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OCR for page 47
SUMMARY 49
2.1 SOLAR WIND-MAGNETOSPHERE INTERACTIONS: THE REALM OF MAGNETIZED PLASMAS 53
Introduction 53
The Fourth State of Matter 53
Reconnection 54
Flowing Magnetized Plasmas 55
The Storage and Release of Energy
Coupling in a Collisionless Plasma
Impact and Relevance 58
Summary 58
57
57
2.2 MAG N ETOSPH ERES AN D TH El R PARTS 58
Overview 58
Bow Shock 59
Magnetosheath 62
Magnetopause, Cusp, Boundary Layers 63
Magnetotai 1 66
I n ner Magnetosphere 70
Plasmasphere 72
Sol ar Wi nd I Interactions with Weakly Magnetized Bod ies 73
Outer Planets 83
2.3 PROCESSES 87
Introduction 87
The Creation and Annihilation of Magnetic Fields 87
Magnetospheres as Shields and Accelerators 88
Magnetospheres as Complex Coupled Systems 89
2.4 CURRENT PROGRAM 90
Introduction 90
Programs 91
Critical Needs 98
47
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48
2.5 FUTURE PROJECTS 99
Introduction 99
Addressing the Major Themes 99
Project Summaries 100
Science Traceability 105
Prioritization: NASA and NSF 1 06
Prioritization of Other Agency and Interagency Initiatives 106
THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS
TECHNOLOGY 1 08
Introduction 1 08
Propulsion Technology 1 09
S pacec raft Tech n o l ogy 1 1 1
Science Instrumentation Technology 1 1 3
Information Architecture Technology 1 1 4
Technology for Ground Systems and Operations 1 14
Recommendations and Priorities 1 1 4
2.7 SOLAR Wl N D-MAGN ETOSPHERE I INTERACTIONS: POLICY ISSU ES 1 1 5
I Introduction 1 1 5
Interagency Coordination 1 15
Coordination Between Programs and Divisions Within Agencies: NSF and NASA 117
Opportunities for Space Measurements in Entities Other Than NASA's Office of Space Science 118
Science in the Structure of Project Management 1 19
I International Cooperation 1 1 9
Model ing, Theory, and Data Assimi ration 1 20
Technology Development 1 21
Data Analysis, Dissemination, and Archiving 121
Extended Missions 1 22
ADDITIONAL READI NG 1 22
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PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS
SUMMARY
The study of solar wind-magnetosphere interactions
at the turn of the 21 st century fi nds itself engaged in
exciting exploration of exotic extraterrestrial environ-
ments and consolidating a comprehensive, fundamental
understanding of the terrestrial magnetosphere. To capi-
talize on these discoveries, we need both classic mis-
sions of exploration to the planets and modern multi-
spacecraft probes in the near-Earth environment. This
report summarizes what we now know about planetary
magnetospheres and the processes within them, what
we need to know, and how we should proceed in ob-
tai n i ng th is knowledge.
MAGNETIC FIELDS
Magnetic fields play a crucial role in governing
Earth's space environment. They organize the helio-
spheric and magnetospheric plasmas, shield planetary
bodies, such as Earth, from bombardment with charged
particles, couple energy from one plasma regime to an-
other, store that energy and later release it rapidly. More-
over, they guide the motion of charged particles to re-
gions where they can cause visible displays such as
solar flares on the photosphere or the polar lights in the
atmosphere. Partners in these processes are the plasmas,
energetic particles, waves, and electromagnetic emis-
sions from radio to x-ray wavelengths in the solar wind
and the planetary magnetospheres.
Solar and planetary magnetic fields organize space
into normally well-separated regions. The principal
plasma regimes are the corona, where the solar wind
originates; the solar wind, the outward streaming plasma
that carries the Sun's magnetic field to the outer helio-
sphere; and the magnetospheres of planetary bodies,
intrinsic or induced. The magnetospheres may act as
flexible shields that deflect the solar wind and thereby
protect the planet and its atmosphere from most of the
direct impact of the solar wind particles. However, these
shields are not impenetrable.
One of the principal processes by which the shield
is penetrated is called magnetic reconnection. This pro-
cess is strongly controlled by the relative orientation of
the magnetic fields in adjacent regions, leading to con-
nection between the magnetosphere and the solar wind.
Magnetic reconnection not only breaches the bound-
aries between different plasma and magnetic field re-
gions, it is also the main process involved in the rapid
49
release of magnetic energy in eruptions in the solar
atmosphere and Earth's magnetosphere, in laboratory
plasmas, and, presumably, in astrophysical settings.
Other processes can breach the magnetic shield. In
the case of weakly magnetized bodies such as comets,
Venus and Mars, and the moons lo and Titan, neutral
particle transport across plasma boundaries occurs, with
subsequent ionization. In magnetically noisy environ-
ments, particles can be scattered across the boundaries,
and for small bodies finite gyroradius effects allow pen-
etration.
An important aspect of the plasmas in most of space
is that the magnetic fields that guide the motion of the
charged particles are, in turn, created by the motion of
those very same particles. Thus the magnetized plasma
can be quite nonlinear, enhancing, deflecting, or anni-
hilating the original magnetic field.
MAGNETOSPHERES
Planetary magnetospheres are particularly acces-
sible settings for studying the processes occurring in
magnetized plasmas, providing unique insights into ba-
sic physical processes not amenable to direct probing,
processes such as particle acceleration, shock forma-
tion, and magnetic reconnection. The solar wind inter-
action with a magnetosphere produces thin boundaries,
separating large regions of relatively uniform plasma.
Within these thin boundaries microscale processes
couple to the mesa- and macroscale processes, affecting
the stability and dynamics not only of the thin boundary
layer but also of the entire coupled magnetosphere
system. The magnetospheric shields of planets and
moons vary considerably. Some weakly magnetized
planetary bodies like Earth's moon routinely lose their
atmosphere to the solar wind, while others such asVenus
and Mars have had thei r atmospheres sign if icantly
altered, as indicated by the isotopic ratios of their atmo-
spheric constituents, but not completely removed.
Magnetospheres also exhibit rapid reconfigurations,
such as the ejection of magnetic islands, or plasmoids,
while the inner region collapses, as seen routinely in the
tail regions of Earth and Jupiter. Overall planetary
magnetospheres are complex, coupled systems, con-
nected on one end to a supersonic flowing magnetized
plasma, the solar wind, on the other end to a cold dense
planetary atmosphere and ionosphere, and sometimes
to embedded plasma sources such as satellites and rings.
While each planetary magnetosphere presents great
inter lectual chal lenges and its behavior provides insight
into diverse astrophysical solar and laboratory systems,
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50
the terrestrial magnetosphere is of particular practical
interest. It provides a home to many technological sys-
tems that are increasingly sensitive to magnetospheric
disturbances. Such disturbances affect the quality of
communications, our ability to navigate, the capacity of
power transmission lines, the orbits of low-altitude sat-
ellites, and the operation of geosynchronous spacecraft
carrying TV broadcasts, relaying phone calls, and moni-
toring our weather. Both astronauts and flight crews on
polar air routes can receive undesirable levels of radia-
tion from energetic particles controlled by the magneto-
sphere. Thus, understanding and predicting the response
of the magnetosphere to varying interplanetary condi-
tions, i.e., space weather, has become a particular con-
cern.
THE TERRESTRIAL MAGNETOSPHERE
The study of Earth's magnetosphere began with
ground-based measurements of the time variations of
the magnetospheric magnetic field. These observations
revealed not only the existence of the magnetosphere
but also its variable state of energization. The Interna-
tional Geophysical Year initiated an era of discovery in
which single-spacecraft missions throughout the mag-
netosphere provided an overview of the characteristic
regions, boundaries, and plasma conditions, with some
evidence of the processes therein, but they did not elu-
cidate how the processes in the magnetosphere work.
Therefore, current and future exploration of the terres-
trial magnetosphere concentrates on the use of multi-
spacecraft missions complemented by ground-based
arrays of magnetic, radar, and optical sensors to charac-
terize plasma behavior in a dynamic environment and
to probe cause and effect in a complex system at various
scales. At the other planets, with few exceptions, we
remain in the discovery phase since thus far we have
generally been restricted to single-spacecraft missions,
often si ngle flybys, not orbiters.
There are many success stories in magnetospheric
exploration as well as continuing puzzles. The standing
bow shock is well understood, but it is only the fastest of
three waves that should stand in the solar wind flow.
The other two waves the intermediate mode, which
rotates field and flow, and the slow mode, which
"stretches" field lines could also lead to standing struc-
ture. Reconnection is now known to provide a time-
varyi ng i ntercon nection of the terrestrial magnetosphere
with the magnetized solar wind, driving the circulation
in the magnetosphere, but in a manner that is as yet not
well understood. Reconnection is recognized to be the
THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS
principal mechanism for the violent release of stored
magnetic energy and for magnetic flux return from the
tails of the magnetospheres of both Jupiter and Earth.
Nevertheless there is not agreement on what triggers the
rapid onset of magnetotai I reconnection.
Radial diffusion and pitch angle scattering of ener-
getic particles apparently produce many of the observed
features in the radiation belts of planetary magneto-
spheres, but the driver of the radial diffusion remains
elusive, and the sources and acceleration mechanisms
for the involved energetic particles are not always clear.
At unmagnetized planets the mechanism for the forma-
tion of induced magnetospheres is relatively well under-
stood but the atmospheric loss is poorly understood.
INTRINSIC AND INDUCED
Magnetospheres can be divided into two types: in-
duced, if any intrinsic magnetic field of the body is so
weak that the ionosphere is directly exposed to the flow-
ing solar wind plasma, and intrinsic, if the body has an
internal magnetic field sufficiently strong to deflect the
plasma that flows against it. Induced magnetospheres
form around highly electrically conducting obstacles if
the conductor, generally an ionosphere, can stave off
the solar wind flow. Induced magnetospheres also form
in strong mass-loading environments such as at a rap-
id Iy outgassi ng cometary nucleus. Comets, Venus, Mars,
and some of the moons of the gas giants have magneto-
spheres induced by the rotating magnetospheric plasma.
Mercury, Earth, Ganymede, and the gas giants have in-
trinsic magnetospheres. Circulation inside the intrinsic
magnetospheres can be driven by the externally flowing
plasma or by an internal source such as plasma derived
from the volcanic gases of lo, accelerated by the rapidly
rotating Jovian magnetosphere. The centrifugal force of
this plasma drives a massive circulation pattern in the
Jovian magnetosphere, powering a massive magneto-
spheric "engine." Thus Jupiter acts as a bridge in our
understanding of the terrestrial and astrophysical mag-
netospheres.
For both intrinsic and induced magnetospheres the
supersonically flowing solar wind is deflected by the
magnetosphere, forming a bow shock. Behind the bow
shock, the decelerated shocked plasma flows around
the obstacle in a region known as the magnetosheath. In
intrinsic magnetospheres, the boundary between the
flowing plasma of the solar wind and the plasma, con-
nected by the magnetic field to the planet, is called the
magnetopause. In an induced magnetosphere, the analo-
gous boundary is called an ionopause. Often the mag-
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PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS
netopause and the ionopause are thin layers of current.
Behind the magnetosphere proper the magnetic field
and plasma are stretched by the solar wind flow, form-
ing a long magnetotail. Inside the magnetosphere, differ-
ing regions of plasma can be found, such as the plasma-
sphere in Earth's magnetosphere and the lo torus in
Jupiter's. In an induced magnetosphere, the plasma is
generally relatively cold and affected by the external
flow in ways much different than in an intrinsic mag-
netosphere. For these i educed magnetospheres, the so-
lar wind interaction acts to scavenge the atmosphere
and may be responsible for the loss of water from the
atmospheres of Venus and Mars and for alteration of
isotopic ratios over the eons since the formation of the
solar system.
THE PRESENT PROGRAM
The present program of studies of magnetized space
plasmas is robust. There is a vigorous program of
ground-based measurements, theory, modeling, and data
analysis, supported jointly by NASA, NSF, and, to a
lesser extent, by other agencies. Data are being returned
from the solar wind, magnetotail, magnetosphere, and
low Earth orbit. Galileo has recently completed its ex-
ploration of the Jovian magnetosphere and Cassini is on
its way to Saturn. The data are being analyzed promptly,
and significant scientific discoveries are being made.
Several important projects are under development and
moving toward their launch opportunities. Nevertheless,
there is still much to do.
UNIFYING THEMES
The outstanding questions that need to be addressed
in planetary magnetospheres can be divided into three
themes: the creation and annihilation of magnetic fields;
magnetospheres as shields and accelerators; and mag-
netospheres as complex, coupled systems. The first
theme includes the formation of the major magneto-
spheric current systems: the magnetopause, the tail cur-
rent, the ring current, and the field-aligned currents. This
theme also includes the disruption of some of these cur-
rents and reconnection of the magnetic field across cur-
rent layers, at the magnetopause, in the magnetotail,
and in planetary magnetodisks.
Under the second theme is the role that induced
and intrinsic magnetic fields play in deflecting the solar
wind and the energetic particle populations coming from
the Sun. These magnetospheres also store energy for
51
later release, leading to sudden energization of the
plasma in the magnetosphere and acceleration of mag-
netospheric energetic particles. In the inner magneto-
sphere, trapped charged particles are also accelerated
slowly to high energies by stochastic processes. None of
these processes is well understood. Even less well un-
derstood are the interactions of flowing magnetized
plasma with the remanent fields of bodies like Mars.
The third theme encompasses some of the most dif-
ficult areas of magnetospheric research: the interactions
among the d isparate pi asma regi mes with i n a magneto-
sphere. The bow shock interacts with the incoming solar
wind upstream and the magnetosheath and magneto-
pause downstream. Reconnection changes the topology
of magnetic field lines, connecting interplanetary and
terrestrial magnetic field lines so that the plasmas from
the two regimes mix, and allowing momentum and
energy to flow into the magnetosphere from the solar
wind. The ionosphere interacts with the polar magneto-
sphere and the magnetospheric regions at lower
latitudes. Planetary magnetospheres have their own
unique twists on these processes. In the Jovian magneto-
sphere the ionosphere enforces co-rotation of the plasma
over enormous scales and a giant circulation pattern is
set up within the magnetosphere. At the unmagnetized
planets there is direct coupling of the solar wind with
the neutral atmosphere.
RECOMMENDATIONS
The discipline of space physics and the subdisci-
pline of solar wind-magnetosphere interactions have
experienced an explosion of knowledge and understand-
ing in recent years. Still there are some very basic pro-
cesses that we do not understand, especially at a predic-
tive level. If we cannot predict the rate of reconnection
at our own magnetopause or in the magnetotail (and
today we cannot), we have little hope of extending our
knowledge to planetary and astrophysical systems. Thus
we recommend that the future exploration of the ter-
restrial and extraterrestrial magnetospheres should be
directed toward the deeper understanding of the funda-
mental physical processes and the global coupled sys-
tems, supported and guided by theoretical investigations
and simulation efforts. This requires multisatellite mis-
sions and the optimal use of simultaneous, coordinated,
and overlapping spacecraft missions. The global coupled
system extends all the way down to the upper atmo-
sphere and ionosphere. Thus in the terrestrial magneto-
sphere ground-based facilities play an important part in
the exploration of the coupled system.
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52
In planning for the next decade of studies of solar
wind-magnetosphere interactions we have been guided
by four essentials. We must understand the physical pro-
cesses involved and therefore need measurements with
high resolution, capable of studying three-dimensional
structure with support from theory and modeling. Our
models must be predictive from knowledge of external
cond itions. Th is req u i res gl obal, mu Iti poi nt observations
and is best achieved with deep, theoretical insight rather
than empirical models. We must investigate how re-
gions couple, not simply how they work in isolation,
and we must continue to explore new settings to de-
velop greater understanding.
Critical scientific objectives in the future exploration
of solar system magnetospheres include the following:
· A deeper physical understanding of fundamental
plasma processes, such as particle acceleration, mag-
netic reconnection, and the role of turbulence. Achieve-
ment of this objective should be at the core of present
and future space exploration, and the panel endorses
the planned Magnetospheric Multiscale mission.
· Understanding the scale sizes of the solar wind
structures that power Earth's magnetosphere. Achieving
this objective, which is needed for predictive purposes,
wi I I requ i re mu Itispacecraft missions near 1 astronomi-
cal unit (AU) with spacecraft separations measured in
tenths of astronomical units.
· Understanding the dynamics of the coupled mag-
netospheric system and of space weather. Achievement
of this objective requires arrays of instruments in space
as well as on the ground (just as readings from ground
weather stations are complemented by readings from
space). A magnetospheric constellation of up to 100
spacecraft to monitor a significant volume of the
magnetosphere is strongly recommended, along with
complementary ground-based measurements.
· Understanding the complex interaction between
the solar wind and the polar ionosphere. Achievement
of this objective requires the establishment at high lati-
tudes of the long-awaited Advanced Modular Incoher-
ent Scatter Radar (formerly known as the Relocatable
Atmospheric Observatory). Th is faci I ity cou Id be en-
hanced by many possible space missions, such as a
stereo imager or a polesitter auroral imager.
· Measurement of the density of the invisible popu-
lations within the magnetosphere. To achieve this objec-
tive, the panel recommends the establishment of mag-
netometer arrays that can perform magnetoseismology,
in analogy to terrestrial and solar seismology, recording
transient waves and the ringing of the magnetosphere.
THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS
· Understanding the energization of the radiation
belts. This long-sought objective requires knowledge of
the radial swaths of the particle and field environment
simultaneously at different local times and under differ-
ent geomagnetic conditions to learn how and why par-
ticle populations intensify and decay.
· Understanding the complex interactions of the
solar wind and planetary magnetospheres and atmo-
spheres. To achieve this objective, particles and fields
instruments will need to be flown on both Discovery-
class and major space missions.
· Understanding planetary magnetospheres. The
exploration of planetary magnetospheres is in its infancy,
yet comparisons between these magnetospheres and the
terrestrial magnetosphere and with each other are criti-
cal to fully understanding the processes taking place.
Missions to study atmospheric loss fromVenus and Mars,
the occurrence of lightning at Venus and Jupiter, the
dynamics of Mercury's magnetosphere, and the joint
control of the jovian aurora by lo and the solar wind are
some of the many missions that could contribute to our
understanding of planetary magnetospheres.
TECHNOLOGY DEVELOPMENT
While some of these objectives are already techni-
cally within our grasp, additional technology develop-
ment is needed for others. For example, several missions
could be undertaken most effectively with a solar sail.
Improved ion propulsion, nuclear-powered propulsion,
and mid-size expendable launchers would also increase
access to space. Smaller spacecraft systems and instru-
ments would enable the constellation missions that are
planned and would allow greater return from resource-
limited planetary missions. Finally, attention needs to be
given to the entire data chain, from operations to data
transmission to their assimilation in models to reduce
manpower and the total expense of the data chain.
CHANGES IN POLICY
Many of our programs would be enabled and en-
hanced with some simple changes in policy. In some
cases, this simply requires better coordination between
or even within agencies. Sometimes data are obtained
but funds are required for data access or archiving. We
need to have processes to determine when a technique
has moved from the research arena into the space-
forecasting arena. We need to coordinate opportunities
for access to space so that all such opportunities are
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PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS
utilized, and we need to ensure that funding for space
experiments is available when possible flight opportuni-
ties arise. Presently, missions of opportunity are solicited
far too seldom and on time scales incongruent with the
duration of the opportunity. We also have to guard
against using the space science budget to cover short-
falls in other programs such as the space station. Budget
raids can devastate smaller programs. Moreover, we
need to find ways to reduce regulatory burdens, such as
International Traffic in Arms Regulations (ITAR) and
information technology security regulations, which have
led to more and more obstacles to international collabo-
ration and to university participation. These policies
often have results much different than originally
intended. High-level communication and coordination
between regulatory agencies and NASA are needed to
achieve reasonable implementation standards and
procedures.
SYNOPSIS
In short, the research enterprise in solar wind-mag-
netosphere interactions is strong, and much has been
accomplished. Nevertheless, some very fundamental
understanding is still needed to reach the quantitative
level of a fully predictive science. Fortunately, the means
to attain this understanding now exist. In some cases an
investment in technology will bring us to the threshold
of the needed breakthroughs. The next decade of this
discipline, launched with the momentum of the last
decade's discoveries, fueled by an exciting series of new
observations, and supported by a strong program of
theory and modeling, promises to usher in a new, quan-
titative level of understanding of the Sun-Earth connec-
tion.
In the next section, the panel provides an overview
of the workings of planetary magnetospheres. This over-
view is followed in Section 2.2 by a detailed discussion
of current understanding of the processes in the terres-
trial magnetosphere and the environments of the plan-
etary magnetospheres. This description is needed to
understand why the panel has chosen the paths it rec-
ommends, but it may be skipped by those seeking only
to learn the recommendations. Section 2.3 is an attempt
to provide three unifying themes that order the remain-
ing tasks. Section 2.4 summarizes the existing program
and presents recommendations. Sections 2.5, 2.6, and
2.7 describe, respectively, the recommended future pro-
gram, the recommended technology developments, and
the recommended policy changes that will enable the
progress needed i n th is field.
53
2.1 SOLAR WIND-
MAGNETOSPHERE INTERACTIONS:
THE REALM OF MAGNETIZED
PLASMAS
INTRODUCTION
Our i ncreasi ngly technological society rel ies more
and more on assets launched into space. In addition,
our investigations extend well past the local space plas-
mas into those of the solar and astrophysical systems.
Understanding the behavior of magnetized plasmas has
become increasingly important. We must understand the
environment in which our satellites operate. We need to
predict how the solar wind affects the terrestrial mag-
netosphere. We require insight into how the Sun gener-
ates explosive events, and we desire to comprehend the
workings of distant astrophysical systems that are clearly
also affected by magnetic processes.
The solar wind's interaction with the terrestrial and
planetary magnetospheres allows us to treat a problem
of much practical importance and learn how these plas-
mas work in a most general manner. We can then ex-
tend this knowledge to other plasma systems in regions
we cannot probe directly.
For users of this report who are not familiar with the
physics of space plasmas, this section offers some brief
insight into the basic plasma processes that occur in
space. This section also provides a preview of the themes
introduced in Section 2.3 and the rationale for the rec-
ommendations made in subsequent sections.
THE FOURTH STATE OF MATTER
Plasmas are often referred to as the fourth state of
matter. The behavior of this state, especially of magne-
tized plasmas, can be nonintuitive. We are most familiar
with the other three states solid, liquid, and gas-
whose dynamical properties are governed by the differ-
ing intermolecular forces of each state. Our intuition
usually serves us well here. In an ideal gas, the forces
between the molecu les are transmitted through col I i-
sions. The random motions of the gas are characterized
by a temperature and, in collisional equilibrium, all con-
stituents come to the same temperature. The pressure in
the gas is proportional to the product of the density and
the temperature. A pressure gradient exerts a force. For
example, in Earth's atmosphere we know that the pres-
sure decreases with altitude. The force associated with
OCR for page 54
~ do.
JO
this pressure gradient acts on a parcel of air to support it
agai nst the force of gravity.
In space plasmas there often are no collisions in the
usual sense. Thus, different components of the plasma
can have d ifferent temperate res. Fu rther, temperate res
along the magnetic field and across it can differ. Still,
and counterintuitively, a plasma can exert pressure
forces not only through the thermal motion of its par-
ticles but also through its magnetic (and electric) fields.
These fields do have pressure (proportional to the square
of the field strength) and, as in the case of a gas, the
gradient of that pressure exerts a force. In a magnetized
plasma, the magnetic field orders the charged particle
motion, the energy of the gyrating particles provides the
plasma pressure perpendicular to the field, and the par-
al lel thermal motions provide pressure along the field.
An example of the interplay between these pres-
sures is provided by the boundary between the solar
wind flow and the magnetosphere. This region, the mag-
netopause, is often treated as a boundary between a
plasma with at most a weak magnetic field (the shocked
solar wind) and a strong magnetic field (Earth's mag-
netosphere) containing very little plasma. The pressure
on the solar wind side is contained in the thermal mo-
tions of the plasma. At the boundary of the plasma the
thermal-pressure gradient exerts a force toward the mag-
netosphere. The pressure in the magnetic field similarly
exerts a force into the solar wind plasma, where the
magnetic pressure decreases. Thus there is force bal-
ance, and the magnetosphere and the solar wind estab-
lish a pressure equilibrium in the absence of collisions.
The ratio of the proton mass to the electron mass is
1,836. Thus an electron at the same temperature as a
proton moves at 43 times the speed of the proton, and
for this reason electrons can communicate rapidly in a
plasma. Protons, though, have all the inertia and mo-
mentum, and electrons tend to follow the dynamics of
the protons, setting up small ambipolar electric fields to
maintain quasi-neutrality in the plasma. As a result, ex-
cept on the microscale, the electric field seldom builds
up to such a degree that its pressure is important. How-
ever, when electric fields do arise parallel to the mag-
netic field they can be very important to the processes in
the plasma, so much so that it is critical to be able to
observe such generally small electric fields. The panel
notes that these electric fields are frame independent,
while the electric field in the direction perpendicular to
the magnetic field is frame dependent, so that the per-
pendicular electric field detected depends on the veloc-
ity of the observer relative to the magnetic plasma. Thus
the flowing solar wind has an electric field as seen in
THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS
Earth's reference frame. In short, while a plasma has
many similarities to fluids and gases, it is different
enough that our physical intuition is often ill-prepared
to understand processes that occur therein.
RECONNECTION
The very large mass ratio between the proton and
electron affects their gyromotion as well as their typical
speeds. Because of the nature of the Lorentz force, which
keeps a charged particle in orbit about a magnetic field,
the radii of gyration of different charged particles are
proportional to their mass times their velocity and in-
versely proportional to their charge. If protons and elec-
trons have the same energy perpendicular to the mag-
netic field, the radius of the gyrating electron is 2.3
percent of that of the proton. In a collisionless plasma,
the gyrating particles define the magnetic field lines.
Particles in orbit about a magnetic field line stay with
that magnetic field line. The ability of a charged particle
to orbit a magnetic field line depends on the scale size
for changes in the field. A small change can cause drift
motion, and too large a change in the field on the scale
of a gyroradius can cause a charged particle to become
unmagnetized and move to orbit another field line.
Owing to their smaller gyroradii, electrons can follow
small-scale field variations to sizes roughly 43 times
smaller than protons. One might think that this is moot
for a system as large as Earth's magnetosphere, because
its scale sizes are vast compared with those of the gyro-
radii. In fact, it is standard practice to average over the
gyromotion and treat the magnetized plasma as a mag-
netic fluid. This formulation is known as magnetobydro-
dynamics (MHD). However, the vast scale of the mag-
netosphere does not allow us to completely ignore the
kinetic motion of its particles. It just reduces the region
in which that kinetic motion is crucial to small areas
called neutral points. Close to these points the protons
first become unmagnetized, and then closer yet the elec-
trons become unmagnetized.
This process in which charged particles lose their
ability to define a magnetic field line is called reconnec-
tion. If they are antiparallel, two neighboring magnetic
field lines, say one that starts and ends on Earth and
another that starts and ends on the Sun, can become
connected so that two new field lines are created, both
of which have one end on Earth and one end on the
Sun. This topological change enables the plasmas in the
two regions (terrestrial and solar in this case) to mix. It
also allows momentum and energy to be supplied from
one plasma to the other. Figure 2.1 illustrates the
OCR for page 55
PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS
geometry of this situation. The magnetic field at the re-
connection point forms an x-type configuration with
plasma flowing into it from the left (Earth's magneto-
spheric plasma) and from the right (Sun's solar wind
plasma). Field lines switch partners at the x-point, and
plasma and cojoined field lines flow rapidly outward
(top and bottom). Since an electric field is frame depen-
dent, these moving magnetized plasmas have electric
fields in the frame of the reconnection point, as sketched
in Figure 2.1.
Collisions, either particle-particle or wave-particle,
can also demagnetize orbiting charged particles, and in
numerical simulations numerical dissipation can mimic
the reconnection process. Thus it is not always clear
how the magnetosphere undergoes this most critical pro-
cess. Much continued in situ study with high temporal
Exhaust
ZNIF
.\ .
Magnetosphere <~ \ \
YN ~ For
i/
XNIF
Exhaust
Magnetosheath
ME MINX N I F
inflow
FIGURE 2.1 The geometry of reconnecting antiparallel mag-
netic fields at a neutral point. Courtesy of J.D. Scudder.
55
and spatial resolution is required, as well as investiga-
tion with state-of-the-art numerical codes.
From the above it is obvious that magnetic recon-
nection is the crucial process enabling plasma (and
momentum, energy, and magnetic flux) to cross
magnetospheric boundaries. In addition, the partner-
swapping process in reconnection can dramatically alter
the stress balance in a plasma, leading to catastrophic
energy release processes, such as solar flares and mag-
netospheric substorms, discussed below.
FLOWING MAGNETIZED PLASMAS
A solid can support both compressional and trans-
verse oscillations but a normal liquid and a gas cannot.
Thus the dynamics of the flow around an object in a
flowing gas is dominated by compressions and rarefac-
tions. However, in a magnetized plasma that otherwise
resembles a gas, there are transverse oscillations as well
as two compressional waves. These three waves are usu-
ally called fast, intermediate, and slow. They are all
necessary to transmit an arbitrarily shaped perturbation
through a magnetized plasma. For example, in the inter-
action of the flowing solar wind with Earth's magneto-
sphere, the fast mode slows, heats, and deflects the flow
and magnetic field, but in general the intermediate mode
is needed for additional field and flow deflection, and
the slow mode is needed to prevent a density pileup at
the subsolar point. Just as in a solid or gas, perturbations
travel at a finite velocity, and in the magnetized plasma
as in many other situations the velocity of each wave
Separator mode is different.
When it arrives at each of the planets the solar wind
flow is supersonic, moving faster than the speed of the
-YNIF compressional (fast mode) wave that could deflect it
around the planetary obstacle. The momentum flux of
the solar wind represents a dynamic pressure that con-
fines the planetary magnetic field, but in order for it to
be applied to the magnetosphere, the flow must pass
through a bow shock that slows, deflects, and heats the
flow, making it subsonic. Then the three wave modes
(fast, intermediate, and slow) can act on the plasma to
cause the deflection of the flow and alter the plasma
conditions at the boundary of the magnetosphere.
It is very important to magnetospheric processes that
there are finite propagation times and finite transport
times in the magnetosphere. When the solar wind mag-
netic field reconnects with the subsolar magnetospheric
magnetic field, it begins to transport magnetic flux to the
geomagnetic tail, as illustrated in Figure 2.2. The tail
may increase in size for about an hour. Then, when
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56
Inter~lanetarv
Polar
Cusp
-Magnetopause Current
FIGURE 2.2 A cutaway diagram of Earth's magnetosphere.Cour-
tesy of C.T. Russell, University of California, Los Angeles.
reconnection begins in the tail, it may require another
hour to transport the magnetic flux out of the tail, but the
signals denoting the onset of each reconnection event
can travel through the magnetosphere within minutes.
Other manifestations of the finite travel time of perturba-
tions in the plasma include the resonant behavior of
individual dipole flux tubes, with waves bouncing back
and forth in the magnetic tube and the propagation of
shock-initiated disturbances through the magnetosphere.
These signals can be used to probe the magnetosphere
much as seismology uses waves triggered by earth-
quakes to probe the structure of Earth.
The waves discussed above have very long wave-
lengths, generally a large fraction of the dimension of
the system. Waves at shorter wavelengths are also pres-
ent in magnetized plasmas. Often these waves are
responsible for releasing free energy from the plasma,
ultimately into heating of the system. Examples of such
waves include those caused by the upstream ions
reflected at the bow shock and ion cyclotron waves
produced both in the solar wind interaction with comets
and in the flow of the lo torus past the mass-loading
region at lo.
THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS
When a flowing magnetized plasma interacts with
an unmagnetized planet there are important simi rarities
and differences from the magnetized case. First, if the
unmagnetized planet has an atmosphere, then it will be
ionized by the solar UV and EUV radiation. The solar
wind magnetic field will be draped across this electri-
cally conducting ionosphere and will pile up in front of
it, as illustrated in Figure 2.3. This pileup region deflects
the solar wind particles around the ionospheric obstacle
while the magnetic field begins to diffuse into the iono-
sphere. However, the solar wind magnetic field is quite
variable in direction, and the long-term (days) vector
average field is close to zero. The diffusion time into the
interior of the ionosphere, high in the collisionless exo-
sphere, is long. Thus, the deep ionosphere does not
generally become strongly magnetized by this external
magnetic field. Second, the neutral atmosphere often
has a great enough extent that the neutral density is
significant on solar wind stream tubes that are flowing
rapidly. When the neutral atoms and molecules of the
atmosphere become ions they are accelerated by the
solar wind and lost to the planet. This loss can lead to
significant changes in a planetary atmosphere over the
age of the solar system. Hence the magnetic field is both
an accelerator and a shield at unmagnetized planets.
tow Shank
Shock
s (Outside Tail)
FIGURE 2.3 The solar wind interaction with an unmagnetized
planet, illustrating the effect of the shock on the field and the
stretching of the field to form an induced magnetotail. Courtesy
of C.T. Russell, University of California, Los Angeles.
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PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS
THE STORAGE AND RELEASE OF ENERGY
The interaction of the solar wind with a planet cre-
ates a magnetic cavity with a long magnetic tail. Since a
magnetic field has an energy density, the formation of a
planetary magnetotail or its expansion involves the stor-
age of energy. The additional energy in a magnetotail
can be provided externally when the magnetosphere
reconnects with the interplanetary magnetic field at the
dayside if the fields are in opposite directions. The stored
energy is extracted from the mechanical energy of the
solar wind flow as the magnetic field lines joining the
magnetosphere to the solar wind slow its flow.
The two nearly antiparallel magnetic lobes of the
tail can also reconnect in a manner similar to the fields
at the magnetopause, and energy can be released rap-
idly from the tail lobes, in a process called a substorm.
Some of this energy accelerates the bulk of the plasma,
some heats the plasma, and some energizes a few par-
ticles to very high energies. On Earth these high-energy
particles help populate the radiation belt. Over the au-
roral zones beams of particles are created that cause the
auroral emissions during the reconfiguration of the night
magnetosphere associated with these acceleration pro-
cesses. On the Sun, solar flares are seen when highly
energetic particles strike the solar surface after a re-
connection event in the magnetic field above the photo-
sphere.
The creation of a small number of energetic par-
ticles (a "high-energy tail," as these energetic particles
are often called) is another nonintuitive phenomenon in
a collisionless plasma. In a collisional environment the
number of particles as a function of energy follows a
Maxwellian distribution in which there are very few par-
ticles that deviate much from the bulk of the distribu-
tion. However, in a collisionless plasma, the high-en-
ergy particles can receive a disproportionate amount of
the energy being put into the plasma. Understanding the
conditions under which this occurs is important, be-
cause these high-energy particles can be deleterious to a
spacecraft system in orbit in the magnetosphere. In the
broader astrophysical setting, we are interested in un-
derstanding how cosmic rays reach energies much,
much higher than those of particles accelerated by pro-
cesses in the solar system.
COUPLING IN A COLLISIONLESS PLASMA
The large proton-to-electron mass ratio not only
affects the relative speeds of the two particles under
normal circumstances and also their relative gyroradii,
17
.~ .
but also the charge separation. In general, electrons will
stay close to the ions to maintain charge neutrality. Al-
though in the absence of collisions charged particles
will stay on a single magnetic field line, any charge
imbalances that arise can generally be removed by mo-
tion along the magnetic field. Thus most plasmas are
quasi-charge-neutral.
The magnetic field in most space plasmas is strong
enough to divide space into different plasma regimes
with little communication across the boundaries be-
tween them. When the magnetic fields in two adjacent
regions are in nearly opposite directions, reconnection,
discussed above, can occur, linking the two regions. If
one of these regions is flowing past the other, this link-
age can transfer momentum from the flowing plasma to
the initially stationary plasma. This is the way in which
Earth's magnetosphere is stirred by the solar wind that
flows past it. Surface waves can also transfer momentum
across such a boundary if the system is dissipative. The
waves may be generated at the bow shock and blown
back against the magnetopause, or they may arise in the
interaction due to a velocity-shear instability, such as
the Kelvin-Helmholtz instability, that is akin to the pro-
cess by which the wind creates ocean waves.
An important coupling occurs between the plasma
and the neutral gas in Earth's magnetosphere at the foot
of the field line. The stress that is applied at the interface
between the solar wind and the magnetosphere must
eventually be taken up by Earth, and this occurs ulti-
mately through collisional transfer between the ions and
the neutrals. To get the ions moving at the feet of mag-
netic field lines and overcome the drag of the neutral
atmosphere, the magnetosphere sets up a large current
system that connects the outer magnetosphere with the
ionosphere along magnetic field lines. The closure of
the current across field lines in the collisional, electri-
cal Iy conducting ionosphere accelerates the low-altitude
ions via the J x B force or ponderomotive force. This
force is the macroscale manifestation of the Lorentz
force, which maintains charged particles in their orbits
around magnetic field lines. It can also transfer stress
from the ionosphere to the magnetosphere such as to
enforce co-rotation of the cold magnetospheric ions.
This mechanism is especially important in the jovian
magnetosphere. In the terrestrial auroral ionosphere, the
chain of momentum transfer is completed when the
moving ionospheric ions transfer their momentum to the
neutral gas, generating high-altitude winds. The overall
coupling from the solar wind down to the ionosphere is
a very complex process, and it is fair to say that while
we now understand this process much better than even
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THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS
INFORMATION ARCHITECTURE TECHNOLOGY
As the density and complexity of information in-
crease, science missions are driven by the need to
handle and ingest the resultant data. New technologies
and concepts are required to make massive amounts of
data accessible to science and scientists. The science
and engineering community, with support from NASA,
needs to continuously evolve this process. There is no
clear-cut separation between data processing, data com-
pression, data retrieval, data storage, and data distribu-
tion and assimilation onboard the spacecraft and like
processing on the ground. Different missions will require
that different parts of the process be done in different
places. For example, a deep space or outer planet mis-
sion may send back mostly highly processed data to
reduce bandwidth requirements. In future missions there
may be a need to assimilate the observational data into
models onboard the spacecraft and telemeter the results
to the ground. The different technologies used must be
migrated to the appropriate platforms.
The constel ration-type missions, i n particu far, have
a recognized need to assimilate the data into global
models. This requires development of software and sys-
tems that can take the data inputs in real time indepen-
dent of their quality and completeness. The concept is
to use adaptive physical models (like adaptive MHD
codes) to provide connectivity between the indepen-
dent in situ observations and to generate a complete
moving picture of the dynamic system being studied.
This is not a current capability except for single-point
interplanetary data inputs to MHD codes. What the con-
stellation science teams envision is to dynamically
modify the MHD codes to best represent the totality of
the observations. Data from constel ration missions im-
pose constraints on the modeling codes. An intensive
effort is required to construct codes with appropriate
data assimilation technologies.
Model development itself is driven by a need for
new tech nology, i n some cases a new computational
tech nology. I n other cases it is the development of new
software technology and numerical techniques. Model
development is a cross-enterprise issue, and theoretical
and modeling missions should be considered to be as
important as experimental missions.
TECHNOLOGY FOR GROUND SYSTEMS
AND OPERATIONS
As noted above, the cost of ground systems and
operations can be very large, especially for long-dura-
tion missions. Technologies that reduce and simpl ify the
systems free resources that can be redi rected i nto the
science component of the missions. Just as autonomous
satellites can reduce the burden on ground and opera-
tions systems, autonomous ground systems can reduce
the manpower required for mission support. With sev-
eral upcoming missions being multisatellite or constel-
lation missions, it is imperative that new operation con-
cepts and technology be developed and implemented.
Examples of possible technologies can be found in com-
mercial communications satellite organizations. Many
of these companies run large numbers of satellites with
relatively small crews. In fact, these organizations are
ahead of NASA in the area of autonomous operations of
systems of spacecraft. Their technologies should be stud-
ied and emulated where appropriate; otherwise they
should be used as a starting point for the development of
the operations and ground systems that will be required
by MagCon and other mu Itisatel I ite science missions.
A separate issue is the retrieval and handling of the
science data from modern missions. Again, in the past
this has been a manpower-intensive and costly effort
throughout the mission. Intensive support is required to
find and implement architectures and technologies able
to handle the massive increase in data that new missions
will generate. New technologies are required to make
such massive amounts of data easily accessible by all
scientists. The science and engineering community as a
whole needs to continuously evolve this process. The
different technologies must be migrated to the appropri-
ate platforms.
RECOMMENDATIONS AND PRIORITIES
As noted above, there needs to be a significant fo-
cus on developing new instrument, satellite, propulsion,
operations, data assimilation, and processing technolo-
gies. The top priority in each of these areas is addressed
below:
· Science measurements. A space-science enter-
prise-wide instrument development program is needed
that is separate from SR&T budgets. This issue clearly
needs to be addressed if quality measurements are to be
made on the smaller micro/nanosatellites being envi-
sioned for future multisatellite and constellation mis-
sions. In addition, new management and team struc-
tures must be generated for developing highly integrated,
micro/nano, science-craft-type spacecraft. This must be
done on a time scale that meets the development needs
and implementation of multisatellite missions.
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PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS
· Propulsion. The top priority is to push the devel-
opment of solar sail technology to support missions that
require non-Keplerian orbits in the interplanetary me-
d i u m to make the necessary science measu remeets at
appropriate locations. NASA has a good start in this area
but needs to move forward with hardware development
and spaceflight demonstrations so the technology is
flight qualified in this decade.
· Satellite technology. The push shou Id be to de-
velop fabrication, integration, and testing technology for
micro/nanosatel I ites that wi 11 enable constel ration mis-
sion science. This requires bringing together new mate-
rial and electronics technologies, new management
techniques and structures, and new integration and test-
ing processes. A way of integrating the development of
spacecraft and science instruments is needed. Test-bed-
type development programs that take multiple micro/
nanosatellite systems from concept to flight should be
considered as one way of developing and testing the
new processes. This work needs to be done soon if it is
to support missions that are identified in NASA's Strate-
gic Plan.
· Operations and data handling. The focus should
be on implementing evolving technologies that reduce
manpower and costs for all missions. In particular, at-
tention should be given to the complete data chain,
from the operations required to get the science data to
the manner in which the data are brought to Earth, as-
similated into models, and ultimately presented to sci-
entists. This must be started now if the needed capabili-
ties are to mature on a time scale that meets planned
schedules for complex new interplanetary observatories
and mu Itisatel I ite constel ration missions.
2.7 SOLAR WIND-
MAGNETOSPHERE INTERACTIONS:
POLICY ISSUES
INTRODUCTION
The need to observe coupled dynamics of the large
magnetospheric system in response to solar wind condi-
tions has several implications. First, the difficulty of ad-
equately specifying the state of the dynamic system
means that the number of observing platforms should be
as large as possible. This goal would be best achieved
by interagency coordination to maximize the opportuni-
1 1 5
ties for instrumentation on non-NASA platforms. Sec-
ond, because ground-based observations provide dis-
tributed knowledge of convection and boundaries that
cannot be achieved in space, ground-based capabilities
must not only be sustained but enhanced, and coordina-
tion between space- and ground-based observations
needs to be exploited worldwide to the fullest extent
possible. Third, it is now firmly established that knowl-
edge of solar wind conditions is critical to developing
further understanding of the magnetosphere-ionosphere
system, so that sustained, continuous monitoring of solar
wind input is essential. Fourth, global physics-based
computer si mu ration codes are an essential component
of the research program because they will play a central
role in maximizing the information that can be extracted
from the observations and unifying disparate data sets
within a comprehensive framework. These implications
in turn have specific ramifications for the policies that
should be pursued to achieve the key science objectives
identified above.
INTERAGENCY COORDINATION
Because the resources required to make the neces-
sary observations exceed those available from any one
agency and because the societal impact of the science
return is relevant to a variety of agencies and interests,
efficient coordination between agencies is a preeminent
policy concern. In fact, one could argue that the coordi-
nated system is much more valuable than the sum of its
components.
NOAA:Transitioning New Operational Observing
Platforms and Models
The National Oceanic and Atmospheric Adminis-
tration has two roles to play. First, the transitioning of
space instrument platforms from basic science research
to operational systems needs to be anticipated and
implemented in a timely fashion. Since space-based sci-
ence platforms are almost exclusively the substance of
NASA programs, these transitions will require coordina-
tion with NASA. Typically, this is done by taking the key
instrumentation and data reduction techniques devel-
aped under NASA research programs and implementing
them under NOAA. Observations that are or should be
planned for transition i ncl ude the fol lowi ng:
· Interplanetary magnetic field and solar wind ob-
servations analogous to those provided in rea/ time from
OCR for page 116
1 1 6
the ACE spacecraft at the L1 point. The necessity for
continuous IMF/solar wind observations from L1 has
been abundantly demonstrated scientifically and opera-
tionally. Nearly all predictive models of magnetospheric
response depend pri nci pal Iy on IMF/sol ar wi nd i nputs.
Since the ACE spacecraft is operating beyond its design
life, it would be prudent to implement a new L1 plat-
form for this purpose in the very near future (<2 years).
Steps should be taken to continue such L1 in situ moni-
toring as an operational system.
· Solar coronal observations. The dramatic ad-
vance in our appreciation and understanding of the
causative link between coronal dynamics, coronal mass
ejections, and high-speed streams, in particular, and
major geomagnetic disturbances made possible by re-
sults from the instrumentation on YOHKOH and SOHO
has motivated both the Solar Dynamics Observatory of
the LWS program and the STEREO mission. There is
now little doubt that observations of this class will play
a central role in operational space forecasting, and steps
should be taken to coordinate operations with NASA in
the short term (next 5 years) and to deploy a line of
operational monitors in the medium term (5-8 years).
The GOES SXI instrument is an important first step.
· Auroral imaging. The advances i n u nderstand i ng
magnetospheric dynamics, particularly nightside/mag-
netotail processes made possible with global auroral
imaging, demonstrate the value of these observations for
monitoring intrinsic magnetospheric dynamics as well
as energy transport to the ionosphere. New results from
the IMAGE mission promise to increase our understand-
ing of the physical correlates of these observations,
thereby improving their operational value. It is already
clear that real-time auroral imaging will prove opera-
tionally valuable, and plans should be laid for NOAA to
provide global auroral imaging on an operational plat-
form in the long term (8-10 years).
The second role for NOAA concerns the transition-
ing of models from science research tools to operational
resources. NOAA, NSF, DOD, and NASA al I have theory
and modeling efforts in magnetospheric physics. The
Space Environment Center of NOAA supports a small
effort to transition science models to operational use.
Discussions with both agency and scientific community
personnel indicate that this transition effort is
undersupported. They note that there appear to be many
more models available in the community that could be
useful to space weather than are being actively con-
verted to operational status.
THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS
A parallel concern exists in the research commu-
nity, which has a separate need for standard models. In
the next decade, the research community also will re-
quire access to realistic global models, including MHD
simulations. The Coordinated Community Modeling
Center at GSFC5 is providing the first community access
to such global models, in part in coordination with the
NSF's GEM program. The challenge of providing sophis-
ticated models to the community is a significant one and
will require an advisory structure to prioritize and ensure
maximum efficiency in coordination between agencies
and to minimize duplication of effort. The panel recom-
mends that these transitioning efforts be supported
aggressively to meet the science and applications objec-
tives of the space environment community.
DOD-DOE: Coordinated Planning for
Launch and Flight Opportunities and
Access to Relevant Data Sets
The Department of Defense and the Department of
Energy conduct operational flight and observation pro-
grams that are directly relevant to the science objectives
of magnetospheric-solar wind interactions. Ensuring ap-
propriate use of these resources is an extraordinarily
cost-effective means of achieving several of the observa-
tional goals described above. Launch opportunities will
be avai lable, particularly for sending smal ler payloads,
~300 kg, into geosynchronous transfer and low Earth
orbits on DOD vehicles. These are key regions for
magnetospheric dynamics, particularly radiation belt
dynamics. H istorical Iy, launches of opportunity have
proven problematic in practice because of cost con-
cerns associated with launch schedules. (The cost
growth and resulting cancellation of IMEX were due in
part to probl ems of th is sort.) Mechan isms for accom-
modating NASA payloads on DOD launch schedules
without raising NASA mission costs via prolonged
launch delays need to be studied. In addition, agree-
ments between NASA and DOD regarding launch op-
portunities need to be formalized so that the availability
of these opportunities and the mutual commitment to
support the programs that use them do not hinge on
agency personnel remaining in key positions. Discus-
sions with DOD representatives indicate that launch
opportunities will continue to be available, but at
present arrangements to use these opportunities are
made only on an ad hoc or informal basis.
5See .
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PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS
Current, future, and planned DOD and DOE plat-
forms obtain or will obtain measurements for opera-
tional purposes that are directly relevant to magneto-
sphere-solar wind interaction science objectives. These
measu remeets i ncl ude data from DMSP and N POESS
particle and fields detectors and from auroral imagers,
particle data from DOD and GPS satellites, and total
electron content from ground-based GPS receivers.
These data sets provide valuable augmentation of space-
based observations and need to be exploited to the full-
est extent possible. Support for preliminary processing
and archiving of these data for retrospective scientific
analyses is essential if the scientific community is to
make meaningful use of these assets. NASA and NSF are
urged to coordinate with DOD and DOE to facilitate
preliminary processing and archiving activities so that
these resources are leveraged to the fullest extent possible.
NSF: Central Role for
Ground-Space Coordinated Observations
Global magnetospheric dynamics are reflected in
ground processes. Our understanding of the specific
correlation between ground observables and magneto-
spheric configuration and dynamics has dramatically
increased in recent years, in part through the efforts of
NSF's GEM campaign. We now understand the relation-
ship between convection distribution and the underly-
ing magnetopause reconnection geometry. We also ap-
preciate how to identify time variations in reconnection
in ground observations. We now know how to relate
auroral spectra to the precipitating source population
and critical bou ndaries i n the magnetosphere, both i n
the magnetotail and on the dayside.
Ground-based observations provide distributed
measurements of quantities that are difficult or impos-
sible to measure with comparable distribution in space.
These measurements include radar and ground magne-
tometer observations of convection; multispectral and
high time and spatial resolution auroral imagery, merid-
ian scanning photometers, and ULF pulsations. The
ground observations therefore provide a means of moni-
toring the enormous system with remarkable efficiency.
Historical Iy, NSF assumed the central role in both estab-
lishing the ground-based observatories and coordinat-
ing these observations with measurements from space.
Mai ntai n i ng and expand i ng these grou nd observa-
tion assets is critical for two reasons. First, ground obser-
vation assets will prove even more valuable as new
space measurements are made and as models become
able to assimilate these data. Recent advances in identi-
1 1 7
tying ground signatures with specific magnetosphere-
solar wind interaction phenomena make the obser-
vations even more valuable because they provide quan-
titative contextual and distributed information that is key
to specifying the system and unavailable any other way.
Second, some impediments remain to establishing an
unambiguous link between phenomena that can be ob-
served from the ground and magnetospheric-solar wind
dynamics. These impediments include our incomplete
understanding of ionospheric conductivities and the dif-
ficulty of specifying the net result of auroral processes
that couple the high-altitude magnetosphere to the iono-
sphere. Only by comparing extensive ground observa-
tions with space-based observations will it be possible
to further improve the power of ground-based observa-
tions.
COORDINATION BETWEEN PROGRAMS AND
DIVISIONS WITHIN AGENCIES: NSF AND NASA
Because magnetospheric physics is one of a number
of priorities in NSF and NASA space science programs,
and because responsibility for it is split between NASA
and NSF, it is important to recognize and eliminate un-
necessary compartmentalization. The panel encourages
cooperation and coordination between agencies and
between programs within each agency. Several areas in
which coordination is desirable are discussed next.
Comparative Magnetospheres
and Planetary Exploration
Comparative magnetospheres remains a vital prov-
ing ground for theories of magnetospheric dynamics,
because different systems present configurations and
conditions not found at Earth. Our understanding of
magnetosphere-solar wind physics will be seriously de-
ficient unless these extraterrestrial systems are explored
in ways that allow us to test our theories of their dynami-
cal behavior. Solar system exploration therefore needs
to provide avenues for observations of other magneto-
spheres.
In the past, major solar system missions could ac-
commodate planetary geology and atmospheric and
magnetospheric science payloads. This has not proven
to be true under the Discovery program, whose missions
are more highly focused. Instruments whose purpose is
to further the understanding of comparative magneto-
spheres have less appeal in the Discovery mission envi-
ronment than instruments that provide new information
on a particular solar system body. The Solar Terrestrial
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1 1 8
Probe missions and, to a greater degree, the Living With
a Star missions tend to focus on the Sun-Earth Connec-
tion rather than on comparative magnetospheres. The
panel encourages the Solar System Exploration and Sun-
Earth Con nection programs to coord i n ate thei r programs
in the upper atmospheres and magnetospheres of the
planets and to develop missions that address the out-
standing problems in these areas.
Ties Across Organizational Boundaries
As the science of the solar wind, magnetospheres,
and ionospheres matures and a new emphasis on serv-
ing the needs of space weather emerges, it is natural that
new programmatic structures are being adopted within
and between funding agencies. As these new structures
are adopted, however, it must be recognized that the
physical systems whose study is being overseen are
closely linked and do not respect administrative bound-
aries. These structures may be reasonable and based on
general distinctions between disciplines, but are none-
theless artificial constructions. Science disciplines must
be allowed the freedom to explore the linkages between
these physical systems. As new organizational structures
are adopted, the ties between subdisciplines must not
be lost, and research that spans administratively differ-
ent areas must not be allowed to fall through the cracks.
Often, cross-discipl inary research is not given priority
by either discipline and therefore languishes. Planetary
magnetospheres and solar wind interactions are an ex-
ample of disciplines where such a lacuna occurs. A for-
mal mechanism to fairly evaluate and support cross-
discipl inary research should be adopted. Broadening the
categories within NASA's SR&T program to allow mag-
netospheric and ionospheric research to be considered
together is commendable in this regard.6 Similar coordi-
nation between NSF's magnetospheric program i n its
Division of Atmospheric Sciences and its planetary mag-
netospheres and atmospheres research in its Division of
Astronomical Sciences would also be most welcome.
Steps appropriate to each case and agency need to be
taken to ensure healthy cross-disciplinary research in
other areas, including comparative magnetospheres,
solar wind-magnetospheric physics, and the transition-
ing of research to application tools.
6See NRC, 2000, "Interim Assessment of Research and DataAnalysis
in NASA's Office of Space Science," letter report, Sept. 22.
THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS
OPPORTUNITIES FOR SPACE MEASUREMENTS
IN ENTITIES OTHER THAN
NASA'S OFFICE OF SPACE SCIENCE
NASA's Office of Space Science provides the most
regu Iar opportunities to gain access to space through its
major missions and principal-investigator-led missions,
but there are other opportunities to have instrumenta-
tion carried into space. One of these is payloads at-
tached to the International Space Station (ISS); another is
launches by DOD or its foreign partners. In this section
the panel discusses issues related to these two opportu-
. .
n Itles.
Space Station Attached Payloads
The knowledge gained in studies of the interaction
of the solar wind with the magnetosphere and the en-
suing understanding of the entry of solar energetic par-
ticles into the magnetosphere is particularly beneficial
to the ISS and its occupants.7 On the other hand, the ISS
is not a natural or optimum platform for observing mag-
netosphere-solar wind interactions. It provides at best a
limited opportunity for space physics research, owing to
its orbit and facility configuration constraints. Fur-
thermore, the additional qualification and safety issues
pertaining to flight aboard a crowed vehicle add sig-
nificantly to the cost of development, further diluting
research resources. For these reasons, ISS is not a pre-
ferred platform for conducting magnetospheric physics
research.
The panel emphasizes that continued progress in
magnetosphere-solar wind interactions is of importance
to ISS. The space environment plays a significant role in
constraining ISS operations, as it does in constraining all
space-based technology assets. Thus, continued basic
research on the science of Earth's space environment is
a high priority for ISS even when space physics instru-
ments cannot be attached to the ISS per se.
Missions of Opportunity
Theabilitytooptimize the return on launch oppor-
tunities by funding individual researchers to build in-
struments for opportunities on non-NASA missions is an
excellent concept. Nevertheless, as presently executed,
it is not achieving its full potential.
7See NRC, 2000, Radiation and the international Space Station:
Recommendations to Reduce Risk, National Academy Press, Wash-
ington, D.C.
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PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS
To achieve the greatest science return, adjustments
need to be made in the mission of opportunity (MOO)
program to increase the frequency of these opportuni-
ties. The panel strongly endorses a change that would
separate MOO from the SMEX and MIDEX announce-
ments of opportunity, thereby allowing more frequent
consideration and implementation of MOO proposals.
To accomplish this change, the cost cap for MOO, in-
cluding attached payloads on the ISS, should be ap-
proximately halved, from $35 million to approximately
$15 million. This is one mechanism whereby launches
of opportunity with DOD could be more effectively le-
veraged for science. (The panel recommends semian-
nual considerations to provide a better match with the
frequency of such opportunities and with the develop-
ment times of the missions.)
SCIENCE IN THE STRUCTURE
OF PROJECT MANAGEMENT
The principal investigator model for missions has
proven highly successful in terms of science return on
the investment. One important reason for this success is
that science issues are given the same weight as space-
craft and mission design issues. Strategic missions such
as Solar Terrestrial Probes and Living with a Star mis-
sions could benefit from emulating some of the manage-
ment structure of these missions. A position of science
manager, equal in importance to the project manager,
should be established for future strategic missions. To
ensure the highest quality leadership, this position
should be selected competitively. The panel believes
that a science consortium lead by a competitively
selected PI would be another way to infuse science into
the management process.
INTERNATIONAL COOPERATION
Historically, research in space science, especially in
solar wind-magnetosphere interactions, has had a strong
international element. This international element arises
first from the need for globally situated, ground-based
measurements and then from the immensity of the task,
which requires a cooperative effort to obtain the critical
mass for its successful outcome. Recently, barriers have
arisen to mean i ngfu I i International cooperation.
ITAR and Export Controls
The International Traffic in Arms Regulations govern
the export of both information and equipment that might
1 19
be used by foreign entities against the United States. All
space-associated investigations are now included under
these regulations, which as implemented by the State
Department have placed substantial burdens on the
nation's space science community. These burdens are
manifested in two ways. Space physics missions have
always been conducted in close collaboration with our
international colleagues in Europe and Asia, primarily
Japan, and in Canada. The ITAR restrictions have made
it extremely difficult to continue working with these col-
leagues on U.S. missions like STEREO, in which interna-
tional contributions to the science payload are major
elements of the design. Even rudimentary essential infor-
mation concerning mission design concepts and space-
craft design plans has been subject to control, making it
extremely difficult, if not impossible, to involve our for-
eign colleagues in making fully informed scientific
judgements. It is simply impossible to properly design
and build a scientific instrument without free access to
relevant data on the spacecraft and mission design. The
problem is even more acute in cases where instrument
subsystems are provided by ou r foreign partners. One
cannot team effectively if the instrument designs to
which the team members contribute are sequestered.
The latter point suggests the second debilitating
effect that these new restrictions are having on the
nation's scientific community. The tremendous return to
the United States from participation in foreign missions
is illustrated by the SOHO (ESA), GEOTAIL (ISAS), and
CLUSTER (ESA) missions, which were implemented by
foreign agencies in Europe and Japan with significant
NASA instrumentation, operations, and science partici-
pation. Now, however, the burdensome impact on foreign
collaborating agencies has jeopardized opportunities to
participate in foreign missions in the future. It is even
harder to build an instrument jointly with our foreign col-
laborators. Clearly, the U.S. science community would
not be on an equal footing with its international colleagues
had it not been able to join them in these missions.
The ITAR situation is serious. Research scientists
have been subjected to criminal charges and penalties.
Consequently, some universities have refused to allow
their researchers to accept grants and contracts with
restrictive ITAR clauses. The inability to share informa-
tion among partners in a mission could lead to mistakes
and mission fai I u res.
An amended ITAR rule was published on March 29,
2002, which applies only to university-based space re-
search. The rule attempts to clarify the regulations and
to remove obstacles to the conduct of university-based
fundamental research in space. However, there remain
OCR for page 120
1 20
a number of serious practical problems with the new
rule, including continued restrictions on which students
and staff at a university can have access to information
and who in partner nations can gain access. The univer-
s iti es sti I I may fi n d the regu I ati ons too restri ctive and
ban thei r staff from enter) ng i nto such programs. More-
over, the revised statutes do not address the equally
serious problem namely, that U.S. universities cannot
work with the U.S. space industry without being subject
to ITAR regulations. Here the restrictions are even greater
than the restrictions on foreign collaborations.
Information Security
Presentfederal policies require all personnel having
access to NASA spacecraft and science payload com-
mand systems to have background security cheeks. This
is enforced by ensuring that contracts with universities
are consistent with NPG 281 0. This regulation requires
that any individual having access to a spacecraft or its
subsystems (such as science payloads) above a certain
value, including the computers used to command sci-
ence payloads, must be so screened. The universities
are not generally convinced that they can require this of
employees, especially those already hired. NASA's rul-
ing means that university computer systems managers,
project managers, and certain technicians and program-
mers must submit to background checks as part of their
i nstitution's contractual agreement with NASA on fl ight
projects. However, university mission participants typi-
cally have no access to spacecraft system commands or
controls. Firewalls are generally placed between the
external workstations from which commands are sent to
the science payload and the mission operations center
that sends them. The investigation teams historically as-
sume responsibility for the correctness of the commands
sent to their instruments on board the spacecraft, and
this has not presented a security problem in the past. For
a few low-cost missions, some academic institutions
have assumed ful I responsibility for operations and com-
manding. These missions present an information tech-
nology security conundrum, for they have been
extremely successful.
MODELING,THEORY, AND DATA ASSIMILATION
Modeling and theory need to be integrated into on-
going research. Because the terrestrial magnetosphere's
reconfiguration time scale is tens of minutes, far shorter
than satellite orbit periods of hours to tens of hours, data
sampl i ng i n Earth's magnetosphere wi 11 always be sparse
THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS
and i Incomplete. For th is reason, theoretical models and
global simulations play a crucial role by forming a
framework of understanding and context for the obser-
vations. The modeling and observations need to be wed-
ded closely via data assimilation in global models. This
willensure that the modelsareproperlyconstrainedby
the observations and can provide a suitable basis for
extrapolating the observations to characterize the state
and dynamics of the whole system.
Because of the central role that theory and global
models play, support for them needs to be robust and
sustained. Global simulation codes require teams of re-
searchers, each with specialized expertise in the under-
lying physics, in numerical techniques, in visualization,
and in user interfaces. To attract and maintain qualified
researchers for efforts of this scope, the efforts cannot be
supported by small (<$100,000) 3-year research grants
but must be supported by larger grants (>$300,000 per
year) for longer durations (5 years). It is also critical that
more than one code be developed and used, because
different techniques can sometimes lead to different be-
havior in the simulations, and comparisons between dif-
ferent codes are essential to identify consistent behavior
potentially reflecting the real behavior of the system.
Theory, simulation, and modeling will also become
increasingly instrumental in the planning and implemen-
tation of future missions. Understanding the character of
the measurements required and the degree of improved
understanding afforded by them and assessing the num-
bers and locations of observations to most efficiently
achieve definitive results will require detailed analysis
with models. Mission definition and design will there-
fore need to draw on the modeling resources of the
community. Reliance on models will continue through-
out each phase of future missions, including data analy-
sis and assimilation. The dependence of future mission
success on modeling underscores the need for sustained
and substantial support for this effort.
The increasingly integral role played by models in
data analysis implies that community access to models
is another aspect of the theory, modeling, and simula-
tion work that needs to be supported. As discussed in
the section "NOAA: Transitioning New Operational Ob-
serving Platforms and Models," modeling is an area that
is very appropriate for coordination with NOAA, which
needs operational models. Furthermore, even in the
arena of pure scientific inquiry, such coordination and
community availability are important. Under NSF the
GEM program has made initial strides in this direction,
but making state-of-the-art models available to the com-
munity remains a challenging task that requires re-
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PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS
sources as well. Attempting to achieve this objective by
requiring modeling teams to make their models avail-
able is manifestly the wrong approach, since these teams
are already hard-pressed to develop robust models.
Rather, the effort to convert research models into
community models is a separate task, which requires
support for developing interfaces and computational
architecture. In many ways the community model is an
intermediate step in the conversion of models to opera-
tional use. Support for this task should therefore derive
not only from basic science, which is its primary pur-
pose, but also from those agencies with an interest in
developing more robust, physics-based models for pre-
diction and forecasting.
Support for theory and model ing is therefore a natu-
ral area for interagency coordination. The benefits of
modeling extend across all areas of interest, from basic
science to prediction and forecasting, to mission devel-
opment and planning. All of the relevant agencies-
NSF, NASA, NOAA, and DOD have a vested interest
in maintaining a strong theory and modeling effort, and
they should find ways to coordinate their activities to
ensure that support is provided in a coherent way that
add resees the concerns descri bed above.
TECHNOLOGY DEVELOPMENT
The technological challenges for future solar-terrestrial
missions are substantial and will require an effort dis-
tinct from SR&T, including an SEC program similar to
the Planetary Instrument Definition and Development
Program of the planetary community. The primary chal-
lenge for future magnetospheric missions will be meeting
the need for constellation-class observations. For a new
generation of spacecraft, the task is to design and develop
a spacecraft architecture that can realize dramatic
economies of scale even in limited production runs (tens
of units). How low the ultimate cost per unit can go is not
known, but the cost of the Iridium satellites, which were
quite large, was ultimately reduced almost to $5 mil-
lion. There are no fundamental technological reasons
why a smaller platform could not be designed to cost
less than this, but the task faces significant systems engi-
neeri ng, management, i Integration, and testi ng problems.
It is not insurmountable, however, and is the type of ambi-
tious but achievable goal that should be a focus for NASA
or DOD. The New Millennium Program has been suc-
cessful in developing spacecraft technologies and it would
seem most appropriate for it to focus some of its techno-
logical investments on enabling constellation missions.
1 21
A comparable development effort is required to gain
the ability to deliver tens of calibrated scientific instru-
ments. Similar challenges of system engineering, man-
power management, integration, and testing activities
confront instrument builders contemplating the delivery
of large quantities of instruments. Again, although the
task is not an easy one, it does not appear to be impos-
sible, and an instrument incubator program would pro-
vide a mechanism to fund the long-lead-time develop-
ment of instrument technologies for this purpose.
For both the spacecraft and instrumentation devel-
opment efforts, proper consideration must be given to
an inherent feature of constellation-class missions-
namely, that the large number of spacecraft and mea-
surement points mitigates risk concerns and relieves the
demands on instrument performance. The risk to the
mission posed by the failure of a single spacecraft unit is
extremely low, because the science return from, say, 45
satellites is nearly the same as that from 50. Because the
redundancy is built into the constellation concept itself,
one can accept single-string concepts in the spacecraft
design.
In a similar way, the science return is enhanced
primarily by the large number of distributed measure-
ments rather than by the high precision of the measure-
ments, so that the requ i remeets for i nstru ment perfor-
mance relative to that demanded for single-satellite
missions should be critically examined. Experience with
non-science-grade instrumentation strongly suggests that
individual instruments performing at a much lower level
can yield dramatic scientific advances when deployed
in constellations.
Finally, innovative and commercial solutions to
spacecraft communications should be encouraged to
reduce mission operations costs. Requiring the use of
already overloaded systems such as the Deep Space
Network for satellite tracking and communications for
constel I ation missions is patently u nworkable because
of the enormous operating costs that such an approach
necessarily entails. Innovative, automated communica-
tions approaches exist for Earth-orbiting satellites; such
approaches were used very successfully for missions
such as Freja and FAST and are being applied for other
programs. These low-cost approaches to satellite com-
munications and tracking need to be expanded aggres-
sively to support constel ration missions.
DATA ANALYSIS, DISSEMINATION, AND ARCHIVING
The analysis, dissemination, and archiving of data
acquired from NASA and non-NASA missions as well as
OCR for page 122
1 22
from ground observatories and networks are of para-
mou nt i mportance to successfu I Iy ach ievi ng the science
advances descri bed above. G iven that the i nterrel ated
data sets to be acquired will be complex and more diffi-
cult to analyze than any acquired previously, the re-
sources devoted to their analysis will need to be more
substantial than those for earlier missions. The man-
power that needs to be brought to bear will be corre-
spondingly greater, and the best way of mobilizing this
expertise will be to ensure that the data are available
community-wide.
Data dissemination is therefore a key element of
future research that advances in information technology
have made much easier than in the past. The experience
with missions such as ACE, SOHO, and IMAGE demon-
strate that electronic dissemination of data works ex-
tremely well and facilitates community involvement in
their analysis. There is no reason this success cannot
carry over into the next decade with equal or greater
success.
Given that the missions envisioned in the coming
decade will not be superseded or repeated in the fore-
seeable future, the preservation of their data for
subsequent analysis is critically important. The standard-
ization system developed for the ISTP data exemplifies
the level of commonality that will be needed for these
new data sets. The standardization should be extended
to ground data sets as well, so that their community use
can be equally widespread. Standardization is also cru-
cial for preservation of the data sets. While it is expected
that the distributed data systems associated with differ-
ent investigators and investigations will be maintained
for some period of time after the prime mission or obser-
vation campaign, a centralized repository for the data
will also be required and needs to be supported. It is
almost certain that the number of basic issues that these
data can be used to resolve will not be exhausted in the
normal mission or observation lifetime of the spacecraft
or the facilities used to obtain the data.8
EXTENDED MISSIONS
It is widely recognized that extended missions can
provide a high science return for modest additional
investment, and they are strongly encouraged. The panel
See NRC, 2002, Assessment of the Usefulness and Avai/abi/ity of
NASA's Earth and Space Science Mission Data, National Academy
Press, Washington, D.C., pp. 41~4.
THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS
endorses the practice of giving priority to those candi-
dates for extension that most clearly support new
research missions and strengthen or expand the science
achieved. However, the costs of mission operations and
data acquisition could be reduced considerably if track-
ing and communications for extended missions could
be transferred to commercial or academic institutions at
the discretion of the mission PI or project management.
The use of this option is consistent with the philosophy
of extended missions since their prime mission objec-
tives would already have been achieved, fulfilling their
intended charter. If the cost of extending missions could
be significantly reduced and the pressure on mission
operations and data analysis resources relieved to allow
more simultaneous operations, a broader array of pro-
ductive observatories could be maintained for magneto-
sphere-solar wind interaction science.
ADDITIONAL READING
A strategy for the conduct of space physics research
has been set down in a number of reports by the NRC's
Space Studies Board and its predecessor, the Space Sci-
ence Board. These reports i nc I ude the fo I I owi ng:
Space Science Board, National Research Counci 1.
1985. An Implementation Plan for Priorities in
Solar-System Space Physics. National Academy
Press, Washington, D.C.
Space Science Board, National Research Counci 1.
1983. The Role of Theory in Space Science.
National Academy Press, Washington, D.C.
Space Science Board, National Research Counci 1.
1 980. Solar-System Space Physics in the 1980's: A
Research Strategy. National Academy of Sciences,
Washington, D.C.
Space Studies Board, National Research Counci 1.
1995. A Science Strategy for Space Physics.
National Academy Press, Washington, D.C.
Space Stud ies Board and Board on Atmospheric
Sciences and Climate, National Research Council.
1991. Assessment of Programs in Solar and Space
Physics—1991. National Academy Press,
Washington, D.C.
The research in this field is summarized in both
textbooks and conference proceedings, including the
fol lowi ng:
OCR for page 123
PANEL ON SOLAR WIND AND MAGNETOSPHERE INTERACTIONS
M.G. Kivelson and C.T. Russell (eds.~. 1995.
Introduction to Space Physics. Cambridge
U n ive rs ity P ress, N ew Yo rk.
A. Nishida, D.N. Baker, and S.W.H. Cowley (eds.~.
1998. New Perspectives on the Earth's
Magnetotail. Monograph 105. American
Geophysical Union, Washington, D.C.
B. Hultqvist, M. Oieroset, G. Paschmann, and R.
Treumann (eds.~. 1999. Magnetospheric Plasma
1 23
Sources and Losses. Kluwer Academic Publishers,
Dordrecht.
S.l. Ohtani, R. Fujii, M. Hesse, and R.L. Lysak. 2000.
Magnetospheric Current Systems. Monograph 118.
American Geophysical U n ion, Wash i ngton, D.C.
P. Song, H.J. Singer, and G.L. Siscoe (eds.~. 2001.
Space Weather. Monograph 1 25. American
Geophysical U n ion, Wash i ngton, D.C.
Note added in proof: New Horizons, the first Pluto probe, has been
selected as the first mission in NASA's New Frontiers program and is
now in development. The probe, which will arrive at Pluto in 2015,
carries solar wind plasma and energetic particle detectors in addition
to its suite of remote sensing instruments and a dust experiment. In
addition to its reconnaissance of the Pluto-Charon system, the probe is
expected to encounter one or more Kuiper Belt objects.
OCR for page 124
Representative terms from entire chapter:
magnetic field
