Plasmas and Fluids (1986) / Chapter Skim
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5. Space and Astrophysical Plasmas
5. Space and Astrophysical Plasmas
Pages 243-282
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From page 243... ...
As a result, the study of space plasmas has become one of the primary motivations and experimental arenas for basic plasma research. The solar system is the primary laboratory in which astrophysical plasma processes of great generality can be studied in situ.
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From page 244... ...
Observations, measurements, and experiments, in space and on the ground, are the key to productive research in space and astrophysical plasma physics. We recommend implementation of the comprehensive research strategy outlined in Solar-System Space Physics in the 1980's (Space Science Board, 1980~.
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From page 245... ...
Between 1930 and 1950, the foundations of plasma physics were created as a by-product of ionospheric, solar-terrestrial, and astrophysical research, motivated by such diverse concerns as understanding how radio waves propagate in the ionosphere, how solar activity leads to magnetic storms and auroral displays at Earth, and the role of magnetic fields in the behavior of stars and galaxies. By the 1940s, it had become clear that, unlike ordinary gases, fully ionized plasmas at high temperatures are collision free an essential property that highlights the collective processes that are fundamental to plasmas.
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From page 246... ...
The subject of plasma astrophysics includes the generation of magnetic fields in planets, stars, and galaxies; the plasma phenomena occurring in stellar atmospheres, in the interstellar and intergalactic media, in neutron-star magnetospheres, in active radio galaxies, and in quasars; and the acceleration and transport of cosmic rays. Astrophysical questions motivate the study of relativistic plasmas.
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From page 247... ...
This is especially true in the field of wave-particle interactions, where non-Maxwellian particle distributions, and the plasma waves they create, have been measured with such high resolution that theoretical instability models had to be increased significantly in precision. Relationship Between Space and Astrophysical Plasma Research The study of plasmas beyond the solar system has developed more slowly than space plasma physics for a fundamental reason: the microscopic plasma processes that regulate the behavior of astrophysical systems cannot be observed directly, as they can in space and in the laboratory.
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From page 248... ...
l his wind carries outward not only plasma and energy but also the embedded magnetic fields and angular momentum. Thus, the solar wind carries energy away from the solar corona and decreases the Sun's mass, magnetic flux, and angular momentum.
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From page 249... ...
Within it, the Earth's magnetic field organizes the behavior of the plasma; it traps energetic particles to form radiation belts; and it transmits MHD stresses between the magnetosphere and atmosphere, a process that leads to auroras. The solar wind interacts with the magnetosphere to set the plasma inside in motion and to stretch the Earth's field into a long magnetic tail.
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From page 250... ...
Standing ahead of the magnetosphere in the solar wind is a bow shock. The solar wind stretches the Earth's magnetic field into a long tail downstream.
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From page 251... ...
SPACE AND ASTROPHYSICAL PLASMAS 251 \ MERCURY \ / \ \ \ \ \ ~_ ~4 km PULSAR EARTH SATURN J UPITER - - ' ~ lOe kit ,,~ .~\ ~ .. ~__ ~ \ \ \ \ N G C 1265 1 1 -, V i, FIGURE 5.2 Planetary and astrophysical magnetospheres.
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From page 252... ...
At the opposite extreme in size are the so-called "tailed radio galaxies." It has been proposed that their magnetic fields might have been stretched into a long tail by the moving galaxy's interaction with intergalactic plasma, in roughly the same way that the Earth's magnetic tail is created. Our understanding of pulsars and tailed radio galaxies has certainly benefited from our awareness of analogous magnetospheric processes in the solar system.
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From page 253... ...
Reconnection between solar wind and originally closed magnetospheric field lines opens some Earth field lines to interplanetary space. Energetic particles that ordinarily would not hit the Earth can be guided along open field lines into the Earth's polar atmosphere.
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From page 254... ...
PARTICLE ACCELERATION AND COSMIC RAYS An astonishingly large fraction of the energy in space and astrophysical plasmas is in the form of energetic particles. For example, cosmic rays comprise about one third of the total energy density of the interstellar medium.
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From page 255... ...
The Unifying Physical Problems Laboratory, space, and astrophysical plasma research is unified by a shared set of physical problems of true intellectual significance. The Committee on Space Plasma Physics (Space Science Board, 1978)
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From page 256... ...
Open magnetic regions-solar coronal holes generate fast streams in the solar wind. Solar flares occur in closed magnetic structures solar coronal loops.
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From page 257... ...
The solar-wind magnetic field proved to reverse direction across a time-variable, warped neutral sheet, and strong, corotating shocks were found in the distant solar wind, thereby altering our picture of how galactic cosmic rays are modulated by the solar wind. MAGNETOSPHERES OF NEUTRON STARS Phase-resolved spectroscopy identified an x-ray line plausibly at the electron cyclotron frequency in two x-ray sources.
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From page 258... ...
The twin lobe pattern of radio emissions from extragalactic radio sources has been familiar since the 1950s. These lobes have a scale of millions of parsecs, much larger than the parent galaxy.
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From page 259... ...
A laboratory experiment diagnosed with high precision the turbulent transport processes occurring in a strong guide field plasma regime appropriate, perhaps, to solar-flare conditions. Problem 2: Interaction of Turbulence with Magnetic Fields The Einstein observatory discovery that most stars exhibit solarlike surface activity proved that plasma processes are central to the physics of stellar chromospheres and coronas.
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From page 260... ...
A coherent program of active and passive radar experiments, chemical releases, rocket measurements, analytic theory, and numerical simulations devoted to the equatorial ionosphere led to the most complete analysis of the nonlinear development of the Rayleigh-Taylor instability in plasma physics. Problem 4: Acceleration of Energetic Particles Measurements of the i0Be/9Be ratio in galactic cosmic rays showed that their age the time since they had been accelerated is about 10 million years, thereby defining the efficiency required of any cosmicray acceleration mechanism.
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From page 261... ...
Problem 6: Collisionless Shocks The strong dependence of the Earth's bow-shock structure on the parameters of the upstream solar wind was demonstrated by synopsis of individual detailed case studies, by the ISEE and other spacecraft. A clear understanding of the discontinuous change in shock structure at the so-called first critical Mach number was achieved by a combination of spacecraft measurements, analytic theory, and numerical simulation.
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From page 262... ...
A nonlinear theory of Type III radio emissions was created and supported by observation in the solar wind. Detailed studies of pulsar radio emissions provided the most complete diagnosis to date of the microstructure of a highly relativistic plasma.
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From page 263... ...
The Galileo mission will diagnose Jovian magnetospheric plasma as completely as any space plasma to date. A Pioneer or Voyager spacecraft might leave the heliosphere by the end of the decade and thus detect interstellar matter and galactic cosmic rays directly.
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From page 264... ...
IMPACT OF RESEARCH ON SPACE AND ASTROPHYSICAL PLASMAS Research in this century has revealed a chain of interactions, almost all of which involve plasma physics, that connects activity at the surface of the Sun to the solar wind, and then on to the magnetosphere and atmosphere. The most spectacular manifestation of this solarterrestrial interaction chain is the magnetic storm.
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From page 265... ...
One of the most important by-products of today's research on solar-system plasmas will be a generation of students and practitioners familiar with the space environment whose work will enrich tomorrow's space science and technology. The Astronomy Survey Committee (1982)
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From page 266... ...
Smaller-scale Explorer-class space missions were addressed in a subsequent CSSP report, A Strategy for the Explorer Program in Solar and Space Physics (Space Science Board, 19841. There it is recommended that "The size, complexity, and management of future Ex
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From page 267... ...
We have examined each of the proposed programs and find that the majority will contribute to the observational basis of astrophysical plasma physics. We now discuss four examples from the diversity of measurements that can contribute to research on astrophysical plasmas.
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From page 268... ...
From the joint perspective of solar system and astrophysical plasma physics, in situ measurements of a stellar corona could be as significant as the discoveries of the solar wind or the Earth's magnetosphere. For this reason: We recommend that NASA ask its advisory committees to evaluate a mission to the Sun.
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From page 269... ...
THE ROLES OF LABORATORY AND ACTIVE SPACE EXPERIMENTS Laboratory Experiments Many experiments help to develop basic theory, and so contribute indirectly to space and astrophysical research. A few have been designed specifically to elucidate processes occurring in space and astrophysical plasmas.
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From page 270... ...
Laboratory experiments demonstrated how plasmas moving through a neutral background at the so-called critical ionization velocity suddenly bring the neutrals to full ionization and produce substantial momentum coupling. Past experience indicates that laboratory experiments will continue to contribute to space and astrophysical plasma physics.
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From page 271... ...
Theoreticians have led the effort to unify space and astrophysical plasma physics. Space Plasma Theory The Study Committee on Space Plasma Physics (Space Science Board, 1978)
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From page 272... ...
The Astronomy Survey Committee recommended a program like the Solar-Terrestrial Theory Program for theoretical astrophysics. The Theory Study Panel of the Space Science Board (1983)
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From page 273... ...
Twenty years ago, imaginative drawings cartoons guided space plasma research. Despite their naivete, they were important.
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From page 274... ...
Because we cannot detect the underlying astrophysical plasma processes directly, we believe that the best strategy will be to create numerical models at the system level that postulate plasma and radiation processes and iterate between the system and process levels until quantitative agreement with observation is achieved. It is our perception that the present level of development of numerical technology, theory, and observations gives such a strategy a significant chance of success for the first time.
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From page 275... ...
The above recommendations reflect the increasingly widespread perception that theory, just as experiment, depends crucially on technology. System Models and Process Simulations in the Next Decade Realistic MHD system models will include the effects of collective plasma processes as regulating subelements; these processes can be individually simulated in idealized form.
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From page 276... ...
Similarly, the first generation of models of bipolar jets is currently being constructed. PROCESS SIMUEATIONS Twenty years of experience in fusion plasma physics, and 10 years in space plasma physics, indicate that numerical simulations are one of the best ways to gain insight about nonlinear plasma processes.
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From page 277... ...
The electron-positron recombination line has been detected from the nucleus of our Galaxy, suggesting the existence of relativistic plasma processes there. The superstrong magnetic fields in neutron stars can lead to an unusual pair-production process that is thought to populate pulsar magnetospheres with positronic plasma.
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From page 278... ...
In some cases, American researchers have had to journey to Europe or Japan in order to perform large-scale computations. In view of the arguments above, and in view of the many space and astrophysical systems ready for systematic modeling: We recommend a national computational program dedicated to basic plasma physics, space physics, and astrophysics, which will provide and maintain state-of-the-art technology appropriate to largescale theoretical models and simulations.
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From page 279... ...
Because the National Center for Atmospheric Research deals with large-scale hydrodynamic calculations, its experience may be equally useful. 4 THE ROLE OF PLASMA PHYSICS IN THE UNIVERSITY CURRICULUM Space Plasma Physics Studies of the space sciences are at present concentrated in a few major institutions and a somewhat larger number of smaller schools.
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From page 280... ...
Courses are taught and graduate degrees are granted in Departments of Physics, Astronomy, Physics and Astronomy, Electrical Engineering, Space Sciences, Earth Sciences, and Atmospheric Sciences, among others. The course content and sequence differ from department to department, resecting the diverse historical origins and motivations for space plasma research.
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From page 281... ...
To the extent that space and astrophysical plasma physics, at the graduate level, are not viewed as integral parts of plasma physics and astrophysics, the intellectual vitality of space science and astrophysics is bound to surer. We recommend that graduate teaching programs in space science and astrophysics include plasma physics as part of their basic course requirements.
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From page 282... ...
Space Science Board, National Research Council, Space Plasma Physics: The Study of Solar-System Plasmas, Vol. 1, Reports of the Study Committee and the Advocacy Panels, S
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