Bulletin of the American Physical Society
2008 APS April Meeting and HEDP/HEDLA Meeting
Volume 53, Number 5
Friday–Tuesday, April 11–15, 2008; St. Louis, Missouri
Session 7HE: Shocks in the Universe and Laboratory |
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Sponsoring Units: HEDP HEDLA Chair: Hideaki Takabe, Osaka University Room: Hyatt Regency St. Louis Riverfront (formerly Adam's Mark Hotel), Promenade F |
Saturday, April 12, 2008 2:00PM - 2:25PM |
7HE.00001: Shock Waves in the Large Scale Structure of the Universe Invited Speaker: Cosmological shock waves result from the supersonic flow motions induced by hierarchical formation of nonlinear structures in the universe. Like most astrophysical shocks, they are collisionless shocks which form in the tenuous intergalactic plasma via collective electromagnetic interactions between particles and electromagnetic fields. The gravitational energy released during the structure formation is transferred by these shocks to the intergalactic gas in several different forms: in addition to the gas entropy, cosmic rays are produced via diffusive shock acceleration, magnetic fields are generated via the Biermann battery mechanism and Weibel instability, and vorticity is generated at curved shocks. Here I review the properties, roles, and consequences of the shock waves in the context of the large scale structure of the universe. [Preview Abstract] |
Saturday, April 12, 2008 2:25PM - 2:50PM |
7HE.00002: Effects of Pre-Existing Upstream Turbulence on Magnetic Fields and Particle Acceleration at Astrophysical Shocks Invited Speaker: We consider effects of pre-existing, large-scale turbulence upstream of a shock on the magnetic field and the acceleration of charged particles. Turbulent magnetic-field-line mixing plays a large role in particle transport. Also, turbulent {\it density} fluctuations upstream of the shock have a large effect on the magnetic field downstream (Giacalone and Jokipii, Ap. J., 633, L41, 2007). For high Alfv\`en-Mach-number shocks, the downstream magnetic field is amplified considerably above the value obtained from the shock jump conditions. These effects may provide a robust and natural understanding of recent observations at astrophysical shocks. The magnetic-field amplification implied by our simulations should exceed factors of $100$, consistent with observed X-rays from supernova remnants, which require magnetic fields of $100 \mu$G. These are much larger than expected from the shock jump conditions. In this case, the upstream field is not amplified, so cosmic-rays with energies approaching the ``knee'' in the spectrum require rapid acceleration, which can occur at the quasi-perpendicular part of the supernova blast wave, where the turbulent field-line mixing plays a large role. Further, recent observations by the Voyager 1 spacecraft downstream of the heliospheric termination shock show that the magnetic field has large magnitude fluctuations. We suggest that these and other effects of pre-existing turbulence play an important role in many astrophysical and heliospheric shocks. [Preview Abstract] |
Saturday, April 12, 2008 2:50PM - 3:15PM |
7HE.00003: Collisionless Shocks and Particle Acceleration Invited Speaker: In recent years, cosmic ray physics has made a transition from being a semi-detached part of astrophysics to become an essential part of observational astronomy. This increased prominence is due to the development of gamma-ray and x-ray astronomy which detect emission produced by TeV particles, the recognition that cosmic rays are probably responsible for the observed large magnetic fields accompanying shocks, the deduction of highly relativistic motion in gamma-ray bursts connected with supernovae, and the Auger project to identify the source of the very highest energy cosmic rays. Observational developments constrain the theory of cosmic ray acceleration by shocks and encourage consideration of acceleration in a wider range of environments. A non-resonant interaction between cosmic rays and the thermal plasma (Bell, MNRAS 353 550 (2004)) generates large magnetic fields and increases the maximum energy to which cosmic rays can be accelerated. This resolves important issues surrounding cosmic ray acceleration, and it also points to the possibility that forces exerted by cosmic rays may be dynamically important on a macroscopic scale with consequences for supernovae and gamma-ray bursts. Laboratory investigation may supplement observation and theory in this intriguing intersection of astrophysics and plasma physics. [Preview Abstract] |
Saturday, April 12, 2008 3:15PM - 3:40PM |
7HE.00004: Turbulent shock processing, relevant to shock-cloud interactions Invited Speaker: The evolution of interstellar clouds following the passage of a supernova shock is an important astrophysical phenomenon; the shock passage may trigger star formation and the post-shock flow surrounding the clouds will strip them of material, effectively limiting cloud life times. Experiments conducted at the Omega laser attempt to (a) quantify the mass-stripping of a single cloud, and (b) simulate the effects of nearby clouds interacting with each other. A strong shock is driven (using 5 kJ of the 30 kJ Omega laser) into a cylinder filled with low-density foam with embedded 120 $\mu $m Al spheres simulating interstellar clouds. The density ratio between Al and foam is $\sim $9. Material is continuously being stripped from a cloud at a rate which is inconsistent with laminar models for mass-stripping; the cloud is fully stripped by 80 ns-100 ns, ten times faster than the laminar model. A new model for turbulent mass-stripping is developed [1,2] that agrees with the observed rate and which should scale to astrophysical conditions. Two interacting spherical clouds are observed to turn their upstream sections to face each other, a result that is completely opposite of earlier work [3] on two interacting cylinders. The difference between these two cases is explained by the relative strength of shocks reflected from the clouds. [1] J.F. Hansen et al, ``Experiment on the Mass-Stripping of an Interstellar Cloud Following Shock Passage,'' \textit{Astrophys. J. }\textbf{662,} 379-388 (2007). [2] J.F. Hansen et al, ``Experiment on the mass-stripping of an interstellar cloud in a high Mach number post-shock flow,'' \textit{Phys. Plasmas }\textbf{14,} 056505 (2007). [3] C. Tomkins et al, ``A quantitative study of the interaction of two Richtmyer-Meshkov-unstable gas cylinders,'' \textit{Phys. Fluids.} \textbf{15,} 986 (2003). [Preview Abstract] |
Saturday, April 12, 2008 3:40PM - 4:05PM |
7HE.00005: Recent progress on particle acceleration at supernova remnants Invited Speaker: Supernova remnants in the Milky Way galaxy are most promising accelerators of cosmic rays showering on the Earth. I will review current unresolved problems and recent progress of both observational and theoretical works on the cosmic-ray acceleration at supernova remnants. I will focus on the fact that recent X-ray and very-high-energy gamma-ray observations tell us important information on this issue, especially on acceleration efficiency, evidence for proton acceleration, and so on. [Preview Abstract] |
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