Sujets de stage de Master 2 pour 2014

Nous proposons deux sujets de stage de Master 2 pour le printemps 2014. Ces deux sujets ont pour thématique la physique des plasmas astrophysiques.

  • stage 1 : La nongyrotropie électronique permet-elle d’identifier un site de reconnexion ?
  • stage 2 : Equilibre cinétique : quelle condition initiale pour la reconnexion magnétique ?

Les deux stages ont une forte composante “simulation numérique”, le second demande un plus gros effort de programmation (fortran et C) car il requiert de modifier d’avantage le code de simulation. Il est peut-être également un peu plus théorique sur le plan conceptuel que le second stage. Le premier stage en revanche comporte également un volet observationnel que le second n’a pas. Les deux stages sont des sujets actuels de recherche, ils sont suffisament ciblés pour garantir un résultat (pas d’inconnue sur le plan technique).

Note importante : Une thèse au sein de l’équipe est possible sur les mêmes thématiques, avec un financement ANR déjà acquis.

Electron nongyrotropy in the context of collisionless magnetic reconnection

Electron nongyrotropy in the context of collisionless magnetic reconnection

Published in Physics of Plasmas

Nicolas Aunai, Michael Hesse

Collisionless magnetized plasmas have the tendency to isotropize their velocity distribution function around the local magnetic field direction, i.e., to be gyrotropic, unless some spatial and/or temporal fluctuations develop at the particle gyroscales. Electron gyroscale inhomogeneities are well known to develop during the magnetic reconnection process. Nongyrotropic electron velocity distribution functions have been observed to play a key role in the dissipative process breaking the field line connectivity. In this paper, we present a new method to quantify the deviation of a particle population from gyrotropy. The method accounts for the full 3D shape of the distribution and its analytical formulation allows fast numerical computation. Regions associated with a significant degree of nongyrotropy are shown, as well as the kinetic origin of the nongyrotropy and the fluid signature it is associated with. Using the result of 2.5D Particle-In-Cell simulations of magnetic reconnection in symmetric and asymmetric configurations, it is found that neither the reconnection site nor the topological boundaries are generally associated with a maximized degree of nongyrotropy. Nongyrotropic regions do not correspond to a specific fluid behavior as equivalent nongyrotropy is found to extend over the electron dissipation region as well as in non-dissipative diamagnetic drift layers. The localization of highly nongyrotropic regions in numerical models and their correlation with other observable quantities can, however, improve the characterization of spatial structures explored by spacecraft missions.

Results of my research proposals

Last March/April I have written 3 research proposals to fund my next post doc at IRAP, Toulouse, France. I got some results, here they are:

  • CNES: I was selected for a CNES fellowship, duration is two years.
  • AXA: My proposal has been selected by AXA Research Funds for a 2 years duration. They fund 30 proposal per year world wide, global selection rate is 10-15%. Here is a list of the selected proposals.
  • ANR Retour Postdoc: My proposal is among the 34 selected to be funded out of the 149 multidisciplinary applications. Duration of the project is 3 years, including the salary of a phd student. (Edit: 08/29)

Aspects of collisionless magnetic reconnection in asymmetric systems

Aspects of collisionless magnetic reconnection in asymmetric systems

Published in Physics of Plasmas

Michael Hesse, Nicolas Aunai, Seiji Zenitani, Maria Kuznetsova and Joachim Birn

Asymmetric reconnection is being investigated by means of particle-in-cell simulations. The research has two foci: the direction of the reconnection line in configurations with nonvanishing magnetic fields; and the question why reconnection can be faster if a guide field is added to an otherwise unchanged asymmetric configuration. We find that reconnection prefers a direction, which maximizes the available magnetic energy, and show that this direction coincides with the bisection of the angle between the asymptotic magnetic fields. Regarding the difference in reconnection rates between planar and guide field models, we demonstrate that a guide field can provide essential confinement for particles in the reconnection region, which the weaker magnetic field in one of the inflow directions cannot necessarily provide.

Influence of the dissipation mechanism on collisionless magnetic reconnection in symmetric and asymmetric current layers

Influence of the dissipation mechanism on collisionless magnetic reconnection in symmetric and asymmetric current layers



Published in Physics of Plasmas

Nicolas Aunai, Michael Hesse, Carrie Black, Rebekah Evans, Maria Kuznetsova

Numerical studies implementing different versions of the collisionless Ohm’s law have shown a reconnection rate insensitive to the nature of the non-ideal mechanism occurring at the X line, as soon as the Hall effect is operating. Consequently, the dissipation mechanism occurring in the vicinity of the reconnection site in collisionless systems is usually thought not to have a dynamical role beyond the violation of the frozen-in condition. The interpretation of recent studies has, however, led to the opposite conclusion that the electron scale dissipative processes play an important dynamical role in preventing an elongation of the electron layer from throttling the reconnection rate. This work re-visits this topic with a new approach. Instead of focusing on the extensively studied symmetric configuration, we aim to investigate whether the macroscopic properties of collisionless reconnection are affected by the dissipation physics in asymmetric configurations, for which the effect of the Hall physics is substantially modified. Because it includes all the physical scales a priori important for collisionless reconnection (Hall and ion kinetic physics) and also because it allows one to change the nature of the non-ideal electron scale physics, we use a (two dimensional) hybrid model. The effects of numerical, resistive, and hyper-resistive dissipation are studied. In a first part, we perform simulations of symmetric reconnection with different non-ideal electron physics. We show that the model captures the already known properties of collisionless reconnection. In a second part, we focus on an asymmetric configuration where the magnetic field strength and the density are both asymmetric. Our results show that contrary to symmetric reconnection, the asymmetric model evolution strongly depends on the nature of the mechanism which breaks the field line connectivity. The dissipation occurring at the X line plays an important role in preventing the electron current layer from elongating and forming plasmoids.

Comparison between hybrid and fully kinetic models of asymmetric magnetic reconnection: Coplanar and guide field configurations

Comparison between hybrid and fully kinetic models of asymmetric magnetic reconnection: Coplanar and guide field configurations



ADS Link, DOI

Aunai, NicolasHesse, MichaelZenitani, Seiji,  Kuznetsova, MariaBlack, CarrieEvans, Rebekah and Smets, Roch

Magnetic reconnection occurring in collisionless environments is a multi-scale process involving both ion and electron kinetic processes. Because of their small mass, the electron scales are difficult to resolve in numerical and satellite data, it is therefore critical to know whether the overall evolution of the reconnection process is influenced by the kinetic nature of the electrons, or is unchanged when assuming a simpler, fluid, electron model. This paper investigates this issue in the general context of an asymmetric current sheet, where both the magnetic field amplitude and the density vary through the discontinuity. A comparison is made between fully kinetic and hybrid kinetic simulations of magnetic reconnection in coplanar and guide field systems. The models share the initial condition but differ in their electron modeling. It is found that the overall evolution of the system, including the reconnection rate, is very similar between both models. The best agreement is found in the guide field system, which confines particle better than the coplanar one, where the locality of the moments is violated by the electron bounce motion. It is also shown that, contrary to the common understanding, reconnection is much faster in the guide field system than in the coplanar one. Both models show this tendency, indicating that the phenomenon is driven by ion kinetic effects and not electron ones.

Electric and magnetic contributions to spatial diffusion in collisionless plasmas

Electric and magnetic contributions to spatial diffusion in collisionless plasmas

ADS link, DOI Smets, R.Belmont, G.Aunai, N. We investigate the role played by the different self-consistent fluctuations for particle diffusion in a magnetized plasma. We focus especially on the contribution of the electric fluctuations and how it combines with the (already investigated) magnetic fluctuations and with the velocity fluctuations. For that issue, we compute with a hybrid code the value of the diffusion coefficient perpendicular to the mean magnetic field and its dependence on the particle velocity. This study is restricted to small to intermediate level of electromagnetic fluctuations and focuses on particle velocities on the order of few times the Alfvén speed. We briefly discuss the consequences for cosmic ray modulation and for the penetration of thermal solar wind particles in the Earth magnetosphere.

Kinetic equilibrium for an asymmetric tangential layer

Kinetic equilibrium for an asymmetric tangential layer

(ADS Link, DOI) Belmont, G.Aunai, N., and Smets, R. Finding kinetic (Vlasov) equilibria for tangential current layers is a long standing problem, especially in the context of reconnection studies, when the magnetic field reverses. Its solution is of pivotal interest for both theoretical and technical reasons when such layers must be used for initializing kinetic simulations. The famous Harris equilibrium is known to be limited to symmetric layers surrounded by vacuum, with constant ion and electron flow velocities, and with current variation purely dependent on density variation. It is clearly not suited for the “magnetopause-like” layers, which separate two plasmas of different densities and temperatures, and for which the localization of the current density j=nδv is due to the localization of the electron-to-ion velocity difference δv and not of the density n. We present here a practical method for constructing a Vlasov stationary solution in the asymmetric case, extending the standard theoretical methods based on the particle motion invariants. We show that, in the case investigated of a coplanar reversal of the magnetic field without electrostatic field, the distribution function must necessarily be a multi-valued function of the invariants to get asymmetric profiles for the plasma parameters together with a symmetric current profile. We show also how the concept of “accessibility” makes these multi-valued functions possible, due to the particle excursion inside the layer being limited by the Larmor radius. In the presented method, the current profile across the layer is chosen as an input, while the ion density and temperature profiles in between the two asymptotic imposed values are a result of the calculation. It is shown that, assuming the distribution is continuous along the layer normal, these profiles have always a more complex profile than the profile of the current density and extends on a larger thickness. The different components of the pressure tensor are also outputs of the calculation and some conclusions concerning the symmetries of this tensor are pointed out.

Energy budgets in collisionless magnetic reconnection: Ion heating and bulk acceleration

Energy budgets in collisionless magnetic reconnection: Ion heating and bulk acceleration (ADS Link, DOI) Aunai, N.; Belmont, G.; Smets, R. This paper investigates the energy transfer in the process of collisionless antiparallel magnetic reconnection. Using two-dimensional hybrid simulations, we measure the increase of the bulk and thermal kinetic energies and compare it to the loss of magnetic energy through a contour surrounding the ion decoupling region. It is shown, for both symmetric and asymmetric configurations, that the loss of magnetic energy is not equally partitioned between heating and acceleration. The heating is found to be dominant and the partition ratio depends on the asymptotic parameters, and future investigations will be needed to understand this dependence.