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OCR for page 243
5
Space and Astrophysical
Plasmas
PRINCIPAL CONCLUSIONS
1. The increasing precision of measurements, numerical modeling,
and theory applied to space plasma problems amounts to a revolution
in technique relative to 10 years ago. 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.
2. Many practical systems, both civilian and defense, must operate
in the highly variable and potentially hostile plasma environment of the
Earth and solar system. Plasma processes in this environment also
influence and even disrupt important ground-based systems over local
and regional scales.
3. Because of the wealth of pertinent information flowing from
solar-system plasma physics, and continuing advances in large-scale
numerical modeling, magnetohydrodynamics and plasma physics are
becoming central to the interpretation of many astronomical observa-
tions.
Studies of plasma behavior in extreme astrophysical environments,
such as pulsars, enrich basic theory and may suggest future laboratory
investigations and technology development.
4. Cosmic-ray observations provide important information about
243
OCR for page 244
244 PLASMAS AND FLUIDS
space and astrophysical plasmas. The plasma physics of cosmic-ray
acceleration and transport has made especially rapid progress in the
past decade.
The improved precision of cosmic-ray-composition measurements
now makes possible quantitative tests of theories of nucleosynthesis
and galactic chemical evolution.
PRINCIPAL RECOMMENDATIONS
To the federal agencies and advisory panels concerned with space
and astrophysical plasma physics we make the following recommen-
dations:
1. Observations, measurements, and experiments, in space and on
the ground, are the key to productive research in space and astrophys-
ical plasma physics.
We recommend implementation of the comprehensive research
strategy outlined in Solar-System Space Physics in the 1980's (Space
Science Board, 1980~. These programs, and especially the International
Solar-Terrestrial Physics Program and the Solar Optical Telescope, are
the primary ones that will explicitly contribute to our knowledge of the
physical processes in large-scale plasmas.
We endorse the programs proposed in Astronomy and Astrophysics
for the 1980's (Astronomy Survey Committee, 1982) because they
could make significant contributions to many problems in plasma
astrophysics.
2. We recommend a national computational program dedicated to
basic plasma physics, space physics, and astrophysics that will main-
tain the state of the art in the technology appropriate to large-scale
theoretical models and simulations and provide access to users on the
basis of peer review.
To the academic community
1. We recommend that plasma physics become a regular part of the
university science curriculum in view of the increasing precision of its
experimental and theoretical techniques, and in view of its many
applications to space physics, astrophysics, and technology.
OCR for page 245
SPACE AND ASTROPHYSICAL PLASMAS 245
INTRODUCTION
The advances in understanding the plasmas in the laboratory, in
space, and in astrophysics have reinforced one another throughout the
twentieth century. In the 1920s, plasma oscillations were discovered in
the laboratory and radio waves were reflected from the plasma in the
Earth's ionosphere-the very edge of space. 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.
Modern plasma physics began in the l950s. Two events symbolizing
the deeper intellectual currents of those years were the first launch of
an artificial Earth satellite by the Soviet Union and the revelation,
through declassification, that both the United States and the Soviet
Union had been attempting to harness the energy source of the Sun-
thermonuclear fusion for peaceful purposes. Then as now, the obsta-
cles to achieving controlled fusion lay not in ignorance of nuclear
physics but of plasma physics. By 1960, the Van Allen radiation belts
and the solar wind had both been discovered by spacecraft. These
discoveries demonstrated that future understanding of the space envi-
ronment of the Earth and Sun would also be expressed in terms of
plasma physics.
Two powerful motivations stimulated the growth of plasma physics
after 1960. Fusion research seeks a source of energy accessible to
human use that will last for a time comparable with the present age of
the Earth. Space research seeks useful comprehension of nature's
processes on a global and, indeed, solar-system scale, in recognition of
man's dependence on his environment.
It is significant that the same discipline of physics plasma physics-
is central to both fusion and space research. Moreover, the plasma
phenomena in the solar system have proven to be examples of general
astrophysical processes. Not only does plasma physics describe both
solar-system and astrophysical phenomena, but the solar system has
become a laboratory in which astrophysical processes of great gener-
ality can be studied in situ.
OCR for page 246
246 PLASMAS AND FLUIDS
RELATIONSHIP BETWEEN LABORATORY, SPACE, AND
ASTROPHYSICAL PLASMA RESEARCH
Definition of Space and Astrophysical Plasma Physics
Space and astrophysical plasma physics comprise many subjects
with distinct historical origins. Space plasma physics includes solar and
solar-wind physics, planetary ionospheric and magnetospheric phys-
ics, cometary physics, and the study of cosmic-ray acceleration and
transport in the solar system. Solar research stands at the interface
between space physics and astrophysics. The Sun's proximity makes it
possible to make measurements, pertinent to the Sun's interior struc-
ture and to the plasma phenomena in its surface layers, that are
obtainable for no other star. 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 magneto-
spheres, in active radio galaxies, and in quasars; and the acceleration
and transport of cosmic rays. Astrophysical questions motivate the
study of relativistic plasmas. Each of these subjects depends on, and
contributes to, laboratory plasma physics. Each has traditionally been
pursued independently. Only recently has there been a tendency to
view them as one unified discipline.
Relationship Between Laboratory and Space Plasma Physics
The Study Committee on Space Plasma Physics (Space Science
Board, 1978) expressed this relationship as follows:
Space and laboratory experiments are complementary. They explore different
ranges of dimensionless physical parameters. Space plasma configurations
usually contain a much larger number of gyroradii and Coulomb mean-free
paths than is achieved in the laboratory plasma configurations. In the labora-
tory, special plasma configurations are set up intentionally, whereas space
plasmas assume spontaneous forms that are recognized only as a result of
many single-point measurements. Space plasmas are free of boundary effects;
laboratory plasmas are not, and often suffer severely from surface contamina-
tion. Because of the differences in scale, probing a laboratory plasma disturbs
it; diagnosing a space plasma usually does not. The pursuit of static equilibria
is central to high-temperature laboratory plasma physics, whereas space
physics is concerned with large-scale time-dependent flows....
OCR for page 247
SPACE AND ASTROPHYSICAL PLASMAS 247
Certain problems are best studied in space.... Certain problems could be
more conveniently addressed in the laboratory.... Theory should make the
results of either laboratory or space experiments available for the benefit of the
whole field of plasma physics.
The recent strengthening of theoretical space plasma physics, to-
gether with the increasing capability of space plasma instrumentation
and the superiority of the space environment for certain types of
measurements, means that the experimental diagnosis and theoretical
interpretation of some space-plasma processes now matches in preci-
sion the best of current laboratory practice. 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 astrophys-
ical systems cannot be observed directly, as they can in space and in
the laboratory. Now, however, the modern theoretical and computa-
tional techniques developed to understand laboratory and space mea-
surements have opened the door to modeling of the plasmas in the still
larger and more exotic environments of astrophysics, where observa-
tion suggests primarily the starting point in model development.
The interplay between small- and large-scale processes is character-
istic of space and astrophysical plasmas. Magnetohydrodynamics
(MHD) describes large-scale fluid systems and identifies, locates, and
characterizes the small-scale plasma processes that regulate their
global dynamics. In general, the MHD flow and the associated plasma
processes must be modeled simultaneously to achieve complete and
self-consistent understanding.
Many of the MHD systems studied in the solar system have
important analogs in astrophysics. Space and astrophysical systems
naturally involve similar plasma processes. We illustrate these remarks
by discussing two types of MHD systems, winds and magnetospheres,
and two plasma processes, particle acceleration and magnetic-field
reconnection, that occur in them.
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248 PLASMAS AND FLUIDS
MAGNETOHYDRODYNAMIC ATMOSPHERES AND WINDS
The outer layers of the Sun are a convective heat engine, whose
motions produce both large- and small-scale magnetic fields. These
magnetic fields do not spread uniformly over the solar surface, but
instead concentrate into intense, small-scale flux tubes. The evolution
of these surface magnetic fields occurs on time scales millions of times
shorter than predicted by classical kinetic theory. The underlying
processes compressible convection, the interaction of turbulent con-
ducting fluids with magnetic fields in a convecting atmosphere, mag-
netic buoyancy are ubiquitous and yet poorly understood. Most of
the planets have magnetic fields whose origins are due to dynamo
action in their interiors; the vast majority of stars in our Galaxy are
now believed to have magnetic fields much like the Sun's; and our
Galaxy contains interstellar magnetic fields that are also believed to
arise because of dynamo action.
The presence of magnetic fields in the Sun's outer layers has yet
another consequence: the interaction between the surface magnetic
fields and turbulent motions heats the solar corona. Although we do not
yet know the precise mechanism responsible for the heating, we do
know that most of the heated plasma is trapped by the solar magnetic
fields and is observed to emit vigorously in the UV and in x rays; some
of it, however, escapes into interplanetary space from open magnetic
structures in the solar corona. This escaping hot gas is subsonic near
the Sun but becomes supersonic as it flows outward to become the
solar wind. 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. The transport of
angular momentum is sufficiently vigorous to account entirely for the
Sun's loss of angular momentum since it reached the main sequence.
The solar wind is finally decelerated to subsonic speeds when it en-
counters the interstellar medium. The solar-wind injects both nuclear-
processed matter and magnetic fields into the interstellar medium.
Magnetized atmospheres and winds of the kind just described are
exceedingly common. Plasma streams out into space from the planets'
polar ionospheres in miniature versions of the solar wind polar winds.
The Einstein observatory has shown that stars with convecting outer
layers have x-ray-emitting coronas, indicating surface magnetic activ-
ity and winds much like the Sun's; these stars constitute the vast ma-
jority of stars in our Galaxy. UV and x-ray observations have also
shown that highly evolved stars with convecting outer layers do not
OCR for page 249
SPACE AND ASTROPHYSICA ~ PLASMAS 249
trap the heated plasma to form x-ray-emitting coronas, but rather eject
the gas in the form of extremely massive winds. Indeed, much of the
interstellar medium is filled with the blended wind material from these
evolved stars and from supernova remnants. Because the densities,
velocities, temperatures, and magnetic-field strengths in the interstellar
medium are similar to those in the solar wind, many in situ observa-
tions of the interplanetary medium are automatically relevant to
astrophysics. The interstellar plasma may also expand out of our
Galaxy as a wind.
Winds that are confined by surrounding gas pressure take the form of
collimated bipolar jets, which are observed to flow away from such
diverse systems as stars in the early phases of formation, the exotic
compact stellar system SS-433, and radio galaxies and quasars. Super-
high-energy, relativistic plasma winds appear to flow away from
pulsars and active galactic nuclei.
The solar and atmospheric winds are the only astrophysical fluids
accessible to detailed diagnostics and in situ measurement. Since the
solar wind in particular has been as completely diagnosed as any
laboratory plasma, a detailed theoretical understanding of it is being
developed.
PLANETARY AND ASTROPHYSICAL MAGNETOSPHERES
The Earth has an atmosphere above the one we breathe that is made
of plasma the magnetosphere. Beyond the magnetosphere, the
plasma behavior is controlled by the solar wind. 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. Figure 5.1, a drawing of various regions of the
magnetosphere alluded to above, does not convey how variable and
dynamic this MHD system really is.
Each planetary body in the solar system has a distinctive magneto-
sphere, and we learn much by comparing their properties. A planet's
size, rotation rate, magnetic field, satellites, and distance from the Sun
influence the type of magnetosphere that it will have. Since the moon
has no dynamo magnetic field, the solar wind interacts directly with its
surface. The solar wind interacts with Venus' ionosphere. Mars may
have a mixed magnetic and ionospheric interaction. Mercury has a
magnetic field but no ionosphere. The Earth has a strong
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250 PLASMAS AND FLUIDS
i:
C~ _ _ _ 3~
~ ,. IONO~HE~
Bee
CUSP
in..
GEOM AGNETOSPHERE
FIGURE 5.1 The Earth's magnetosphere. This imaginative drawing of the Earth's
magnetosphere is a collective creation of the space research community, based on 25
years of measurements and theoretical modeling. It shows various features of the
magnetosphere that will be discussed in this report. 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. The northern and southern lobes of the tail are
divided by a sheet of hot plasma. Impulsive plasma acceleration, probably due to
reconnection, occurs in the tail and is probably related to violent disturbances in the
pattern and strength of the auroras in the Earth's upper atmosphere and ionosphere.
These disturbances are called substorms. The properties of the geomagnetically trapped
energetic particles, the Earth's radiation belts, are determined by the balance of
substorm acceleration and particle diffusion and transport. Such drawings cannot
communicate the dynamism of this magnetohydrodynamic system.
OCR for 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. This figure illustrates the
enormous range of spatial scales to which the concepts of magnetospheric physics apply.
Mercury's magnetosphere, the smallest in the solar system, has a size of a few thousand
kilometers. It is about a factor of 10 smaller than the Earth's magnetosphere and pulsar
magnetospheres. These, in turn, are about one hundredth the size of Jupiter's and
Saturn's giant magnetospheres. All magnetospheres in the solar system are dwarfed by
those of tailed radio galaxies, a trillion times larger than Jupiter' s.
magnetosphere but it rotates slowly. Jupiter's rapidly rotating magnetic
field couples with heavy-ion plasma from the satellite to to form a
binary magnetosphere. Saturn is an aligned rotator, whose spin and
magnetic dipole axes are parallel. Recent ultraviolet observations
indicate that Uranus has auroras, a strong indicator of magnetospheric
processes; its magnetosphere may be pole-on to the the solar wind,
unlike all others known. Neptune's, if it exists, may be affected by
interstellar neutral atoms. Finally, the solar wind interacts with neutral
gases expanding from the nuclei of comets to form cometary
magnetospheres.
Figure 5.2 sketches the magnetospheres of Mercury, Earth,-Jupiter,
and Saturn; they range in size from Mercury's (103 km) to Jupiter's (106
km), the largest MHD object in the solar system other than the solar
wind itself. Figure 5.2 also shows two kinds of astrophysical
/
OCR for page 252
252 PLASMAS AND FLUIDS
magnetospheres. Pulsars are rapidly rotating highly magnetized neutron
stars that generate and expel highly relativistic plasmas from their
magnetospheres. There are striking similarities between pulsar
magnetospheres and Jupiter's magnetosphere, both in their rotationally
driven flows and in their pulsed, periodic radio emissions. Pulsar
magnetospheres are comparable with the Earth's in size. 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. However, the parameters of space and astrophys-
ical magnetospheres can differ so much that the day-to-day problems
faced by researchers in these fields are quite different, and we observe
them in very different ways. Nonetheless, the fact that both types of
magnetosphere present similar questions about plasma dynamics gives
us confidence that their physics is basically unified.
MAGNETIC-FIELD RECONNECTION
Suddenly the dark polar sky is pierced by a brilliant flash of light.
Within minutes, a dazzling array of auroral forms stretches from
horizon to horizon, million-ampere currents surge through the Earth's
atmosphere and out into space, and 100 billion (10~) watts of power are
dissipated in the Earth's atmosphere a magnetospheric substorm has
begun (Figure 5.31. On the Sun, a burst of x rays near a dark sunspot
signals the beginning of a catastrophic disruption of the solar corona
a solar flare. Relativistic flare electrons heat the chromosphere to x-ray
temperatures. A strong shock wave moves through the corona and
begins a journey into interplanetary space that will carry it beyond all
the planets of the solar system. The optical and x-ray luminosities start
to build up in a distant quasar. Within a day, the quasar's luminosity
will exceed the total power of a thousand galaxies. A sudden plasma
loss occurs in a tokamak fusion device. These diverse phenomena
seem unrelated. Nonetheless, they may share a common origin the
release of stored magnetic energy by the mixed MHD and plasma
process of reconnection.
Violent reconnection can lead to spectacular events such as those
above, but even in its more quiescent forms, reconnection can deter-
mine the behavior of MOD systems. Consider the interaction between
OCR for page 253
SPACE AND ASTROPHYSICAL PLASMAS 253
FIGURE 5.3 The aurora from the ground. An observer in the Earth's polar regions can
often look upward at the fiery auroral displays 100 km above him in the upper
atmosphere. Their colors, complex patterns, and violent motions have fascinated
observers for centuries. It has been given to this generation of space plasma physicists
to understand how the aurora is made. Turbulent plasma processes some 5000 km above
the Earth accelerate a beam of electrons downward. When they hit the upper atmo-
sphere, these electrons cause the molecules and atoms there to radiate. The auroral
acceleration processes may be activated by violent events in the Earth's magnetic tall,
caused by reconnection.
the magnetized solar wind and the Earth's magnetosphere. Reconnec-
tion 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. Thus, reconnection
changes the topology of the Earth's magnetic field. More importantly,
reconnection enables the solar wind to do work on the magnetosphere,
to set the plasma inside in motion. The basic energetics of the
magnetosphere are in large part determined by the rate of reconnec-
tion. Or consider the magnetic fields in the solar corona. It is thought
that a balance is set by the creation of magnetic fields by turbulent
convection below the solar surface and its destruction by reconnection
OCR for page 272
272 PLASMAS AND FLUIDS
Theory has to play an increasingly central role in the planned development of
solar-system space physics. Moreover, theory and quantitative modeling
should guide its entire information chain data acquisition, reduction, dissem-
ination, correlation, storage, and retrieval to a higher level of sophistication,
to provide prompt availability of coordinated data of diverse origins.
NASA's Solar-Terrestrial Theory Program, initiated in view of the
above recommendations, has been one reason why solar-system
plasma research has reached a new level of precision, whereby it now
makes strong contributions to both general plasma physics and to the
interpretation of space data.
We heartily recommend continuance of the excellent support that
space plasma theory has received in the past 5 years and, especially,
of the Solar-Terrestrial Theory Program.
Theoretical Astrophysics
The Astronomy Survey Committee (1982) recommended as a pre-
requisite for new research initiatives:
[augmentation of theory and data analysis, to facilitate the rapid analysis and
understanding of observational data; ....
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) made a similar
recommendation:
We recommend that NASA establish independent theoretical re-
search programs in planetary sciences and astrophysics, with objec-
tives similar to those of the solar-system plasma-physics theory
program.
Experience suggests that such a theory program could be highly
successful and, in particular, that it might transform plasma astrophys-
~cs.
Theory and numerical modeling must both be strengthened in order
that plasma physics play the central role in the interpretation of
astronomical observations warranted by the fact that most of the
universe is in the plasma state.
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SPACE AND ASTROPHYSICAL PLASMAS 273
THE ROLE OF NUMERICAL MODELS AND SIMULATIONS
Why Quantitative Models Are Essential
We must picture the entire magnetosphere of the Earth before we
can deduce where and how its plasma processes operate, yet we must
understand what the processes do before we can determine the
structure and dynamics of the magnetosphere. This essential difficulty
is repeated through space and astrophysical plasma physics: plasma
processes both determine, and are determined by, their parent
system's global MHD configuration.
Twenty years ago, imaginative drawings cartoons guided space
plasma research. Despite their naivete, they were important. Even
then, fairly detailed information about the local behavior of plasmas in
the magnetosphere was being acquired. The significance of this infor-
mation was evaluated with the help of drawings, which provided a
conceptual link between local measurements taken at different points
in space and time. As our picture of the magnetosphere grew more
complete, so also did our grasp of the plasma processes regulating its
behavior. Until this finally occurred, many scientific controversies
would have been settled had it been possible to photograph the
magnetosphere.
We can photograph astrophysical systems. However, our photo-
graphs detect photons that are usually generated by mechanisms
indirectly related to the MHD and plasma processes that regulate the
structure and energetics of the systems under study. We have no in situ
measurements, as we do in the solar system, to tell us even the most
basic plasma parameters. These must be inferred using our knowledge
about how the light we observe was generated. Our photographs
provide only a two-dimensional picture at one instant of time of
three-dimensional, evolving systems. Thus, we study classes of related
objects of different ages to deduce how they evolve in time, and we use
drawings to elucidate the relationships between their structure, dynam-
ics, evolution, and the radiation that we measure.
To achieve quantitative agreement between theory and observation,
it is essential to progress beyond the cartoon approximation to quan-
titative models. It is less obvious, but no less true, that the process of
model building is also a process of discovery. By constructing a series
of models we are led to appreciate the relationships between the parts
and the whole of the time-variable, three-dimensional systems that we
observe and to perceive how microscopic processes regulate their
OCR for page 274
274 PLASMAS AND FLUIDS
structure and behavior. Models also suggest new measurements that
then clarify the physics underlying the models.
In the past decade, our studies of solar-system plasmas have
achieved a measure of quantitative understanding though the system-
atic use of analytic and, more recently, numerical models. The first
generation of numerical models of astrophysical plasma systems is
being created at this time. 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 observa-
tion is achieved. It is our perception that the present level of develop-
ment of numerical technology, theory, and observations gives such a
strategy a significant chance of success for the first time.
The increasing urgency of the need for advanced numerical simula-
tions and models may be perceived from the phrasing of successive
recommendations of National Research Council and NASA panels.
The Report on Space Science 1975 by the Space Science Board
(1976) simply noted for all the space sciences that
Results from . . . theoretical modeling have been of critical importance in
planning and supporting space missions.
without commenting on the needed computational facilities.
The Advocacy Panels in their unified recommendations to the Study
on Space Plasma Physics (Space Science Board, 1978) recommended
the following:
Strengthening theoretical solar-system plasma physics and, to aid in achieving
this goal, support for computer modeling....
The International Magnetospheric Study Working Conference on
Magnetospheric Theory made the following explicit recommendation
for this field (Committee on Solar-Terrestrial Research, 1979~:
Future theoretical progress must involve the use of plasma simulation and
large-scale numerical modeling of magnetospheric dynamics in parallel with
the development of pure theory.
The Committee on Solar and Space Physics of the Space Science
Board, in Solar System Space Physics in the 1980's (NAS, 1980), made
a much more general recommendation:
. . . theory and quantitative modeling should guide [the] entire information
chain tof solar-system plasma physics] to a higher level of sophistication....
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SPACE AND ASTROPHYSICAL PLASMAS 275
Recent recommendations for advanced computations for the broad
field of astrophysics have been directed toward the computing facilities
that will be needed. In A Strategy for Space Astronomy and Astro-
physics for the 1980's, the Committee on Space Astronomy and
Astrophysics of the Space Science Board (1979), among other recom-
mendations for theory, advocated that:
NASA should make available time on its largest computers for theoretical
problems of great complexity, which are often beyond the capacity of
university-scale computers.
By 1982, the Astronomy Survey Committee felt it necessary to
recommend, as a prerequisite for new research initiatives in astron-
omy:
Computational facilities, to promote data reduction, image processing, and
theoretical calculations.
The Initial Report of the NASA/University Relations Study Group
(NASA, 1983) recommended that NASA should provide to researchers
in fields sponsored by NASA, including space plasma physics and
astrophysics:
Major facilities [such as] . . . large, fast computer facilities of the Cray class,
which would be used by several investigators and jointly by investigators at
several institutions.
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. System models are conceptu-
alized and executed at the fluid level, process simulations at the
microscopic, kinetic level. Here we present examples of global models
and local simulations that will be needed in the next 10 years.
SYSTEM MODELS
An entire space project, the International Solar-Terrestrial Physics
Program (ISTP), has been designed and recommended with the idea
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276 PLASMAS AND FL UlDS
that an MHD magnetospheric model with realistic microscopic ele-
ments will also be created and tested by the data. The multiple
spacecraft and ground facilities associated with the ISTP will measure
key parameters pertinent to the MHD model, together with the local
plasma processes that regulate the dynamics of the magnetosphere.
The ISTP project will provide the first systematic experimental test
of a comprehensive magnetohydrodynamic model of a large-scale
flowing system. Testing the model will force the development of
innovative methods of data analysis and dissemination.
The ISTP magnetospheric model will be the first MHD system model
in space physics and astrophysics to include all known, pertinent
plasma process elements.
The ISTP magnetospheric model will be a prototype of what must be
done if hydrodynamic and MHD models are to play their potentially
powerful role in the interpretation of remote astronomical observa-
tions. Solar-terrestrial models that successfully meet the test of de-
tailed measurements at both large- and small-scale processes would
substantially increase our confidence in models of more distant astro-
physical systems.
The first large-scale astrophysical plasma models are being created
for solar physics, as hydrodynamic models of the turbulent convection
zones of the Sun and similar stars have been extended to include MHD.
These models will allow us to test our understanding of fluid-magnetic
field interactions as the data from the next generation of high-resolution
solar instruments become available. (The first and most important of
these is the Solar Optical Telescope.) System models will be crucial to
the interpretation of these anticipated high-resolution data because of
the intimate connection between the processes that determine mor-
phology and those that produce the photons that we observe.
System models analogous to those just described are being devel-
oped for accretion disks near neutron stars and black holes and are
being used to interpret data from galactic x-ray sources and active
galactic nuclei. 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. For
example, simulations have illuminated how H + and O + ions are
accelerated by auroral plasma turbulence and then ejected into space.
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SPACE AND ASTROPHYSICAL PLASMAS 277
The output of such microscopic plasma calculations must be fed back
into system models. In the above example, the H+ and O+ ions add
mass to, and decelerate, the MHD flow in the magnetosphere.
Advanced numerical simulations will be very important to reconnec-
tion, where three-dimensional kinetic simulations in a time-dependent
magnetically complex configuration are required to resolve our out-
standing theoretical questions. Such problems as wave-particle inter-
actions, generation of radiation in plasmas, collisionless shock struc-
ture, strong heat conduction, and many more will continue to benefit
from advanced numerical simulations.
Another application of detailed modeling is to plasmas with super-
high-energy densities. For example, the ultra-high plasma tempera-
tures believed to prevail at the centers of quasars and active galaxies
are far more extreme than those encountered either in the laboratory or
the solar system and probably lead to some fascinating phenomena
associated with the creation of electrons and positrons from heat
energy. 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. Detailed
models of these processes can be checked by x- and gamma-ray
observations of pulsars. From the meager theoretical work to date, it is
already clear that an understanding of such plasmas, and the complex
role played by electron-positron pairs, will provide novel theoretical
constraints on the sizes, luminosities, and temperatures of some of the
most energetic astrophysical objects.
OVERALL CONCLUSIONS
Our review of the models and simulations needed in the next 10 years
has led us to the following conclusions:
1. Many problems in space and astrophysical plasma physics have
evolved to the point where numerical system modeling is a next logical
step. These problems include planetary magnetospheric structure,
solar convection and coronal structure, three-dimensional structure of
the solar wind, supernova remnants, astrophysical jets, pulsar
magnetospheres, and accretion onto neutron stars and black holes.
2. Many microscopic plasma problems that arise in the study of
space and astrophysical systems would benefit by coordinated simula-
tion efforts.
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278 PLASMAS AND Ft UlDS
Proposal for a Dedicated, Advanced Computational Program
Plasma physics was a pioneer in the successful utilization of large-
scale computations, for fluid, MHD, hybrid, and kinetic models. Most
of the progress since the late 1960s has been in fusion research and
nuclear weapons phenomenology. The computational facility dedi-
cated to magnetic fusion energy (MFE) has critically advanced under-
standing of magnetic-confinement systems and of fusion and basic
plasma processes. The establishment, in 1979, of NASA's theory
program in solar-terrestrial plasma physics made numerical models and
simulations regular tools in solar-system plasma research and has
prepared that research community for the next, more advanced stage.
The continued development of numerical technology will advance
many branches of science. In our own fields, we foresee that many
problems of a scale that requires today's national computing facilities
will soon be addressable by local university and laboratory facilities.
This will only increase the importance of numerical modeling to our
subjects. Nonetheless, we believe that the leading research on many of
the models discussed above will continue to be done on the most
advanced computing facilities existing at any given time, because these
models involve a complex interplay between large- and small-scale
processes.
Thus far, the responsibility for the maintenance and advancement of
state-of-the-art computing facilities has been a national one, because it
is beyond the capability of single institutions and because a national
scope provides an adequate pool of users. America's existing advanced
computational facilities, devoted to defense, fusion research, and
meteorology, have been used on a piecemeal basis for space and
astrophysics problems. These busy facilities do not have space physics
and astrophysics as an institutional objective, and researchers in these
fields must make individual agreements to secure access to advanced
computing. 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 large-
scale theoretical models and simulations. Such a program should
ensure ready access to advanced computing on the basis of peer
review.
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SPACE AND ASTROPHYSICAL PLASMAS 279
We estimate that present U.S. expenditures on space plasma com-
puting are about $2 million to $3 million per year. About $1 million per
year is provided by the National Science Foundation (NSF), and a
large fraction of NASA's approximately $2.2 million per year solar-
terrestrial theory budget is devoted to numerical modeling. A some-
what smaller sum is spent on astrophysical modeling. The effort
represented by these expenditures provides a reasonable basis on
which the more ambitious program that we are proposing could be
constructed. Our role has been to point out that a large number of
problems central to space and astrophysical plasma research are ready
for advanced numerical modeling. If these problems are combined with
others in hydrodynamics and general astrophysics that should be
included in the program, in a few years the scientific demand, and more
importantly, the scientific payoff, will justify the dedicated effort that
we propose.
We further recommend that a study be initiated forthwith that would
address such issues as the following:
1. The scope and evolution of a national computational program for
basic plasma physics, space physics, and astrophysics;
2. The institutional arrangements needed to provide strong scientific
guidance to such a program and to ensure ready access to advanced
computing on the basis of peer review;
3. The appropriate balance between large-scale and mid-scale com-
putations and between national and local facilities;
4. The ability of existing national facilities to meet the needs of basic
plasma physics, space physics, and astrophysics in the near future.
Because problems in magnetic fusion are similar to those in space
physics and astrophysics, the experience with the MFE-dedicated
facility may prove valuable in considering the questions above. Be-
cause 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.
For example, approximately 75 percent of graduate students in solar
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280 PLASMAS AND FF UlDS
terrestrial physics and the related field of aeronomy are enrolled in 24
United States institutions, which graduate 50 to 60 Ph.D.s each year.
Within solar-terrestrial research, most students are supported in the
subdisciplines of magnetospheric and solar physics. About 25 percent
of the students involved in solar-terrestrial research appear to be doing
theoretical plasma physics, and a larger fraction uses plasma concepts
in the interpretation of experimental data. In aeronomy, which deals
with the upper atmospheres and ionospheres of the Earth and other
planets, about 15 percent of the graduate students are pursuing
plasma-related topics. We believe that these sample figures illustrate
the recent emergence of plasma physics as an important conceptual
tool in the older fields of solar-terrestrial research and agronomy.*
The place of solar-system plasma physics in the teaching curriculum
differs from institution to institution. Courses are taught and graduate
degrees are granted in Departments of Physics, Astronomy, Physics
and Astronomy, Electrical Engineering, Space Sciences, Earth Sci-
ences, 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. In
those few institutions with major programs in both fusion and space
plasma research, the two specialties are not always well integrated into
a single course curriculum.
We view the current fragmentation of the space plasma curriculum
with concern but not with alarm. It appears to be a natural stage in the
evolution of our new discipline. However, a more unified plasma-
physics curriculum that takes into account the achievements of, and
applications to, solar-system and astrophysical plasma physics is an
important objective for the immediate future.
Astrophysical Plasma Physics
Because astrophysical plasma physics has not yet become a well-
organized subdiscipline of astrophysics, it is difficult to pinpoint the
number of graduate students working in astrophysics who are pursuing
plasma research. However, an increasing number of research topics in
astrophysics involves the use of plasma concepts wholly or in part. At
many universities with graduate programs in astrophysics, including
*The figures in this paragraph were compiled by D. S. Peacock, Program Director for
Solar-Terrestrial Research at NSF, for a joint European-U.S. Workshop on Space
Plasma Physics, held at Hilton Head, South Carolina, September 20-23, 1983.
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SPACE AND ASTROPHYSICAL PLASMAS 281
several with distinguished programs, the teaching of plasma physics is
inadequate to prepare students for research in plasma astrophysics.
Because the role of plasma physics in astrophysics is destined to
grow, this relative lack of university involvement limits both man-
power and progress. 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.
Plasma Physics in General
The following remarks are meant to apply to all of plasma physics,
and the recommendation is directed to colleges and universities
whether or not they currently teach plasma physics.
Well-developed scientific disciplines are characterized by deep
philosophical motivations, a unified body of powerful theoretical and
experimental techniques, and a wide range of applications. It is our
conviction that because of the growing integration of space and
astrophysics plasma physics with one another, and with laboratory and
fusion research, plasma physics is maturing. When a scientific disci-
pline matures, technological innovation soon follows. Plasma physics
is only beginning to have its impact.
It is only one generation since plasma physics became a highly
developed discipline. During this time, a handful of universities,
primarily those with federally funded projects in fusion or space
physics, developed graduate programs in plasma physics. Because
graduate training in plasma physics is excellent preparation for a
variety of careers in science and technology, and in universities,
government laboratories, and industry, it is now important to introduce
undergraduate students to plasma physics, so that they may make an
informed choice of graduate specialty. At present, it is primarily those
universities with graduate programs in plasma physics that teach the
subject at the undergraduate level.
In view of the increasing precision of its experimental and theoret-
ical techniques, and in view of its many applications to space physics,
astrophysics, and technology, we recommend that plasma physics now
become a regular part of the university science curriculum. A one-year
junior- or senior-level elective course in plasma physics would be an
excellent response to our recommendation.
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282 PLASMAS AND FLUIDS
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Representative terms from entire chapter:
solar wind
