Publications (92)245.39 Total impact
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ABSTRACT: Magnetic reconnection is the topological reconfiguration of the magnetic field in a plasma, accompanied by the violent release of energy and particle acceleration. Reconnection is as ubiquitous as plasmas themselves, with solar flares perhaps the most popular example. Other fascinating processes where reconnection plays a key role include the magnetic dynamo, geomagnetic storms and the sawtooth crash in tokamaks. Over the last few years, the theoretical understanding of magnetic reconnection in largescale fluid systems has undergone a major paradigm shift. The steadystate model of reconnection described by the famous SweetParker (SP) theory, which dominated the field for ∼ 50 years, has been replaced with an essentially timedependent, bursty picture of the reconnection layer, dominated by the continuous formation and ejection of multiple secondary islands (plasmoids). Whereas in the SP model reconnection was predicted to be slow, a major implication of this new paradigm is that reconnection in fluid systems is fast (i.e., independent of the Lundquist number), provided that the system is large enough. This conceptual shift hinges on the realization that SPlike current layers are violently unstable to the plasmoid (tearing) instability — implying, therefore, that such current sheets are supercritically unstable and thus can never form in the first place. This suggests that the formation of a current sheet and the subsequent reconnection process cannot be decoupled, as is commonly assumed. This paper provides an introductorylevel overview of the recent developments in reconnection theory and simulations that led to this essentially new framework. We briefly discuss the role played by the plasmoid instability in selected applications, and describe some of the outstanding challenges that remain at the frontier of this subject. Amongst these are the analytical and numerical extension of the plasmoid instability to (i) 3D and (ii) nonMHD regimes. New results are reported in both cases.  [Show abstract] [Hide abstract]
ABSTRACT: Energy dissipation is highly intermittent in turbulent plasmas, being localized in coherent structures such as current sheets. The statistical analysis of spatial dissipative structures is an effective approach to studying turbulence. In this paper, we generalize this methodology to investigate fourdimensional spatiotemporal structures, i.e., dissipative processes representing sets of interacting coherent structures, which correspond to flares in astrophysical systems. We develop methods for identifying and characterizing these processes, and then perform a statistical analysis of dissipative processes in numerical simulations of driven magnetohydrodynamic turbulence. We find that processes are often highly complex, longlived, and weakly asymmetric in time. They exhibit robust powerlaw probability distributions and scaling relations, including a distribution of dissipated energy with powerlaw index near 1.75, indicating that intense dissipative events dominate the overall energy dissipation. We compare our results with the previously observed statistical properties of solar flares.  [Show abstract] [Hide abstract]
ABSTRACT: We carry out a systematic study of the dispersion relation for linear electrostatic waves in an arbitrarily degenerate quantum electron plasma. We solve for the complex frequency spectrum for arbitrary values of wavenumber $k$ and level of degeneracy $\mu$. Our finding is that for large $k$ and high $\mu$ the real part of the frequency $\omega_{r}$ grows linearly with $k$ and scales with $\mu$ only because of the scaling of the Fermi energy. In this regime the relative Landau damping rate $\gamma/\omega_{r}$ becomes independent of $k$ and varies inversly with $\mu$. Thus, damping is weak but finite at moderate levels of degeneracy for short wavelengths.  [Show abstract] [Hide abstract]
ABSTRACT: We live in an age in which highperformance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in astrophysics and space physics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgridscales (SGS) that are not explicitly resolved. In addition to the parameter regime, astrophysical and heliophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) flows. After discussing the nature of MHD turbulence and the smallscale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several specific applications in astrophysics and heliophysics, assessing triumphs, challenges, and future directions.  [Show abstract] [Hide abstract]
ABSTRACT: Energy dissipation in magnetohydrodynamic (MHD) turbulence is known to be highly intermittent in space, being concentrated in sheetlike coherent structures. Much less is known about intermittency in time, another fundamental aspect of turbulence which has great importance for observations of solar flares and other space/astrophysical phenomena. In this Letter, we investigate the temporal intermittency of energy dissipation in numerical simulations of MHD turbulence. We consider fourdimensional spatiotemporal structures, "flare events", responsible for a large fraction of the energy dissipation. We find that although the flare events are often highly complex, they exhibit robust powerlaw distributions and scaling relations. We find that the probability distribution of dissipated energy has a power law index close to 1.75, similar to observations of solar flares, indicating that intense dissipative events dominate the heating of the system. We also discuss the temporal asymmetry of flare events as a signature of the turbulent cascade.Physical Review Letters 01/2015; 114(6). DOI:10.1103/PhysRevLett.114.065002 · 7.51 Impact Factor 
Article: Magnetic Reconnection Onset via Disruption of a Forming Current Sheet by the Tearing Instability
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ABSTRACT: The recent realization that SweetParker current sheets are violently unstable to the secondary tearing (plasmoid) instability implies that such current sheets cannot occur in real systems. This suggests that, in order to understand the onset of magnetic reconnection, one needs to consider the growth of the tearing instability in a current layer as it is being formed. Such an analysis is performed here in the context of nonlinear resistive MHD for a generic timedependent equilibrium representing a gradually forming current sheet. It is shown that two regimes, singleisland and multiisland, are possible, depending on the rate of current sheet formation. A simple model is used to compute the criterion for transition between these two regimes, as well as the reconnection onset time and the current sheet parameters at that moment. For typical solar corona parameters this model yields results consistent with observations.  [Show abstract] [Hide abstract]
ABSTRACT: Using twodimensional particleincell simulations, we characterize the energy spectra of particles accelerated by relativistic magnetic reconnection (without guide field) in collisionless electronpositron plasmas, for a wide range of upstream magnetizations $\sigma$ and system sizes $L$. The particle spectra are wellrepresented by a power law $\gamma^{\alpha}$, with a combination of exponential and superexponential highenergy cutoffs, proportional to $\sigma$ and $L$, respectively. For large $L$ and $\sigma$, the powerlaw index $\alpha$ approaches about 1.2.  [Show abstract] [Hide abstract]
ABSTRACT: Certain classes of astrophysical objects, namely magnetars and central engines of supernovae and gammaray bursts (GRBs), are characterized by extreme physical conditions not encountered elsewhere in the Universe. In particular, they possess magnetic fields that exceed the critical quantum field of 44 teragauss. Figuring out how these complex ultramagnetized systems work requires understanding various plasma processes, both smallscale kinetic and largescale magnetohydrodynamic (MHD). However, an ultrastrong magnetic field modifies the underlying physics to such an extent that many relevant plasmaphysical problems call for building QEDbased relativistic quantum plasma physics. In this review, after describing the extreme astrophysical systems of interest and identifying the key relevant plasmaphysical problems, we survey the recent progress in the development of such a theory. We discuss how a supercritical field modifies the properties of vacuum and matter and outline the basic theoretical framework for describing both nonrelativistic and relativistic quantum plasmas. We then turn to astrophysical applications of relativistic QED plasma physics relevant to magnetar magnetospheres and central engines of supernovae and long GRBs. Specifically, we discuss propagation of light through a magnetar magnetosphere; largescale MHD processes driving magnetar activity and GRB jet launching and propagation; energytransport processes governing the thermodynamics of extreme plasma environments; microscale kinetic plasma processes important in the interaction of intense magnetospheric electric currents with a magnetar's surface; and magnetic reconnection of ultrastrong magnetic fields. Finally, we point out that future progress will require the development of numerical modeling capabilities.Reports on Progress in Physics 03/2014; 77(3):036902. DOI:10.1088/00344885/77/3/036902 · 15.63 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The Crab Nebula was formed after the collapse of a massive star about a thousand years ago, leaving behind a pulsar that inflates a bubble of ultrarelativistic electronpositron pairs permeated with magnetic field. The observation of brief but bright flares of energetic gamma rays suggests that pairs are accelerated to PeV energies within a few days; such rapid acceleration cannot be driven by shocks. Here, it is argued that the flares may be the smoking gun of magnetic dissipation in the Nebula. Using 2D and 3D particleincell simulations, it is shown that the observations are consistent with relativistic magnetic reconnection, where pairs are subject to strong radiative cooling. The Crab flares may highlight the importance of relativistic magnetic reconnection in astrophysical sources.Physics of Plasmas 01/2014; 21(5). DOI:10.1063/1.4872024 · 2.25 Impact Factor 
Article: Threedimensional relativistic pair plasma reconnection with radiative feedback in the Crab Nebula
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ABSTRACT: The discovery of rapid synchrotron gammaray flares above 100 MeV from the Crab Nebula has attracted new interest in alternative particle acceleration mechanisms in pulsar wind nebulae. Diffuse shockacceleration fails to explain the flares because particle acceleration and emission occur during a single or even subLarmor timescale. In this regime, the synchrotron energy losses induce a drag force on the particle motion that balances the electric acceleration and prevents the emission of synchrotron radiation above 160 MeV. Previous analytical studies and 2D particleincell (PIC) simulations indicate that relativistic reconnection is a viable mechanism to circumvent the above difficulties. The reconnection electric field localized at Xpoints linearly accelerates particles with little radiative energy losses. In this paper, we check whether this mechanism survives in 3D, using a set of large PIC simulations with radiation reaction force and with a guide field. In agreement with earlier works, we find that the relativistic drift kink instability deforms and then disrupts the layer, resulting in significant plasma heating but few nonthermal particles. A moderate guide field stabilizes the layer and enables particle acceleration. We report that 3D magnetic reconnection can accelerate particles above the standard radiation reaction limit, although the effect is less pronounced than in 2D with no guide field. We confirm that the highest energy particles form compact bunches within magnetic flux ropes, and a beam tightly confined within the reconnection layer, which could result in the observed Crab flares when, by chance, the beam crosses our line of sight.The Astrophysical Journal 11/2013; 782(2). DOI:10.1088/0004637X/782/2/104 · 6.28 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In this paper, we consider two outstanding intertwined problems in modern highenergy astrophysics: (1) the verticalthermal structure of an optically thick accretion disk heated by the dissipation of magnetohydrodynamic turbulence driven by the magnetorotational instability (MRI), and (2) determining the fraction of the accretion power released in the corona above the disk. For simplicity, we consider a gaspressuredominated disk and assume a constant opacity. We argue that the local turbulent dissipation rate due to the disruption of the MRI channel flows by secondary parasitic instabilities should be uniform across most of the disk, almost up to the disk photosphere. We then obtain a selfconsistent analytical solution for the vertical thermal structure of the disk, governed by the balance between the heating by MRI turbulence and the cooling by radiative diffusion. Next, we argue that the coronal power fraction is determined by the competition between the Parker instability, viewed as a parasitic instability feeding off of MRI channel flows, and other parasitic instabilities. We show that the Parker instability inevitably becomes important near the disk surface, leading to a certain lower limit on the coronal power. While most of the analysis in this paper focuses on the case of a disk threaded by an externally imposed vertical magnetic field, we also discuss the zero net flux case, in which the magnetic field is produced by the MRI dynamo itself, and show that most of our arguments and conclusions should be valid in this case as well.The Astrophysical Journal 09/2013; 775(2):103. DOI:10.1088/0004637X/775/2/103 · 6.28 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A secondorder accurate semiimplicit Lorentz force ions, fluid electrons δfδf hybrid model has been developed using a “current closure” scheme. The model assumes quasineutrality and is fully electromagnetic. The implicit field solver improves numerical accuracy by separating the equilibrium terms in the presence of small perturbations. The equilibrium part of the generalized Ohm’s law is solved by direct matrix inversion along the direction of gradients for every Fourier mode in the other two directions, while the nonlinear part is solved iteratively. The simulation has been benchmarked on Alfvén waves, ion sound waves and whistler waves against analytical dispersion relation in a slab. In particular, the firstorder and secondorder schemes are compared by studying the numerical damping of whistler waves. The full evolution of the resistive tearing mode using the Harris sheet equilibrium is also investigated. The linear growth rate and mode structure are compared with the resistive MHD theory. Important tearing mode nonlinear phenomena such as the Rutherford regime and saturation are demonstrated. We also presented systematic study of Rutherford growth rates and saturation island width, which is consistent with previous MHD studies.Journal of Computational Physics 07/2013; 245:364–375. DOI:10.1016/j.jcp.2013.03.017 · 2.49 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Magnetic reconnection converts magnetic field energy into particle kinetic energy, accelerating particles to sufficient energies to emit gammaray synchrotron radiation in astrophysical contexts, possibly including pulsar wind nebulae, GammaRay Bursts, and blazar jets. A balance between acceleration (by the electric field E) and synchrotron braking (while orbiting a Bfield line) limits particle energy so that synchrotron processes cannot emit photons above about 160 MeV, unless E > B. However, short, intense gammaray flares of much higher energies have recently been observed in the Crab nebula. This work demonstrates, using 2D simulations, that reconnection in relativistic electronpositron pair plasmas can accelerate particles in Speiser orbits around the magnetic null (where E > B) such that the particles can emit synchrotron photons above the 160 MeV limit. Furthermore, reconnection bunches particles and focuses them into beams; highenergy synchrotron radiation is also strongly beamed, and the sweeping of the beam across the observer's line of sight can explain the fast time variability of the flares.  [Show abstract] [Hide abstract]
ABSTRACT: It is generally accepted that astrophysical sources cannot emit synchrotron radiation above 160 MeV in their rest frame. This limit is given by the balance between the accelerating electric force and the radiation reaction force acting on the electrons. The discovery of synchrotron gammaray flares in the Crab Nebula, well above this limit, challenges this classical picture of particle acceleration. To overcome this limit, particles must accelerate in a region of high electric field and low magnetic field. This is possible only with a nonideal magnetohydrodynamic process, like magnetic reconnection. We present the first numerical evidence of particle acceleration beyond the synchrotron burnoff limit, using a set of 2D particleincell simulations of ultrarelativistic pair plasma reconnection. We use a new code, Zeltron, that includes selfconsistently the radiation reaction force in the equation of motion of the particles. We demonstrate that the most energetic particles move back and forth across the reconnection layer, following relativistic Speiser orbits. These particles then radiate >160 MeV synchrotron radiation rapidly, within a fraction of a full gyration, after they exit the layer. Our analysis shows that the highenergy synchrotron flux is highly variable in time because of the strong anisotropy and inhomogeneity of the energetic particles. We discover a robust positive correlation between the flux and the cutoff energy of the emitted radiation, mimicking the effect of relativistic Doppler amplification. A strong guide field quenches the emission of >160 MeV synchrotron radiation. Our results are consistent with the observed properties of the Crab flares, supporting the reconnection scenario.The Astrophysical Journal 02/2013; 770(2). DOI:10.1088/0004637X/770/2/147 · 6.28 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We develop a framework for studying the statistical properties of current sheets in numerical simulations of 3D magnetohydrodynamic (MHD) turbulence. We describe an algorithm that identifies current sheets in a simulation snapshot and then determines their geometrical properties (including length, width, and thickness) and intensities (peak current density and total energy dissipation rate). We then apply this procedure to simulations of reduced MHD turbulence and perform a statistical analysis on the obtained population of current sheets. We evaluate the role of reconnection by separately studying the populations of current sheets which contain magnetic Xpoints and those which do not. We find that the statistical properties of the two populations are different in general. We compare the scaling of these properties to phenomenological predictions obtained for the inertial range of MHD turbulence. Finally, we test whether the reconnecting current sheets are consistent with the SweetParker model.The Astrophysical Journal 02/2013; 771(2). DOI:10.1088/0004637X/771/2/124 · 6.28 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A twodimensional (2D) linear theory of the instability of SweetParker (SP) current sheets is developed in the framework of reduced magnetohydrodynamics. A local analysis is performed taking into account the dependence of a generic equilibrium profile on the outflow coordinate. The plasmoid instability [Loureiro et al., Phys. Plasmas 14, 100703 (2007)] is recovered, i.e., current sheets are unstable to the formation of a largewavenumber chain of plasmoids (k_{max}L_{CS}∼S^{3/8}, where k_{max} is the wave number of fastest growing mode, S=L_{CS}V_{A}/η is the Lundquist number, L_{CS} is the length of the sheet, V_{A} is the Alfvén speed, and η is the plasma resistivity), which grows super Alfvénically fast (γ_{max}τ_{A}∼S^{1/4}, where γ_{max} is the maximum growth rate, and τ_{A}=L_{CS}/V_{A}). For typical background profiles, the growth rate and the wave number are found to increase in the outflow direction. This is due to the presence of another mode, the KelvinHelmholtz (KH) instability, which is triggered at the periphery of the layer, where the outflow velocity exceeds the Alfvén speed associated with the upstream magnetic field. The KH instability grows even faster than the plasmoid instability γ_{max}τ_{A}∼k_{max}L_{CS}∼S^{1/2}. The effect of viscosity (ν) on the plasmoid instability is also addressed. In the limit of large magnetic Prandtl numbers Pm=ν/η, it is found that γ_{max}∼S^{1/4}Pm^{5/8} and k_{max}L_{CS}∼S^{3/8}Pm^{3/16}, leading to the prediction that the critical Lundquist number for plasmoid instability in the Pm≫1 regime is S_{crit}∼10^{4}Pm^{1/2}. These results are verified via direct numerical simulation of the linearized equations, using an analytical 2D SP equilibrium solution.Physical Review E 01/2013; 87(11):013102. DOI:10.1103/PhysRevE.87.013102 · 2.33 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report on the first study of energetic particles and radiation angular distributions generated in relativistic collisionless pair plasma reconnection, using 2.5dimensional particleincell simulations. We have discovered that the energetic particles are focused within a small solid angle, and bunched into compact regions inside magnetic islands. In addition, we find that the synchrotron radiation emitted by these particles, as seen by an external observer, is tightly beamed and variable on time scales much shorter than the lightcrossing time of the system. This energy dependent "kinetic beaming" differs fundamentally from the achromatic Doppler beaming usually ascribed to relativistic jets. Our findings can account for the puzzling discoveries of bright, short flares seen in highenergy gamma rays, especially from the Crab Nebula and from blazars.12/2012; DOI:10.1063/1.4772339  [Show abstract] [Hide abstract]
ABSTRACT: A white paper submitted to the Astro2010 Decadal Survey  [Show abstract] [Hide abstract]
ABSTRACT: The magnetosphere of a rotating pulsar naturally develops a current sheet beyond the light cylinder (LC). Magnetic reconnection in this current sheet inevitably dissipates a nontrivial fraction of the pulsar spindown power within a few LC radii. We develop a basic physical picture of reconnection in this environment and discuss its implications for the observed pulsed gammaray emission. We argue that reconnection proceeds in the plasmoiddominated regime, via an hierarchical chain of multiple secondary islands/flux ropes. The interplasmoid reconnection layers are subject to strong synchrotron cooling, leading to significant plasma compression. Using the conditions of pressure balance across these current layers, the balance between the heating by magnetic energy dissipation and synchrotron cooling, and Ampere's law, we obtain simple estimates for key parameters of the layers  temperature, density, and layer thickness. In the comoving frame of the relativistic pulsar wind just outside of the equatorial current sheet, these basic parameters are uniquely determined by the strength of the reconnecting upstream magnetic field. For the case of the Crab pulsar, we find them to be of order 10 GeV, $10^{13} cm^{3}$, and 10 cm, respectively. After accounting for the bulk Doppler boosting due to the pulsar wind, the synchrotron and inverseCompton emission from the reconnecting current sheet can explain the observed pulsed highenergy (GeV) and VHE (~100 GeV) radiation, respectively. Also, we suggest that the rapid relative motions of the secondary plasmoids in the hierarchical chain may contribute to the production of the pulsar radio emission.The Astrophysical Journal 10/2012; 780(1). DOI:10.1088/0004637X/780/1/3 · 6.28 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Magnetic reconnection is one of a few astrophysical mechanisms that can accelerate particles to energies sufficient to emit observable highenergy radiation. This work reports on 2D simulations of reconnection in relativistic electronpositron pair plasmas, which may power gammaray emission from pulsar wind nebulae (PWNe), GammaRay Bursts (GRBs), and blazar jets. The most important new discovery is the strong, energydependent angular anisotropy and spatial inhomogeneity of accelerated particles: highenergy particles are bunched in space and focused into beams mostly confined to the reconnection layer midplane. Another important advance is the calculation of the associated radiative signatures (spectra and light curves) seen by a distant observer. The synchrotron and inverse Compton radiation from the highenergy particles is likewise focused in narrow beams. The beams sweep back and forth within the midplane, so that an observer sees intense bursts (only) when a beam crosses the line of sight. The resulting rapid variability, on timescales much shorter than the lightcrossing time of the reconnection region, could explain the short, intense gammaray flares observed in blazar jets and PWNe, including the GeV flares recently discovered in the Crab nebula.
Publication Stats
1k  Citations  
245.39  Total Impact Points  
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Institutions

2010–2015

University of Colorado at Boulder
 Department of Physics
Boulder, Colorado, United States


2012

University of Colorado
 Department of Physics
Denver, Colorado, United States


2009

University of California, Berkeley
Berkeley, California, United States


1996–2009

Princeton University
 • Department of Astrophysical Sciences
 • Princeton Plasma Physics Laboratory
Princeton, New Jersey, United States


2008

University of Chicago
 Department of Astronomy and Astrophysics
Chicago, IL, United States


2002–2005

University of California, Santa Barbara
 Kavli Institute for Theoretical Physics
Santa Barbara, California, United States
