Effect of guide field on three dimensional electron shear flow instabilities in collisionless magnetic reconnection

We examine the effect of an external guide field and current sheet thickness on the growth rates and nature of three dimensional unstable modes of an electron current sheet driven by electron shear flow. The growth rate of the fastest growing mode drops rapidly with current sheet thickness but increases slowly with the strength of the guide field. The fastest growing mode is tearing type only for thin current sheets (half thickness $\approx d_e$, where $d_e=c/\omega_{pe}$ is electron inertial length) and zero guide field. For finite guide field or thicker current sheets, fastest growing mode is non-tearing type. However growth rates of the fastest 2-D tearing mode and 3-D non-tearing mode are comparable for thin current sheets ($d_e <$half thickness $< 2\,d_e$) and small guide field (of the order of the asymptotic value of the component of magnetic field supporting electron current sheet). It is shown that the general mode resonance conditions for electron-magnetohydrodynamic (EMHD) and magnetohydrodynamic (MHD) tearing modes depend on the effective dissipation mechanism (electron inertia and resistivity in cases of EMHD and MHD, respectively). The usual tearing mode resonance condition ($\mathbf{k}.\mathbf{B}_0=0$, $\mathbf{k}$ is the wave vector and $\mathbf{B}_0$ is equilibrium magnetic field) can be recovered from the general resonance conditions in the limit of weak dissipation. Necessary conditions (relating current sheet thickness, strength of the guide field and wave numbers) for the existence of tearing mode are obtained from the general mode resonance conditions.Comment: The following article has been submitted to Physics of Plasmas. After it is published, it will be found at http://scitation.aip.org/content/aip/journal/pop. Authors gratefully acknowledges the support of the German Science Foundation CRC 96

Analysis of fast turbulent reconnection with self-consistent determination of turbulence timescale

We present results of Reynolds-averaged turbulence model simulation on the problem of magnetic reconnection. In the model, in addition to the mean density, momentum, magnetic field, and energy equations, the evolution equations of the turbulent cross-helicity $W$, turbulent energy $K$ and its dissipation rate $\varepsilon$ are simultaneously solved to calculate the rate of magnetic reconnection for a Harris-type current sheet. In contrast to previous works based on algebraic modeling, the turbulence timescale is self-determined by the nonlinear evolutions of $K$ and $\varepsilon$, their ratio being a timescale. We compare the reconnection rate produced by our mean-field model to the resistive non-turbulent MHD rate. To test whether different regimes of reconnection are produced, we vary the initial strength of turbulent energy and study the effect on the amount of magnetic flux reconnected in time.Comment: 10 pages, 7 figure

Spontaneous current-layer fragmentation and cascading reconnection in solar flares: II. Relation to observations

In the paper by B\'arta et al. (arXive:astro-ph:/1011.4035, 2010) the authors addressed some open questions of the CSHKP scenario of solar flares by means of high-resolution MHD simulations. They focused, in particular, on the problem of energy transfer from large to small scales in decaying flare current sheet (CS). Their calculations suggest, that magnetic flux-ropes (plasmoids) are formed in full range of scales by a cascade of tearing and coalescence processes. Consequently, the initially thick current layer becomes highly fragmented. Thus, the tearing and coalescence cascade can cause an effective energy transfer across the scales. In the current paper we investigate whether this mechanism actually applies in solar flares. We extend the MHD simulation by deriving model-specific features that can be looked for in observations. The results of the underlying MHD model showed that the plasmoid cascade creates a specific hierarchical distribution of non-ideal/acceleration regions embedded in the CS. We therefore focus on the features associated with the fluxes of energetic particles, in particular on the structure and dynamics of emission regions in flare ribbons. We assume that the structure and dynamics of diffusion regions embedded in the CS imprint themselves into structure and dynamics of flare-ribbon kernels by means of magnetic-field mapping. Using the results of the underlying MHD simulation we derive the expected structure of ribbon emission and we extract selected statistical properties of the modelled bright kernels. Comparing the predicted emission and its properties with the observed ones we obtain a good agreement of the two.Comment: 7 pages, 5 figure

Electron Energy-Loss Spectroscopy: A versatile tool for the investigations of plasmonic excitations

The inelastic scattering of electrons is one route to study the vibrational and electronic properties of materials. Such experiments, also called electron energy-loss spectroscopy, are particularly useful for the investigation of the collective excitations in metals, the charge carrier plasmons. These plasmons are characterized by a specific dispersion (energy-momentum relationship), which contains information on the sometimes complex nature of the conduction electrons in topical materials. In this review we highlight the improvements of the electron energy-loss spectrometer in the last years, summarize current possibilities with this technique, and give examples where the investigation of the plasmon dispersion allows insight into the interplay of the conduction electrons with other degrees of freedom

Preferential acceleration of heavy ions in magnetic reconnection: Hybrid-kinetic simulations with electron inertia

Solar energetic particles (SEPs) in the energy range 10s KeV/nucleon - 100s MeV/nucleon originate from Sun. Their high flux near Earth may damage the space borne electronics and generate secondary radiations harmful for the life on Earth and thus understanding their energization on Sun is important for space weather prediction. Impulsive (or ${}^{3}$He-rich) SEP events are associated with the acceleration of charge particles in solar flares by magnetic reconnection and related processes. The preferential acceleration of heavy ions and the extra-ordinary abundance enhancement of ${}^3$He in the impulsive SEP events are not understood yet. In this paper, we study ion acceleration in magnetic reconnection by two dimensional hybrid-kinetic plasma simulations (kinetic ions and inertial electron fluid). All the ions species are treated self-consistently in our simulations. We find that heavy ions are preferentially accelerated to energies many times larger than their initial thermal energies by a variety of acceleration mechanisms operating in reconnection. Most efficient acceleration takes place in the flux pileup regions of magnetic reconnection. Heavy ions with sufficiently small values of charge to mass ratio ($Q/M$) can be accelerated by pickup mechanism in outflow regions even before any magnetic flux is piled up. The energy spectra of heavy ions develop a shoulder like region, a non-thermal feature, as a result of the acceleration. The spectral index of the power law fit to the shoulder region of the spectra varies approximately as $(Q/M)^{-0.64}$. Abundance enhancement factor, defined as number of particles above a threshold energy normalized to total number of particles, scales as $(Q/M)^{-\alpha}$ where $\alpha$ increases with the energy threshold. We discuss our simulation results in the light of the SEP observations.Comment: Submitte