Magnetic Reconnection

In this work, we study the propagation and spin oscillations of neutrinos in their scattering by a supermassive black hole (SMBH) surrounded by a realistic accretion disk. We use a semi-analytical model of a thick accretion disk which can co-rotate and counter-rotate with respect to BH. Moreover, we assume that a disk contains only a toroidal magnetic magnetic field of moderate strength. Spin precession of neutrinos, which are supposed to be Dirac particles, is caused by the interaction of the neutrino magnetic moment with the magnetic field in the disk. We consider the incoming flux of neutrinos having an arbitrary angle with respect to the BH spin since the recent results of the Event Horizon Telescope indicate that the BH spin in the galactic center is not always perpendicular to the galactic plane. For our study, we consider a large number of incoming test neutrinos. We briefly discuss our results and their applications in the observations of astrophysical neutrinos.

A multi-hierarchy simulation model aimed at magnetic reconnection studies has been developed, in which macroscopic and microscopic physics are solved self-consistently and simultaneously. In this work, the previous multi-hierarchy model by these authors is extended to a more realistic one with non-uniform space grids. Based on the domain decomposition method, the multi-hierarchy model consists of three parts: a magnetohydrodynamics algorithm to express the macroscopic global dynamics, a particle-in-cell algorithm to describe the microscopic kinetic physics, and an interface algorithm to interlock macro and micro hierarchies. For its verification, plasma flow injection is simulated in this multi-hierarchy model and it is confirmed that the interlocking method can describe the correct physics. Furthermore, this model is applied to collisionless driven reconnection in an open system. Magnetic reconnection is found to occur in a micro hierarchy by injecting plasma from a macro hierarchy. V

The anomalous energization of plasma ions during magnetic reconnection has long been observed in numerous laboratory and astrophysical plasmas. In the MST reversed-field pinch, the reconnection impulsively heats the ions, more than doubling their temperature in ˜100 musec. A critical outstanding question has been whether or not this heating is isotropic with respect to the magnetic field, which we attempt

The quantum physics of light is a most fascinating field. Here I present a very personal viewpoint, focusing on my own path to quantum entanglement and then on to applications. I have been fascinated by quantum physics ever since I heard about it for the first time in school. The theory struck me immediately for two reasons: (1) its immense mathematical beauty, and (2) the unparalleled precision to which its predictions have been verified again and again. Particularly fascinating for me were the predictions of quantum mechanics for individual particles, individual quantum systems. Surprisingly, the experimental realization of many of these fundamental phenomena has led to novel ideas for applications. Starting from my early experiments with neutrons, I later became interested in quantum entanglement, initially focusing on multi-particle entanglement like GHZ states. This work opened the experimental possibility to do quantum teleportation and quantum hyper-dense coding. The latter became the first entanglement-based quantum experiment breaking a classical limitation. One of the most fascinating phenomena is entanglement swapping, the teleportation of an entangled state. This phenomenon is fundamentally interesting because it can entangle two pairs of particles which do not share any common past. Surprisingly, it also became an important ingredient in a number of applications, including quantum repeaters which will connect future quantum computers with each other. Another application is entanglement-based quantum cryptography where I present some recent long-distance experiments. Entanglement swapping has also been applied in very recent so-called loophole-free tests of Bell's theorem. Within the physics community such loophole-free experiments are perceived as providing nearly definitive proof that local realism is untenable. While, out of principle, local realism can never be excluded entirely, the 2015 achievements narrow down the remaining possibilities for local realistic explanations of the quantum phenomenon of entanglement in a significant way. These experiments may go down in the history books of science. Future experiments will address particularly the freedom-of-choice loophole using cosmic sources of randomness. Such experiments confirm that unconditionally secure quantum cryptography is possible, since quantum cryptography based on Bell's theorem can provide unconditional security. The fact that the experiments were loophole-free proves that an eavesdropper cannot avoid detection in an experiment that correctly follows the protocol. I finally discuss some recent experiments with single-and entangled-photon states in higher dimensions. Such experiments realized quantum entanglement between two photons, each with quantum numbers beyond 10 000 and also simultaneous entanglement of two photons where each carries more than 100 dimensions. Thus they offer the possibility of quantum communication with more than one bit or qubit per photon. The paper concludes discussing Einstein's contributions and viewpoints of quantum mechanics. Even if some of his positions are not supported by recent experiments, he has to be given credit for the fact that his analysis of fundamental issues gave rise to developments which led to a new information technology. Finally, I reflect on some of the lessons learned by the fact that

Context. Solar observations suggest that some of the most dynamic active regions are associated with complex photospheric magnetic configurations such as quadrupolar regions, and especially those that have a δ-spot configuration and a strong polarity inversion line (PIL). Aims. We study the formation and eruption of magnetic flux ropes in quadrupolar regions. Methods. We performed 3D magnetohydrodynamics simulations of the partial emergence of a highly twisted flux tube from the solar interior into a non-magnetised stratified atmosphere. We introduced a density deficit at two places along the length of the subphotospheric flux tube to emerge as two Ω-shaped loops, forming a quadrupolar region. Results. At the photosphere, the emerging flux forms two initially separated bipoles, which later come in contact, forming a δ-spot central region. Above the two bipoles, two magnetic lobes expand and interact through a series of current sheets at the interface between them. Two recurrent confined eruptions are produced. In both cases, the reconnection between sheared low-lying field lines forms a flux rope. The reconnection between the two lobes higher in the atmosphere forms field lines that retract down and push against the flux rope, creating a current sheet between them. It also forms field lines that create a third magnetic lobe between the two emerged lobes, that later acts as a strapping field. The flux rope eruptions are triggered when the reconnection between the flux ropes and the field above the ropes becomes efficient enough to remove the tension of the overlying field. These reconnection events occur internally in the quadrupolar system, as the atmosphere is non-magnetised. The flux rope of the first, weaker, eruption almost fully reconnects with the overlying field. The flux rope of the second, more energetic, eruption is confined by the overlying strapping field. During the second eruption, the flux rope is enhanced in size, flux, and twist, similar to confined-flare-to-flux-rope observations. Proxies of the emission reveal the two erupting filaments channels. A flare arcade is only formed in the second eruption owing to the longer lasting and more efficient reconnection at the current sheet below the flux rope.

A unique filament is identified in the Herschel maps of the Orion A giant molecular cloud. The filament, which we name the Stick, is ruler-straight and at an early evolutionary stage. Transverse position–velocity diagrams show two velocity components closing in on the Stick. The filament shows consecutive rings/forks in C18O (1−0) channel maps, which is reminiscent of structures generated by magnetic reconnection. We propose that the Stick formed via collision-induced magnetic reconnection (CMR). We use the magnetohydrodynamics code Athena++ to simulate the collision between two diffuse molecular clumps, each carrying an antiparallel magnetic field. The clump collision produces a narrow, straight, dense filament with a factor of >200 increase in density. The production of the dense gas is seven times faster than freefall collapse. The dense filament shows ring/fork-like structures in radiative transfer maps. Cores in the filament are confined by surface magnetic pressure. CMR can...

We study the linear and non-linear development of the Kruskal-Schwarzchild Instability in a relativisitically expanding striped wind. This instability is the generalization of Rayleigh-Taylor instability in the presence of a magnetic field. It has been suggested to produce a selfsustained acceleration mechanism in strongly magnetized outflows found in active galactic nuclei, gamma-ray bursts, and micro-quasars. The instability leads to magnetic reconnection, but in contrast with steady-state Sweet-Parker reconnection, the dissipation rate is not limited by the current layer's small aspect ratio. We performed two-dimensional (2D) relativistic magnetohydrodynamic (RMHD) simulations featuring two cold and highly magnetized (1 σ 10 3 ) plasma layers with an anti-parallel magnetic field separated by a thin layer of relativistically hot plasma with a local effective gravity induced by the outflow's acceleration. Our simulations show how the heavier relativistically hot plasma in the reconnecting layer drips out and allows oppositely oriented magnetic field lines to reconnect. The instability's growth rate in the linear regime matches the predictions of linear stability analysis. We find turbulence rather than an ordered bulk flow near the reconnection region, with turbulent velocities up to ∼ 0.1c, largely independent of model parameters. However, the magnetic energy dissipation rate is found to be much slower, corresponding to an effective ordered bulk velocity inflow into the reconnection region v in = β in c, of 10 -3 β in 5 × 10 -3 . This occurs due to the slow evacuation of hot plasma from the current layer, largely because of the Kelvin-Helmholtz instability experienced by the dripping plasma. 3D RMHD simulations are needed to further investigate the non-linear regime.

The energy dissipation mechanism within gamma-ray burst (GRB) outflows, driving their extremely luminous prompt γ -ray emission is still uncertain. The leading candidates are internal shocks and magnetic reconnection. While the emission from internal shocks has been extensively studied, that from reconnection still has few quantitative predictions. We study the expected prompt-GRB emission from magnetic reconnection and compare its temporal and spectral properties to observations. The main difference from internal shocks is that for reconnection one expects relativistic bulk motions with Lorentz factors a few in the jet's bulk frame. We consider such motions of the emitting material in two antiparallel directions (e.g. of the reconnecting magnetic-field lines) within an ultrarelativistic (with 1) thin spherical reconnection layer. The emission's relativistic beaming in the jet's frame greatly affects the light curves. For emission at radii R 0 < R < R 0 + R (with = const), the observed pulse width is T ∼ (R 0 /2c 2 ) max (1/ , R/R 0 ), i.e. up to ∼ times shorter than for isotropic emission in the jet's frame. We consider two possible magnetic reconnection modes: a quasi-steady state with continuous plasma flow into and out of the reconnection layer, and sporadic reconnection in relativistic turbulence that produces relativistic plasmoids. Both of these modes can account for many observed prompt-GRB properties: variability, pulse asymmetry, the very rapid declines at their end and pulse evolutions that are either hard to soft (for 2) or intensity tracking (for > 2). However, the relativistic turbulence mode is more likely to be relevant for the prompt sub-MeV emission and can naturally account also for the peak luminosity -peak frequency correlation.

The quantum physics of light is a most fascinating field. Here I present a very personal viewpoint, focusing on my own path to quantum entanglement and then on to applications. I have been fascinated by quantum physics ever since I heard about it for the first time in school. The theory struck me immediately for two reasons: (1) its immense mathematical beauty, and (2) the unparalleled precision to which its predictions have been verified again and again. Particularly fascinating for me were the predictions of quantum mechanics for individual particles, individual quantum systems. Surprisingly, the experimental realization of many of these fundamental phenomena has led to novel ideas for applications. Starting from my early experiments with neutrons, I later became interested in quantum entanglement, initially focusing on multi-particle entanglement like GHZ states. This work opened the experimental possibility to do quantum teleportation and quantum hyper-dense coding. The latter became the first entanglement-based quantum experiment breaking a classical limitation. One of the most fascinating phenomena is entanglement swapping, the teleportation of an entangled state. This phenomenon is fundamentally interesting because it can entangle two pairs of particles which do not share any common past. Surprisingly, it also became an important ingredient in a number of applications, including quantum repeaters which will connect future quantum computers with each other. Another application is entanglement-based quantum cryptography where I present some recent long-distance experiments. Entanglement swapping has also been applied in very recent so-called loophole-free tests of Bell's theorem. Within the physics community such loophole-free experiments are perceived as providing nearly definitive proof that local realism is untenable. While, out of principle, local realism can never be excluded entirely, the 2015 achievements narrow down the remaining possibilities for local realistic explanations of the quantum phenomenon of entanglement in a significant way. These experiments may go down in the history books of science. Future experiments will address particularly the freedom-of-choice loophole using cosmic sources of randomness. Such experiments confirm that unconditionally secure quantum cryptography is possible, since quantum cryptography based on Bell's theorem can provide unconditional security. The fact that the experiments were loophole-free proves that an eavesdropper cannot avoid detection in an experiment that correctly follows the protocol. I finally discuss some recent experiments with single-and entangled-photon states in higher dimensions. Such experiments realized quantum entanglement between two photons, each with quantum numbers beyond 10 000 and also simultaneous entanglement of two photons where each carries more than 100 dimensions. Thus they offer the possibility of quantum communication with more than one bit or qubit per photon. The paper concludes discussing Einstein's contributions and viewpoints of quantum mechanics. Even if some of his positions are not supported by recent experiments, he has to be given credit for the fact that his analysis of fundamental issues gave rise to developments which led to a new information technology. Finally, I reflect on some of the lessons learned by the fact that

It is shown that a magnetic-pressure-dominated, supersonic jet which expands (or contracts) in response to variations in the confining external pressure can dissipate magnetic energy through field-line reconnection as it relaxes to a minimum-energy configuration. In order for a continuous dissipation to take place, the effective reconnection time must be a fraction € < 1 of the expansion time. For a force-free jet (with a magnetic field satisfying \ x B = ¡xB) in the axisymmetric (m = 0) minimum-energy state, the energy per unit length that is dissipated on the characteristic reconnection time scale (assuming conservation of magnetic helicity) is given by (l/1152R 2 )(T / /47r) 2 e 2 (/iR) 6 (where R is the jet radius and T 1 is the axial magnetic flux), to lowest order in e and piR. On the basis of this result, it is concluded that magnetic energy dissipation could, in principle, power the observed synchrotron emission in extragalactic radio jets such as NGC 6251. However, just as in the case of analogous coronal heating models, this mechanism is only viable if the reconnection time is substantially shorter than the nominal resistive tearing time in the jet. Subject headings: galaxies: jets -hydromagnetics -radiation mechanisms I. INTRODUCTION It has long been recognized that the synchrotron-radiating plasma in extragalactic radio jets must undergo continuous particle reacceleration in order to overcome the adiabatic and radiative losses sustained by the expanding flow (see, e.g., the review by . The energy for the acceleration process has customarily been assumed to come from the bulk kinetic motion of the jet. Specifically, it has been suggested that a fraction of the bulk kinetic energy might be tapped through surface shear instabilities which could lead to a turbulent energy cascade in the jet and to the formation of internal shocks (e.g., . It has also been argued that the dissipation of the energy derived in this way could lead to efficient particle acceleration through the Fermi mechanism or by means of resonant interactions with MHD waves (e.g., . Recently, however, an alternative energy source for particle acceleration has been proposed in the context of the force-free-field model for magnetized supersonic jets hereafter Paper I). According to this model, once a jet becomes magnetic-pressure dominated, it tends to settle down to that equilibrium field configuration which has the lowest magnetic energy for the given magnetic helicity. This unique configuration can be expressed as a superposition of the first two modes (m = 0 and m = 1) in the Chandrasekhar-Kendall representation of linear force-free fields . Any element of the jet which propagates through a region of varying external pressure has to undergo continuous field redistribution in order to satisfy the pressurebalance condition at the boundary while maintaining a minimum-energy configuration. It was suggested that this rearrangement process might be accompanied by field-line reconnection, and that the energy liberated in this fashion could then power the synchrotron emission from the jet. The energy-dissipation scheme just described bears a strong analogy to certain recent proposals regarding the heating mechanism in the solar corona (Norman and Heyvaerts 1983;. According to these models, the energy for the heating is derived from photospheric fluid motions which build up stresses within magnetic flux tubes that extend into the corona; some of this energy can then be released in the magnetically dominated region above the photosphere as the flux tubes relax to the lowest accessible energy state that is compatible with conservation of magnetic helicity. In much the same vein, one can envision the magnetic field lines in a jet being braided and twisted at the base of the flow (which is in fact, how they, acquire a nonvanishing helicity) and subsequently releasing the accumulated stresses through reconnection at large distances from the source. The main difference between these two scenarios lies in the fact that the field relaxation process in a jet is triggered by the changes in the external pressure which confines the super-Alfvénic flow, whereas in the solar case it is initiated by the footpoint motions of the quasistatic flux tubes. As a consequence of this, the evolution of a jet can be modeled as a steady state process in which the magnetic helicity per unit length remains constant along the flow (see Paper I), whereas the simplest footpoint motions considered in the coronal heating models generally involve a change in the magnetic helicity of the associated flux tubes. The basic principle underlying the energy dissipation mechanism remains, however, the same in both cases.

The recent detection of a 3-hr X-ray flare by the Chandra Observatory has raised the possibility of enhanced emission over a broad range of wavelengths from Sgr A*, the suspected 2.6 x 10 6 M ⊙ black hole at the Galactic Center, during a flaring event. We have, therefore, reconstructed 3-hr data sets from 2µm speckle and adaptive optics images (θ core = 50 -100 mas) obtained with the W. M. Keck 10-m telescopes between 1995 and 2001. In 25 separate observations, no evidence of any significant excess emission associated with Sgr A* was detected. The lowest of our detection limits gives an observed limit for the quiescent state of Sgr A* of 0.09±0.005 mJy, or, equivalently, a dereddened value of 2.0±0.1 mJy, which is a factor of 2 lower than the best previously published quiescent value. Under the assumption that there are random 3-hr flares producing both enhanced X-ray and near-infrared emission, our highest limit constrains the variable state of Sgr A* to 0.8 mJy (observed) or 19 mJy (dereddened). These results suggest that the model favored by , in which the flare is produced through local heating of relativistic particles surrounding Sgr A* (e.g., a sudden magnetic reconnection event), is unlikely, because it predicts peak 2µm emission of ∼300 mJy, well above our detection limit.

Se presenta un metodo numerico eficiente y estable para calcular flujos cuasi-bidimensionales a superficie libre. Se trata de un esquema fuertemente impl!cito de direcciones alternadas. El metodo numerico es preciso cuando se tratan ondas relativamente largas con respecto al paso de discretizacion espacial. Su aplicacion a la simulacion del escurrimiento del R!o Parana en la zona de Corpus resulto satisfactoria, y proveyo resultados practicos de interes. An efficient and stable ADI numerical method, for the calculation of nearly two-dimensional flows with a free surface, is presented. It provides a high accuracy when treating waves with long wave -Unghts relative to the spatial discretization step. Its performance in the numerical simulation of the flow in Parana River, close to the site where Corpus Dam will be built, was satisfactory, and provided interesting practical results.

Reconnection physics at micro-scales is investigated in an electron magnetohydrodynamics frame. A new process of collapse of the neutral current sheet is demonstrated by means of analytical and numerical solutions. It shows how at scales smaller than ion inertia length a compression of the sheet triggers an explosive evolution of current perturbation. Collapse results in the formation of a intense sub-sheet and then an X-point structure embedded into the equilibrium sheet. Hall currents associated with this structure support high reconnection rates. Nonlinear static solution at scales of the electron skin reveals that electron inertia and small viscosity provide an efficient mechanism of field lines breaking. The reconnection rate does not depend on the actual value of viscosity, while the maximum current is found to be restricted even for space plasmas with extremely rare collisions. The results obtained are verified by a two-fluid large-scale numerical simulation.

The general concept of the magnetic reconnection converter (MRC) is considered, based on the cyclic combination of two
physical processes: 1) controlled turbulence using super-linear Richardson diffusion and/or self-generated/self-sustaining
physical processes increase the stochasticity of the magnetic field (MF) in a limited volume of plasma and, accordingly, the
global helicity H through the processes of twisting, writhing, and linking of the MF flow tubes to the level of a local maximum
(optimally global), which is determined by the plasma parameters, boundary conditions, magnetic tension of the field lines, etc.
At this stage of the MF turbulent pumping, the β of plasma will decrease to the minimum possible value with a corresponding
increasing in the accumulated "topological" MF energy; 2) upon reaching the local (if possible global) maximum of MF
stochasticity, turbulent magnetic reconnection (TMR) occurs in the plasma, which reduces the state of the local (if possible
global) maximum of MF stochasticity and increases the kinetic stochasticity of plasma particles, accelerating and heating them,
which is used in direct converters of electrical power. At this stage of turbulent discharge, the β of plasma will be increasing to the
maximum possible value with a corresponding increasing in its kinetic and thermal energy; 3) when the kinetic stochasticity of
plasma particles subsequently decreases and reaches a local minimum, the control system repeats the MF turbulent pumping in
the plasma and the cycles are repeated. Practically, the basis of the MRC can be the fusion scheme of two anti-spiral spheromaks,
the helicity of which is increased in a cycle with the help of controlled turbulence before their fusion and the creation of a
field-reversed configuration (FRC) to increase the efficiency of the annihilation of their toroidal and poloidal magnetic fields into
kinetic and thermal energy of plasma particles with its subsequent direct transformation into electrical power for industrial use or
single-volume plasma (spheromak) with changing beta at turbulent pumping/discharge phases of the working cycle.

The application of a mixture of nitrogen and deuterium for the gas-puffing along the anode axis in deuterium plasma-focus discharges, as carried out at megaampere-level currents, enabled observations of the filamentary structure, and the decrease in the transformation velocity of the plasma column to be performed. It made possible to investigate the instability evolution during the production of hard X-rays and fast neutrons in more detail. The constriction of a plasma column transforms itself during the final phase of the compression into one or more small dense plasmoid-like structures which are separated by narrow necks. During the next phase, these structures start to decay by an expansion, in which a part of the plasma volume maintains its compactness. This evolution is explained by an increase and later decrease in the internal poloidal current component by reconnections of the associated magnetic lines, which are responsible for the acceleration of electron and ion beams.

The present experiments were performed on the PF-1000 plasma focus device at a current of 2 MA with the deuterium injected from the gas-puff placed in the axis of the anode face. The XUV frames showed, in contrast with the interferograms, the fine structure: filaments and spots up to 1 mm diameter. In the deuterium filling, the short filaments are registered mainly in the region of the internal plasmoidal structures and their number correlates with the intensity of neutron production. The longer filamentary structure was recorded close to the anode after the constriction decay. The long curve-like filaments with spots were registered in the big bubble formed after the pinch phase in the head of the umbrella shape of the plasma sheath. Filaments can indicate the filamentary structure of the current in the pinch. Together with the filaments, small compact balls a few mm in diameter were registered by both interferometry and XUV frame pictures. They emerge out of the dense column and t...

A Lagrangian Remap (LareXd) Code is employed to investigate the shock wave formation in the current sheet of a solar coronal magnetic loop and its effect on the magnetic reconnection. We constructed the slow shock structure in the presence of viscosity and heat conduction parallel and perpendicular to the magnetic field and pairs of slow shocks propagate away from the central current sheet, the so-called diffusion region. Significant jumps in plasma density, pressure, velocity and magnetic field occur across the main shock while the temperature appears in the foreshock. In the presence of dissipative effects, the distinct jumps disappear and the shock profiles show smooth transition between the downstream and the upstream regions while the plasma density and the pressure show very narrow and a sharp decrease with time. These results can be applied to the heating of the solar corona, the structure of the magnetic reconnection and the solar wind.

Magnetic reconnection occurs ubiquitously in the universe and is often invoked to explain fast energy release and particle acceleration in high-energy astrophysics. The study of relativistic magnetic reconnection in the magnetically dominated regime has surged over the past two decades, revealing the physics of fast magnetic reconnection and nonthermal particle acceleration. Here we review these recent progresses, including the magnetohydrodynamic and collisionless reconnection dynamics as well as particle energization. The insights in astrophysical reconnection strongly connect to the development of magnetic reconnection in other areas, and further communication is greatly desired. We also provide a summary and discussion of key physics processes and frontier problems, toward a better understanding of the roles of magnetic reconnection in high-energy astrophysics.

College -During magnetically dominated relativistic reconnection, inflowing plasma depletes the initial relativistic pressure at the x -line, and collisionless plasma heating inside the diffusion region appears to be insufficient to overcome this pressure loss. The resulting significant pressure drop causes a collapse at the xline, essentially a localization mechanism of the diffusion region necessary for fast reconnection. The extension of this low-pressure region (into the outflow) further explains the bursty nature of antiparallel reconnection because a once opened outflow exhaust can also collapse, which repeatedly triggers secondary tearing islands. However, a stable single x -line reconnection can be achieved when an external guide field exists, since the reconnecting magnetic field component rotates out of the reconnection plane at outflows, providing additional magnetic pressure to keep the exhaust open.

Goddard Space Flight Center -Kinetic simulations of magnetically dominated reconnection (plasma beta ≪ 1) with closed or periodic simulation domains have shown the formation of hard power-law distribution with spectral index p ∼ 1. However, for most of applications observations have inferred significantly softer spectra. Here we present 2D and 3D fully kinetic simulations with open boundary conditions that allow the escape of accelerated particles from the reconnection acceleration region. While the primary acceleration mechanism is still the Fermi acceleration through curvature drift motion of particles, we show that the energy spectrum can be significantly softer than "-1." We further examine the effect of domain size, plasma beta, and magnetization parameters.

Using fully kinetic simulations, we study the suppression of asymmetric reconnection in the limit where the diamagnetic drift speed Alfvén speed and the magnetic shear angle is moderate. We demonstrate that the slippage between electrons and the magnetic flux facilitates reconnection, and can even result in fast reconnection that lacks one of the outflow jets. Through comparing a case where the diamagnetic drift is supported by the temperature gradient with a companion case that has a density gradient instead, we identify a robust suppression mechanism. The drift of the x-line is slowed down locally by the asymmetric nature of the current sheet and the resulting tearing modes, then the x-line is run over and swallowed by the faster-moving following flux.

Magnetic reconnection is thought to be the driver for many explosive phenomena in the universe. The energy release and particle acceleration during reconnection have been proposed as a mechanism for producing high-energy emissions and cosmic rays. We carry out two-and three-dimensional kinetic simulations to investigate relativistic magnetic reconnection and the associated particle acceleration. The simulations focus on electron-positron plasmas starting with a magnetically dominated, force-free current sheet (σ ≡ B 2 /(4πn e m e c 2 ) 1). For this limit, we demonstrate that relativistic reconnection is highly efficient at accelerating particles through a first-order Fermi process accomplished by the curvature drift of particles along the electric field induced by the relativistic flows. This mechanism gives rise to the formation of hard power-law spectra f ∝ (γ -1) -p and approaches p = 1 for sufficiently large σ and system size. Eventually most of the available magnetic free energy is converted into nonthermal particle kinetic energy. An analytic model is presented to explain the key results and predict a general condition for the formation of power-law distributions. The development of reconnection in these regimes leads to relativistic inflow and outflow speeds and enhanced reconnection rates relative to non-relativistic regimes. In the three-dimensional simulation, the interplay between secondary kink and tearing instabilities leads to strong magnetic turbulence, but does not significantly change the energy conversion, reconnection rate, or particle acceleration. This study suggests that relativistic reconnection sites are strong sources of nonthermal particles, which may have important implications to a variety of high-energy astrophysical problems. Subject headings: acceleration of particles -magnetic reconnection -relativistic process -gamma-ray bursts: general -galaxies: jets -pulsars: general 1. INTRODUCTION Magnetic reconnection is a fundamental plasma process that rapidly rearranges magnetic topology and converts magnetic energy into various forms of plasma kinetic energy, including bulk plasma flow, thermal and nonthermal plasma distributions . It is thought to play an important role during explosive energy release processes of a wide variety of laboratory, space and astrophysical systems including tokamak, planetary magnetospheres, solar flares, and high-energy astrophysical objects. Relativistic magnetic reconnection is often invoked to explain high-energy emissions and ultra-high-energy cosmic rays from objects such as pulsar wind nebulae (

Magnetic reconnection is a primary driver of particle acceleration processes in space and astrophysical plasmas. Understanding how particles are accelerated and the resulting particle energy spectra is among the central topics in reconnection studies. We review recent advances in addressing this problem in nonrelativistic reconnection that is relevant to space and solar plasmas and beyond. We focus on particle acceleration mechanisms, particle transport due to 3D reconnection physics, and their roles in forming power-law particle energy spectra. We conclude by pointing out the challenges in studying particle acceleration and transport in a large-scale reconnection layer and the relevant issues to be addressed in the future.

Magnetic reconnection is a leading mechanism for dissipating magnetic energy and accelerating nonthermal particles in Poynting-flux dominated flows. In this letter, we investigate nonthermal particle acceleration during magnetic reconnection in a magnetically-dominated ion-electron plasma using fully kinetic simulations. For an ion-electron plasma with the total magnetization σ 0 = B 2 /(4πn(m i + m e )c 2 ), the magnetization for each species is σ i ∼ σ 0 and σ e ∼ (m i /m e )σ 0 , respectively. We have studied the magnetically dominated regime by varying σ e = 10 3 -10 5 with initial ion and electron temperatures T i = T e = 5 -20m e c 2 and mass ratio m i /m e = 1 -1836. The results demonstrate that reconnection quickly establishes power-law energy distributions for both electrons and ions within several (2 -3) light-crossing times. For the cases with periodic boundary conditions, the power-law index is 1 < s < 2 for both electrons and ions. The hard spectra limit the power-law energies for electrons and ions to be γ be ∼ σ e and γ bi ∼ σ i , respectively. The main acceleration mechanism is a Fermi-like acceleration through the drift motions of charged particles. When comparing the spectra for electrons and ions in momentum space, the spectral indices s p are identical as predicted in Fermi acceleration. We also find that the bulk flow can carry a significant amount of energy during the simulations. We discuss the implication of this study in the context of Poynting-flux dominated jets and pulsar winds especially the applications for explaining the nonthermal high-energy emissions.

The American Physical Society Orientation of x-lines in asymmetric magnetic reconnection YI-HSIN LIU, MICHAEL HESSE, MASHA KUZNETSOVA, NASA/GSFC -At Earth's magnetopause, reconnection proceeds asymmetrically between magnetosheath plasmas, namely solar wind plasmas compressed by Earth's bow shock, and magnetospheric plasmas. In an asymmetric configuration, it is unclear if there is a simple principle to determine the orientation of the x-line. Using fully kinetic simulations, we study this issue and a spatially localized perturbation is employed to induce a single x-line, that has sufficient freedom to choose its orientation in threedimensional systems. The effect of ion to electron mass ratio is investigated, and the x-line appears to bisect the magnetic shear angle across the current sheet in the large mass ratio limit. The deviation from the bisection angle in the lower mass ratio limit can be explained by the physics of tearing instability. 1 The local physics control of the x-line orientation studied in this slab geometry could potentially interplay with global geometrical effects to determine the location of collisionless magnetic reconnection at Earth's magnetopause.

Using 2-dimensional (2D) magnetohydrodynamics (MHD) simulations, we show that Petschek-type magnetic reconnection can be induced using a simple resistivity gradient in the reconnection outflow direction, revealing the key ingredient of steady fast reconnection in the collisional limit. We find that the diffusion region self-adjusts its halflength to fit the given gradient scale of resistivity. The induced reconnection x-line and flow stagnation point always reside within the resistivity transition region closer to the higher resistivity end. The opening of one exhaust by this resistivity gradient will lead to the opening of the other exhaust located on the other side of the x-line, within the region of uniform resistivity. Potential applications of this setup to reconnection-based thrusters and solar spicules are discussed. In a separate set of numerical experiments, we explore the maximum plausible reconnection rate using a large and spatially localized resistivity right at the x-line. Interestingly, the resulting current density at the x-line drops significantly so that the normalized reconnection rate remains bounded by the value 0.2, consistent with the theoretical prediction.

In strongly magnetized astrophysical plasma systems, magnetic reconnection is believed to be the primary process during which explosive energy release and particle acceleration occur, leading to significant high-energy emission. Past years have witnessed active development of kinetic modeling of relativistic magnetic reconnection, supporting this magnetically dominated scenario. A much less explored issue in studies of relativistic reconnection is the consequence of three-dimensional dynamics, where turbulent structures are naturally generated as various types of instabilities develop. This paper presents a series of three-dimensional, fully-kinetic simulations of relativistic turbulent magnetic reconnection (RTMR) in positron-electron plasmas with system domains much larger than kinetic scales. Our simulations start from a force-free current sheet with several different modes of long wavelength magnetic field perturbations, which drive additional turbulence in the reconnection region. Because of this, the current layer breaks up and the reconnection region quickly evolves into a turbulent layer filled with coherent structures such as flux ropes and current sheets. We find that plasma dynamics in RTMR is vastly different from their 2D counterparts in many aspects. The flux ropes evolve rapidly after their generation, and can be completely disrupted due to the secondary kink instability. This turbulent evolution leads to superdiffusion behavior of magnetic field lines as seen in MHD studies of turbulent reconnection. Meanwhile, nonthermal particle acceleration and energy-release time scale can be very fast and do not strongly depend on the turbulence amplitude. The main acceleration mechanism is a Fermi-like acceleration process supported by the motional electric field, whereas the non-ideal electric field acceleration plays a subdominant role. We also discuss possible observational implications of three-dimensional RTMR in high-energy astrophysics.

shear extending to the very low shear limit. Here we report on our study of the effect of the magnetic shear on details of reconnection such as its structure and rate, using 2D and 3D kimetic simulations and analytical theory. Contrary to all previous theories, we find that the electron diffusion region bifurcates into two or more distinct layers in regimes with weak magnetic shear. 1 This new morphology is explained by oblique tearing modes which produce flux ropes while simultaneously driving enhanced current at multiple resonance surfaces. This physics persists into the nonlinear regime leading to multiple electron layers embedded within a larger Alfvénic inflow and outflow, a feature that could be observed by NASA's up-coming Magnetospheric Multiscale mission. We have extended the study to lower shear cases and found a new regime where the rate becomes much smaller and the properties of the reconnection changes significantly. We will discuss this new regime and offer a new analytical model that predicts key aspects of this regime.

Using 2.5-dimensional particle-in-cell (PIC) simulations of magnetotail dynamics, we investigate the onset of reconnection in realistic tail configurations. Reconnection onset is preceded by a driven phase, during which magnetic flux is added to the tail at the high-latitude boundaries, followed by a relaxation phase, during which the configuration continues to respond to the driving. We found a clear distinction between stable and unstable cases, dependent on the deformation amplitude and ion/electron mass ratio. The threshold appears consistent with electron tearing. The evolution prior to onset as well as the evolution of stable cases, are largely independent of the mass ratio, governed by the integral entropy conservation as imposed in MHD. This suggests that ballooning instability in the tail should not be expected prior to the onset of tearing and reconnection. The onset time and other onset properties depend on the mass ratio, consistent with expectations for electron tearing. At onset, we found electron anisotropies T ⊥ /T ∥ = 1.1 -1.3, raising growth rates and wave numbers. Our simulations have provided a quantitative onset criterion that is easily evaluated in MHD simulations, provided the spatial resolution is sufficient. 1

Particle acceleration/energization during reconnection Key unknowns • Primary acceleration mechanism • Resulting energy spectra • The role of 3D Physics 3D instabilities (self-generated turbulence) external turbulence • The role of different boundary conditions Beresnyak & Li

Este trabajo se suma a los múltiples esfuerzos que se han realizado en México y en el mundo para documentar las experiencias de los profesores universitarios en el abrupto cambio de educación presencial a educación remota ocasionado por la pandemia. Se presentan los resultados de una encuesta que respondieron 647 profesores de la Facultad de Química de la Universidad Nacional Autónoma de México. El cuestionario tiene 17 preguntas cerradas y 6 preguntas abiertas que fueron analizadas con estadística descriptiva y análisis de contenido. Los resultados muestran que la mayoría de los profesores vivió la situación más como un reto que como un problema. Si bien la mayoría no tenía experiencia con la docencia en línea, la enfrentaron utilizando distintos recursos. La experiencia de los docentes es muy diversa y muestra la necesidad de compartir formas de plantear las clases y recursos para transformar las prácticas de enseñanza en la universidad.

Using magnetometer and electron observations from the Mars Global Surveyor (MGS) and theWind spacecraft we show that the region of magnetic field pile-up and density decrease located between the Martian ionosphere and bow shock exhibit strong similarities with the plasma depletion layer (PDL) observed upstream of the Earth's magnetopause in the absence of magnetic reconnection when the magnetopause is a solid obstacle in the solar wind. A PDL is formed upstream of the terrestrial magnetopause when the magnetic field piles up against the obstacle and particles in the pile-up region are squeezed away from the high magnetic pressure region along the field lines as the flux tubes convect toward the magnetopause. We here discuss the possibility that at least part of the region of magnetic field pile-up and density depletion upstream of Mars may be formed by the same physical processes which generate the PDL upstream of the Earth's magnetopause. More complete ion, electron, and neutral measurements are needed to conclusively determine the relative importance of the plasma depletion process versus exospheric processes.

Magnetic reconnection leads to energy conversion in large volumes in space but is initiated in small diffusion regions. Because of the small sizes of the diffusion regions, their crossings by spacecraft are rare. We report four-spacecraft observations of a diffusion region encounter at the Earth's magnetopause that allow us to reliably distinguish spatial from temporal features. We find that the diffusion region is stable on ion time and length scales in agreement with numerical simulations. The electric field normal to the current sheet is balanced by the Hall term in the generalized Ohm's law, E n j B=ne n, thus establishing that Hall physics is dominating inside the diffusion region. The reconnection rate is fast, 0:1. We show that strong parallel currents flow along the separatrices; they are correlated with observations of high-frequency Langmuir/upper hybrid waves.

We report direct measurements of high-energy particles in a rare crossing of the diffusion region in Earth's magnetotail by the Wind spacecraft. The fluxes of energetic electrons up to 300 keV peak near the center of the diffusion region and decrease monotonically away from this region. The diffusion region electron flux spectrum obeys a power law with an index of ÿ3:8 above 2 keV, and the electron angular distribution displays strong field-aligned bidirectional anisotropy at energies below 2 keV, becoming isotropic above 6 keV. These observations indicate significant electron acceleration inside the diffusion region. Ions show no such energization.

We report an accelerated plasma flow event detected by Wind at the low-latitude dawn tail magnetopause (XGSE =-10 RE) when the local magnetic shear across the magnetopause was high (-180 ø) and the GSM y component of the interplanetary magnetic field was positive and much larger than the z component. High time resolution (3 s) three-dimensional ion and electron distributions were obtained for this event. We have performed rigorous tests of fluid and particle predictions of reconnection. Consistent with rcconnection, we observed at the magnetopause (1) jetting of plasma, with a flow speed, measured in the deHoffmann-Teller frame, at 98% of the Alfv6n speed; (2) mixing of a nearly isotropic hot plasma sheet ion distribution with a field-aligned magnetosheath distribution having a low-energy cutoff at the predicted deHoffmann-Teller velocity; (3) opposite streaming of magnetosheath and plasma sheet electrons consistent with open field topology; and (4) a finite inward (earthward) directed normal magnetic field (BN<0) at and an associated earthward plasma flow (at 94% of the normal Alfv6n speed) across the magnetopause. The combined fluid and particle signatures provide a comprehensive set of evidence for reconnection at the magnetopause. These reconnection signatures were observed in the tail flank magnetopause, where the presence of fast plasma flow (at almost twice the local Alfv6n speed) in the adjacent magnetosheath has been predicted to suppress reconnection. The sense of the flow enhancement, the direction of the electron heat flux, and the polarity of B N are all consistent with each other and with the spacecraft being located tailward and northward of the reconnection site. Our analysis places the reconnection site --7 Earth radii south of the magnetic equator. The dimensionless reconnection rate at this flank magnetopause is estimated to be in the range of 0.1 to 0.2, which is similar to values reported for the subsolar region. In essence, the present event provides unambiguous evidence for reconnection and shows that reconnection signatures at the tail flank magnetopause are not noticeably different from those predicted and observed in the subsolar region.

Solar atmosphere is filled with plasma and magnetic field. Activities in the atmosphere are due to plasma instabilities in the magnetic field. To understand the physical mechanisms of activities / instabilities, it is necessary to know the physical conditions of magnetized plasma, such as temperature, density, magnetic field, and their spatial structures and temporal developments. Multi-wavelength imaging is essential for this purpose. Imaging observations of the Sun at microwave, X-ray, EUV and optical ranges are routinely going on. Due to free exchange of original data among solar physics and related field communities, we can easily combine images covering wide range of spectrum. Even under such circumstances, we still do not understand the cause of activities in the solar atmosphere well. The current standard model of solar activities is based on magnetic reconnection: release of stored magnetic energy by reconnection is the cause of solar activities on the Sun such as solar flares. However, recent X-ray, EUV and microwave observations with high spatial and temporal resolution show that dense plasma is involved in activities from the beginning. Based on these observations, I propose a high-beta model of solar activities, which is very similar to high-beta disruptions in magnetically confined fusion experiments.

Our knowledge about the origin and transformation mechanisms of the bright points in the solar network has a significant role in understanding the ejection of materials and the transfer of energy into the solar corona. Outside the active region of the Sun (AR), although it is called the Quiet Sun (QS), various types of small-scale bright phenomena constantly occur within the boundary of the super granular cells above the magnetic network. Knowing the bright points is an effective key in considering the solar spicules. In this research, we study the solar transition region bright points and examine their apparent velocities with the local correlation tracking Fourier (FLCT) method. The results illustrate that these points differ in apparent velocity direction and brightness. Their lifetime and average horizontal velocity were estimated at 100 s and 4 kms-1, respectively. Recently, a new group of solar spicules has been observed, those lifetimes are around 100 s, and show a typical horizontal velocity of 3-4 kms-1. According to the analysis of the two-dimensional, apparent velocity of the bright points on the rosettes of the network, these points can be the disk counterpart of the type II spicules. In addition, the analysis of the two-dimensional field of velocities shows rotations that can cause the excitation of Alfvenic pulses.

X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14% in the 2-10 keV band in 10 5 s for pointed observations, and 0.6% for an X10 class solar

This contribution makes a review and compares the current competitive theories, based on triggering mechanisms, in order to explain the episodic phenomena of disconnection events (DEs), i.e., when the plasma tail appears disconnected from the cometary head, namely: the ion production effects, the pressure effects and the magnetic reconnection effects are analysed.

This contribution makes a review and compares the current competitive theories, based on triggering mechanisms, in order to explain the episodic phenomena of disconnection events (DEs), i.e., when the plasma tail appears disconnected from the cometary head, namely: the ion production effects, the pressure effects and the magnetic reconnection effects are analysed.

There have been many significant advances in understanding magnetic field reconnection as a result of improved space measurements and two-dimensional computer simulations. While reviews of recent work have tended to focus on symmetric reconnection on ion and larger spatial scales, the present review will focus on asymmetric reconnection and on electron scale physics involving the reconnection site, parallel electric fields, and electron acceleration.