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Magnetic reconnection in collisionless turbulent plasmas : Astrophysical applications

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Thesis advisor : Olivier AGULLO
Email and address : olivier.agullo
Tel : 04 91 28 82 51
Co-advisor : Matteo FAGANELLO

Subject description :
Magnetic reconnection represents a textbook example of the multiple-scale behaviour of magnetized plasmas. Although occurring locally at small scale (electron skin depth for collisionless space plasmas or resistive scale for low-collisional laboratory plasmas), reconnection is capable of changing the global topology of the magnetic field and so allows different kind of plasmas (previously "unconnected") to mix-up together [1].
Furthermore reconnection allows the system to reach lower energy states that, before reconnection, were forbidden by topology constrains. Thus this local phenomenon, driven by kinetic effects, is thus able to affect the global transport properties and the global energy balance and, in this way, is able to drive the (fluid-like) dynamics of the whole system. This situation occurs at the boundary between the solar wind and the magnetosphere. Here, the mechanism by which the solar wind can enter the magnetosphere is strongly related to magnetic reconnection : due to the collisionless behaviour of the space medium (low density plasma) and to the presence of a large-scale (magnetospheric and interplanetary) magnetic field, the diffusive processes in this magnetized plasma are believed to be too weak to explain the efficient mixing between the magnetosphere and the solar wind [2]. For this reason, magnetic reconnection has been invoked as the only mechanism able to explain the penetration of the solar wind plasma into the magnetosphere, and finally has been observed by satellites [2, 3].
Until now, these phenomena have been mostly described as a laminar evolution of the system. In contrast, in the solar wind strong temporal and spatial variations are observed. Usually turbulence is investigated using statistical tools in order to understand its role in fundamental processes such as energy dissipation, or more correctly in a collisionless medium, energy transfer from flows and waves to particles (i.e. particle acceleration to high-energies). On the other hand the large turbulent fluctuactions can provide the correct conditions and/or the seed perturbations for the development of "laminar" instabilities that have been reported by satellite observations (e.g. conditions for turbulent reconnection in the magnetosheath [4]).
As a consequence, a correct approach for reconnection at the magnetosphere boundary should account for the turbulent behaviour of the plasma at its external side and focus on the interplay between laminar and turbulent phenomena.
This investigation will be done with the aid of a numerical code that includes, in a fluid
framework, two-fluid effects as finite electron inertia allowing for collisionless reconnection and dominant kinetic correction as Finite Larmor Radius effects. This code is able to run on massively parallel machines and, for the present project, will be used in its reduced 2D version [5] and in its full 3D version [6], gradually increasing the complexity of the system description.
One of the key features of this code are the boundary conditions, which are based on the projected MHD characteristics. In this way, although the bulk of the simulation domain is described by more accurate equations, the user has the total control on what can enter or leave the domain, as in simpler MHD description. In particular large scale waves or alfvenic turbulence can be injected from the boundaries allowing us to study the interaction between laminar large-scale dynamics at the magnetospheric boundary and solar-wind turbulence fluctuations.
Bibliography :
[1] B. Coppi, Phys. Lett. 11, 226 (1964).
[2] G. Le et al., J. Geophys. Res. 99, 23723 (1994).
[3] T.D. Phan et al., Phys. Rev. Lett. 99, 255002 (2007).
[4] A. Retin_o et al., Nature Physics 3, 236 (2007).
[5] S.S. Cerri et al., Phys. Plasmas 20, 112112 (2013).
[6] M. Faganello et al., Europhys. Lett. 100, 69001 (2012).