Session UI2: Space and Astrophysical Plasmas II

Formation and Interaction of Flux Ropes in 3D Collisionless Magnetic Reconnection

Magnetic reconnection is important in a diverse range of applications including solar flares, geomagnetic substorms and a variety of astrophysical problems. For collisionless regimes, most kinetic studies have considered ion-scale current sheets using 2D models. These layers are unstable to the tearing instability which gives rise to the onset of reconnection and the formation of magnetic islands. As the dynamics develops, new electron-scale current layers are formed extending outwards from the diffusion regions, and these are often unstable to the formation of secondary islands. In this work, we demonstrate there are some profound differences in extending these previous 2D results to real 3D systems. With a finite guide field, tearing modes are unstable at resonant surfaces across the initial layer, corresponding to oblique angles relative to the standard 2D geometry. The 2D models artificially suppress these oblique modes and greatly restrict the manner in which magnetic islands can interact. In real 3D systems, both primary and secondary islands correspond to extended flux ropes, which can interact in a variety of complex ways not possible in 2D. Here, we address these challenges using Vlasov theory and 3D kinetic simulations. The linear stability of collisionless tearing is calculated using an exact integro-differential treatment, and these results are compared with an asymptotic approach to gain insight into the range of unstable oblique modes and the main parametric dependencies. The results are used to guide and interpret 3D kinetic simulations performed on two petascale computers ({\em Roadrunner} and {\em Kraken}). These unprecedented simulations, using up to ~1.3 trillion particles, have revealed an inherently 3D evolution featuring the formation and interaction of flux ropes within the initial current layer, followed by the subsequent generation of secondary flux ropes within the elongated current sheets extending outward from the diffusion region as well as along the separatrices. These results may have far-reaching implications for a range of basic issues, including the structure of the exhaust, the dissipation rate of magnetic energy, the generation of stochastic magnetic fields and the transport of particles.   

Fast and slow two-fluid magnetic reconnection

A two-fluid magnetohydrodynamics (MHD) model of quasi-stationary, two-dimensional magnetic reconnection in an incompressible plasma composed of electrons and ions is presented. Two distinct regimes of slow and fast reconnection are found. The presence of these two regimes can provide a possible explanation for an initial slow build up and the subsequent rapid release of magnetic energy frequently observed in cosmic and laboratory plasmas.   

Magnetic Braking of Massive Stars: Observation and Theory

Massive stars are not expected to harbor magnetic fields, owing to the absence of a sub-surface convection zone with which to drive a field-generating dynamo. Nevertheless, it is known that a small ($\sim 5\%$) subset of these stars possess kilogauss-strength, ordered, stable fields. These fields greatly enhance the loss of angular momentum in the stars' radiation-driven winds, to such an extent that direct measurement of changes in the rotation periods {\it of individual objects} becomes a possibility. The past few years have witnessed the first realizations of this possibility, with the discovery of braking in at least two magnetic massive stars. In this presentation I will present these discoveries, and explain the underlying observational techniques that enable us to measure tiny changes in rotation periods. I will also review the complementary recent progress made in understanding the theoretical principles behind magnetic braking of massive stars.   

Cyclotron Maser Emission - Stars, Planets and Laboratory

X-ray and radio observations of active stars over many years have shown that they frequently generate X-ray bursts that are quickly followed by radio bursts. In many cases the radio bursts are highly polarised. More recently, the star CU Virginis has been found to exhibit pulsar-like behaviour. In both these situations we believe that the radio emission can be best explained by a cyclotron maser type instability initiated by electron beams funnelling down converging magnetic field configurations typical of a dipole magnetic topology. Just such a geometry also exists in the Earth's auroral zone and so our model can explain the Earth's auroral kilometric radiation (AKR). Via a similar process, all the gas giant/magnetised planets in the solar system also emit radio emission. We have established a laboratory-based facility that has verified many of the details of our original theoretical description. The experiment has demonstrated, for example, that an electron beam entering a strongly converging magnetic field geometry does indeed produce a ``horse-shoe'' (or crescent-shaped) distribution in velocity space. It is the generation of this horse-shoe distribution, also observed in the Earth's auroral zone, which is vital for our theoretical model. It leads to a population inversion in the perpendicular velocity distribution and generation of electromagnetic waves close to the cyclotron frequency. We will discuss recent developments in the theory and simulation of the instability and relate these to the laboratory, space and astrophysical observations. The research was supported by UK Engineering and Physical Sciences Research Council. The input of R.A. Cairns, R. Bingham, B.J. Kellett and the experimental and computer modelling team at Strathclyde University, Glasgow is gratefully acknowledged.   

Structure in Radiating Shocks

The basic radiative shock experiment is a shock launched into a gas of high-atomic-number material at high velocities, which fulfills the conditions for radiative losses to collapse the post-shock material to over 20 times the initial gas density. This has been accomplished using the OMEGA Laser Facility by illuminating a Be ablator for 1 ns with a total of 4 kJ, launching the requisite shock, faster than 100 km/sec, into a polyimide shock tube filled with Xe. The experiments have lateral dimensions of 600 $\mu$m and axial dimensions of 2-3 mm, and are diagnosed by x-ray backlighting. Repeatable structure beyond the one-dimensional picture of a shock as a planar discontinuity was discovered in the experimental data. One form this took was that of radial boundary effects near the tube walls, extended approximately seventy microns into the system. The cause of this effect - low density wall material which is heated by radiation transport ahead of the shock, launching a new converging shock ahead of the main shock - is apparently unique to high-energy-density experiments. Another form of structure is the appearance of small-scale perturbations in the post-shock layer, modulating the shock and material interfaces and creating regions of enhanced and diminished aerial density within the layer. The authors have applied an instability theory, a variation of the Vishniac instability of decelerating shocks, to describe the growth of these perturbations. We have also applied Bayesian statistical methods to better understand the uncertainties associated with measuring shocked layer thickness in the presence of tilt. Collaborators: R. P. Drake, H. F. Robey, C. C. Kuranz, C. M. Huntington, M. J. Grosskopf, D. C. Marion.   

Experimental Study of Equilibrium and Stability of Partial Toroidal Plasma Discharges

We present detailed laboratory studies of stability and equilibrium characteristics of partially toroidal flux ropes which we consider relevant to solar coronal activities. At the existing Magnetic Reconnection Experiment Facility (MRX [1]) a set of electrodes are inserted to generate a variety of plasma flux ropes which contain variable toroidal guide field. Three dimensional evolution of the simulated flares is monitored by an ultra fast framing camera and magnetic structures of the flux ropes are monitored by a variety of magnetic probes on Alfven time scales. The time evolution of discharges with Argon, Helium and Hydrogen with peak currents of 10-30 kA show the stability condition for line-tied plasma flux ropes. The q value, which describes the rotational transform of field lines, is the key for characterizing the global stability. The stability condition is found to be the same as Kruskal-Shafranov limit for the external kink mode with the modified line -tied boundary condition. This limit is verified for various plasma lengths. Flux ropes maintain their equilibrium for time scales much longer than the Alfven time even in the absence of a strapping field. Internal relaxation of flux ropes are observed even after the flux rope stabilizes to the external kink mode. The basic features of this internal relaxation events will also be presented.\\[4pt] [1] E. Oz et al submitted to PRL 2010. In collaboration with M. Yamada, H. Ji, R. Kulsrud, C. E. Myers, and J. Xie.