Professors Howes, Mutel, Spangler
Many of the most familiar astronomical objects consist of matter that is ionized, and has a large enough density of charges to produce collective behavior, or motion of many charges in response to electric and magnetic fields. As such, these objects are composed of matter in a plasma state. Among the very many examples of astrophysical plasmas are the atmospheres of stars, including the Sun, the interstellar medium, or gas between the stars, accretion disks around black holes, radio galaxies and quasars. A beautiful example of an astronomical plasma is the supernova remnant S147 shown at right. Our efforts to understand these objects require input from the field of plasma physics, just like the understanding of stellar structure and evolution requires a good knowledge of nuclear physics.
The University of Iowa is in an unusually advantageous position as regards plasma astrophysics. We have an internationally-recognized group in plasma physics. The faculty, research scientists, and students in the plasma physics group are interested in fundamental questions in plasma physics, as well as problems related to astronomy and astrophysics. In addition, the space physics group is primarily interested in plasma processes in the interplanetary medium, the Earth's atmosphere and magnetosphere, and the atmospheres of other planets. This group studies processes, such as plasma turbulence and particle acceleration, which occur throughout the universe. In the solar system, we can make direct measurements with spacecraft instruments that are impossible in more remote astronomical plasmas. We can then apply that knowledge to stars, the Galaxy, and quasars. Finally, the astronomers at the University of Iowa are particularly interested in astronomical objects and processes which have a plasma physical basis.
Some examples of the investigations we are undertaking are as follows.
The Nature of Plasma Turbulence. Huge amounts of energy are released in many astronomical media, such as supernova remnants. This energy release should drive random, turbulent motions in the gas, just as we see on a more modest scale on Earth in hurricanes and tornados. We have evidence from astronomical observations that this turbulence is present in the interstellar medium, stellar atmospheres, and elsewhere. It is possible, and even probable, that this turbulence plays an important role in the dynamics and thermodynamics of these objects.
We have a number of ongoing projects on this turbulence. Professor Gregory Howes and his group are studying the way in which this turbulence dissipates, in other words, how the turbulent motions are turned into heat or other forms of energy. To understand this, one has to study how electrons and ions of different speeds interact with the electric and magnetic fields in astrophysical plasma turbulence. Professor Howes and his group use powerful computers to solve the equations describing this wave-particle interaction in turbulence.
Generation of Intense Radio Waves by Unstable Plasmas. Radio Astronomy is devoted to the study of radio waves which are produced by natural radio transmitters in the universe. Some of these natural transmitters are very efficient, and produce extremely intense emission. Even after several decades of research in radio astronomy, we are not certain what physical processes are responsible for the most extreme forms of this emission, such as that from pulsars, other stars including the famous binary Algol, and planets in our solar system.
Professor Mutel and colleagues have made important discoveries about the radio waves which are produced in the auroral regions of the Earth, and similar radiation from the planet Saturn. Professor Mutel and colleagues find that mechanism responsible is a plasma instability called the cyclotron maser instability. As in the case of the plasma turbulence study, it is crucial to take account of the distribution of charged particles (in this case electrons) in velocity space. When a plasma has a hole in its velocity space distribution, it can respond by amplifying radio waves to very high levels. Professor Mutel and colleagues have convincingly demonstrated this mechanism in the case of the Earth's auroral regions, where satellites have measured the electron distribution in velocity space. They have also applied it to radio emission from Saturn, and it seems a promising candidate for radio emission from stars as well.
The Plasma Structure of the Interstellar Medium. The interstellar medium is the space between the stars. It forms the atmosphere of the Milky Way Galaxy. The interstellar medium is filled with gas possessing a huge range of densities and temperatures. It ranges from being almost entirely ionized, to being almost completely neutral. However, even in much of the mainly neutral parts of the interstellar medium, there is enough ionization to cause the gas to behave like a plasma. At Iowa we are engaged in many studies of the interstellar medium (see separate section on this theme), including interstellar processes in which plasma physics is the main driver. Professors Mutel and Spangler are involved in measuring properties of the turbulence that exists in the background interstellar medium, as well as that in dense HII regions, which are bright nebulae surrounding hot stars. In these investigations, we search for a radio equivalent of the twinkling of starlight. These interstellar scintillations and interstellar scattering give us information on the interstellar turbulence between our radio telescopes and the distant radio sources. These astronomical observations complement the theoretical work of Professor Howes.
We are also interested in the way in which young, hot, and massive stars modify and 'texture' the interstellar medium. Hot stars can ionize huge volumes around themselves; the radii of the ionized regions can be 50 - 200 light years. This ionized gas is also heated, and as it expands it both compresses and is acted on by the surrounding interstellar gas. An example of our observations is shown in the accompanying picture of a part of the sky near a cluster of massive young stars. The picture displays measurements of a quantity called the Faraday rotation measure, which can be measured from radioastronomical polarization observations, and gives information on the magnetic field in the interstellar medium.
Mutel, R. L. et al., 2010, CMI Growth Rates for Saturnian Kilometric Radiation, Geophys. Res. Letters, in press, doi:10.1029/2010GL044398.
Ingleby, L.D., Spangler, S.R., and Whiting, C.A. 2007, Probing the Large Scale Plasma Structure of the Solar Corona with Faraday Rotation Measurements'', Astrophysical Journal 668, 520, 2007
Fey, A.L. and Mutel, R.L., Observations of the Compact Double Radio Source 2050+364-Constraints on Interstellar Scattering', Astrophysical Journal 404, 197, 1993
Howes, G.G., Cowley, S.C., Dorland, W., Hammett, G.W., Quataert, E., and Schekochihin,A.A., A Model for Turbulence in Magnetized Plasmas: Implication for the Dissipation Range in the Solar Wind'', Journal of Geophysical Research 113, A05103, 2008
Mutel, R.L., Peterson, W.M., Jaeger, T.R., and Scudder, J.D., Dependence of Cyclotron Maser Instability Growth Rates on Electron Velocity Distributions and Perturbations by Solitary Waves, Journal of Geophysical Research 112, A07211, 2007
Spitler, L.G. and Spangler, S.R. 2009, Limits on Enhanced Radio Waves Scattering by Supernova Remnants, Astrophysical Journal 632, 932, 2005
Whiting, C.A., Spangler, S.R., Ingleby, L.D., and Haffner, L.M. 2009, Confirmation of a Faraday Rotation Measure Anomaly in Cygnus, Astrophysical Journal 694, 1452, 2009
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.
Theoretical and computational plasma physics.
- Turbulence in the magnetized plasmas found in laboratories, space and astrophysics
- Analysis of spacecraft data from the turbulent solar wind
- Students develop skills including high-performance computing on the nation's fastest supercomputers, analysis of simulation and observational data, and development of simple analytical models to interpret results
- Students also interact with group members including a postdoc and collaborators around the world
Radio astronomy; plasma astrophysics; space plasma physics.
- Solar corona, solar wind, interstellar medium
- Students use the Very Large Array (VLA) radio telescope
- Students also encouraged to carry out instrumentation-development projects with the 4.5 meter instructional radio telescope on roof of Van Allen Hall
- Students develop skills in numerical methods, writing code in Python and other languages