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A Science Strategy for Space Physics: Chapter 3
A Science Strategy for Space Physics
3
The Structure and Dynamics of Magnetospheres and Their
Coupling to Adjacent Regions
SCIENTIFIC BACKGROUND
When the solar wind reaches a magnetized planet such as Earth, this collisionless
plasma interacts with the magnetic field, initiating a complex process through which the
solar wind can ultimately influence the upper reaches of the atmosphere. The elements of
the magnetosphere and its space environment are presented schematically in Figure 8.
The streaming solar wind compresses the magnetosphere on the dayside and stretches it
out on the nightside in a long anti-sunward tail structure. The interaction between the solar
wind, the magnetosphere, and the atmosphere produces a number of plasma and
magnetic field structures, including the magnetopause, magnetosheath, boundary layers,
cusp region, plasmasphere, ring current, plasma sheet, magnetotail, and magnetotail
REPORT MENU lobes. Each of these regions maps along magnetic field lines into the atmosphere-
NOTICE
ionosphere system and is the origin of energetic inputs that perturb the state of the
MEMBERSHIP
plasmas and neutral gases. The transport of mass, momentum, and energy into and
SUMMARY
through magnetospheres and ionospheres is a fundamental yet poorly understood
PART I
process. The study of the coupling between magnetospheres and ionospheres can be
PART II
characterized as a synthesis of in situ data collected over a period of years into an
CHAPTER 1
intriguing picture of a basic astrophysical process. Yet space-time ambiguities and the
CHAPTER 2
problems associated with having only single or limited multipoint observations to study a
CHAPTER 3
large and highly dynamic system have resulted in a view of the magnetosphere-
CHAPTER 4
ionosphere system that is necessarily schematic. A global perspective requires that the
CHAPTER 5
local small-scale plasma processes be placed in the context of the large-scale system,
PART III
and ideally, provides a connection between small-scale processes occurring
APPENDIX
simultaneously in widely separated parts of the magnetosphere-ionosphere system.
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FIGURE 8 A diagram of the Earth's magnetosphere, showing the flow
of the solar wind through the bow shock (where its flow goes from
supersonic to subsonic) and around the current sheet known as the
magnetopause, which forms the boundary between the magnetosphere
and the solar wind. Through interconnection of magnetic fields and
through entry of particles across the magnetopause, solar wind mass,
momentum, and energy are transferred to the magnetosphere. Some of
this energy ultimately finds its way into the Earth's atmosphere.
(Courtesy of Donald Mitchell, Applied Physics Laboratory, Johns
Hopkins University.)
The magnificent images of vast astrophysical plasma systems, a familiar example
being the Crab nebula, clearly laced with magnetic fields, hot gasses, and highly
energized charged particles, have captured the interest of both the public and scientists.
However, those images and spectral data are the only means of studying the complex
processes operating in those objects, because in situ measurements in the nebula are not
possible. In contrast, quite the opposite condition holds in the case of planetary
magnetosphere-ionosphere systems. Over the past 38 years, many measurements have
been made of the constituent gases, fields, and plasmas within the Earth's and other
planets' magnetospheres. However, with only a few exceptions, the global structures of
those astrophysical plasma systems have remained invisible, and the external influences
on and interrelationships between the highly differentiated regions of those systems have
been only partially understood.
Computer modeling and simulations are a valuable aid to deducing the global
structure of the magnetosphere-ionosphere system from the point-by-point observational
data. Although these models are invaluable for providing a global setting for local satellite
measurements and suggesting directions for future studies, there are not yet global
observations to assess the accuracy of the data syntheses and the models. The first step
in improving this situation has been the use of satellite auroral images (such as those from
DMSP, Dynamics Explorer, Viking, and Freja) to get a near-global view of the polar
atmosphere that functions as the sink for a significant fraction of the energy transferred
from the solar wind to the magnetosphere.
Data from several satellites (in particular, ISEE and AMPTE) have enabled
identification of many of the important links in the transfer of energy, mass, and
momentum from the solar wind and have highlighted the complex, nonsteady nature of the
physical processes. Space physicists now know that transport of solar wind energy and
momentum takes place across narrow boundary layers that separate regions with very
different plasma conditions. The importance of these boundary layers in transport
processes and particle acceleration in several regions of the magnetosphere, such as the
plasma sheet, has also been identified. Phenomenological models of the global structure
and dynamic processes developed from these satellite data are the basis of our present
understanding of the magnetosphere.
The outer boundary of the magnetosphere (see Figure 8), called the
magnetopause, is characterized by a number of transient phenomena and by two major
boundary layers, one at low latitudes and one near the poles. Both of the boundary layers
are important sources of plasma for the magnetosphere, but the mechanisms forming the
boundary layers themselves are still not fully understood. Possible plasma entry
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A Science Strategy for Space Physics: Chapter 3
mechanisms into the magnetosphere include diffusion and magnetic reconnection.
Transient phenomena, such as Kelvin-Helmholtz waves, flux transfer events, and
magnetic field perturbations due to small-scale pressure changes in the solar wind, also
contribute to the entry of mass, momentum, and energy into the magnetosphere.
The steady merging of solar wind and magnetospheric magnetic fields, once
thought to be the predominant entry process, does not seem to be sufficient to account for
all of the features suggested by the data. The simple picture of steady-state merging has
been replaced by one of transient, patchy, and small-scale merging with associated
current systems that map to the polar cap and churn up the ionosphere. The evidence for
reconnection at the magnetopause is very strong, although the microphysics of the
reconnection process is still not understood. The interaction of the solar wind with the
magnetosphere also sets up a convection system whereby plasma and "frozen-in"
magnetic field lines flow from the dayside of the magnetosphere to the tail and then return
to the dayside. While the large-scale flow features have been confirmed by observations
in the polar ionosphere, the specifics of the flow patterns throughout the magnetosphere
are lacking. As would be expected when the input is transient, the convection is not
steady. In fact, recent studies indicate that plasma flow in the magnetotail is usually bursty
and irregular.
Two examples of dynamic processes that involve dramatic changes in the entire
magnetosphere are magnetic storms and substorms. A magnetic storm is a period of
enhanced geomagnetic activity, typically lasting many hours to days. During this period,
particles are injected into the outer Van Allen belts to form an intense magnetospheric ring
current that depresses the geomagnetic field at low latitudes. A portion of the ring current
connects with the ionosphere, where it produces intense magnetic perturbations. Large
magnetic storms can cause significant changes in the inner magnetosphere that are
intimately associated with the lowest-latitude occurrences of the aurora. For example,
recent observations by CRRES of the sudden formation of a "belt" of very energetic
particles (tens of MeV) show that large magnetic storms can cause major, long-lived
changes deep in the inner magnetosphere. SAMPEX continues to study the creation of
transient belts as well as their decay via precipitation into the atmosphere.
Magnetic storms and substorms are known to cause premature loss of
communication satellites, disruption of radio communications, currents flowing in pipelines
and cables, and interruption of electric power to consumers due to power-grid failures.
Until recently, little attention was paid to magnetic storm events by the magnetospheric
community, but observations from near-geosynchronous spacecraft such as CRRES now
indicate that many aspects of magnetic storms are either poorly understood or not known.
These include magnetic storm effects on the geomagnetic tail configuration, the temporal
evolution in the processes of solar wind entry, and the energization of plasma from the
ionosphere during storms.
In a substorm, the energy stored in the magnetic field of the tail is released
explosively. The most frequently discussed substorm model has been magnetic
reconnection in the region between 10 and 30 Earth radii downstream of the Earth.
However, recent observations of substorms that are triggered on field lines at distances of
6 to 10 Earth radii seriously challenge this model and raise important theoretical
questions. Moreover, the recent plasma observations at synchronous orbit show very
different features from the familiar energetic-particle injections during substorms and are
providing new views of the dynamics of energy transport into the inner magnetosphere.
These observations are not fully understood. The types of physical processes that occur in
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the near-Earth magnetotail region during substorms remain one of the major unresolved
issues of magnetospheric physics.
During substorms about 10% of the energy transferred from the solar wind into the
magnetosphere is deposited in the auroral ionosphere. A significant fraction is deposited
in the ring current and the midlatitude atmosphere, and some is also convected through
the magnetopause into the magnetosheath. Limited studies of the deep tail made by ISEE-
3 indicate that the rest of the energy flows down the tail and eventually back into the solar
wind. Recent results from the Geotail mission are clarifying and expanding upon this initial
picture. Because the structures in the distant tail can be complicated due to the tail's
"memory" of the recent history of solar wind/ magnetosphere interactions and substorms,
much work remains to be done to understand the physics of this important, yet largely
unexplored, region.
The coupling process between the magnetosphere and the ionosphere is complex
and highly variable. Large-scale current systems, precipitating charged particles, and
electric fields are the intermediaries that transport a significant portion of the energy and
momentum resulting from the solar wind/magnetosphere interaction into the ionosphere.
Precipitating particles from the magnetosphere are the major source of ionization in
nightside midlatitude and polar regions. Those particles consist of keV ions and electrons
in the auroral zone, lower-energy ions and electrons in the polar cap regions, and
relativistic electrons and ions at tens to hundreds of keV at subauroral latitudes.
Currents and electric fields in the magnetosphere also play an important role in the
magnetosphere-ionosphere (MI) coupling. The million-ampere magnetospheric currents
flow into the ionosphere where they heat the thermal plasma and neutral gas. The
ionization level governs the conductivity of the ionosphere and establishes the relationship
between current and electric field. Magnetospheric energy is dissipated in the neutral
atmospheric constituents in three ways: (1) heating due to particle precipitation, (2)
heating from the dissipation of currents, and (3) energy and momentum transfer to the
neutral molecules from collisions with ionospheric ions drifting in electric fields. The
resulting changes can lead to feedback effects in the magnetosphere. For example, the
large-scale winds of neutral gas that persist even after the magnetospheric driver is turned
off can drag ions across magnetic fields and may modify and even create new electric
fields and current patterns in the magnetosphere. Another MI coupling effect comes from
the flow of a large number of ionospheric ions up along the magnetic field lines into the
magnetosphere. These ions become energized by processes as yet unidentified and
could play important roles in MI coupling and magnetic storm and substorm dynamics.
Current understanding of these processes is based on many prior missions, including the
Dynamics Explorer mission. Other MI coupling issues related to feedback of atmospheric
and ionospheric effects into the magnetosphere are discussed in Chapter 4.
Not much is known about the transport processes in other planetary
magnetospheres. The intense auroral emission observed from the jovian polar regions
indicates robust MI coupling, but the identity of the precipitating particles (electrons,
protons, oxygen or sulfur ions from Io) responsible for those emissions and the energy
distribution of the particles have not yet been discovered. Researchers are even further
away from knowing what acceleration mechanisms operate. The dependence of outer
planetary aurora on solar wind coupling as opposed to internal dynamics, such as
planetary rotation and interaction with satellites, is not yet established. The Cassini
mission should help explain some of these unique interactions at Saturn; Figure 9 displays
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an artist's conception of the saturnian magnetosphere based on the data from several
previous flyby missions.
FIGURE 9 Artist's conception of the saturnian
magnetosphere, including major plasma structures
and sources and sinks of ions from the solar wind,
Saturn, and its rings and satellites. (Courtesy David
McComas, Los Alamos National Laboratory.)
The following questions address both the flow of mass, momentum, and energy
from the solar wind into and through the magnetosphere-ionosphere system and the
impacts on the global structure and dynamics of this coupled system.
How does the magnetic reconnection process operate under different
boundary conditions? What role does reconnection play in the formation of the
magnetopause boundary layers?
How does the solar wind plasma enter the magnetosphere? How important is
reconnection compared to diffusion and direct entry processes?
How does the magnetospheric convection respond to changes in the solar
wind and interplanetary magnetic field? How is the magnetospheric convection affected by
the highly variable ionization in the ionosphere and winds in the upper atmosphere?
Where and how are substorms triggered and magnetic storms driven? What
are the relative roles of ionospheric and solar wind plasma sources? How do small and
mesoscale auroral features result from such global processes?
How do the properties of both the near-Earth and the distant geomagnetic tail
change during substorms and magnetic storms? Do local processes affect the global
structure of the magnetosphere?
What are the physical links between the component parts of the
magnetosphere, shown in Figure 8, such as the auroral electron precipitation regions,
auroral upflowing ion regions, the plasmasphere, the ring current, boundary layers, and
plasma sheet regions?
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What processes transport plasma between the ionosphere and the
magnetosphere? Do the transport processes involve turbulent dynamics? Are electrons
and ions transported by similar or different processes? How is ionospheric plasma
energized in the magnetosphere?
What are the global distributions of electric fields, current systems, and
charged particles in the MI coupling region? How are magnetospheric and ionospheric
electric fields and currents set up and how do they evolve over time?
What is the accuracy of existing global models and simulations? What
processes or structures do they fail to model? Does the magnetosphere have a ground,
lowest-energy quasi-static state? If so, what is it?
CURRENT PROGRAM
The International Solar-Terrestrial Physics (ISTP) program was specifically
designed to study the flow of mass, energy, and momentum through the combined solar
wind and magnetosphere systems. ISTP will employ an armada of spacecraft in the solar
wind, magnetotail, and regions that cover the auroral zones and polar caps as shown in
Figure 10. ISTP encompasses such spacecraft as NASA's Wind and Polar, the Japanese
Geotail, and the European Cluster and SOHO. Additional contributions to ISTP will come
from NASA's FAST and the Russian Interball-Aurora and Interball-Tail; from the older
operating spacecraft, Akebono, Freja, IMP-8, DMSP, and GOES; and from the NOAA and
Los Alamos geosynchronous satellites.
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FIGURE 10 The locations of some elements of the International Solar-Terrestrial Physics program relative to major
features of the Earth's magnetosphere (compare with Figure 8). Each spacecraft is designed to measure and enhance our
understanding of successive links in that energy, mass, and momentum flow through the magnetospheric and
atmospheric system. (Courtesy of Donald Mitchell, Applied Physics Laboratory, Johns Hopkins University.)
The Wind spacecraft will characterize the conditions in the solar wind. The Polar
mission has the capability to continuously image the global auroral oval, and FAST will
observe and characterize microphysical processes taking place in the auroral ionosphere.
In addition, ISTP includes coordinated ground-based observations of ionospheric
convection using radar, and mission-oriented theory teams to tackle the physics and
model the processes. There will be special emphasis on determining the connection
between the microphysics and the physics on global scales.
Some of the ISTP spacecraft can change orbit to provide sequential coverage in
more than one region. In particular, the Geotail spacecraft will be moved closer to Earth so
that it has an apogee near 30 Earth radii in order to examine reconnection processes in
the near-tail region. It will be supported by Interball-Tail at 30 to 40 Earth radii and by
spacecraft in geosynchronous orbit. By the time Geotail gets to the near-Earth tail, it is
planned that Polar will be orbiting above the polar caps and will provide observations of
auroral particles and fields as well as imaging. In addition, the FAST spacecraft to be
launched in 1995 will provide in situ observations of the particles and fields at low polar
altitudes.
Geotail and Interball-Tail will provide observations of the low-latitude boundary
layer on the flanks of the geomagnetic tail. Cluster and Polar will provide observations of
the high-latitude boundary layer. On the dayside parts of its near-tail orbit, Geotail will
observe the dayside low-latitude boundary layer and provide information on transient low-
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latitude phenomena at the dayside magnetopause. An extended Polar mission would also
observe the dayside magnetopause. Taken together, the set of Wind, Polar, Geotail, and
Interball spacecraft, along with low-Earth-orbiting and synchronous-orbit spacecraft, will
address the mechanisms involved in the global flow of mass, momentum, and energy
across the boundaries of the system. The four co-orbiting Cluster spacecraft will enable
researchers to observe the gradients associated with the boundaries. This will provide
observations of the boundary structures, how they evolve in time, and the processes
operating there.
Concurrent with the ISTP program, the NSF's CEDAR and GEM programs will use
theory and available ground-based and satellite data to develop a global geospace
environment model. GEM and CEDAR, along with NASA's Space Physics Theory
program, will provide global models and simulations for investigating magnetospheric
behavior. All will require stringent observational tests of their global predictions.
Several international and national organizations have established programs to
increase the value returned from magnetospheric programs through the coordination of in
situ observations, ground-based observations, data exchanges, and modeling and
simulation studies. Among those efforts are the campaigns being organized by the
Interagency Consultative Group and the international Solar-Terrestrial Energy Program.
Although the ISTP is potentially the most productive magnetospheric program ever
mounted, there are some concerns. The ISTP concept will work only if the required
simultaneous multipoint observations can be made. Also, the program has had many
delays in key elements, and it is not clear whether the spacecraft will all fly in time to make
the required simultaneous observations. In addition, several key observations will be
missing. The regions originally chosen for detailed in situ sampling formed the minimum
set required to infer large-scale features, but with the loss of the equatorial satellite
(Equator), this minimum set no longer exists and there is a serious gap in the observing
strategy. At this writing no replacement spacecraft is planned for this key region in the
near-Earth equatorial tail between 8 and 15 Earth radii. This gap represents a double loss.
It will not be possible to make observations in an important region where the substorm
current disruption may be occurring in the tail. Adding to the science loss, the orbit that
would have allowed a spacecraft to probe the near-Earth tail would also have enabled the
spacecraft to spend much of its time on the dayside near the magnetopause where it
could observe transient events at the dayside magnetopause and in the low-latitude
boundary layer.
The ISTP and FAST are also expected to make major contributions to our
knowledge of MI coupling. The ISTP will add to our understanding of the magnetospheric
input to MI coupling, of global electrical currents, and of the relation between global
magnetospheric dynamics and conditions in the solar wind. FAST, passing through the
low-altitude auroral acceleration region, will be able to address specific questions
concerning what accelerates auroral particles, how parallel electric field and currents are
established and regulated, and what specific instabilities and wave-particle interactions
are involved.
The Earth's magnetosphere is not the only magnetospheric focus. Observations of
the jovian aurora from the International Ultraviolet Explorer, the Hubble Space Telescope,
and the Rosat x-ray satellite contribute to the study of MI coupling at Jupiter. The Galileo
mission should also help clarify our understanding of the magnetospheric end of the jovian
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MI chain. Similarly, the Cassini mission to Saturn and its satellite Titan will yield new data
on MI coupling for both Saturn and the interaction of Titan's ionosphere with the
magnetosphere of Saturn (see Figure 9). In addition to in situ measurements of particles
and fields, images of some parts of Saturn's magnetosphere will be obtained. Besides the
optical imagers, the Cassini payload also contains an instrument that will provide images
of the regions of the saturnian magnetosphere that emit energetic neutral atoms. Those
data will in turn yield large-scale views of Saturn's magnetospheric energetic particle
populations and their time variations. Although Cassini will not reach Saturn until 2004, it
may obtain the first magnetospheric observations that are truly global in nature.
FUTURE DIRECTIONS
By far the highest priority for understanding the processes that mediate the
transport of mass, energy, and momentum through the magnetosphere-ionosphere
system is the successful completion of the ISTP program. This includes ensuring that the
planned satellite elements are launched in time for the required simultaneous
measurements. The committees also strongly support efforts to recover the science lost
when the original Equator satellite was canceled. It is also vital that the data analysis
phase be sufficiently long and adequately funded, including support of scientists not
currently on the instrument teams (i.e., "guest investigators") and extending well beyond
the end of the mission operations. The data obtained in this program will be extraordinarily
rich and complex, and the full scientific value will not be realized if any aspect-mission
operations, the ground-based component, or data analysis-is cut short.
Global imaging of the magnetosphere is the next-highest priority for
magnetospheric investigations. Its importance has long been recognized and it should be
the next observational thrust in the field. Current techniques developed for imaging and
remotely sensing the magnetopause, the plasmasphere, and other plasma and energetic
particle distributions and boundaries in the magnetosphere as well as in the aurora have
the potential to provide important new insights about the workings of planetary
magnetospheres as well as making this research more intelligible to the public. With the
exception of auroral imaging, however, the technology has not received the flight
opportunity it deserves. It is clear that remote sensing of the Earth's magnetosphere will
provide information on its global dynamics and structure. It will also generate the data for
refinement of inversion schemes and neutral geocoronal models needed to process the
images. Figure 11 is the result of a simulation of magnetospheric images taken under
storm-time conditions.
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FIGURE 11 Simulations of images of the Earth's magnetosphere during the main phase of a magnetic
storm, along with an auroral image taken during the main phase of the magnetic storm which provided the
model input for the magnetospheric images. The auroral image was taken by the University of Iowa far-
ultraviolet auroral imager on the Dynamics Explorer 1 spacecraft on April 22, 1988 (courtesy of L.A. Frank).
Clockwise from upper right, the simulated images (which include a black circle indicating the position of
Earth) are as follows: (1) an image of the plasmasphere taken in the emission of singly ionized helium at 304
angstroms, using a plasmaspheric model that includes a feature known as a plasma tail at the
plasmaspause. The Sun is to the upper left of the box, resulting in reduced emission from the shadowed
region behind the Earth. (2) the magnetosphere as imaged in low-energy neutral atoms. This simulation
shows the emission of ~5-keV hydrogen atoms produced in charge exchange interactions between plasma
protons and the cold hydrogen exosphere/geocorona. Signatures of ion heating in the auroral zone and
beams of up-flowing ions are indicated. (3) the magnetosphere as imaged in energetic neutral atoms. This
simulation shows the emission of ~30-keV hydrogen produced by the same mechanisms as (2). The region
labeled "transport" indicates a feature in the image produced by the dynamics of the drifting ion distribution
in the Earth's ring current, a transient feature of the magnetic storm. (Simulation images courtesy of Edmond
C. Roelof, Johns Hopkins University, Applied Physics Laboratory.)
Detailed measurements are needed for MI coupling studies in the critical altitude
regime from 5000 to 20,000 km, where the bulk of the auroral acceleration occurs. Active
experiments might be considered to investigate how natural processes work. For example,
a combination of an electron gun and wave-particle instrument package for diagnostics
launched on a rocket could provide detailed information on electric field structures for
accelerating auroral electrons along the geomagnetic field and might also shed light on
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causes of the complex and dynamic mesoscale structures that are observed in auroral
arcs. Other methods that hold promise for unraveling the details of the magnetospheric
response to solar inputs center on the interpretation of ionospheric signatures of large-
and small-scale currents, precipitation patterns, and plasma drifts. These ionospheric
manifestations of important magnetospheric processes can be monitored and studied
using strategically placed ground-based radars, magnetometers, and spectrometers.
Coordinating these observations with spacecraft observations that supply information on
the linkages of these ionospheric signatures, and the magnetospheric processes that
generate them, can provide important global information on the properties of the
magnetopause, magnetotail, and boundary layer. The relatively low cost and high
scientific return of programs of this nature make their development and implementation a
high scientific priority.
New knowledge about physical processes can also be obtained by studying
magnetospheres where one or more of the boundary conditions are different from those at
Earth. For example, the role of MI coupling in substorms should be examined at Mercury,
which does not have an Earth-like ionosphere. In addition, the smaller spatial and shorter
temporal scales of the hermean system are such that intrinsic time scales of processes
such as tail instability and reconnection at the dayside can be examined more effectively,
because the solar wind is more likely to remain constant for the duration of the event than
is the case for the Earth's magnetosphere. Outer planets, such as Jupiter or Saturn,
provide situations where plasma sources other than the ionosphere and solar wind are
important and where energy sources other than the solar wind may dominate. In situ and
global imaging observations of the magnetospheres of other planets will be important in
furthering our understanding of space plasma processes. Radio techniques offer another
ground-based diagnostic for monitoring radiation belts and thermal plasma properties;
spaceborne lightning detectors are capable of making contributions related to auroral
kilometric radiation as well as providing a method for remote sensing of auroral processes
and atmospheric electricity on other planets.
Several regions of the Earth's magnetosphere are currently underexplored. For
example, multispacecraft missions to skim the dayside magnetopause near the equator
from dawn to dusk or from pole to pole would greatly enhance our understanding of the
important physical processes contributing to the flow of energy, mass, and momentum
from the solar wind into the magnetosphere on many different plasma scale lengths.
Similarly, the auroral acceleration region (near altitudes of ~1 Earth radius) requires
multispacecraft measurements with orbit maneuver capabilities, as does the near-Earth
tail (8 to 12 Earth radii). These regions will not be addressed by the ISTP mission,
especially not on the small scale where microphysics is addressed. Global imaging, in
conjunction with in situ observations, leads to a more comprehensive view of any
magnetosphere than multipoint satellite observations alone can provide.
All future magnetospheric observations would benefit from new technology
development including design of smaller, lighter, more capable instruments and
sophisticated data compression schemes. For example, new types of instruments are
being developed that will provide enhanced global imaging of the magnetosphere. These
include experiments to attempt radio sounding of magnetospheric boundaries. It is
important that opportunities for new instrument development and flight testing be
available.
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