Signatures of Cold Ions in a Kinetic Simulation of the Reconnecting Magnetopause
Dargent, J., Aunai, N., Lavraud, B., Toledo‐Redondo, S., Califano, F. ( 2019). Journal of Geophysical Research: Space Physics, 124, 2497– 2514.
At the Earth’s magnetopause, a low‐energy ion population of ionospheric origin is commonly observed at the magnetospheric side. In this work we use a 2‐D fully kinetic simulation to identify several original signatures related to the dynamics of cold ions involved in magnetic reconnection at the asymmetric dayside magnetopause. We identify several original signatures of the cold ions dynamics driven by the development of magnetic reconnection at the asymmetric dayside magnetopause. We find that cold ions tend to rarefy in the diffusion region, while their density is enhanced as a result of compression along magnetospheric separatrices. We also observe the formation of crescent‐shaped cold ion distribution functions along the separatrices in the near‐exhaust region, and we present an analytical model to explain this signature. Finally, we give evidence of a localized parallel heating of cold ions. These signatures should be detected with the magnetospheric multiscale mission high‐resolution observations.
Smilei : A collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation
Computer Physics Communications Volume 222, January 2018, Pages 351-373
DOI : https://doi.org/10.1016/j.cpc.2017.09.024
J.Derouillat, A. Beck, F. Pérez, T.Vinci, M. Chiaramello, A. Grassi, M.Flé, G.Bouchar, I. Plotnikov, N.Aunai, J.Dargent, C.Riconda, M.Grech
Smilei is a collaborative, open-source, object-oriented (C++) particle-in-cell code. To benefit from the latest advances in high-performance computing (HPC), Smilei is co-developed by both physicists and HPC experts. The code’s structures, capabilities, parallelization strategy and performances are discussed. Additional modules (e.g. to treat ionization or collisions), benchmarks and physics highlights are also presented. Multi-purpose and evolutive, Smilei is applied today to a wide range of physics studies, from relativistic laser–plasma interaction to astrophysical plasmas.
Kinetic simulation of asymmetric magnetic reconnection with cold ions
Journal of Geophysical Research: Space Physics, Volume 122, Issue 5, pp. 5290-5306 DOI : http://dx.doi.org/10.1002/2016JA023831
Dargent, J.; Aunai, N.; Lavraud, B.; Toledo-Redondo, S.; Shay, M. A.; Cassak, P. A.; Malakit, K.
At the dayside magnetopause, the magnetosphere often contains a cold ion population of ionospheric origin. This population is not always detectable by particle instruments due to its low energy, despite having an important contribution to the total ion density and therefore an impact on key plasma processes such as magnetic reconnection. The exact role and implications of this low-temperature population are still not well known and has not been addressed with numerical simulation before. We present 2-D fully kinetic simulations of asymmetric magnetic reconnection with and without a cold ion population on the magnetospheric side of the magnetopause, but sharing the same total density, temperature, and magnetic field profiles. The comparison of the simulations suggests that cold ions directly impact signatures recently suggested as a good marker of the X line region: the Larmor electric field. Our simulations reveal that this electric field, initially present all along the magnetospheric separatrix, is related to the bounce of magnetosheath ions at the magnetopause magnetic field reversal through Speiser-like orbits. Once reconnection widens the current sheet away from the X line, the bouncing stops and the electric field signature remains solely confined near the X line. When cold ions are present, however, their very low temperature enables them to E × B drift in the electric field structure. If their density is large enough compared to other ions, their contribution to the momentum equation is capable of maintaining the signature away from the X line. This effect must be taken into account when analyzing in situ spacecraft measurements.
Orientation of the X-line in asymmetric magnetic reconnection
Journal of Plasma Physics, Volume 82, Issue 4, article id. 535820401, 15 pp.DOI : http://dx.doi.org/10.1017/S0022377816000647
Aunai, N.; Hesse, M.; Lavraud, B.; Dargent, J.; Smets, R.
Magnetic reconnection can occur in current sheets separating magnetic fields sheared by any angle and of arbitrarily different amplitudes. In such asymmetric and non-coplanar systems, it is not yet understood what the orientation of the X-line will be. Studying how this orientation is determined locally by the reconnection process is important to understand systems such as the Earth magnetopause, where reconnection occurs in regions with large differences in upstream plasma and field properties. This study aims at determining what the local X-line orientation is for different upstream magnetic shear angles in an asymmetric set-up relevant to the Earth’s magnetopause. We use two-dimensional hybrid simulations and vary the simulation plane orientation with regard to the fixed magnetic field profile and search for the plane maximizing the reconnection rate. We find that the plane defined by the bisector of upstream fields maximizes the reconnection rate and this appears not to depend on the magnetic shear angle, domain size or upstream plasma and asymmetries.
On the electron diffusion region in planar, asymmetric, systems
Published in Geophysical Research Letters DOI : http://dx.doi.org/10.1002/2014GL061586
Hesse, Michael; Aunai, Nicolas; Sibeck, David; Birn, Joachim
simulations and analytical theory are employed to study the electron diffusion region in asymmetric reconnection, which is taking place in planar configurations without a guide field. The analysis presented here focuses on the nature of the local reconnection electric field and on differences from symmetric configurations. Further emphasis is on the complex structure of the electron distribution in the diffusion region, which is generated by the mixing of particles from different sources. We find that the electric field component that is directly responsible for flux transport is provided not by electron pressure-based, “quasi-viscous,” terms but by inertial terms. The quasi-viscous component is shown to be critical in that it is necessary to sustain the required overall electric field pattern in the immediate neighborhood of the reconnection X line.
On the relationship between quadrupolar magnetic field and collisionless reconnection
Published in Physics of Plasmas DOI : http://dx.doi.org/10.1063/1.4885097
R. Smets, N. Aunai, G. Belmont, C. Boniface and J. Fuchs
Using hybrid simulations, we investigate the onset of fast reconnection between two cylindrical magnetic shells initially close to each other. This initial state mimics the plasma structure in High Energy Density Plasmas induced by a laser-target interaction and the associated self-generated magnetic field. We clearly observe that the classical quadrupolar structure of the out-of-plane magnetic field appears prior to the reconnection onset. Furthermore, a parametric study reveals that, with a non-coplanar initial magnetic topology, the reconnection onset is delayed and possibly suppressed. The relation between the out-of-plane magnetic field and the out-of-plane electric field is discussed.
The relation between reconnected flux, the parallel electric field, and the reconnection rate in a three-dimensional kinetic simulation of magnetic reconnection
Published in Physics of Plasmas
D.E. Wendel, D.K. Olson, M. Hesse, N. Aunai, M. Kuznetsova, H. Karimabadi, W. Daughton and M. L. Adrian
We investigate the distribution of parallel electric fields and their relationship to the location and rate of magnetic reconnection in a large particle-in-cell simulation of 3D turbulent magnetic reconnection with open boundary conditions. The simulation’s guide field geometry inhibits the formation of simple topological features such as null points. Therefore, we derive the location of potential changes in magnetic connectivity by finding the field lines that experience a large relative change between their endpoints, i.e., the quasi-separatrix layer. We find a good correspondence between the locus of changes in magnetic connectivity or the quasi-separatrix layer and the map of large gradients in the integrated parallel electric field (or quasi-potential). Furthermore, we investigate the distribution of the parallel electric field along the reconnecting field lines. We find the reconnection rate is controlled by only the low-amplitude, zeroth and first–order trends in the parallel electric field while the contribution from fluctuations of the parallel electric field, such as electron holes, is negligible. The results impact the determination of reconnection sites and reconnection rates in models and in situ spacecraft observations of 3D turbulent reconnection. It is difficult through direct observation to isolate the loci of the reconnection parallel electric field amidst the large amplitude fluctuations. However, we demonstrate that a positive slope of the running sum of the parallel electric field along the field line as a function of field line length indicates where reconnection is occurring along the field line.
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
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
ADS Link, DOI
Aunai, Nicolas, Hesse, Michael, Zenitani, Seiji,
Kuznetsova, Maria, Black, Carrie, Evans, 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.