Burning Plasma Research Group, Politecnico di Torino, Italy




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Magnetic Fields in the Universe: From Laboratory and Stars to the Primordial Structures (Talks)

Talk Abstracts


I. Basic Plasma Processes and Numerical Methods


THE NONLINEAR ALPHA-OMEGA DYNAMO


Ethan Vishniac1

Johns Hopkins University, USA


The kinematic dynamo is limited by "alpha-quenching", in which the accumulation of magnetic helicity in small-scale structures turns off the dynamo process. Continued dynamo activity depends on the small-scale magnetic helicity current. I will discuss how this works for disk systems, concentrating in particular on systems where the disk turbulence is driven by the magnetorotational instability. This leads to strong large-scale magnetic fields in accretion disks systems, and possibly galactic disks.


LABORATORY STUDY OF MAGNETIC RECONNECTION: RECENT PROGRESS


M. Yamada*

Princeton University, USA


MAGNETIC RECONNECTION


A. Lazarian*

University of Wisconsin, USA


Collisionless magnetic reconnection


Franco Porcelli*

Burning Plasma Research Group, Politecnico di Torino, Italy




Recent progress in magnetic reconnection theory in weakly collisional and collisionless regimes is reviewed. Magnetic reconnection can often be considered as a two-dimensional phenomenon in which oppositely directed field lines spontaneously merge together. In many cases of interest, there is also an essentially uniform magnetic guide field, directed perpendicular to the merging field lines. Most fluid treatments of the reconnection process adopt reduced MHD models, where the compressional Alfven wave is decoupled from the reconnective dynamics. This standard reduction procedure is normally valid in the limit of a strong guide field. A new set of reduced equations governing 2D, two-fluid, collisionless magnetic reconnection is presented. These equations are valid for arbitrary values of the magnetic guide field. This represents a significant advance in magnetic reconnection theory, as it allows to bridge the limiting regimes where collisionless reconnection is mediated by either whistler waves at low guide fields (where the Hall term in the generalized Ohm law plays an important role) or by kinetic Alfven waves (where electron compressibility along the field lines becomes important). Indeed, the model exhibits a single scale length, which we denote by , where , is the plasma beta parameter (kinetic pressure/magnetic pressure) based on the magnetic guide field (i.e., is large when the guide field is weak) and is the ion skin depth. In the strong guide field limit, and , the ion sound Larmor radius. In the opposite limit of weak guide field, . On the basis of this model, basic questions on the theory of collisionless reconnection are revisited. These questions include the scaling of the reconnection rate with microscopic parameters, the non-dissipative transfer of magnetic energy through a phase mixing process, and the difference in reconnection with and without a guide field.


II. Basic Plasma Processes and Numerical Methods


THERMAL INSTABILITIES IN PLASMAS


Shu-ichiro Inutsuka(1)*, Hiroshi Koyama (2)

(1) Kyoto University, Japan

(2) Kobe University


The role of thermal instability in the generation and maintenance of tubulence in magnetized interstellar medium is investigated. The analysis of the propagation of a shock wave into atomic interstellar medium shows that the thermal instability in the post-shock gas in the interstellar medium produces high-density molecular cloudlets embedded in warm neutral medium. The molecular cloudlets have velocity dispersion which is supersonic with respect to the sound speed of the cold medium but is subsonic with respect to the warm medium. The stability of the interface between warm medium and cold medium is studied in detail, and the dissipation processes of the turbulence in interstellar medium are analyzed.


FLUID DESCRIPTION FOR DISPERSIVE MHD WAVES IN A COLLISIONLESS PLASMA


T. Passot* and P.L. Sulem
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