Session NI2: Space and Astrophysical Plasmas: Energetic Phenomena

A New Approach in Modeling the Large Scale Structure of Magnetically Dominated Astrophysical Jets

Formation of supermassive black holes ($\sim 10^8 M_{\odot}$) at the centers of massive galaxies leads to the release of significant amount of gravitational energies, part of which comes out in magnetic fields mixed with highly energetic plasmas. Powerful astrophysical jets and giant radio lobes from extra-galactic radio galaxies are examples of such processes, as revealed by multi-wavelength observations. Magnetic fields are believed to play an important role in determining the overall structure of astrophysical jets, though many fundamental questions remain. We will give a brief overview of the different approaches in modeling these jets. We will describe a new global model of the electromagnetic structure of the jets/helixes using a semi-analytical theory, motivated both by models of the astrophysical accretion disk dynamo and by helicity injection experiments (such as spheromak) in laboratory experiments. In this model, the poloidal current forms a closed circuit, with the central poloidal current producing a collimated magnetic helix in the middle and the return current around an expanded ``lobe''. The size of the lobe is determined by the balance between the toroidal magnetic pressure produced by the central poloidal current and the surrounding plasma pressure. We will present global 3-D ideal magnetohydrodynamics simulations of the formation, propagation, and termination of large scale magnetic jets, confirming the basic theoretical framework. This global solution is also subject to the 3-D Kink instability, which increases the inductance of the magnetic structure and causes current filamentation in the lobe, giving some resemblance to the observed inhomogeneities in astrophysical lobes. This instability, however, does not completely disrupt the propagation of the magnetic helix, partly because the dynamic expansion of the helix relaxes the pinch $q(r,z)$ profile on a fast timescale. The magnetic field lines become chaotic, as shown by their Poincare plots. This has important implications on the energetic particle acceleration and transport since these particles may be the primary contributor to the extragalactic cosmic rays, including the ultra-high energy cosmic rays. Comparisons between our 3-D MHD simulations will also be made with laboratory experiments studying the jet formation.   

The Evolution of Magnetic Tower Jets in the Laboratory and in Astrophysics

Collimated, powerful jets are found in a large variety of astrophysical objects, ranging from those associated with proto-stars to galactic jets powered by black-holes. A frequent element found in many evolutionary jet models is the presence, at least in some regions, of a dominant toroidal field responsible for accelerating and confining the plasma to narrow channels which transport angular momentum and energy away from the source. In this context we present 3D MHD simulations of scaled laboratory experiments that not only reproduce the important physical processes thought to exist in the astrophysical systems but also provide new insights into the formation, evolution and stability of magnetically produced jets. The laboratory jets are produced using radial wire arrays driven by a 1 MA current pulse. The general outflow structure of a magnetic tower comprises an expanding magnetic cavity, largely collimated by the pressure of an extended plasma background medium, and a magnetically confined jet which forms within the magnetic cavity itself. A shell of swept-up shocked plasma surrounds the cavity. Although this structure is intrinsically transient and instabilities in the jet and disruption of the magnetic cavity ultimately lead to its break-up, a well collimated, radiatively cooled, ``clumpy'' jet still emerges from the system; notably such morphology is reminiscent of that observed in many astrophysical jets. We also investigate the effects on the laboratory jets of poloidal fields and rotation, which are thought to stabilize the jets observed in space. In collaboration with S. V. Lebedev, A. Frank, E. G. Blackman, D. J. Ampleford, C. A. Jennings, J. P. Chittenden, S. N. Bland, S. C. Bott, G. N. Hall, F. A. Suzuki Vidal, and A. Marocchino.   

Magnetically-Dominated Jets inside Collapsing Stars as a Model for Gamma-Ray Bursts and Supernova Explosions

It has been suggested that magnetic fields play a dynamically-important role in core-collapse explosions of massive stars. In particular, they may be important in the collapsar scenario for gamma-ray bursts (GRB), where the central engine powering the explosion is a hyper-accreting black hole or a millisecond magnetar. We here focus on the magnetar scenario, with an emphasis on the interaction of a newly-born magnetar with the infalling stellar envelope. We first introduce the ``Pulsar-in-a-Cavity'' problem as a basic-physics paradigm for a magnetar inside a collapsing star. We briefly describe the basic set-up of this fundamental plasma-physics problem, outlining its main features and deriving simple estimates for the evolution of the magnetic field and the magnetic luminosity. We find that, at first, the ram pressure of the infalling plasma acts to confine the neutron-star magnetosphere, enabling a gradual build-up of the magnetic pressure. At some point, the growing magnetic pressure overtakes the (decreasing) ram pressure of the surrounding gas, resulting in a magnetically-driven explosion. The explosion should be highly anisotropic, as the hoop-stress of the toroidal field, confined by the surrounding stellar matter, collimates the magnetically-dominated outflow into two beamed magnetic tower jets. This provides an attractive scenario for the creation of a clean narrow channel for the escape of energy from the central engine through the star, as required by GRB observations. In addition, the delayed onset of the collimated phase of the explosion has interesting consequences for the production of Nickel-56, and hence for the GRB-Supernova connection. Finally, we describe the results of recent numerical MHD simulations related to this scenario.   

Experiment on mass-stripping of interstellar cloud following shock passage

The interaction of supernova shocks and interstellar clouds is an important astrophysical phenomenon which can lead to mass-stripping (transfer of material from cloud to surrounding flow, ``mass-loading'' the flow) and possibly increase the compression in the cloud to high enough densities to trigger star formation. Our experiments attempt to simulate and quantify the mass-stripping as it occurs when a shock passes through interstellar clouds. We drive a strong shock using 5 kJ of the 30 kJ Omega laser into a cylinder filled with low-density foam with an embedded 120 $\mu $m Al sphere simulating an interstellar cloud. The density ratio between Al and foam is $\sim $9. Time-resolved x-ray radiographs show the cloud getting compressed by the shock (t$\approx $5 ns), undergoing a classical Kelvin-Helmholtz roll-up (12 ns) followed by a Widnall instability (30 ns), an inherently 3D effect that breaks the 2D symmetry of the experiment. Material is continuously being stripped from the cloud at a rate which is shown to be inconsistent with laminar models for mass-stripping (the cloud is fully stripped by 80ns-100ns, ten times faster than the laminar model). We present a new model for turbulent mass-stripping that agrees with the observed rate and which should scale to astrophysical conditions, which occur at even higher Reynolds numbers than the current experiment. The new model combines the integral momentum equations, potential flow past a sphere, flat plate skin friction coefficients, and Spalding's law of the wall for turbulent boundary layers. \newline \newline In collaboration with H. F. Robey, R. I. Klein, A. R. Miles, Lawrence Livermore National Laboratory; C. F. McKee, University of California Berkeley.   

Modeling the Magnetospheres of Luminous Stars: Interactions Between Supersonic Radiation-Driven Winds and Stellar Magnetic Fields

Hot, luminous stars (spectral types O and B) lack the hydrogen recombination convection zones that drive magnetic dynamo generation in the sun and other cool stars. Nonetheless observed rotational modulation of spectral lines formed in the strong, radiatively driven winds of hot-stars suggest magnetic perturbations analogous to those that induce ``corotating interaction regions'' in the solar wind. Indeed, recent advances in spectropolarimetric techniques have now led to direct detection of moderate to strong (100-10,000 G), tilted dipole magnetic fields in several hot stars. Using a combination of analytic and numerical MHD models, this talk will focus on the role of such magnetic fields in channelling, and sometimes confining, the radiatively driven mass outflows from such stars. In particular, I discuss how the resulting ``magnetically confined wind shocks'' can explain the moderately hard X-ray emission seen from the O7V star Theta-1 Ori C, and how the trapping of material in a ``rigidly rotating magnetosphere'' can explain the periodically modulated Balmer line emission seen from the magnetic B2pV star sigma Ori E. I also discuss how magnetic reconnection heating from episodic centrifugal breakout events might explain the occasional very hard X-ray flares seen from the latter star. I conclude with a brief discussion on the generation of hot-star fields and the broader relationship to other types of magnetospheres.   

Creating a Driven, Collapsed Radiative Shock in the Laboratory

We report details of the first experimental campaign to create a driven, planar, radiatively collapsed in laboratory experiment. Radiation hydrodynamics experiments are challenging to realize in a laboratory setting, requiring high temperatures in a system of sufficient extent. The Omega laser at $\sim $10$^{15}$ W/cm$^{2}$ drives a thin slab of low-Z material at $>$100 km/s gas via laser ablation pressure. This slab initially shocks, then continues driving a shock through a cylindrical volume of Xe gas at 6 mg/cc. Simulations predict a collapsed layer in which the density reaches $\sim $45 times initial density. Side-on x-ray backlighting was the principal diagnostic. We have successfully imaged shocks with average velocities between 95-205 km/sec, with measured thicknesses of 45-150 $\mu $m in experiments lasting up to 20 ns and spanning up 2.5 mm in extent. Comparison of the shock position as a function of time from these experiments to 1D radiation hydrodynamic simulation results show some discrepancy, which will be explored. Optical depth before and behind the shock is important for meaningful comparison to these astrophysical systems. This shock is optically thin to emitted radiation in the unshocked region and optically thick to radiation in the shocked, dense region. We compare this system to collapsed shocks in astrophysical systems with similar optical depth profiles. An experiment using a Thomson scattering diagnostic across the shock front is also discussed. This research was sponsored by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through DOE Research Grants DE-FG52-03NA00064, DE-FG53-2005-NA26014, and other grants and contracts.