Narrow Results

This review paper comprises main concepts, available observational data and recent theoretical results related to astrophysical aspects of particle acceleration at/near the Sun and extreme capacities of the solar accelerator(s). We summarize underground and ground-based observations of solar cosmic rays (SCR) accumulated since 1942, direct spacecraft measurements of solar energetic particles (SEP) near the Earth's orbit, indirect information on the SCR variations in the past, and other relevant astrophysical, solar and geophysical data. The list of the problems under discussion includes: upper limit spectrum (ULS) for solar cosmic rays; maximum energy (rigidity), E_m(R_m), of particles accelerated at/near the Sun; production of the flare neutrinos; energetics of SCR and solar flares; production of flare neutrons and gamma rays; charge states and elemental abundances of accelerated solar ions; coronal mass ejections (CME's) and extended coronal structures in acceleration models; magnetic reconnection in acceleration scenarios; size (frequency) distributions of solar proton events (SPE) and stellar flares; occurrence probability of giant flares; archaeology of solar cosmic rays. The discussion allows us to outline a series of interesting conceptual and physical associations of SCR generation with the high-energy processes at other stars. The most reliable estimates of various parameters are given in each of research fields mentioned above; a set of promising lines of future studies is highlighted. A great importance of SCR data for resolving some general astrophysical problems is emphasized.

Some topological aspects of the magnetic reconnection phenomenon are summarized and recent numerical results, derived within a two-fluid model, of two-dimensional collisionless magnetic reconnection in presence of a strong guide field are reported. Both the Alfvèn and the whistler frequency range are investigated by including electron parallel compressibility effects that are related respectively to thermal effects and to density fluctuations. The Hamiltonian character of the system is emphasized as it drives the small scale dynamics through the presence of topological invariants. These determine the formation and the shape of small scale current and vorticity layers inside the magnetic island. Secondary fluid instabilities, mainly of the Kelvin–Helmholtz type, can destabilize these layers when a hydrodynamic type regime is achieved. The inclusion of parallel electron compressibility has stabilizing effects. In view of the limitations of the two-fluid modelling, possible developments are briefly discussed such as the inclusion of Larmor-radius corrections, in lieu of a fully kinetic approach.

In this work, we investigated the magnetic annihilation and reconnection and the resulted hot electron acceleration driven by double-beam intense laser pulses in two-layer near critical density (NCD) plasma target. The results are obtained by performing two-dimensional (2D) particle-in-cell (PIC) simulations. It is found that a quasi-mono-energetic peak can be formed in the energy spectrum of electrons accelerated by the process of magnetic field annihilation (MA) at cutoff energy. Electron spectra feature depends on the length of the second low-density layer. This suggests that the process of relativistic magnetic annihilation may be controlled in experiments by target design.

We introduce topological helicity, an invariant for oriented framed links. Topological helicity provides an elementary means of computing helicity for a magnetic flux rope by measuring its knotting, linking, and twisting. We present an equivalence relation, reconnection-equivalence, for framed links and prove that topological helicity is a complete invariant for the resulting equivalence classes. We conclude by showing that one can use magnetic reconnection to transform one collection of linked flux ropes into another collection if and only if they have the same helicity.

Most of the rotational luminosity of a pulsar is carried away by a relativistic magnetized wind in which the matter energy flux is negligible compared to the Poynting flux. However, observations indicate that most of the Poynting flux is eventually converted into ultrarelativistic particles. The mechanism responsible for this transformation remains poorly understood. Near the equatorial plane of an obliquely rotating pulsar magnetosphere, the magnetic field reverses polarity with the pulsar period, forming a wind with oppositely directed field lines, called a striped wind.

We study the conditions required for magnetic energy release at the termination shock of the striped pulsar wind. Magnetic reconnection is considered via analytical methods and 1D relativistic PIC simulations. An analytical condition on the upstream parameters for partial and full magnetic reconnection is derived from the conservation laws of energy, momentum and particle number density across the relativistic shock. Furthermore, by using a 1D relativistic PIC code, we study in detail the reconnection process at the termination shock for different upstream Lorentz factors and magnetizations, and find good agreement with our analytical criterion. Thus, alternating magnetic fields annihilate easily at relativistic highly magnetized shocks. We apply our criterion for dissipation to deduce bounds on the pair multiplicity, κ, in the pulsar wind.

One of the fundamental properties of astrophysical magnetic fields is their ability to change topology through reconnection and in doing so, to release magnetic energy, sometimes violently. In this work, we review recent results on the role of magnetic reconnection and associated heating and particle acceleration in jet/accretion disk systems, namely young stellar objects (YSOs), microquasars, and active galactic nuclei (AGNs).

Black hole surroundings and relativistic jets host magnetically dominated regions and fast magnetic reconnection are likely to play an important role concerning astrophysical phenomena associated with such regions. In this contribution, we highlight the works related to turbulence-driven reconnection processes. These processes have been studied by us using analytical as well as numerical methods which showed that fast reconnection processes are powerful ways to giving rise to relativistic particles and associated non-thermal emissions around stellar-mass and supermassive black holes. The power released from the reconnection can even compete with those of extraction from black hole spin.

MHD (Magneto-Hydro-Dynamic) linear theory of magnetic reconnection process is studied for fourth-order differential magnetic diffusion effects which may be called hyper-resistivity. In general, the second-order magnetic diffusion, i.e., resistivity, is employed to drive the reconnection process. In this paper, the fourth-order diffusion is examined and compared with the second-order diffusion. In fact, the importance of such a higher-order diffusion is predicted from plasma kinetic particle simulations of reconnection process. Rather than the second-order diffusion, higher-order diffusion may be important to achieve the fast magnetic reconnection. In this paper, firstly, the equilibrium is numerically derived for the magnetic annihilation process in a 1D current sheet by an initial value problem (shooting) technique. It is shown that the equilibrium established by mixing those two types of the magnetic diffusion is simply dominated by the ratio of two Lundquist numbers defined for each magnetic diffusion. Second, on the basis of the equilibrium, the linear growth rate of the tearing instability is studied as an initial value problem technique. As the remarkable point, it is shown that, the linear growth rate is determined by two dimensionless parameters, i.e., the aspect ratio εof the magnetic diffusion region and the ratio of two Lundquist numbers, in addition to the wave length k and upstream boundary condition c.

The noncanonical Hamiltonian formulation of a recently derived four-field model describing collisionless reconnection is presented. The corresponding Lie-Poisson bracket is shown to be a sum of a direct and semi-direct product forms and to possess four infinite independent families of Casimir invariants. Three out of four of these families are directly associated with the existence of Lagrangian invariants of the model. Two of the invariants generalize previously discovered invariants of a two-field model for reconnection in low-β plasmas. Finally a variational principle is given for deriving general equilibrium equations and an example of an equilibrium solution is described explicitely.

In this contribution we present some aspects of the nonlinear dynamics of a model which describes the phenomenon of magnetic reconnection (MR) occurring in plasmas where particle collisions can be neglected. The concept of MR is introduced in the framework of the single-fluid description of a plasma. The model under consideration is then reviewed with focus on its non-canonical Hamiltonian structure. Numerical solutions of the model equations show that in the nonlinear phase a secondary instability of Kelvin-Helmholtz type occurs at moderate values of the parameter β, which indicates the ratio between thermal and magnetic pressure, and also in the presence of finite electron compressibility. This represents a novel feature, in the nonlinear dynamics of reconnection, with respect to previously investigated models valid only for very low values of β.

The idea of ‘magnetic reconnection’ has been around since the work of Ronald Giovanelli who proposed it while searching for an explanation of solar flares. The idea seems to have received a boost from the work of James Dungey but he made it very clear in his writing that magnetic lines of force were not physical entities and shouldn’t be treated as such. Bearing this point in mind, attention is concerned here with noting the non-physicality of the whole notion of ‘magnetic reconnection’ and suggesting an alternative approach to seeking explanations for such phenomena as solar flares via the study of plasmas.