Dmitri A. Uzdensky

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

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Publications (92)245.39 Total impact

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    N F Loureiro, D A Uzdensky
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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 large-scale fluid systems has undergone a major paradigm shift. The steady-state model of reconnection described by the famous Sweet-Parker (SP) theory, which dominated the field for ∼ 50 years, has been replaced with an essentially time-dependent, 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 SP-like current layers are violently unstable to the plasmoid (tearing) instability — implying, therefore, that such current sheets are super-critically 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 introductory-level 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) non-MHD regimes. New results are reported in both cases.
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    Vladimir Zhdankin, Dmitri A. Uzdensky, Stanislav Boldyrev
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    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 four-dimensional 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, long-lived, and weakly asymmetric in time. They exhibit robust power-law probability distributions and scaling relations, including a distribution of dissipated energy with power-law 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.
  • Shane Rightley, Dmitri Uzdensky
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    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.
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    ABSTRACT: We live in an age in which high-performance 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 subgrid-scales (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 small-scale 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.
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    Vladimir Zhdankin, Dmitri A. Uzdensky, Stanislav Boldyrev
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    ABSTRACT: Energy dissipation in magnetohydrodynamic (MHD) turbulence is known to be highly intermittent in space, being concentrated in sheet-like 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 four-dimensional 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 power-law 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
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    Dmitri A. Uzdensky, Nuno F. Loureiro
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    ABSTRACT: The recent realization that Sweet-Parker 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 time-dependent equilibrium representing a gradually forming current sheet. It is shown that two regimes, single-island and multi-island, 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.
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    ABSTRACT: Using two-dimensional particle-in-cell simulations, we characterize the energy spectra of particles accelerated by relativistic magnetic reconnection (without guide field) in collisionless electron-positron plasmas, for a wide range of upstream magnetizations $\sigma$ and system sizes $L$. The particle spectra are well-represented by a power law $\gamma^{-\alpha}$, with a combination of exponential and super-exponential high-energy cutoffs, proportional to $\sigma$ and $L$, respectively. For large $L$ and $\sigma$, the power-law index $\alpha$ approaches about 1.2.
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    Dmitri A Uzdensky, Shane Rightley
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    ABSTRACT: Certain classes of astrophysical objects, namely magnetars and central engines of supernovae and gamma-ray 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 ultra-magnetized systems work requires understanding various plasma processes, both small-scale kinetic and large-scale magnetohydrodynamic (MHD). However, an ultra-strong magnetic field modifies the underlying physics to such an extent that many relevant plasma-physical problems call for building QED-based relativistic quantum plasma physics. In this review, after describing the extreme astrophysical systems of interest and identifying the key relevant plasma-physical problems, we survey the recent progress in the development of such a theory. We discuss how a super-critical field modifies the properties of vacuum and matter and outline the basic theoretical framework for describing both non-relativistic 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; large-scale MHD processes driving magnetar activity and GRB jet launching and propagation; energy-transport processes governing the thermodynamics of extreme plasma environments; micro-scale kinetic plasma processes important in the interaction of intense magnetospheric electric currents with a magnetar's surface; and magnetic reconnection of ultra-strong 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:10.1088/0034-4885/77/3/036902 · 15.63 Impact Factor
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    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 ultra-relativistic electron-positron 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 particle-in-cell 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
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    ABSTRACT: The discovery of rapid synchrotron gamma-ray flares above 100 MeV from the Crab Nebula has attracted new interest in alternative particle acceleration mechanisms in pulsar wind nebulae. Diffuse shock-acceleration fails to explain the flares because particle acceleration and emission occur during a single or even sub-Larmor 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 particle-in-cell (PIC) simulations indicate that relativistic reconnection is a viable mechanism to circumvent the above difficulties. The reconnection electric field localized at X-points 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 non-thermal 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:10.1088/0004-637X/782/2/104 · 6.28 Impact Factor
  • Dmitri A. Uzdensky
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    ABSTRACT: In this paper, we consider two outstanding intertwined problems in modern high-energy astrophysics: (1) the vertical-thermal 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 gas-pressure-dominated 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 self-consistent 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:10.1088/0004-637X/775/2/103 · 6.28 Impact Factor
  • Jianhua Cheng, Scott E. Parker, Yang Chen, Dmitri A. Uzdensky
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    ABSTRACT: A second-order accurate semi-implicit Lorentz force ions, fluid electrons δfδf hybrid model has been developed using a “current closure” scheme. The model assumes quasi-neutrality 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 first-order and second-order 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
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    ABSTRACT: Magnetic reconnection converts magnetic field energy into particle kinetic energy, accelerating particles to sufficient energies to emit gamma-ray synchrotron radiation in astrophysical contexts, possibly including pulsar wind nebulae, Gamma-Ray Bursts, and blazar jets. A balance between acceleration (by the electric field E) and synchrotron braking (while orbiting a B-field line) limits particle energy so that synchrotron processes cannot emit photons above about 160 MeV, unless E > B. However, short, intense gamma-ray flares of much higher energies have recently been observed in the Crab nebula. This work demonstrates, using 2D simulations, that reconnection in relativistic electron-positron 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; high-energy 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.
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    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 gamma-ray 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 non-ideal magnetohydrodynamic process, like magnetic reconnection. We present the first numerical evidence of particle acceleration beyond the synchrotron burnoff limit, using a set of 2D particle-in-cell simulations of ultra-relativistic pair plasma reconnection. We use a new code, Zeltron, that includes self-consistently 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 high-energy 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 cut-off 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:10.1088/0004-637X/770/2/147 · 6.28 Impact Factor
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    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 X-points 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 Sweet-Parker model.
    The Astrophysical Journal 02/2013; 771(2). DOI:10.1088/0004-637X/771/2/124 · 6.28 Impact Factor
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    ABSTRACT: A two-dimensional (2D) linear theory of the instability of Sweet-Parker (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 large-wave-number 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 Kelvin-Helmholtz (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(1-1):013102. DOI:10.1103/PhysRevE.87.013102 · 2.33 Impact Factor
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    ABSTRACT: We report on the first study of energetic particles and radiation angular distributions generated in relativistic collisionless pair plasma reconnection, using 2.5-dimensional particle-incell 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 light-crossing 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 high-energy gamma rays, especially from the Crab Nebula and from blazars.
    12/2012; DOI:10.1063/1.4772339
  • Jonathan Arons, Steven A. Balbus, École Normale Supérieure, Eric G. Blackman, Jeremy Goodman, D. Uzdensky, J. Arons, S. Balbus, E. Blackman, J. Goodman, M. Medvedev, A. Spitkovsky, J. Stone
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    ABSTRACT: A white paper submitted to the Astro-2010 Decadal Survey
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    Dmitri A. Uzdensky, Anatoly Spitkovsky
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    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 spin-down 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 gamma-ray emission. We argue that reconnection proceeds in the plasmoid-dominated regime, via an hierarchical chain of multiple secondary islands/flux ropes. The inter-plasmoid 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 inverse-Compton emission from the reconnecting current sheet can explain the observed pulsed high-energy (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:10.1088/0004-637X/780/1/3 · 6.28 Impact Factor
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    ABSTRACT: Magnetic reconnection is one of a few astrophysical mechanisms that can accelerate particles to energies sufficient to emit observable high-energy radiation. This work reports on 2D simulations of reconnection in relativistic electron-positron pair plasmas, which may power gamma-ray emission from pulsar wind nebulae (PWNe), Gamma-Ray Bursts (GRBs), and blazar jets. The most important new discovery is the strong, energy-dependent angular anisotropy and spatial inhomogeneity of accelerated particles: high-energy 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 high-energy 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 light-crossing time of the reconnection region, could explain the short, intense gamma-ray 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

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