Bulletin of the American Physical Society
APS April Meeting 2012
Volume 57, Number 3
Saturday–Tuesday, March 31–April 3 2012; Atlanta, Georgia
Session L3: Invited Session: The Dynamics of Waves and Energetic Particles: Simulations |
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Sponsoring Units: DPP GPAP Chair: William Heidbrink, University of California, Irvine Room: Hanover CDE |
Sunday, April 1, 2012 3:30PM - 4:06PM |
L3.00001: Particle Acceleration in Relativistic Magnetized Astrophysical Shocks Invited Speaker: Lorenzo Sironi The termination shock of pulsar winds and internal shocks in gamma-ray bursts and AGN jets are likely to be seeded with a substantial magnetic field (ratio $\sigma$ of magnetic to kinetic energy density $>$ few percent), oriented mostly perpendicular to the shock normal. By means of two- and three-dimensional particle-in-cell simulations, we study how the efficiency of particle acceleration in relativistic shocks depends on the magnetization of the pre-shock flow and the geometry of the upstream field. We study both uniform and alternating pre-shock fields. For uniform fields, we find that if $\sigma>0.001$ only nearly-parallel shocks lead to particle acceleration, via the first-order Fermi process. The downstream particle spectrum consists of a relativistic Maxwellian and a high-energy power-law tail with slope around $-2.5$, which typically contains $\sim1\%$ of particles and $\sim10\%$ of flow energy. The scattering is provided by magnetic turbulence generated self-consistently by the shock-accelerated particles that propagate ahead of the shock along the magnetic field lines. On the contrary, in quasi-perpendicular shocks, where relativistic particles cannot outrun the shock along the field, the self-generated turbulence is not strong enough to permit efficient Fermi acceleration, and the downstream particle spectrum is consistent with a thermal distribution. Alternatively, if the pre-shock medium consists of magnetic stripes of alternating polarity and $\sigma\gg1$, as expected in the relativistic wind of oblique pulsars, dissipation of the stripes when compressed at the shock front can transfer energy from the field to the particles, via driven magnetic reconnection. In the limit of long stripe wavelengths, the post-shock spectrum approaches a flat power-law tail with slope around $-1.5$, populated by particles accelerated by the reconnection electric field. Our findings place important constraints on the models of non-thermal radiation from Pulsar Wind Nebulae, gamma-ray bursts and AGN jets that invoke particle acceleration in relativistic magnetized shocks. [Preview Abstract] |
Sunday, April 1, 2012 4:06PM - 4:42PM |
L3.00002: Alfv\'{e}n instabilities and energetic particle physics in toroidal plasmas Invited Speaker: Donald Spong Modeling capabilities and experimental diagnostics for energetic particle-driven Alfv\'{e}n instabilities have advanced significantly in recent years. Simulation tools now range from rapidly applied reduced-dimensionality models and hybrid fluid particle models to more comprehensive gyrokinetic approaches. Alfv\'{e}n mode theory has been applied not only to tokamaks, but also to stellarators and reversed field pinches. Current diagnostic techniques allow direct imaging of the mode structure, fast ion density and loss patterns at the plasma edge, allowing theory/experiment comparisons in greater depth than previously possible. Examples from a variety of tokamak, stellarator and reversed field pinch experiments and the associated theory will be described. These activities are preparing the way for future ignited devices, such as ITER, where energetic alpha particles will provide the dominant plasma heating mechanism. High fidelity models of alpha behavior will be required for predicting their effects on the alpha heating profile, non-diffusive transport, nonlinear feedback loops and localized wall heat loads; in addition, understanding Alfv\'{e}n spectral emissions can provide diagnostic opportunities. Projections of the current models to ITER and future physics needs will be discussed. [Preview Abstract] |
Sunday, April 1, 2012 4:42PM - 5:18PM |
L3.00003: Particle Acceleration at Astrophysical Shocks: Are Self-Excited Magnetic Fluctuations Important? Invited Speaker: Joe Giacalone The physics of particle acceleration by astrophysical shocks is discussed with particular emphasis on the importance of so-called self-excited magnetic fluctuations. The origin of cosmic rays and other high-energy charged particles in space is an important unsolved problem in astrophysics. Acceleration in the converging plasma flows across a collisionless shock is the best mechanism to explain the majority of the observations. While direct comparisons between the simplest version of the acceleration theory -- so-called diffusive shock acceleration -- and in situ observations has revealed some discrepancies, these are reasonably accounted for by extending the simple theory to more realistic spatial and temporal geometries. A particularly important issue in shock acceleration theory is the maximum energy attainable. This is governed by the spatial diffusion of particles near the shock. When the diffusion scale is short, the particles remain near the shock and are accelerated to high energies rapidly. If the diffusion scale is long, the particles spend most of their time away from the shock and the acceleration is slow. The diffusion scale is governed by magnetic turbulence in the vicinity of the shock. It has long been thought that the energetic particles themselves excite magnetic fluctuations near the shock, which reduces their diffusion scale. However, such waves are not commonly observed in heliospheric shocks. It can be shown that pre-existing magnetic fluctuations, which are known to exist in astrophysical plasmas, are usually sufficient to account for the observed high energies for a variety of phenomena ranging from solar-energetic particles, to anomalous cosmic rays, to cosmic rays accelerated by a supernova blast wave. [Preview Abstract] |
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