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Space plasmas turbulence : first in-situ estimation of the dissipation rate at sub-ion scales
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Turbulence is a universal process that is ubiquitous fluids of everyday life and in plasmas. In astrophysics, turbulence plays a key role in a variety of energetic processes such as the accretion of matter around compact objects (e.g., black holes), star formation in the interstellar medium or the heating of the solar corona and wind. This role is all the more important since in these plasmas the usual energy dissipation processes (friction and/or electric resistivity) are nearly inexistent. In the near-Earth space plasmas (i.e., the magnetosphere and the solar wind) it is possible to study turbulence in great detail thanks to in-situ measurements made on board various orbiting spacecraft. Thanks to new theoretical models (developed at the LPP) and multi-point measurements of NASA’s MMS space mission, it was possible for the first time to measure the rate of turbulent energy cascade (or dissipation) at very small scales (<100km) in the terrestrial magnetosheath (part of the solar downstream of the terrestrial bow shock). These measurements highlight the predominant role played by density fluctuations on the small scales even when their effect is negligible on the larger scales. Ultimately, these theoretical and observational works should help resolving (at least partially) the thorny question of the energy partition (resulting from the turbulent cascade coming from large scales) between ions and electrons in weakly collisional plasmas.
The solar wind is a supersonic plasma (flow of charged particles, mainly protons and electrons) that is continuously emitted by the sun. The solar wind propagates in the interplanetary medium and interacts with the planets of the solar system. For planets with an intrinsic magnetic field, a magnetosphere forms around the planet and acts as an obstacle to the solar wind flow. A shock wave is then generated, followed by a very turbulent region called the magnetosheath in which the solar wind slows down, becomes denser and warmer (Figure 1). Plasma turbulence in the magnetosheath is considered a key ingredient for understanding energy transfers and the penetration of particles from the solar wind into the magnetosphere, processes that are responsible for several dynamic phenomena such as the aurora.
Although turbulence in space plasmas has been studied for decades, several of its fundamental properties remain unknown, in particular in the magnetosheath. One of these unknowns is the mean rate at which energy is dissipated in the medium. In a turbulent fluid (air, water in a river), large vortices of comparable size collide with each other, fragment and create vortices of smaller size until reaching the smallest scales of the system where the kinetic energy of the vortices is converted into heat (dissipation). The rate by which this dissipation occurs is the same as that with which the energy of the large vortices is transferred to the smallest (Figure 2). In the magnetosheath, this cascade can cover scales ranging from 100,000 km to 1 km. Unlike neutral fluids, in plasmas the energy involved is that of the electric and magnetic fields (in addition to the kinetic energy of the flow) and its dissipation results in heating of the plasma particles. These processes occur in many astrophysical plasmas (cf. heating of the solar corona, acceleration of cosmic rays, …).
In the article Andrés et al., PRL, 2019 (published end of december), the authors obtained for the first time an estimate of the rate of energy dissipation at kinetic scales, thanks to multipoint measurements of the MMS mission and to new and more complete theoretical models (based on compressible magnetohydrodynamics with Hall effect) recently developed at LPP. The application of these models to a large sample of MMS data in the terrestrial magnetogaine has made it possible to highlight the predominant role played by density fluctuations on small scales even when their effect is negligible on the larger scales. Ultimately, this theoretical and observational work should help to provide new insights into a fundamental question relevant to several astrophysical plasmas, namely what is the quantity of energy (resulting from the turbulent cascade coming from large scales) that is dissipated separately into ions and electrons, when the cascade reaches the kinetic scales. This question, as well as that related to the processes by which this absorption takes place, remain open problems which future work will attempt to answer.

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