Plasma Astrophysics

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S147

An optical image of the supernova remnant S147. The bright filaments are apparently tubes of plasma that were heated by the shock wave, and are now glowing via line emission.  One of the many scientific mysteries is how the shocked plasma "curdles'' into these filaments. (Spitler et al. 2005)

S4 

Plasma astrophysical measurements of a part of the sky near an OB association of young stars.  Positive and negative Rotation Measures are indicated by solid and open circles, respectively. The brightness of the gray scale is a measure of the intensity of hydrogen alpha light, which is an indicator of the amount of ionized gas in this region.  The large changes in rotation measure on relatively small angular scales are believed due to the action of the young stars on the surrounding interstellar medium. This illustration is taken from Whiting et al. (2009)

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.  

Representative publications

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


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