arXiv subjects

Giovanni Lapenta

Publications and source records attributed to Giovanni Lapenta.

85 records · Page 5Linked to original sources

arXiv2010

Jet deflection by very weak guide fields during magnetic reconnection

Simulations of antiparallel reconnection have shown collimated electron jets outflowing from the x-point, and associated highly elongated "outer electron diffusion regions." New PIC simulations with ion/electron mass ratios as large as 1836 show that jets are deflected towards the magnetic separatrix by out-of-plane guide fields, Bg, as small as 0.05 times the asymptotic reconnecting field, B0. The outer electron diffusion region is distorted and broken up, but the diffusion rate is unchanged. These results are interpreted in terms of electron dynamics and are compared to recent measurements of reconnection jets in the magnetosheath.

physics.plasm-ph↗

arXiv2010

Investigation of Dynamics of Self-Similarly Evolving Magnetic Clouds

Magnetic clouds (MCs) are "magnetized plasma clouds" moving in the solar wind. MCs transport magnetic flux and helicity away from the Sun. These structures are not stationary but feature temporal evolution. Commonly, simplified MC models are considered. The goal of the present study is to investigate the dynamics of more general, radially expanding MCs. They are considered as cylindrically symmetric magnetic structures with low plasma β. In order to study MC`evolution the self-similar approach method and a numerical approach are used. It is shown that the forces are balanced in the considered self-similarly evolving, cylindrically symmetric magnetic structures. Explicit analytical expressions for magnetic field, plasma velocity, density and pressure within MCs are derived. These solutions are characterized by conserved values of magnetic flux and helicity. We also investigate the dynamics of self-similarly evolving MCs by means of the numerical code "Graale". In addition, their expansion in a medium with higher density and higher plasma β is studied. It is shown that the physical parameters of the MCs maintain their self-similar character throughout their evolution. Conclusions. A comparison of the different self-similar and numerical solutions allows us to conclude that the evolving MCs are quite adequately described by our self-similar solutions - they retain their self-similar, coherent nature for quite a long time and over large distances from the Sun.

astro-ph.SR↗

arXiv2009

Analysis of the energy release for different magnetic reconnection regimes within the solar environment

A 2.5-dimensional magnetohydrodynamics simulation analysis of the energy release for three different reconnection regimes is presented. The system under investigation consists in a current-sheet located in a medium with a strong density variation along the current layer: such system is modeled as it were located in the high chromosphere/low solar corona as in the case of pre- flare and coronal mass ejection (CME) configurations or in the aftermath of such explosive phenomena. By triggering different magnetic-reconnection dynamics, that is from a laminar slow evolution to a spontaneous non-steady turbulent reconnection [1,2,3], we observe a rather different efficiency and temporal behavior with regard to the energy fluxes associated with each of these reconnection-driven evolutions. These discrepancies are fundamental key-properties to create realistic models of the triggering mechanisms and initial evolution of all those phenomena requiring fast (and high power) magnetic reconnection events within the solar environment. 1. G. Lapenta, Phys. Rev. Lett. 100, 235001 (2008). 2. L. Bettarini, and G. Lapenta, ApJ Submitted (2009). 3. M. Skender, and G. Lapenta, Phys. Plasmas submitted (2009).

astro-ph.SR↗

arXiv2009

Spontaneous non-steady magnetic reconnection within the solar environment

This work presents a 2.5-dimensional simulation study of the instability of current-sheets located in a medium with a strong density variation along the current layer. The initial force-free configuration is observed to undergo a two-stage evolution consisting of an abrupt regime transition from a slow to a fast reconnection process leading the system to a final chaotic configuration. Yet, the onset of the fast phase is not determined by the presence of any anomalous enhancement in plasma's local resistivity, but rather is the result of a new mechanism discovered in Lapenta (2008)* and captured only when sufficient resolution is present. Hence, the effects of the global resistivity, the global viscosity and the plasma beta on the overall dynamics are considered. This mechanism allowing the transition from slow to fast reconnection provides a simple but effective model of several processes taking place within the solar atmosphere from the high chromosphere up to the low corona. In fact, the understanding of a spontaneous transition to a self-feeding fast reconnection regime as well as its macroscopic evolution is the first and fundamental step to produce realistic models of all those phenomena requiring fast (and high power) triggering events (* Lapenta G. 2008, Phys. Rev. Lett., 100, 235001).

astro-ph.SR↗

arXiv2008

Adaptive Multi-Dimensional Particle In Cell

Kinetic Particle In Cell (PIC) methods can extend greatly their range of applicability if implicit time differencing and spatial adaption are used to address the wide range of time and length scales typical of plasmas. For implicit differencing, we refer the reader to our recent summary of the implicit moment PIC method implemented in our CELESTE3D code [G. Lapenta, Phys. Plasmas, 13, 055904 (2006)]. Instead, the present document deals with the issue of PIC spatial adaptation. Adapting a kinetic PIC code requires two tasks: adapting the grid description of the fields and moments and adapting the particle description of the distribution function. Below we address both issues. First, we describe how grid adaptation can be guided by appropriate measures of the local accuracy of the solution. Based on such information, grid adaptation can be obtained by moving grid points from regions of lesser interest to regions of higher interest or by adding and removing points. We discuss both strategies. Second, we describe how to adapt the local number of particles to reach the required statistical variance in the description of the particle population. Finally two typical applications of adaptive PIC are shown: collisionless shocks and charging of small bodies immersed in a plasma.

physics.plasm-ph↗

arXiv2008

Self-Feeding Turbulent Magnetic Reconnection on Macroscopic Scales

Within a MHD approach we find magnetic reconnection to progress in two entirely different ways. The first is well-known: the laminar Sweet-Parker process. But a second, completely different and chaotic reconnection process is possible. This regime has properties of immediate practical relevance: i) it is much faster, developing on scales of the order of the Alfvén time, and ii) the areas of reconnection become distributed chaotically over a macroscopic region. The onset of the faster process is the formation of closed circulation patterns where the jets going out of the reconnection regions turn around and forces their way back in, carrying along copious amounts of magnetic flux.

physics.plasm-ph↗

arXiv2008

Spontaneous large scale momentum exchange by microinstabilities: an analogy between tokamaks and space plasmas

Based on a recent theory (Coppi, Nuclear Fusion, 42, 1, 2002) of spontaneous toroidal rotation in tokamaks (Lee et al, Phys Rev Lett, 91, 205003, 2003) and in astrophysical accretion disks, we propose that an analogous process could be at play also in the Earth space environment. We use fully kinetic PIC simulations to study the evolution of drift instabilities and we show that indeed a macroscopic velocity shear is generated spontaneously in the plasma. As in tokamaks, the microscopic fluctuations remain limited to the edge of the plasma channel but the momentum spreads over the whole macroscopic system.

physics.plasm-ph↗

arXiv2008

The algorithms of the implicit moment method for plasma simulation

We discuss the fundamentals of the implicit moment method for Particle In Cell (PIC) simulation as presently implemented in the CELESTE3D code. We present the method in its fully electromagnetic and fully kinetic version. The application of the method is to problems with multiple temporal and spatial scales, common in all space, astrophysical and laboratory plasmas.

physics.comp-ph↗

arXiv2007

Blob formation and acceleration in the solar wind: role of converging flows and viscosity

The effect of viscosity and of converging flows on the formation of blobs in the slow solar wind is analysed by means of resistive MHD simulations. The regions above coronal streamers where blobs are formed (Sheeley et al., 1997) are simulated using a model previously proposed by Einaudi et al. (1999). The result of our investigation is twofold. First, we demonstrate a new mechanism for enhanced momentum transfer between a forming blob and the fast solar wind surrounding it. The effect is caused by the longer range of the electric field caused by the tearing instability forming the blob. The electric field reaches into the fast solar wind and interacts with it, causing a viscous drag that is global in nature rather than local across fluid layers as it is the case in normal uncharged fluids (like water). Second, the presence of a magnetic cusp at the tip of a coronal helmet streamer causes a converging of the flows on the two sides of the streamer and a direct push of the forming island by the fast solar wind, resulting in a more efficient momentum exchange.

astro-ph↗

arXiv2004

Influence of the Lower Hybrid Drift Instability on the onset of Magnetic Reconnection

Two-dimensional and three-dimensional kinetic simulation results reveal the importance of the Lower-Hybrid Drift Instability LHDI to the onset of magnetic reconnection. Both explicit and implicit kinetic simulations show that the LHDI heats electrons anisotropically and increases the peak current density. Linear theory predicts these modifications can increase the growth rate of the tearing instability by almost two orders of magnitude and shift the fastest growing modes to significantly shorter wavelengths. These predictions are confirmed by nonlinear kinetic simulations in which the growth and coalescence of small scale magnetic islands leads to a rapid onset of large scale reconnection.

physics.plasm-ph↗

arXiv2003

Plasma sheet structure in the magnetotail: kinetic simulation and comparison with satellite observations

We use the results of a three-dimensional kinetic simulation of an Harris current sheet to propose an explanation and to reproduce the ISEE-1/2, Geotail, and Cluster observations of the magnetotail current sheet structure. Current sheet flapping, current density bifurcation, and reconnection are explained as the results of the kink and tearing instabilities, which dominate the current sheet evolution.

physics.plasm-ph↗

arXiv2003

Electron acceleration and heating in collisionless magnetic reconnection

We discuss electron acceleration and heating during collisionless magnetic reconnection by using the results of implicit kinetic simulations of Harris current sheets. We consider and compare electron dynamics in plasmas with different βvalues and perform simulations up to the physical mass ratio. We analyze the typical trajectory of electrons passing through the reconnection region, we study the electron velocity, focusing on the out-of-plane velocity, and we discuss the electron heating along the in-plane and out-of-plane directions.

astro-ph↗

arXiv2003

Soliton-Like Solutions of the Grad-Shafranov Equation

A new class of soliton-like solutions is derived for the Grad-Shafranov (GS) equations. A mathematical analogy between the GS equation for MHD equilibria and the cubic Schrödinger (CS) equation for non-linear wave propagation forms the basis to derive the new class of solutions. The soliton-like solutions are considered for their possible relevance to astrophysics and solar physics problems. We discuss how a soliton-like solution can be generated by a repetitive process of magnetic arcade stretching and plasmoid formation induced by the differential rotation of the solar photosphere or of an accretion disk.

astro-ph↗