Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session DI3: Magnetic Reconnection and Astrophysical Plasmas |
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Chair: William Daughton, Los Alamos National Laboratory Room: Ballroom AC |
Monday, November 14, 2011 3:00PM - 3:30PM |
DI3.00001: Magnetic Reconnection in high-Lundquist-number plasmas Invited Speaker: Magnetic reconnection is the driver of explosive phenomena in both laboratory and astrophysical contexts. Sawtooth crashes in fusion experiments and solar flares are prominent examples of fascinating events where reconnection plays a key role. Over the past few years, the basic understanding of this fundamental process has undergone profound changes. The validity of the most basic, and widely accepted, reconnection paradigm -- the famous Sweet-Parker (SP) model, which predicts that, in MHD, reconnection is extremely slow, its rate scaling as $S^{-1/2}$, where S is the Lundquist number of the system -- has been called into question as it was analytically demonstrated that, for $S\gg1$, SP-like current sheets are violently unstable to the formation of a large number of secondary islands, or plasmoids. Subsequent numerical work has confirmed the validity of the linear theory, and shown that plasmoids quickly grow to become wider than the thickness of the original SP current sheet, thus effectively changing the underlying reconnection geometry. Ensuing numerical work has revealed that the process of plasmoid formation, coalescence and ejection from the sheet drastically modifies the steady state picture assumed by Sweet and Parker, and leads to the unexpected result that MHD reconnection is actually fast (i.e., independent of S). In this talk, we review these recent developments and present a novel theoretical model of MHD reconnection in high Lundquist number plasmas. The results of a detailed numerical study are presented, validating the main predictions of this theory, which we thus suggest as valid replacement of the SP paradigm. In particular, we discuss the formation of so-called monster plasmoids (whose widths are 10\% of the system size, and thus not only detectable but also potentially disruptive), predicted by the theory and observed in our simulations. [Preview Abstract] |
Monday, November 14, 2011 3:30PM - 4:00PM |
DI3.00002: Experimental observation of instability cascade from ideal MHD to kinetic scale culminating in magnetic reconnection Invited Speaker: Magnetic reconnection underlies the critical dynamics of magnetically confined plasmas both in nature and in the lab. Reconnection necessarily involves processes outside ideal MHD, a model in which magnetic field lines are frozen into the plasma frame and so cannot reconnect even if energetically favorable. Finite resistivity, the most obvious non-ideal MHD mechanism that might enable reconnection, gives dynamics far too slow to explain observations. Consequently, contemporary models invoke microscale dynamics beyond the scope of MHD. However, these models do not in general explain how MHD couples to the required microscale, nor why magnetic reconnection is typically observed to be very sudden, i.e., impulsive. We present detailed experimental observations that identify a macro to microscale cascade of instabilities giving rise to impulsive reconnection. High-speed imaging of a magnetized plasma jet shows that about 20 $\mu$s after initiation the jet develops an ideal MHD kink instability; the initially straight jet becomes an exponentially growing helix. Images also show that one segment of this helix develops a localized, sharply defined, fast Rayleigh-Taylor (RT) instability with short axial wavelength. The observed RT growth rate agrees with the theoretically predicted RT growth rate calculated using the effective gravity produced by the measured kink-induced lateral acceleration. The fast-growing RT destroys the jet segment on which it resides in about 1-2 $\mu$s, thereby accomplishing an impulsive localized reconnection. The jet radius has shrunk to the ion skin depth scale (i.e., non-MHD scale) when the RT instability occurs. These observations clearly demonstrate an ``instability of an instability" which achieves impulsive reconnection via a coupling of the ideal MHD scale to the ion skin depth scale. [Preview Abstract] |
Monday, November 14, 2011 4:00PM - 4:30PM |
DI3.00003: On the Mechanism for Breaks in the Cosmic Ray Spectrum Invited Speaker: Recent observations of galactic supernova remnants by the Fermi spacecraft observatory strongly support the idea that the bulk of galactic cosmic rays are accelerated in such remnants by a Fermi mechanism, also known as diffusive shock acceleration. However, the remnants most visible in gamma rays expand into weakly ionized dense gas, and so a significant revision of the basic mechanism is required. In this talk, I provide the necessary modifications and demonstrate that strong ion- neutral collisions in the remnant lead to steepening of the energy spectrum of accelerated particles by exactly one power. The spectral break is caused by Alfven wave evanescence leading to fractional particle losses. The gamma-ray spectrum generated in collisions of the accelerated protons with the ambient gas is also calculated and successfully fitted to the Fermi data. The parent proton spectrum is best represented by a classical test particle power law $E^{-2}$, steepening to $E^{-3}$ at $E_{br}$=7GeV due to deteriorated particle confinement. [Preview Abstract] |
Monday, November 14, 2011 4:30PM - 5:00PM |
DI3.00004: A Magnetically Induced Spiral Instability in the Princeton MRI Experiment Invited Speaker: The rapid angular momentum transport commonly observed in astrophysical disks and stars is a topic of intense theoretical and numerical study. It is widely believed that this transport is facilitated by MHD instabilities, especially the magnetorotational instability (MRI), which can be destabilized when an electrically conducting fluid has a radially-decreasing azimuthal velocity profile and is exposed to a magnetic field. This and other shear-flow instabilities are studied using the Princeton MRI experiment, a Taylor-Couette device with independently-rotating split endcaps, which uses the gallium eutectic GaInSn as its working fluid. Here we report the observation of a new magnetic-field-induced shear-flow instability. The fluid's velocity field is measured using an ultrasonic Doppler velocimetry system, which has indicated a dramatic destabilization of the background flow when there is sufficiently large applied magnetic field and differential rotation of the endcaps. The saturated state is a coherent, vertically independent, large-amplitude m=1 azimuthal mode, with a distinct spiral structure. Though the instability requires an applied magnetic field, it also exists as the magnetic Reynolds number approaches zero, indicating an inductionless mechanism. Characterization of the instability will be presented and compared with 3D numerical simulations. The relationship of this instability to the MRI and other shear-flow instabilities, as well as the resultant angular momentum transport and its possible astrophysical and geophysical applications, will be discussed. [Preview Abstract] |
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