Using fully kinetic simulations, we demonstrate that magnetic reconnection in relativistic plasmas is highly efficient at accelerating particles through a first-order Fermi process resulting from the curvature drift of particles in the direction of the electric field induced by the relativistic flows. This mechanism gives rise to the formation of hard power-law spectra in parameter regimes where the energy density in the reconnecting field exceeds the rest mass energy density σ ≡ B 2 /(4πnmec 2 ) > 1 and when the system size is sufficiently large. In the limit σ 1, the spectral index approaches p = 1 and most of the available energy is converted into non-thermal particles. A simple analytic model is proposed which explains these key features and predicts a general condition under which hard power-law spectra will be generated from magnetic reconnection.PACS numbers: 52.27. Ny, 52.35.Vd, 98.54.Cm, 98.70.Rz Introduction -Magnetic reconnection is a fundamental plasma process that allows rapid changes of magnetic field topology and the conversion of magnetic energy into plasma kinetic energy. It has been extensively discussed in solar flares, Earth's magnetosphere, and laboratory applications. However, magnetic reconnection remains poorly understood in high-energy astrophysical systems [1]. Magnetic reconnection has been suggested as a mechanism for producing high-energy emissions from pulsar wind nebula, gamma-ray bursts, and jets from active galactic nuclei [2][3][4][5][6]. In those systems, it is often expected that the magnetization parameter σ ≡ B 2 /(4πnmc 2 ) exceeds unity. Most previous kinetic studies focused on the non-relativistic regime σ < 1 and reported several acceleration mechanisms such as acceleration at X-line regions [7][8][9] and Fermi-type acceleration within magnetic islands [8][9][10][11]. More recently, the regime σ = 1-100 has been explored using pressure-balanced current sheets and strong particle acceleration has been found in both diffusion regions [12][13][14][15] and island regions [16,17]. However, this initial condition requires a hot plasma component inside the current sheet to maintain force balance, which may not be justified for high-σ plasmas.

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.

We have studied electronic excited states in films of poly(p-phenylenevinylene) using picosecond transient and cw photomodulation, photoluminescence, and their excitation spectra, as well as electroabsorption spectroscopy. %'e have determined all the important energy levels of singlet excitons with odd and even parity, the onset of the continuum band, the two-electron (biexciton) states, and the two relevant triplet states, and show that good agreement exists with models involving electron correlation. PACS numbers: 78.47.+p, 72.20.3v, 78.55.Kz, 78.66.gn The photophysics and resonant nonlinear optical properties of conducting polymers are dominated by the locations and natures of the excited-state energy levels.These excited states include singlet excitons with odd (8") and even (As) parity, the continuum band (CB), two-electron (biexciton) states, and the triplet manifold [1,2]. Recent theoretical advances in the area of subgap third-order optical nonlinearity [3,4] provide information about a subset of the excited states, which include the lowest B"exciton (18"),a dominant As exciton (hereafter the mAs), and the CB threshold. The relative locations of the 18"and the lowest As (2As) excitons are determined by a sensitive interplay between electronelectron interaction and alternation (b) in the tr electron transfer integral along the polymer chain [5]. For realistic Coulomb interaction and small 8 [5], the optical gap Eg to the 1 B"exciton is relatively small, the 2Ag lies below the 18",and, due to the dipole forbidden character of the lowest singlet, photoluminescence (PL) is weak. Large b results in larger Es, state ordering E(2As) )E(18"), and consequently high PL efficiency with promising applications in displays [such as light emitting diodes (LED) [6]]. The benzene ring in the backbone structure of poly(p-phenylenevinylene) (PPV) yields an effective 8 for the extended n states that is large [2], and therefore PPV belongs to the latter category. Nevertheless, Coulomb interaction among the tr electrons in PPV leads to behavior qualitatively different from the predictions of single-particle Hiickel or SSH models. Recent subpicosecond PL [7] and site-selection PL [8] have demonstrated that the primary excitation in PPV is to an exciton, and that the associated lattice relaxation energy is small. This already suggests a subsidiary role of the electron-phonon interaction. The location of the mug exciton has been determined by two-photon luminescence [9], whereas long-lived triplet excitons have been found in thin films [10,11] and LEDs [12). In the present work, we present a more complete picture of the various photoexcitations and excited states in PPV, based on a variety of optical probes including picosecond transient and cw photomodulation (PM) and PL and their excita-tion dependence, and the electroabsorption (EA) technique. We have tentatively mapped the most relevant singlet and triplet electronic manifolds, including the CB threshold and the lowest biexciton, that are not seen in direct optical absorpt...

Magnetic reconnection is believed to be the dominant energy release mechanism in solar flares. The standard flare model predicts both downward and upward outflow plasmas with speeds close to the coronal Alfvén speed. Yet, spectroscopic observations of such outflows, especially the downflows, are extremely rare. With observations of the newly launched Interface Region Imaging Spectrograph (IRIS), we report the detection of greatly redshifted (∼125 km s −1 along line of sight) Fe xxi 1354.08Å emission line with a ∼100 km s −1 nonthermal width at the reconnection site of a flare. The redshifted Fe xxi feature coincides spatially with the loop-top X-Ray source observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). We interpret this large redshift as the signature of downward-moving reconnection outflow/hot retracting loops. Imaging observations from both IRIS and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) also reveal the eruption and reconnection processes. Fast downward-propagating blobs along these loops are also found from cool emission lines (e.g., Si iv, O iv, C ii, Mg ii) and images of AIA and IRIS. Furthermore, the entire Fe xxi line is blueshifted by ∼260 km s −1 at the loop footpoints, where the cool lines mentioned above all exhibit obvious redshift, a result that is consistent with the scenario of chromospheric evaporation induced by downward-propagating nonthermal electrons from the reconnection site.

While observations have suggested that power-law electron energy spectra are a common outcome of strong energy release during magnetic reconnection, e.g., in solar flares, kinetic simulations have not been able to provide definite evidence of power-laws in energy spectra of non-relativistic reconnection. By means of 3D large-scale fully kinetic simulations, we study the formation of power-law electron energy spectra in nonrelativistic low-β reconnection. We find that both the global spectrum integrated over the entire domain and local spectra within individual regions of the reconnection layer have power-law tails with a spectral index p ∼ 4 in the 3D simulation, which persist throughout the non-linear reconnection phase until saturation. In contrast, the spectrum in the 2D simulation rapidly evolves and quickly becomes soft. We show that 3D effects such as self-generated turbulence and chaotic magnetic field lines enable the transport of high-energy electrons across the reconnection layer and allow them to access several main acceleration regions. This leads to a sustained and nearly constant accel-