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OCR for page 17
3
Status Expected in 1995
SOLAR AND HE[IOSPHERIC PHYSICS
The subject areas of solar and heliospheric physics encompass
a broad range of physical processes; by 1995 we may anticipate
continued progress in a number of these areas: the structure and
dynamics of the solar interior; variations In the solar luminous
output; the emergence and evolution of solar surface magnetic
fields, including solar flares; energy and momentum input to the
solar corona; global structure and evolution of the heliosphere;
and microscopic plasma processes.
Helioseismological studies of the structure and dynamics of
the convection zone, the temperature and molecular weight dis-
tribution throughout the interior, as well as the radial and lati-
tudinal variation of rotation should be initiated by ground-based
networks of instruments as well as instruments on board the solar
satellite of the ISTP. However, if our most optimistic expectations
of the progress of these projects and their subsequent analysis were
justified, we would be only beginning to investigate the possible
variation of these properties through the activity cycle.
The specification of the solar interior properties from helioseis-
mological techniques will provide crucial new input into models of
17
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18
the solar dynamo. Indeed, progress in such modem fundamental
to our understanding of basic solar cyclic variations will be stimu-
lated by observational progress. The role of dispersal and diffusion
of small-scale magnetic fields in the dynamo process (at least as
reflected by surface fields) awaits clues from extended temporal
observations of the successor to the SOT (see below).
Solar irradiance will have been monitored on an episodic basis
and the solar luminosity will have been monitored on a more con-
tinuous basis over more than a complete solar cycle. However, it is
unlikely that the solar radius will have been monitored accurately;
this, together with the lack of accurate specification of the solar
interior properties makes it unlikely that the issues concerning the
storage and release of luminous energy (essential in understanding
the transport of energy through the solar atmosphere) will have
been resolved in a quantitative way.
Extraordinary progress in our view of the fine structure of
the solar photosphere and chromosphere will be made by Spacelab
2, SunIab, and the successor to SOT. It may be anticipated that
fundamental new observations of the interaction of solar plasmas
and magnetic fields on the spatial and temporal scales at which the
basic physical processes occur will have been obtained. To study
and understand these processes (for example, changes in magnetic
field strength, waves, single pulses, and systematic mass flows),
it will be necessary to resolve spatial scales over which significant
gradients occur in the local magnetic and nonthermal velocity
fields, as well as in the local densities and temperatures. Studies
of the plasma-magnetic field interactions on these spatial scales
will be directed toward understanding these most fundamental
physical processes that can be observed in the solar atmosphere.
The Pinhole Occulter Facility (POF) may be operational by
1995. If so, it will allow hard x-ray observations with a similar
spatial resolution as that of SOT. However, no similar capability
is anticipated for XUV and EUV spectral observations, or for the
crucially -important high-energy flare particle- signatures. As a
result, one may expect limited progress in our understanding of
the flare process, which seems to take place also on spatial scales
of a few hundred kilometers.
In coronal physics, by 1995, several views of the global proton
temperature distribution will have been obtained by new instru-
mentation, from SPARTAN and SOHO, at least in the region be-
yond 1.5 solar radii. Inferred coronal outflow speeds necessarily
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19
less reliable wiD follow. Thus, our understanding of the physics
of the near-solar-w~nd acceleration wait remain largely Incomplete,
as will our understanding of the origin of the energy flux from
the lower solar atmosphere required to drive the wind. Unfortu-
nately, little new information on the three-dimensional structure
of the corona wiD be available by 1995, so all of the long-standing
issues concerning the solar longitudinal density distribution of
the corona and also the newer temperature and outflow speed
distribution~will remain unresolved. Despite intensive efforts,
there wiD remain a paucity of observations of the initiation of
coronal mass ejection phenomena, and suitable diagnostic infor-
mation on the structure of these events will remain unobserved.
Hence, most of these fundamental issues will be unresolved.
Our understanding of the three-dimensional structure of the
heliosphere will be improved by the flight of the ISPM, but a
second passage of the ISPM will be critical to gaining a reason-
able knowledge of latitudinal variations of heliospheric structure
during the period of the solar cycle when these variations are ex-
pected to be significant. We can also expect some improvement in
our knowledge of the latitudinal variation of solar wind mass flux
and flow speed (away from solar maximum) from EUV and radio
observations that remotely sense the interplanetary medium. OF
servations of the Mutant solar wind will be extended to beyond 60
AU, and our present view of distant stream evolution is likely to
be confirmed, but it is not clear whether the termination of super-
sonic. solar wind flow will be observed by the Voyager 1 spacecraft,
which will have gone some 50 AU in the direction of the solar apex.
Significant progress can be expected in two areas of study in-
volving rrucroscopic plasma processes in the interplanetary medi-
um. Continued theoretical work on the problem of heat conduc-
tion and viscosity In a thermally driven stellar wind not domi-
nated by Coulomb collisions should advance to the point where
lack of in situ observations of the inner solar wind would pre-
vent further progress. A combined observational and theoretical
effort toward understanding the acceleration of particles at inter-
planetary shocks should meet with some success and provide an
improved basis for understanding energetic particle populations in
interplanetary space and other astrophysical systems.
We can expect to see some progress in the study of the role
of corotating interaction regions on the interplanetary modulation
of galactic cosmic rays. There ~ no reason to believe that we
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20
will learn much more about any other modulation mechanisms by
1995.
MAGNETOSPHERIC PHYSICS
The solar system offers a variety of magnetospheres for study.
These magnetospheres have sufficiently different internal parame-
ters and boundary conditions to allow testing of the universality
of basic magnetospheric concepts. The solar wind interacts with
planetary magnetic fields to produce diverse phenomena that in-
volve the storage and energ~zation of charged particles. Also, a
small fraction of the energy within a magnetosphere is converted
into a broad assortment of radio emissions. Differences and simi-
larities between magnetospheres, if properly un~lerstood and for-
mulated, yield physical principles that can be applied throughout
the universe.
Sources of Plasma
The solar wind, anmediately after its existence was estate
fished, was thought to be the source of plasma, particularly en-
ergetic plasma, within the Earth's magnetosphere. Nearly two
decades later it was discovered that the ionosphere was a source of
quantitatively significant flows of plasma into the magnetosphere.
Then, consistent with the discovery that the solar wmd is a negligi-
ble source of plasma for the magnetospheres of Jupiter and Saturn,
experimenters have interpreted recent data from the DE satellite
as indicating that perhaps most of the Earth's magnetospheric
plasma may be supplied by the ionosphere. This question may
be partially resolved by AMPTE, which can give a quantitative
indication of the ability of solar wind, magnetosheath, and tad]
plasma to enter the inner magnetosphere, and ISTP, which will
make simultaneous, coordinated measurements in the solar wind
and the magnetosphere.
Sources of Power
Phenomena within the Earth's magnetosphere are driven by
power extracted from the solar wind. The solar wind, as it passes
the Earth, drives plasma within the magnetosphere in a circula-
tion pattern that energizes some of the magnetospheric plasma
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21
and drives strong magnetically field-aligned currents that connect
to the auroral zones. Magnetic merging, in which topologically
separate magnetic regions are connected and magnetic energy is
released, is still a poorly understood process that is central to
most of the discussion of magnetospheric substorrns and power
delivery to the Earth's magnetosphere and other dynamic pro-
cesses. It is expected that ISTP will provide data that is necessary
to more fully understand this phenomenon. In contrast to the
solar wind power source for the Earth's magnetosphere, Jupiter
and probably Saturn draw power for their magnetospheres pri-
marily from the kinetic energy of planetary rotation. The flybys of
Uranus and Neptune by Voyager will give us two more examples
of magnetospheric energ~zation from which to derive general phys-
ical principles governing the transfer of power to magnetospheric
regions.
Generation of Plasma Waves and Radio Emissions
Cosm~c-scale plasma and magnetic field regions are not quies-
cent. One universal form of activity they exhibit is the generation
and amplification of radio waves. Aside from the interest in the
radio emissions themselves, they serve as a means of exchanging
energy between particles in a low-density plasma where Coulomb
collisions are ineffective. Because we can get so close to (even
within) the emitting regions of a planetary magnetosphere, we
hear all of the symphony of electromagnetic emissions they pro-
duce: hms, chorus, narrow- and broad-band electrostatic waves,
and decnnetric, decametric, and kilometric radiations. A common
theoretical framework has been developed so that when account
is taken of differing conditions within the various magnetospheres,
radio emissions in one planetary magnetosphere can be related to
that in another as different manifestations of the same physical
process, although the frequency ranges of the emissions might be
quite different.
The Terrestrial Magnetosphere as a System
The International Solar Terrestrial Physics Program will ac-
quire simultaneous measurements throughout key regions in order
to understand the behavior of the system as a whole. A coor-
dinated network of spacecraft will permit us to investigate the
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22
physical behavior of each key region involved in the analysis of
solar-terrestrial plasma physics. WIND will be stationed in the
upstream solar wind to observe the interplanetary input function
and to observe the escape of energetic magnetosphere particles
back into the solar wind. POLAR will (1) observe directly the en-
try of magnetosheath plasma via the dayside cusp, (2) measure the
flow of hot plasma into and out of the ionosphere on auroral field
lines, and (3) observe the deposition of particle energy into the
ionosphere/atmosphere. CRRES will permit us to observe solar
wind-magnetosphere coupling near the magnetic equator, and it
will directly measure the interaction of ionosphere and tail plasmas
in the ring current and the plasma-sheet storage and transport
regions when the apogee ~ situated over the night hemisphere.
GEOTAIL will provide extensive simultaneous measurements of
entry, storage, acceleration, and transport in the geomagnetic tad!
and the tad! plasma sheet; and it will also measure plasma entry
and transport in magnetopause boundary layers along the dawn
and dusk flanks of the magnetosphere. The four CLUSTER space-
craft will provide detailed information on localized current systems
and magnetohydrodynamic processes, and SOHO will yield addi-
tional information on interplanetary and solar phenomena that
can affect the terrestrial magnetosphere.
These ISTP measurements will allow us to develop an under-
standing of the physical processes occurring in the solar-terrestrial
environment. In addition, theoretical studies will provide the
framework upon which the empirical understanding from the oh
servations can be both systematized and used to further our basic
understanding of other plasma systems.
Solar-Terrestrial Research
In parallel with the major spacecraft efforts addressed above,
progress in understanding the solar-terrestri~1 system as a whole
will also occur through the use of existing and planned ground-
based research programs in the United States and in nations
throughout the world. These include magnetic field and iono-
spheric arrays established for monitoring and/or campaign pur-
poses and large radar systems for studies of high-altitude plasma
convection and transport. During the ISTP program, the inter-
national ground arrays will be considerably enhanced and sum
plemented by more sensitive instruments and, particularly, by
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23
considerably advanced data acquisition, storage, and transrnis-
sion systems. All of these advances will contribute crucially to
understanding the global terrestrial magnetosphere system in the
context of the ISTP program.
In the United States the ongoing Solar-Terrestrial Theory
Program and other theoretical support will be crucial in order to
provide the predictive theoretical models for the global system.
Input data required for the theory and mode} advances will occur
from both the spacecraft and the ground-based experiments.
UPPI:R ATMOSPHE:RE SCIENCE
The upper atmosphere is defined here as the region of the neu-
tral atmosphere above the tropopause (12 km) extending all the
way to the exosphere. During the 1970s a great deal of progress
was achieved in our understanding of mid- and low-latitude ther-
~nospheric photochemistry, mainly as a result of measurements
obtained by the Atmosphere Explorer satellites. At the present
time the dynamics of the thermosphere is being studied extensively
by in situ and remote sensing instruments, which were carried by
the Dynamics Explorer 2 satellite, by ground-based optical and
radar methods, and by large-scale mode} studies. The dynam-
ic~ of the magnetosphere and thermosphere are coupled through
field-aligned currents, electric fields, Joule heating, and particle
precipitation, which both heats the upper atmosphere and alters
the electrical conductivity of the ionosphere. Before the end of this
decade, this multipronged study of thermospheric dynamics is ex-
pected to lead to significant advances in our understanding of the
energy and momentum sources controlling atmospheric motions
in these altitude regions. However, gaps will still remain in our
understanding of how specific ionospheric and auroral phenomena
correspond to magnetospheric sources and/or consequences.
Significant progress in our understanding of the stratosphere,
mesosphere, and lower thermosphere (often referred to as the m~d-
dIe atmosphere) is expected as the result of measurements made
by the instrument complement of the Upper Atmosphere Research
Satellite (UARS), to be launched in the late 1980s. This mission
will carry out a comprehensive set of measurements, particularly
from the viewpoint of atmospheric chemistry. Global measure-
ments of O3 and many of the radical species that destroy it (e.g.,
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24
NO2, C10) will be acquired sunultaneously, allowing for anal-
ysis of their interrelationships. Further, the "reservoirs species
that sequester these radicals in relatively inert forms will also be
measured (e.g., HNO3, HC1), along with the long lived molecules
that are the sources of all these constituents (e.g., N2O, CHIT.
In addition to these chemical measurements, atmospheric winds
will be measured s~rnultaneously from WARS, allowing for a more
complete analysis of transport processes than ever before possi-
ble. An explorer-class missions MELTER, is also planned for the
early 1990s for a focused study of the energetic and dynamic cou-
pling processes between the mesosphere and lower thermosphere.
Finally, atmospheric measurements need to be complemented by
continued laboratory measurements of the rate coefficients of the
important reactions as weD as the atomic and molecular parame-
ters of the relevant absorbing and emitting atmospheric species.
These data will be analyzed and interpreted with the aid of
multidimensional, chemical-dynamical-radiative models that will
include much more complete descriptions of many of the relevant
physical and chemical processes and their coupling. The progress
in all the various numerical modeling studies that are relevant to
atmospheric sciences will be greatly accelerated by advances in
computer technology.
A critical aspect of our understanding of the middle atmo-
sphere ~ the question ot what physical and temporal scales are
involved in the various photochem~cal and transport processes.
The spatial resolution achievable, currently as well as in the near
future, by both satellite observations and numerical models is rel-
atively large and represents a significant barrier to a thorough
understanding of many of the smaller scales on which significant
processes may well occur.
Another important area of study ~ the l990s will continue
to be the question of interactions between the middle atmosphere
and its neighbors, the troposphere and the thermosphere. Ther-
mospheric coupling includes studies of the particle and solar inputs
and corresponding middle atmospheric responses on a global scale.
Finally, the important question of the coupling of the troposphere
and the middle atmosphere will be a central component of the
field as a whole; anthropogenic perturbations to the atmosphere
in general and the ozone layer specifically will continue to be an
important central theme for middle atmospheric studies.
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25
Global Electric Circuit
The existence of a global electric circuit has been known for
many years, and yet we still do not understand the basic processes
that drive and control the behavior of this system. According to
the classical picture, thunderstorms are the sole generators within
the circuit, and, acting together, they maintain a potential dif-
ference of about 200 to 600 kV between the highly conducting
ionosphere and the surface of the Earth. This potential differ-
ence causes a downward conduction current of about ~ to 2 kA
from the ionosphere to the ground in the fair-weather dissipative
portion of the circuit. The global circuit Is believed to be closed
through an upward current flow from the Earth's surface, beneath
a thunderstorm, to the negative charge at the cloud base. Within
a cloud, updrafts and downdrafts and ~rucrophysical and electri-
fication processes maintain the charge separation. Although this
picture is generally the accepted one today, considerable doubts
and uncertainties are associated with many of the macrophysical
and rn~crophysical concepts that have been advocated (e.g., are
thunderstorms the main generators, and what controb charge sep-
aration?~. Simple models of the circuit assume that the ionosphere
is a highly conductive, equipotential upper boundary; however, in
reality, there are significant horizontal potential differences of tens
of kilovolts generated by both the ionospheric neutral wind and
the solar wind/magnetosphere dynamos. The details of the telluric
currents flowing in both the solid earth and oceans are complex
and require comprehensive experimental and theoretical investi-
gations.
The problems associated with the global electric circuit cut
across numerous disciplines, from magnetospheric convective pro-
cesses at one end to soil and ocean conductivity issues at the other
encI.
Some of the more "relevantly ways in which atmospheric elec-
tricity may play a potentially important role are as follows:
~ Causing interference in man-made systems such as com-
munication cables, power lines, and pipe lines.
~ Influencing the spatial distribution and effectiveness of con-
densation nuclei in the atmosphere.
~ Acting as a possible mechanism for the generation of odd
nitrogen compounds through lightning processes.
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26
PLANETARY SPACE PHYSICS
There are basically four different types of interactions of plas-
mas with bodies in the solar system. The plasmas can collide
with solid bodies directly. For example, the solar wind strikes the
lunar surface and is absorbed. Energetic particles are similarly
absorbed by the jovian moons, albeit with sputtering of material
that in turn supplies material to the jovian magnetosphere. The
rings of Saturn are also subject to such processes.
A second interaction is the deflection of a flowing plasma by
a planetary magnetic field. The smallest scale on which such
deflection is known to occur is the deflection of solar wind above
the lunar terminators when magnetized regions of the lunar surface
occur at the lunar limbs. Such deflection occurs on a global scale
at Mercury, the Earth, Jupiter, and Saturn and perhaps other
planets as well.
If a body has no strong magnetic field but does have an
atmosphere, two other interactions can occur, whose effects are
sometimes difficult to separate. First, if an ionosphere can form
due to strong ionization of the neutral atmosphere, a cold plasma
region may form whose pressure is sufficient to exclude the external
nonplanetary plasma. This process occurs at Venus, where the
gravitationally bound ionosphere usually has sufficient pressure to
stand off the solar wind. Fresh comets, strongly outgassing near
the Sun, are also thought to have ionospheres. If the ionospheric
pressure is insufficient to stand off the solar wind, it is essentially
pushed back toward lower altitudes where the atmospheric density
is greater. The resulting ion-neutral coupling due to the relative
drift of the ions and neutrals transmits pressure to the plasma
and aids In the support of the ionosphere. At comets, the outflow
of neutral gas assists in this pressure balance. At the top of the
ionosphere, the ionopause, a tangential discontinuity, separates
the external plasma, the magnetosheath in the case of Venus, from
the ionospheric plasma.
The second process that occurs is the direct interaction of the
neutral gas and the external plasma without the intermediary of
an ionosphere. The neutral atmosphere at a planet such as Venus
extends well above the ionopause. This gas can be photoionized
by solar extreme ultraviolet or charge exchange with the magne-
tosheath or solar wind plasma. At a satellite such as To, the neutral
gas can be ionized also by impact ionization. This new source of
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27
plasma ma" loads the external plasma, slowing it down if it
flowing.- Charge exchange can also lead to the creation of fast
neutrals that remove momentum from the plasma. This process
is thought to create the long tails seen behind Venus and comets
and cause the field-aligned currents and distorted magnetic fields
seen at To and Titan.
Finally, it is noted that the solar wind flows much more rapidly
than the speed of compressional waves in the solar wind plasma.
Hence, when it interacts with a conducting planetary object, shock
waves form to deflect the flow around the object. These collision-
less shocks are very interesting objects that hare been intensively
studied at the Earth. However, planetary studies have much to
add to this investigation because the properties of the plasmas in
the solar system change greatly with heliocentric distance.
Interaction with Unn~agnetized' Atmo~hereless Bodies
The interaction of the solar wind with the Moon ~ understood
only on the most elementary level. Basically we know that the solar
wind is absorbed by the forward hemisphere of the Moon, leaving
a wake behind. Because of the limited plasma instrumentation on
Explorer 35 and the Apollo subsatellites, we know little about the
closure of plasma behind the Moon. Some work has been done
with plasma instruments on the lunar surface and much empirical
understanding obtained.
The interaction of the radiation belt of Jupiter with its satel-
lites ~ understood on an elementary level, although much more
needs to be done on the role of sputtering in providing a source
of jovian plasma. Galileo should contribute greatly in this area.
We also have an elementary understanding of the interaction of
Saturn's radiation belts with its satellites and rings. We expect
new progress in this area through laboratory investigations, the-
ory, and the Saturn orbiter of the Cassini my - ion. Studies of the
interaction of the solar wind with cometary dust are also impor-
tant. Here, we expect some near-term progress through laboratory
data and theoretical treatment.
Deflection by Planetary Magnetospheres
We know little about the solar wind interaction with Mercury
except that Mercury has a magnetosphere, bow shock, and tran-
sient energetic particle population, suggesting that subetorm-like
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28
processes occur. Mercury's magnetosphere is a very important
one because of its lack of a dynamically important atmosphere.
Thus field-aligned currents should not play a dominant role in the
magnetosphere of Mercury.
Our studies of the jovian magnetosphere have proceeded from
the exploratory phase to the beginnings of intensive investigation.
We expect further refinements from the Galileo mission. The jo-
vian magnetosphere is important because of its rapid rotation and
the large mass loading by To deep in the interior of the magneto-
sphere.
Saturn complements the jovian studies tenth its extensive ring
system that both absorbs and supplies charged particles. Titan
is also important as discu~ed below. We expect little change in
our understanding of the saturnian magnetosphere until the data
from the Cassini minion are obtained.
Voyager has provided existence data on the magnetosphere
of Uranus, leaving us with elementary concepts on the physics
of these systems. Similar data will be available from Neptune in
1989.
Plasma-Atmopphere Attractions
The solar wind interaction with Venus is now understood
to first order. The present and future data base from Pioneer
Venus plus computer modeling should give us sufficient insight
to ask more fundamental questions, but large gaps remain in our
observational knowledge.
The magnitude of the Mars intrinsic magnetic moment is not
known. This will be addressed with the Mars Observer in the early
l990s. However, we will still be ignorant of all the basic plasma
processes on Mars, as well as of the processes In the upper neutral
atmosphere.
The interaction of the jovian plasma with To and the inter-
action of the saturnian plasma with Titan are also key elements
in our study of plasma/neutral gas interactions. The former will
be addressed briefly by Galileo in one flyby. This may not be
sufficient. The latter will be addressed by the Cassini mission to
Saturn.
Interest in the interaction of the solar wind with comets ~
undergoing a strong resurgence owing to the high interest in the
Giacobini-Z~nner and Halley missions. Theoretical progress should
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29
be made by the end of the decade, and there should be some
confirmatory data.
SUMMARY
Tables 3.1 and 3.2 provide a graphic summary of the expected
status of research by 1995 on the science objectives defined in
Chapter 2. Table 3.1 depicts in situ investigations; Table 3.2
depicts remote sensing investigations.
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30
TABLE 3.1 Leveb of In Situ Spacecraft Investigation
Disolpline
Awaiting Reconnals- Exploration Intensive Physical
Recon- sance Study Under-
naissance . standing
Solar physics
Sun, solar
corona
Hellospheric physics
Generation of
solar wind
High-latitude
solar wind
In-ccliptic
solar wind
beyond Saturn
In-ecilptle
solar wind
between Bllercury
and Jupller
Hellopause,
Interstellar
medium
Terrestrial,
magnetospheric
physics
Magnetosphere
<60 Rig
Earth magnetic
tall, wake
Terrestrial,
atmospheric,
ionospheric physics
Thermosphere,
ionosphere
>150 km
M exosphere
_-
Solar Probe
1~ -
Solar Probe
- ~0~ -
ISPM
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31
TABLE S.1 (continued)
Disclpllne
=on~l~ ~p~-on licensee Phil
Recon- sance Study under-
-~-e amid
Panda,
_-ed
nos~ed~
magn~osphedc
phyla
Comas
Solar wind
Oregon
of Ears
Atmosphere of
Bars
Soar and
l~em~lon
of Venus
~agna10sphere
of ~ ercu ~
magnetosphere
of Junker
~agne10spbere
of Saturn
~agne10sphere
of ursnus, Neptune
upper atmosphere
Ionosphere of
Judged ~urn,
Than
Galileo
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32
TABLE 3.2 Levels of Remote Sensing Investigation
Discipline
Awaiting Preliminary Global Intensive Physical
Preliminary Survey Survey Study Under-
Survey standing
Solar physics
Coronal holes,
large-scale
magnetic fields
Radio bursts
Global oscillation
i-fares
Internal structure
and dynamics
solar dynamo
Surface plasma-
magnetic field
interactions
Energy storage
and release
Atmospheric
heating
Structure and
dynamics of
corona and
solar wind
Heliospheric
physics
Interstellar
neutrals
Terrestrial,
magnetospheric
physics
Global auroral
morphology
Remote sensing of
magnetospheric
structure
Terrestrial,
atmospheric,
ionospheric physics
Middle atmosphere
SOT/ASO
SOT/ASO
SOT/ASO
1'
ISTP
Representative terms from entire chapter:
magnetic fields
