Colloquia are held in Kigsbury S145 at 4:00pm on either Mondays or Thursdays.
"Bubble Wrap For Bullets (and Bubbles): Draping of the Magnetic Fields of Galaxy Clusters"
Dr. Jonathan Dursi, Canadian Institute for Theoretical Astrophysics, University of Toronto
February 4, 2008
S145 Kingsbury, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
Clusters of galaxies are the largest bound objects in the Universe, and most of their normal matter exists in the form of extremely hot, diffuse, gas. High-resolution X-ray observations have revealed cavities with sharp edges in temperature, density, and metallicity in this gas. Their presence poses a puzzle since these features are not expected to be hydrodynamically stable, or to remain sharp in the presence of diffusion.
The mergers of galaxy clusters can be the most energetic events in the Universe, and even `minor' mergers are very dynamic events, which eventually lead to the stripping and dissolution of the smaller core. However, where this occurs is important -- it helps determine the thermal and compositional history of the evolution of the cluster (and thus its component galaxies). However, with large 3D adaptive-mesh MHD simulations, and smaller more focused 2D simulations, we show how a moving core or bubble in even a very weakly magnetized plasma necessarily sweeps up enough magnetic field to build up a dynamically important sheath around the object; the layer's strength is set by a competition between `plowing up' of field and field lines slipping around the core, and to first order depends only on the ram pressure seen by the moving object.
"Simulations of Type Ia Supernovae and Buoyancy-Driven Turbulent Nuclear Combustion"
Dr. Robert Fisher, Department of Astronomy and Astrophysics, University of Chicago
February 11, 2008
S145 Kingsbury, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
In recent years, advances in computational science have led to fundamental insights into physical processes, particularly under extreme conditions of temperature and density. These advances have placed computation alongside theory and experiment as one of the foundational pillars of the physical sciences. In this colloquium, I will discuss one of the best examples of physical insight gleaned from computation -- full-star simulations of Type Ia supernovae. I will introduce Type Ia supernovae as astronomical events, highlighting their importance as standard candles and their use in the determination of cosmological parameters, including the equation of state of dark energy. I will present recent, exciting, fully three-dimensional simulations of Type Ia supernovae which have -- for the first time -- self-consistently detonated. I will then conclude with current simulations which we are now undertaking on a new generation of petascale supercomputers which will improve our understanding of buoyancy-driven turbulent nuclear burning -- one of the key fundamental physical processes which must be modeled in full-star Ia simulations.
Magnetic reconnection and turbulent transport in astrophysical and laboratory plasmas
Dr. Paolo Ricci, Ecole Polytechnique Federale de Lausanne, Switzerland
February 14, 2008
MUB Theater I, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
Magnetic reconnection and turbulence-driven plasma transport are phenomena of fundamental importance in nearly all plasma systems, including the Earth's magnetosphere, the solar corona, the interstellar medium, molecular clouds, accretion disks, and laboratory fusion experiments. Magnetic reconnection allows a plasma to rapidly convert magnetic energy into high speed flows and thermal energy. Solar flares are a stunning example of this phenomenon, in which stored magnetic energy approaching a billion megatons is released in a matter of minutes. In magnetic fusion devices, turbulence driven by small-scale instabilities leads to a transport of density and heat from the hot dense core of the machine to its periphery, making confinement of the plasma a challenging endeavor. Despite the vital importance of these phenomena to our understanding of a wide range of plasma systems, basic physics questions still remain: Can magnetic reconnection really account for the observed explosively fast release of magnetic energy? Can turbulent transport in a magnetically confined plasma be reduced to levels compatible with fusion energy production? This talk will address my ongoing investigations of these topics and the hope for future progress.
Singularities in fluids and plasmas: Drivers of fast reconnection and turbulence
Prof. Kai Germaschewski, College of Staten Island, City University of New York
February 18, 2008
S145 Kingsbury, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
Computer simulations have become a new tool in physics and other sciences that supplement the theoretical and experimental approaches. I will present two examples where numerical simulations have enabled major progress. In both of these examples, computer simulations using Adaptive Mesh Refinement have enabled the study of singularities that may lie beyond the capability of experimental diagnostics, and yet are crucial in developing a fundamental understanding of the underlying physics. The question whether finite-time-singularities develop from smooth initial conditions in the Navier-Stokes and Euler equations is one of the important open problems in fluid dynamics and also of importance for understanding turbulence in fluids. I will show high-resolution numerical work investigating the occurrence of a finite time singularity in a high-symmetry initial condition introduced by Kida. Plasmas are also amenable to a fluid description, using the equations of magnetohydrodynamics (MHD), or two-fluid equations that recognize the distinct identity of electrons and ions. Magnetic reconnection is a process which allows magnetic field lines in plasmas to change topology, releasing magnetic energy. In experiments on earth as well as in plasmas in space, the observed reconnection events are typically bursty and impulsive, i.e. intrinsically nonlinear. Computer simulations are an important tool to gain a better understanding of these phenomena, and show that is essential to go beyond the MHD model to include two-fluid effects in order to reproduce the behavior observed in reality.
Kinetic Turbulence in Space and Astrophysical Plasmas
Dr. Gregory Howes, Department of Astronomy, University of California, Berkeley
February 21, 2008
MUB Theater I, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
Plasma is a ubiquitous form of matter in the universe and is nearly always found to be magnetized and turbulent. One must understand this behavior to interpret a large body of astronomical observations. Examples include turbulence in the interstellar medium, which is stirred by violent events like supernova explosions; turbulence in accretion flows around stars and compact objects; and turbulence in the solar wind streaming outward from our Sun. Although the dynamics at large scales is well-described by fluid theory, for scales smaller than the collisional mean free path, the dynamics must be described instead by kinetic theory. Because the energy of the turbulent motions is converted into heat at these small scales, important macroscopic properties are determined by microscopic physics. To advance our understanding of kinetic turbulence requires a concerted effort of analytical modeling, nonlinear kinetic simulation, and analysis of observational data. I will present the first-of-a-kind nonlinear simulations of near-Earth solar wind turbulence; the numerical approach is based on gyrokinetic theory, a reduced kinetic theory of magnetized, low-frequency plasma turbulence applicable to many space and astrophysical plasmas. The energy spectra computed from the simulations show good qualitative agreement with satellite observations in the solar wind, and the entire shape of the numerical spectra are well reproduced by a simple analytical model of kinetic turbulence. These results support the view that the observed break in the magnetic-energy spectrum in the solar wind corresponds to a transition to kinetic-Alfven-wave turbulence, not to the onset of ion cyclotron damping. This study demonstrates that such kinetic simulations of plasma turbulence may be undertaken with some confidence, using existing computational resources, and may guide the journey into the rich yet largely unexplored terrain of kinetic turbulence in space and astrophysical plasmas.
When Magnetized Winds Collide: Probing the Interaction of the Solar System with the Interstellar Medium
Prof. Merav Opher, Department of Physics and Astronomy, George Mason University
February 25, 2008
S145 Kingsbury, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
Magnetic effects are ubiquitous and known to be crucial in space and astrophysical media; these media are excellent plasma laboratories and provide observational data that add valuable constraints to theoretical models. Modern computational techniques such as magnetohydrodynamic (MHD) and kinetic modeling are currently used to explore several fundamental plasma effects such as turbulence, reconnection, shocks etc. It is essential in a strong guided physical approach to use computer simulations in order to obtain novel information on critical research questions. In this talk I will discuss one example of how sophisticated computational work allied with detailed observational data can be guided towards resolving important physical questions. The twin Voyager spacecraft, both approximately 100 AU from the Sun, are providing us with an unexpected view of how stars interact with their surrounding media. For the first time we are able to make in situ measurements of particles and fields at the boundaries of the solar system. Voyager 1 crossed the termination shock in December 2004, and is now in the heliosheath, and in August 2007 Voyager 2 also crossed the shock. Recently, combining radio emission and energetic particle streaming measurements from Voyager 1 and 2 with extensive state-of-the art 3D MHD modeling, we were able to constrain the direction of the local interstellar magnetic field. As a result of the interstellar magnetic field, we have shown that the solar system is asymmetric, being pushed towards the Sun in the southern hemisphere. I will also review our previous work that showed that Kelvin-Helmholtz instabilities and turbulence exist near the current sheets. These effects will be able to be sampled by the Voyager observations in the heliosheath, providing direct testing of the theories and simulations we have developed. I will discuss these results, future work plans and their implications for the local interstellar magnetic turbulence.
In Search of Pentaquarks
Dr. Hovanes Egiyan, UNH Nuclear Physics Group.
March 10, 2008
S145 Kingsbury, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Probing Hot Deconfined QCD Matter with Jets at the Relativistic Heavy Ion Collider
Dr. Anne Sickles, Brookhaven National Laboratory
March 24, 2008
S145 Kingsbury, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
The Relativistic Heavy Ion Collider at Brookhaven is a unique tool to study deconfined QCD matter, the strongly interacting quark gluon plasma, at extremely high energy density. In the first years
of RHIC operation it was established that a new form of matter, incompatible with ordinary hadronic degrees of freedom, was formed in Au+Au
collisions. With recent high luminosity data sets, the RHIC experiments have exploited high momentum transfer quark and gluon scattering--jet production, which can only happen during early stage of the collision before formation of a quark gluon plasma, as calibrated probes of the produced matter. One, two and three particle observables have been used to extract propertiesof the matter including: information on plasma constitutents, it's responseto the propagation of fast quarks and gluons, and it's interaction with heavy charm and bottom quarks. Quantitative extraction of these effects is allowed by comparison of heavy ion results with p+p measurements of the same
observables in the same detectors. I will present recent results from both heavy ion and p+p collisions and discuss their impact on our
understanding of the hot deconfined matter and directions for future measurements.
"The Spin Structure of the Nucleon"
Dr. Carl Slifer, UVA Institute of Nuclear and Particle Physics.
March 26, 2008
S145 Kingsbury, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
Much of our knowledge of the spin structure of the proton and neutron comes from polarized electron scattering. This experimental program has revealed that quarks, which are fundamental constituents of the nucleon, only carry a small fraction of its total spin. In recent years, we have developed a deeper understanding into how the nucleon spin arises from the intrinsic properties of the quarks and their orbital angular momentum, along with the role played by the `glue' that holds the nucleon together. Insight into this topic has been gained by testing fundamental sum rule predictions. Some of these, such as the Bjorken sum rule, provide direct tests of Quantum Chromodynamics (QCD), while others such as the GDH, or the Burkhardt-Cottingham sum rule allow us to test some of the underlying assumptions inherent in QCD.
In this talk, we'll give an overview of the experimental Spin Structure program with emphasis on recent results from Jefferson Laboratory and perspectives for future experiments
"Discovering neutrino properties: a low energy nuclear physics perspective"
Dr. Henninh Back, North Carolina State University
March 31, 2008
S145 Kingsbury, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
It has been more than 75 years since the neutrino was first postulated
and more than 50 years since it was first detected. In that time the
neutrino has been reluctant to give up its secrets. Although large
strides have been made in neutrino physics over the past decade, basic
fundamental properties such as the value of the neutrino mass are still
not known. The neutrino has also been useful as a messenger for other
physics, such as confirming that solar energy production is a nuclear
process. The low energy nuclear physics community has had a major role
in pursuing neutrino properties and for using the neutrino as a tool for
other physics. In this talk I will tell the story of the neutrino, what
we currently know about them, and where the future may take us.
"RHIC discoveries; the perfect liquid and new phases of QCD matter"
Dr. Paul Sorensen, Brookhaven National Laboratory.
April 2, 2008
S145 Kingsbury, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
Collisions at the relativistic heavy ion collider (RHIC) at
Brookhaven National Laboratory lead to energy densities large enough
to create new forms of matter with quark and gluon degrees of
freedom. Data collected with RHIC detectors indicate that the
geometry of the initial overlap region is reflected in correlations
of the produced particles. The correlations can be explained by the
expansion of a liquid like matter with nearly zero viscosity. In this
talk, I'll discuss these conclusions and future efforts to map out
other regions of the QCD phase diagram.
"The shape of the nucleon."
Dr. Nikos Sparveris, MIT
April 14, 2008
S145 Kingsbury, Coffee: 3:40 PM,
Talk: 4:00-5:00 PM
Abstract
Nucleon is the smallest stable building block in nature and thus its role in nuclear physics is fundamental. The exploration of the shape of the nucleon has been the subject of intense scientific activity in recent years. Revealing the shape of the nucleon will provide access to its structure and properties. Results from nuclear physics experiments held at the MIT Bates Linear Accelerator Center and at the MAMI Microtron indicate that the shape of the nucleon deviates from spherical symmetry and have provided access to the reaction mechanisms that lead to the deformation. The Bates and MAMI experimental programs, their results and the most recent theoretical developments will be presented. The plans for a new series of experiments that are scheduled to take place at the Thomas Jefferson National Accelerator Facility will also be discussed.
