Professor Greg Howes
Postdoc: Jason TenBarge
Graduate students: Kevin Nielson and Kris Klein
Cutting-edge research in astrophysics today depends heavily on the use of numerical simulations to act as a bridge between the limited data gathered from ground- and space-based observatories and the predictions of simplified analytical models. Computational astrophysics has, in fact, established itself as the third pillar of scientific investigation of astrophysical systems, alongside observational study and theoretical analysis. Computational studies range from the exploration of simple models on desktop computers to large-scale numerical simulations requiring the petaflop supercomputers (capable of 1015 operations per second) at our nation's high-performance computing facilities.
Researchers at the University of Iowa in astrophysics employ a range of computational approaches in support of federally funded research programs. Examples from some of these investigations follow.
Plasma Turbulence in Astrophysical Systems
Professor Howes, postdoc Jason TenBarge, graduate students Kevin Nielson and Kris Klein
The development of a detailed understanding of turbulence in magnetized plasmas has long been a goal of the broader scientific community, not only for its prominence as a fundamental plasma physics process, but also for its importance in a wide variety of environments. In astrophysical plasmas, turbulence governs the transformation of the energy in large-scale motions to plasma heat, thus exerting a significant influence on the emitted radiation that is observed at Earth. This broadly impacts our ability to interpret both ground- and space-based observations of a wide range of astrophysical systems, from clusters of galaxies to accretion disks around black holes to the birthplace of stars in the turbulent interstellar medium of our Galaxy. Within our solar system, turbulence is likely to play a crucial role in the heating of the solar corona and acceleration of the solar wind. In the laboratory plasmas of the magnetic confinement fusion program, turbulence plays a crucial role in the transport of energy and particles across the confining magnetic field, limiting the efficiency of proposed fusion reactors. Advancing our knowledge of plasma turbulence, therefore, has the potential to impact this very wide range of research frontiers.
Professor Gregory Howes has a NASA and NSF supported program of research into the nature of plasma turbulence and the plasma heating resulting from the kinetic dissipation of this turbulence in space and astrophysical systems. Numerical simulation using the publicly available simulation code AstroGK is the primary method supporting this research program. The nonlinear gyrokinetic simulations of turbulence at small scales employs a five-dimensional kinetic description of the turbulent dynamics, demanding a high-performance computing approach to complete turbulence simulations over the dynamic range required in space and astrophysical problems.
Through the NSF TeraGrid Program, Professor Howes's project, "Kinetic Dissipation of Astrophysical Plasma Turbulence," has been awarded 6,000,000 cpu-hours on the Kraken, the Cray XT5 at the National Institute for Computational Sciences at the University of Tennessee. Kraken, pictured at right, has 99,072 computing cores and boasts a peak performance of 1.03 petaflops, positioning it as one of the top five supercomputers in the world. Professor Howes' collaboration has recently completed the first-of-a-kind fully electromagnetic, kinetic simulations of magnetized turbulence in a homogeneous, weakly collisional plasma at the scale of the ion Larmor radius. This numerical result reproduces the qualitative features found in recent solar wind turbulence observations using the Cluster spacecraft and supports the hypothesis that the frequencies of turbulent fluctuations in the solar wind remain well below the ion cyclotron frequency both above and below the ion gyroscale. This research, recently published in Physical Review Letters, represents the first step in a long-term program to understand the fundamental physics of turbulence and plasma heating in plasmas over a wide range environments, from the solar wind to the solar corona to black hole accretion disks.
In addition, Professor Howes and his graduate student Kevin Nielson have recently completed a study to demonstrate that AstroGK can reproduce the linear physics of Alfven waves measured in experiments using the Large Plasma Device (LAPD) at UCLA. The figures shown at the right present the results of AstroGK simulations compared to LAPD experiments of inertial Alfven waves, relevant to waves occurring in the Earth's magnetosphere. These tests justify the use of AstroGK in the modeling of LAPD Alfven wave experiments and suggest that AstroGK will be a valuable tool in modeling the nonlinear evolution of proposed Alfvenic turbulence experiments on the LAPD.
Tatsuno, T., Dorland, W., Schekochihin, A. A., Plunk, G., Barnes, M. A., Cowley, S. C., and Howes, G. G., "Nonlinear phase mixing and phase-space cascade of entropy in gyrokinetic plasma turbulence," Physical Review Letters 16, 015003, 2009.
Howes, G. G., Cowley, S. C., Dorland, W., Hammett, G. W., Quataert, E., Schekochihin, A. A. and Tatsuno, T. "Kinetic Simulations of Magnetized Turbulence in Astrophysical Plasmas," Physical Review Letters 100, 065004, 2008.
Howes, G. G., "Inertial Range Turbulence in Kinetic Plasmas," Physics of Plasmas 15, 055904, 2008.
Howes, G. G., Cowley, S. C., Dorland, W., Hammett, G. W., Quataert, E., and Schekochihin, A. A., "Astrophysical Gyrokinetics: Basic Equations and Linear Theory," Astrophysical Journal 651, 590, 2006.