Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 9
A Science Strategy for Space Physics: Summary
A Science Strategy for Space Physics
Summary
This report by the Committee on Solar and Space Physics and the
Committee on Solar-Terrestrial Research recommends the major directions for
scientific research in space physics for the coming decade. As a field of science,
space physics has passed through the stage of simply looking to see what is out
beyond Earth's atmosphere. It has become a "hard" science, focusing on
understanding the fundamental interactions between charged particles,
electromagnetic fields, and gases in the natural laboratory consisting of the
galaxy, the Sun, the heliosphere, and planetary magnetospheres, ionospheres,
and upper atmospheres. The motivation for space physics research goes far
beyond basic physics and intellectual curiosity, however, because long-term
variations in the brightness of the Sun vitally affect the habitability of the Earth,
while sudden rearrangements of magnetic fields above the solar surface can
have profound effects on the delicate balance of the forces that shape our
environment in space and on the human technology that is sensitive to that
balance.
REPORT MENU
The several subfields of space physics share the following objectives:
NOTICE
MEMBERSHIP
SUMMARY
To understand the fundamental laws or processes of nature as they
PART I
apply to space plasmas and rarefied gases both on the microscale and in the
PART II
larger, complex systems that constitute the domain of space physics;
CHAPTER 1
CHAPTER 2
CHAPTER 3
To understand the links between changes in the Sun and the resulting
CHAPTER 4
effects at the Earth, with the eventual goal of predicting the significant effects on
CHAPTER 5
the terrestrial environment; and
PART III
APPENDIX
To continue the exploration and description of the plasmas and
rarefied gases in the solar system.
Significant progress has been made in the more than three-decade
history of space research. Many space plasma and rarefied gas phenomena
have been characterized and are well understood, but many others are still under
investigation and new discoveries continue to be made. Space physics asks
fundamental questions about how plasmas are energized; about how the energy
file:///C|/SSB_old_web/strasum.html (1 of 12) [6/18/2004 2:18:02 PM]
OCR for page 10
A Science Strategy for Space Physics: Summary
is redistributed with the result that a few particles are taken out of a near-thermal
distribution and accelerated to superthermal or very high energies; about the
specific roles played by magnetic fields in transferring energy from plasmas to
particles and vice versa; about instabilities and interactions between waves and
particles in a plasma; about the generation of magnetic fields through convection
and rotation; and about the complex physical processes that operate in boundary
layers between regions of different types of plasmas and rarefied gases. Some
plasma configurations or particle distributions are known to be unstable and to
relax spontaneously to a more stable state with the release of free energy, but
there are many others for which the instabilities and wave-particle interactions
are not yet understood. Determining the physics of such relaxation processes is
fundamental to understanding and eventually being able to predict disturbances
such as solar eruptions and geomagnetic storms, both of which can have
important impacts on a technological society.
This strategy identifies five key scientific topics to be addressed in
space physics research in the coming decade. For each of these topics, the
report presents the scientific background and discusses why the topic is
important, describes the current program for research on the topic, and
then recommends, in priority order, research activities for the future. As is
made clear in the main text, each of these five diverse topics is linked by a
number of basic themes. Even though this strategy does not address specific
proposals for future programs or missions, consideration is given to the practical
aspects of carrying out the recommended investigations. The rationale for the
research priorities is driven not only by scientific priority, but also by
considerations such as current plans, near-term budget constraints, technological
readiness, and balance between large- and small-scale endeavors. The five
topics (Box 1), which are not prioritized, and the prioritized recommendations for
research in each topic are briefly summarized as follows.
Box 1
The Key Topics in Space Physics Research
Mechanisms of solar variability
q
The physics of the solar wind and the heliosphere
q
The structure and dynamics of magnetospheres and their coupling to
q
adjacent regions
The middle and upper atmospheres and their coupling to regions
q
above and below
Plasma processes that accelerate very energetic particles and control
q
their propagation
THE KEY TOPICS IN SPACE PHYSICS RESEARCH
Mechanisms of Solar Variability
file:///C|/SSB_old_web/strasum.html (2 of 12) [6/18/2004 2:18:02 PM]
OCR for page 11
A Science Strategy for Space Physics: Summary
The Sun is a variable star on time scales of milliseconds to centuries or
more. These variations occur not only in visible light, but also at radio, ultraviolet,
x-ray, and g-ray wavelengths and in the emission of the solar wind and energetic
particles. Although solar variability ultimately arises from the interaction of
magnetic fields and differential rotation inside the Sun, the Sun's interior
dynamics are largely unknown. The new tool of helioseismology is being used to
probe the solar interior; it has shown that the Sun's rotation rate does not
increase inward as had previously been postulated and thus rules out the
"standard" model of the dynamo that generates the solar magnetic field. At least
at the solar surface, and perhaps in the interior as well, the magnetic fields are
confined to flux tubes—rope-like structures with diameters of about 100 km that
are too small to be resolved from Earth. It has been suggested that the twisting
and shearing of these flux tubes lead to bursts of high-speed solar wind, called
coronal mass ejections, and to solar flares, but the trigger mechanisms for those
violent events are not yet known.
On longer time scales, complacency about the simplicity of the Sun has
been upset by the discovery and documentation in the historical record that the
Sun has undergone periods of low activity. The association of the Maunder
Minimum period (~1645 to 1715), when few sunspots appeared on the solar
surface for roughly 70 years, with the Little Ice Age, when Europe experienced
exceptionally cold winters, has potentially serious implications for society should
a similar episode occur under current conditions. In addition, confidence in
current understanding of the solar interior was upset by the discordantly low flux
of solar neutrinos observed in several experiments. It is not yet known whether
this disagreement derives from errors in neutrino physics or from errors in
understanding of the solar interior.
The research priorities for advancing the understanding of solar variability
are as follows:
Use helioseismology to study the structure and dynamics of the solar
surface and interior over a full solar cycle, to obtain information on the interior
changes that cause solar cycles.
Assure continuity of total and spectral irradiance measurements,
supported by spatially resolved spectrophotometry, to investigate correlations
between solar magnetic activity and solar output variations and thereby to
understand how they are coupled.
Measure high-energy radiation and particles from flares and coronal
mass ejections with good angular resolution, good spectral resolution, and wide
spectral coverage to determine what drives each of those phenomena and how
they contribute to the solar output at high energies.
Observe surface magnetic fields, velocities, and thermodynamic
file:///C|/SSB_old_web/strasum.html (3 of 12) [6/18/2004 2:18:02 PM]
OCR for page 12
A Science Strategy for Space Physics: Summary
properties with enough spatial resolution (<150 km, with an ultimate goal of <100
km) to study small-scale structures such as flux tubes that may play a decisive
role in solar activity and the generation of solar outputs.
Make global-perspective measurements of the solar surface magnetic
and velocity fields and solar oscillations to measure the three-dimensional
structure and long-term evolution of active regions and to detect weak but
coherent global oscillations.
Measure active regions with angular resolution of ~1 arc sec and
temporal resolution of ~10 min for a duration of ~10 days without nighttime gaps
to determine the magnetohydrodynamic history of their emergence, development,
and decay and the physical scenario behind it.
The Physics of the Solar Wind and the Heliosphere
Some of the energy transported from the solar interior goes into heating
the Sun's outer atmosphere, called the corona, to over a million degrees by
processes that are currently the target of intense study. The hot corona in turn
becomes the source of the solar wind, but there are still major questions about
how this occurs. Further observations and numerical simulations are required to
determine the relative importance of magnetic reconnection, explosive jets, tiny
active regions called bright points, hydromagnetic waves, and the topology of the
magnetic fields in the corona in accelerating the quasi-stationary solar wind.
There are additional questions about the acceleration of the nonstationary or
transient solar wind arising from explosive events called coronal mass ejections.
New observations of the variable elemental composition and ion charge states of
the solar-wind plasma are providing valuable clues concerning the acceleration of
both types of solar wind.
As the solar wind flows out through the solar system, it blows a big
bubble, called the heliosphere, in the interstellar medium. Because of its very
large scale (~100 AU), the heliosphere provides a unique laboratory for studying
plasma processes in relative isolation from boundary effects; from heliospheric
studies it is possible to learn much about instabilities in expanding plasmas, the
interaction of colliding plasmas, the generation and evolution of plasma waves
and magnetohydrodynamic turbulence, and the acceleration and propagation of
energetic particles in turbulent fields. Within the decadal time frame considered
by this report, it will be possible to measure the latitudinal variations of
heliospheric particles and fields over the full range of solar activity and to test
theories about the interaction of the solar wind with the interstellar gas and
plasma.
The following priorities are identified for future research on the solar wind
and the heliosphere:
Continue to obtain and synthesize the data from the present
file:///C|/SSB_old_web/strasum.html (4 of 12) [6/18/2004 2:18:02 PM]
OCR for page 13
A Science Strategy for Space Physics: Summary
constellation of heliospheric spacecraft and from the interplanetary cruise phases
of planetary missions in order to characterize the global and solar-cycle-
dependent properties of the heliosphere and its interactions with the interstellar
medium.
Carry out in situ observations of the solar corona to explore and
characterize the region of acceleration of the solar wind and the physical
processes responsible for that acceleration.
Obtain new types of data required to reveal the mechanisms
responsible for the transport of energy, including wave motions (periods of 1 to
10 s), from the solar surface into the chromosphere and corona to understand
how these are heated.
Carry out stereo imaging of the solar corona to reveal the three-
dimensional structure of coronal features without the ambiguity caused by
integration along the line of sight.
Develop and use techniques for the remote sensing of the coronal
magnetic field in order to improve knowledge of the acceleration of the solar wind
and of the initiation of coronal mass ejections.
Make in situ measurements of the outer heliospheric boundaries and
the interstellar medium with instruments specifically designed for those purposes.
The Structure and Dynamics of Magnetospheres
and Their Coupling to Adjacent Regions
Some of the most visually awe-inspiring, yet poorly understood, terrestrial
phenomena are a direct consequence of the interaction of the variable Sun and
solar wind with the Earth. Auroral displays, usually confined to high latitudes,
episodically descend into the temperate zones during periods of extreme solar
activity. The aurora is only one manifestation of the complex chain of physical
events and connections that link the energy output of the Sun with the Earth's
magnetosphere, ionosphere, and atmosphere.
As the solar wind reaches the Earth, some of it enters the magnetosphere
via several different processes and paths and affects the circulation and
dynamics of the plasma within the magnetosphere. The interplanetary magnetic
field can become temporarily connected to the geomagnetic field, but the physics
of the reconnection process is not yet well understood. Once within the
magnetosphere, the energy from the solar wind cascades through the system
and some is released catastrophically in events whose trigger mechanisms and
extent are not well known. Flows of thermal and energetic plasma, large-scale
current systems, magnetic perturbations, and imposed electric fields provide the
basic links between the magnetosphere and the ionosphere. The ionosphere
file:///C|/SSB_old_web/strasum.html (5 of 12) [6/18/2004 2:18:02 PM]
OCR for page 14
A Science Strategy for Space Physics: Summary
provides feedback to the magnetosphere in the form of ion outflow, conductivity
changes, and dynamo fields. There is a continual reconfiguration of this system
as the solar wind and its embedded magnetic field change in response to solar
and interplanetary dynamics and energetics.
The past and current program, based primarily on in situ measurements,
is providing an understanding of the magnetosphere that is strong in terms of
local phenomena and a statistical picture of the global structure, but weak in
terms of global dynamics. Researchers now know that most transport processes
take place within narrow boundary layers connecting regions with very different
plasma conditions. The frontier issues for the future center on the global
magnetospheric dynamics in response to the solar wind driver, and the physical
mechanisms that determine the coupling between regions. Many of the
outstanding questions in magnetospheric physics will be addressed by global
magnetospheric imaging, a new addition to the techniques available for
magnetospheric research. In addition, studies of the magnetospheric
environments of other planetary bodies can also yield important physical insights
into the mechanisms that drive the dynamical behavior of the Earth's
magnetosphere.
The following priorities are identified for future progress on this topic:
Reap the full scientific potential of the International Solar-Terrestrial
Physics program and its coordinated programs to advance understanding of the
transport of mass, momentum, and energy throughout the solar wind, and the
magnetosphere and ionosphere systems.
Simultaneously image the plasma and energetic particle populations
in the aurora, plasmasphere, ring current, and inner plasma sheet to study the
global structure and large-scale interactions of magnetospheric and ionospheric
regions during different levels of solar and geomagnetic activity.
Maintain the full complement of particle and field instruments on
current and future planetary missions to gain increased understanding of the
formation and dynamics of diverse magnetospheres and ionospheres.
Further develop and exploit ground-based facilities that image the
ionospheric manifestations or "footpoints" of solar wind/magnetosphere coupling
processes to complement the magnetospheric imaging initiative aimed at
studying the global properties of the magnetosphere.
Explore Mercury's magnetosphere to understand the role of an
ionosphere in coupling between the solar wind and planetary magnetospheres.
Target localized regions that require greater understanding of the
small-scale physical processes occurring there with high-resolution,
file:///C|/SSB_old_web/strasum.html (6 of 12) [6/18/2004 2:18:02 PM]
OCR for page 15
A Science Strategy for Space Physics: Summary
multispacecraft measurements that take advantage of new smaller, lighter, more
capable instruments and more sophisticated data-compression schemes.
The Middle and Upper Atmospheres
and Their Coupling to Regions Above and Below
A complex interface exists between Earth's space environment and the
lower atmosphere or troposphere where weather and climate occur. This
interface includes the highly variable middle and upper regions of the atmosphere
extending upward from a lower boundary at 10 to 15 km altitude. The middle and
upper atmospheres have considerable practical as well as intellectual interest
because most ozone resides there and because disturbances in the upper
atmosphere and ionosphere caused by solar wind and magnetospheric variations
can disrupt technological systems through their effects on satellite drag,
communications, and induced ground currents.
The middle and upper atmospheres are strongly influenced by inputs of
mass, momentum, and energy from both above and below. The absorption of
variable solar ultraviolet and x-radiation and of energetic particles not only heats
the atmosphere, but also initiates chains of photochemical reactions and ionizes
the upper atmosphere to form the ionosphere. Highly variable electric fields and
currents originating above and below the upper atmosphere are major sources of
energy and momentum to that region. Gravity, planetary, and tidal waves that
originate partly from the lower atmosphere grow in amplitude as they propagate
upward, where they contribute to the momentum and energy budgets of the
middle and upper atmospheres and produce turbulence that influences mixing
processes. There are major deficiencies in our knowledge of many of these
inputs to the middle and upper atmospheres as well as of the multiple interactions
and feedbacks that occur there.
The following priorities are identified for future research aimed at
understanding this important interface between Earth's lower atmosphere and
space:
Exploit the exciting new capabilities of UARS,1 FAST, and CEDAR to
provide the foundation for future advances in our understanding of the middle and
upper atmospheres.
Investigate the reaction of the middle and upper atmospheres to
upward propagating waves from the lower atmosphere and energy inputs from
space so that the sources of important features such as the quasi-biennial and
semiannual oscillations and the causes of mesosphere/lower-thermosphere
structure and variability can be understood.
Study the long-term variations in the middle and upper atmospheres
using a combination of consistent long-term satellite and ground-based
measurements together with three-dimensional radiative-chemical-dynamical
file:///C|/SSB_old_web/strasum.html (7 of 12) [6/18/2004 2:18:02 PM]
OCR for page 16
A Science Strategy for Space Physics: Summary
modeling to understand natural and anthropogenic changes in these regions.
Develop methods to observe the time-dependent electrodynamics
operating on microscales to global scales, both in the upper atmosphere-
ionosphere-magnetosphere coupling regions so that feedback processes can be
characterized, and in the regions above thunderstorms so that the effects of
electrified clouds on the "global circuit" and on middle atmosphere chemistry and
energetics can be characterized.
Take advantage of opportunities to include carefully chosen,
appropriate instruments on planetary orbiter missions to make measurements
critical to understanding planetary aeronomy and its relation to terrestrial
aeronomic processes.
Plasma Processes That Accelerate Very Energetic
Particles and Control Their Propagation
Many of the plasma processes responsible for phenomena within the
heliosphere probably also play a role in determining the properties of galactic
cosmic rays, which are the only available sample of matter from outside the local
solar neighborhood. Mass, charge, and energy spectrometers on existing and
planned spacecraft can make in situ measurements of energetic particles
throughout the heliosphere to study particle acceleration, fractionation, and
transport in a variety of space plasma environments. Theories of acceleration
mechanisms in larger-scale galactic structures such as supernova remnants
make specific predictions about compositional changes that should take place at
the highest energies attainable in those objects, but the theories have not yet
been tested observa-tionally.
Cosmic rays are confined to the galaxy by turbulent magnetic fields.
Measurements of radioactive "clock" nuclei can be used to distinguish diffusive
trapping in the galactic halo from the simpler phenomenological models used for
many years. Gamma ray measurements can be used to trace the radioactive
parent elements of positrons that should be accelerated by the shocks presumed
to be the source of the galactic cosmic ray nuclei. Although fluxes of antiprotons
and positrons produced in collisions of cosmic rays with gas in the interstellar
medium can be calculated with some precision, a full understanding of the
sources of antimatter in the cosmic radiation requires a new generation of
measurements.
Nuclides heavier than nickel are produced by accretion of neutrons either
in supernova explosions or during certain other phases of stellar evolution.
Knowledge of the abundance of different cosmic ray elements and isotopes will
allow the use of nucleosynthesis models to determine quantitatively the fraction
of cosmic rays synthesized in each type of source. The abundance of actinide
elements can be used as a radioactive clock to determine the time delay between
the synthesis of these elements and their acceleration to cosmic ray energies.
file:///C|/SSB_old_web/strasum.html (8 of 12) [6/18/2004 2:18:02 PM]
OCR for page 17
A Science Strategy for Space Physics: Summary
The measurements necessary to address scientific issues concerning
particle acceleration and propagation are as follows, in priority order:
Complete the observations from the current and planned network of
interplanetary spacecraft to study particle acceleration, fractionation, and
transport.
Extend direct composition measurements to 1015 eV to probe the
limits of acceleration and trapping mechanisms.
Measure abundances of radioactive isotopes above 1 GeV/nucleon to
search for evidence of an extended galactic magnetosphere and wind.
Measure the spectra of positrons (10 MeV to 100 GeV) and
antiprotons (100 MeV to 100 GeV) to deter-mine where those particles are
created and how they are accelerated.
Measure isotope abundances for nuclei heavier than nickel and
elemental abundances through the actinides to probe the plasma regions where
the nuclei are synthesized and to measure the time scales involved.
RECOMMENDED RESEARCH EMPHASES
The specific programs required to obtain answers to the questions raised
under each of the five key topics outlined above are quite different. However,
they are united by four common elements or themes that the CSSP and CSTR
consider to be the most important research emphases for space physics in the
next decade.
1. Complete currently approved programs.
The space physics community must reap the benefits of the existing
approved programs. A stable program permits the most efficient management
and execution of high-priority research. In addition to the obvious scientific return,
these ongoing programs provide the basis for developing future research
directions. Space physicists will gain the maximum benefit from ongoing and
approved missions by:
Completing the diverse components of the International Solar-
Terrestrial Physics program;
Enhancing data analysis and interpretation efforts;
file:///C|/SSB_old_web/strasum.html (9 of 12) [6/18/2004 2:18:02 PM]
OCR for page 18
A Science Strategy for Space Physics: Summary
Streamlining mission operations for all space physics missions;
Carrying out extended missions for the uniquely placed Voyager (to
the greatest possible heliocentric distance) and Ulysses (through the solar polar
passes at solar maximum);
Supporting essential observational programs that require long-
duration databases; and
Enhancing the effectiveness of some of the longer-duration programs
by soliciting new ideas and analysis techniques from guest investigators and by
ensuring easy access to archived data resulting from the various programs for
use in "small science" research tasks.
2. Exploit existing technologies and opportunities to obtain new
results in a cost-effective manner.
Much technology is already in place to take the next observational steps
required to address many of the important questions in space physics outlined
here. These steps include:
Adapting existing instrumentation to the new generation of smaller
spacecraft and more focused space missions;
Placing space physics instruments on planetary, Department of
Defense, and other spacecraft of opportunity;
Utilizing suborbital platforms such as rockets and long-duration
balloons for both science objectives and instrument development; and
Supporting, where appropriate, activities at unique ground sites such
as in the polar cap.
3. Develop the new technology required to advance the frontiers of
space physics.
In order to achieve several high-priority objectives, or to lower the cost of
projects, the limits of technology must be pushed in the following ways:
Developing methods to approach the Sun ever more closely to open
one of the most exciting new frontiers of space science;
file:///C|/SSB_old_web/strasum.html (10 of 12) [6/18/2004 2:18:02 PM]
OCR for page 19
A Science Strategy for Space Physics: Summary
Producing new spacecraft and instruments based on lightweight
structure and miniature electronics;
Designing a new generation of instrumentation for remote global
imaging of magnetospheric, ionospheric, and solar wind plasmas;
Extending capabilities in suborbital techniques for both
experimentation and instrument development;
Exploiting infrared instrumentation for solar physics; and
Devising techniques to explore the region between the altitudes
reached by balloons and those reached by spacecraft.
4. Support strongly the theory and modeling activities vital to space
physics.
Special emphasis should be given to the following topics:
Recognizing that synergy between observations, modeling, and theory
provides the optimum way of addressing the principal questions in space physics;
Making numerical simulations of space physics systems more realistic
by extending them to three dimensions, longer time durations, and a greater
range of scale sizes, and by incorporating additional physical and chemical
processes;
Ensuring access to state-of-the-art computational facilities;
Exploiting new insights gained from theory, especially the theory of
nonlinear processes; and
Revisiting earlier efforts to predict solar activity, such as coronal mass
ejections and flares, using simulations combined with solar observations.
1A glossary of acronyms is included as the appendix, and the principal programs
identified by the acronyms are described in the main text.
file:///C|/SSB_old_web/strasum.html (11 of 12) [6/18/2004 2:18:02 PM]
