Form Of Reconnection Research Articles

Properties of the Turbulence Associated with Electron-only Magnetic Reconnection in Earth’s Magnetosheath

Abstract Turbulent plasmas generate intense current structures, which have long been suggested as magnetic reconnection sites. Recent Magnetospheric Multiscale observations in Earth’s magnetosheath revealed a novel form of reconnection where the dynamics only couple to electrons, without ion involvement. It was suggested that such dynamics were driven by magnetosheath turbulence. In this study, the fluctuations are examined to determine the properties of the turbulence and if a signature of reconnection is present in the turbulence statistics. The study reveals statistical properties consistent with plasma turbulence with a correlation length of ∼10 ion inertial lengths. When reconnection is more prevalent, a steepening of the magnetic spectrum occurs at the length scale of the reconnecting current sheets. The statistics of intense currents suggest the prevalence of electron-scale current sheets favorable for electron reconnection. The results support the hypothesis that electron reconnection is driven by turbulence and highlight diagnostics that may provide insight into reconnection in other turbulent plasmas.

Self‐organized Critical Model of Energy Release in an Idealized Coronal Loop

We present and discuss a new avalanche model for solar flares, based on an idealized representation of a coronal loop as a bundle of magnetic flux strands wrapping around one another. The model is based on a two-dimensional cellular automaton with anisotropic connectivity, where linear ensembles of interconnected nodes define the individual strands collectively making up the coronal loop. The system is driven by random deformation of the strands, and a form of reconnection is assumed to take place when the angle subtended by two strands crossing at the same lattice site exceed some preset threshold. Driven in this manner, the cellular automaton produces avalanches of reconnection events characterized by scale-free size distributions that compare favorably with the corresponding size distribution of solar flares, as inferred observationally. Although lattice-based and highly idealized, the model satisfies the constraints Δ B = 0 by design and is defined in such a way as to be readily mapped back onto coronal loops with set physical dimensions. Carrying this exercise for a generic coronal loop of length 1010 cm and diameter 108 cm yields flare energies ranging from 1023 to 1029 erg, for an instability threshold angle of 11° between contiguous magnetic flux strands. These figures square well with both observational determinations and theoretical estimates.

Interaction of an obliquely rising vortex ring with a free surface in a viscous fluid

A vortex ring, which approaches a free surface under various initial conditions and different values of flow parameters, is investigated by means of numerical solutions of the Navier-Stokes equations. The study focuses on the connection process of the vortex ring with the free surface at low Reynolds number. Discrepancies between a numerical solution and experimental observations found recently with regard to the shape of the reconnected vorticity tubes have been resolved. The crucial parameter, which determines the essential differences at the low Reynolds number selected, is the angle of inclination at which the vortex ring rises toward the free surface. At an angle of inclination of 20° circular vorticity tubes at the free surface evolve, while at an angle of 45° the reconnection takes place in the form of an almost circular sheet. At higher Reynolds number this form of reconnection may change to the other form through instability. The possibility of vortex-ring reflection at the free surface is discussed.

Loss of equilibrium and reconnection in tearing of two-dimensional equilibria

Two-dimensional tearinglike behavior is studied in reduced resistive magnetohydrodynamics (MHD) with flux conserving boundary conditions on a rectangular box. The tearinglike perturbations do not destroy the symmetries of the initial state, either discrete or continuous. In such cases, in which the perturbation does not break a symmetry of the equilibrium, linear instability is typically not directly observed. However, there can be a loss of equilibrium associated with the existence of a tearing unstable state. These ideas are illustrated with three examples: a very elongated tokamak, a tokamak with pinching coils to elongate its flux surfaces, and a model for the magnetotail or for solar arcades. The loss of equilibrium is demonstrated by means of a nonlinear energy functional. The importance of the fact that the dynamics shows a loss of equilibrium is that a large amount of free energy can be released, in the form of reconnection, and that there is a possibility of hysteresis.

The magnetopause for large magnetic shear: Analysis of convection electric fields from AMPTE/IRM

A number of magnetopause crossings by the AMPTE/IRM spacecraft, which exhibited large shear in the magnetic field, were studied recently by Paschmann et al. (1986) in order to assess the presence or absence of reconnection processes at the dayside magnetopause. In the present article, eleven of these crossings are reanalyzed by use of methods described by Sonnerup et al. (1987). Although only eight of the cases lend themselves to this more detailed analysis procedure, the results derived when the analysis is successful are considerably more detailed and quantitative than those obtained previously. Four such cases are documented in detail in the paper. For these events it was possible to derive accurate vectors, n, normal to the magnetopause. It was also possible to place bounds on the values of average magnetic field and flow velocity components along n as well as on the average electric field tangential to the magnetopause and on the velocity of the magnetopause itself along n and therefore on the magnetopause thickness. It is particularly noteworthy that in two crossings (on September 4, 1984) it was possible to place lower bounds of 2.8 mV/m and 0.5 mV/m on the tangential (reconnection) electric field and corresponding lower bounds on normal flow and field. In a third crossing (September 8, 1984), located about 1 RE north of the subsolar point, evidence is deduced, indicating that the observed plasma acceleration along the magnetopause had a large temporal component, caused by a tangential gradient in the total pressure, in addition to the usual convective component caused by the Maxwell shear stresses. Finally, in the fourth crossing (October 19, 1984), large magnetic shear was present but, in agreement with a conclusion reached by Paschmann et al. (1986), the analysis shows that no evidence for reconnection was present and that the magnetopause had the properties of a tangential discontinuity. This case provides convincing evidence that the mere existence of a deHoffmann‐Teller frame of reference in which the flow is field aligned (an excellent fit of the data to such a frame could be performed in this case) does not guarantee that the magnetopause is a rotational discontinuity. The remaining events analyzed by use of the new procedure demonstrate that the methodology sometimes fails, or yields unconvincing results, even when there is considerable evidence, in the form of accelerated plasma flows, that some form of reconnection is in progress.

The Buoyancy of U-Loops

AbstractThe problem of the surface migration of solar magnetic fields is discussed. It is shown that, unless some form of reconnection takes place below the surface, magnetic tension should effectively prevent such migration and the possibility of U-loop formation and transport is considered. As a first step, the buoyancy of a magnetic ‘U-loop’ within an isothermal atmosphere is considered. It is shown that such a configuration may form by the reconnection of the arms of an Ω-loop associated with an active region, and is likely to be buoyant initially and rise through the region. However, as it rises, the net buoyancy decreases and, provided that the temperature within the loop is even marginally less than that outside, it will reach a sub-surface configuration in which the net buoyancy is zero. The significance of this result for solar magnetic fields is then assessed.