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Accueil > A propos du LPP > Communication > Actualités archivées > 2012 > Awards Plasma-René Pellat 2011 and 2012 to two former PhD students of the LPP

Awards Plasma-René Pellat 2011 and 2012 to two former PhD students of the LPP

At its 12th Congress held in Orleans late May, Plasma Division of the French Physical Society (SFP) has conducted the awards ceremony "Plasma" for years 2011 and 2012, attributed for the first time under their name Price René Pellat. They were given to Nicolas Aunai and Sedina Tsikata, both of whom prepared their PhD within the LPP. This prize has been awarded since 1992 for young researchers who have produced an exemplary thesis in Plasma Physics. It is for the whole community plasmas (hot, cold, natural) and rewards the work of both fundamental and applied plasma physics. Sedina Tsikata worked on the physics of plasma propulsion before being recruited by the CNRS in the ICARE laboratory (Orleans), while Nicolas Aunai’s work deals with natural plasmas, particularly magnetic reconnection, and the role of microphysical processes on the fluid description of plasma in the case of Earth’s magnetosphere.

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Nicolas Aunai at the Congress of the Plasma Division of SFP on May 23rd, 2012 in Orleans, presenting work for which he earned the René Pellat Award.

Nicolas Aunai’s thesis concerns magnetic reconnection and the role of microphysical processes on the fluid description of plasma.
Nicolas describes his work : "Because of its ability to transfer the energy stored in magnetic field together with the breaking of the flux freezing constraint, magnetic reconnection is considered as one of the most important phenomena in plasma physics. When it happens in a collision less environment such as the terrestrial magnetosphere, it should a priori be modelled within the framework of kinetic physics. However, most of our understanding of the process comes from the more intuitive fluid interpretation. To what extent are these two separate descriptions of the same phenomenon related ? What is the role of kinetic effects in the averaged/fluid dynamics of reconnection ? This thesis addresses these questions for the proton population in the particular case of anti-parallel magnetic reconnection.
It is through the use of a kinetic simulation code that we could understand how the plasma is accelerated in a reconnection site. The study details how the individual particles are accelerated (Figure 1), and how the plasma, as a fluid, is accelerated too (Figure 2). The two approaches, complementary, are interconnected via an understanding of particles collective behavior, which structures the pressure tensor, previously neglected in theoretical models. The plasma acceleration reflects the transfer of magnetic energy to kinetic energy in the process. We showed that the majority of the magnetic energy lost in the nonideal region surrounding the site of reconnection was not communicated to the plasma in the form of convective kinetic energy as commonly accepted, but mostly "lost" as heat flux.

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Figure 1 : The typical trajectory (black line) of a proton is divided into three phases. Two phases of electromagnetic drift, surrounding a stage where the proton oscillates between two electrostatic walls that are the separators (the horizontal component of the electric field is represented in color). During these bouncing, particles gain kinetic energy.

Then the numerical results on plasma acceleration and the role played by the pressure tensor allowed us to set new constraints on the observation of symmetric reconnection sites. These predictions were compared with measurements of the Cluster satellites and validated by the analysis of these data.

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Figure 2 : The fluid trajectory (black) from upstream (top and bottom) of the reconnection site to downstream (right). The fluid is accelerated and deflected downstream by the electric field, whose horizontal component is represented in color code.

The last part of this work was devoted to theoretical modeling of asymmetric current layers such as terrestrial magnetopause. A theoretical model has been developed and allows, in contrast to some previous studies, to construct equilibrium states parametrizable. This is the first time that such a model was proposed and validated by numerical simulation. "
Currently Nicolas Aunai works at Goddard Space Flight Center in Washington as a Post-doctoral fellow. He is continuing there his work on numerical simulation of reconnection, now looking more precisely at the role of electrons. He also participates actively in the scientific preparation of the NASA mission MMS(Magnetospheric Multi Scale, 4 satellites to be launched in 2014), mission for which LPP has built search coil magnetometers to measure the magnetic components of waves.

Photo credit : Alain Roux

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)