not have been made earlier using the classical models of incompressible turbulence widely used in the solar wind. A new and more general theoretical models -based on the magnetohydrodynamics were recently developed at LPP to be able to describe the turbulence in astrophysical plasmas where the effects of compressibility and anisotropy are important.
Applying these models to a large sample of data set collected by the ESA/Cluster and NASA/Themis spacecraft, in the terrestrial magnetosheath, has allowed to obtain a dissipation rate that is at least a hundred times larger than its estimated value in the solar wind, to which density and magnetic fluctuations contribute significantly. The work led to another important result that consists in obtaining a first empirical law that relates the cascade rate to the turbulent sonic Mach number in a compressible medium. The new obtained power-law, if proven to be universal, might be applied to distant astrophysical objects, such as the magnetospheres of other planets or the interstellar medium, where the in-situ measurements are rare or non-existent (Figure 3). The processes by which the energy of the turbulence is dissipated remain an open question to which future work will attempt to answer.

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Figure 3
Artist’s interpretation of the interaction between the edge of the solar system and the interstellar wind forming the heliosheath, a highly turbulent region. This system is very similar to the one shown in Figure 1. © NASA

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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)