By the early 1990s, magnetospheric physics will have progressed primarily through observations made from Explorer-class spacecraft, sounding rockets, ground based facilities, and shuttle based experiments. The global geospace science (GGS) element of the International Solar Terrestrial Physics program, when combined with contributions to the ESA Cluster mission and ground based and computer modeling programs, will form the basis for a major U.S. initiative in magnetospheric physics. The scientific objectives of the GGS program involve the study of energy transport throughout geospace. The Cluster mission will investigate turbulence and boundary phenomena in geospace, particularly at high latitudes on the dayside and in the region of the neutral sheet at geocentric distances of about 20 earth radii on the night side of the earth. The current state of knowledge is reviewed and the goals of these missions are briefly discussed

Anderson, H. R.

Baker, D. N.

Burch, J. L.

Chappell, C. R.

Taylor, W. W. L.

Winningham, J. D.

English

NASA Technical Reports Server

N90-18334
Chapter 5
Report from Magnetospheric Science
Panel: J.L. Bureh, Chair; J.D. Winningham, H.R. Anderson, D.N. Baker,
C.R. Chappell and W.W.L. Taylor
5.1 .Introduction and Background
By the early 1990s, magnetospheric physics
will have progressed primarily through observa-
tions made from Explorer-class spacecraft,
sounding rockets, ground-based facilities, and
shuttle-based experiments. The field is still domi-
nated by in-situ measurements with remote imag-
ing and active experimentation becoming more
prevalent in recent missions. The global geospace
science (GGS) element of the ISTP program is
the first major new start in the field in over 20
years. Although the spacecraft are still in the
class of large Explorers, there are two US space-
craft and instrumentation on a Japanese space-
craft. When combined with contributions to the
ESA Cluster mission and vigorous ground-based
and computer modeling programs, GGS will
form the basis for a major US initiative in mag-
netospheric physics.
The scientific objectives of the GGS pro-
gram involve the study of energy transport
throughout geospace. The Cluster mission will
investigate turbulence and boundary phenomena
in geospace, particularly at high latitudes on the
dayside and in the region of the neutral sheet at
geocentric distances of about 20 earth radii on
the night side of the earth.
After ISTP the achievement of further im-
portant objectives in the discipline will rely on
the development of a three-pronged approach to
magnetospheric experimentation combined with
a complementary theory and modeling program.
The three necessary techniques are: (1) the use of
advanced photon and particle imaging of all of
the plasma populations of the magnetosphere
and the global convection patterns; (2) the use of
arrays of spacecraft measuring crucial parameters
in carefully selected regions of the magneto-
sphere; and (3) the use of active probing and
stimulation of magnetospheric plasmas. The
simultaneous use of these three techniques will be
needed along with a real-time modeling capabil-
ity, which is capable of transforming the mea-
surements into a global perspective of magneto-
spheric processes.
5.2 Current State of Knowledge
The interaction between the solar wind and
the magnetosphere is characterized by a contin-
ual gradation of disturbance levels that manifest
themselves through auroral displays and the
intensities of the driving currents of the system.
There is some doubt now that there exists a
magnetospheric ground state. If there is, it cor-
responds to conditions of a nearly zero north-
south component of the interplanetary magnetic
field (IMF).
Increasingly southward components of the
IMF lead to substorm activity with extensive
auroral displays along an expanded auroral oval.
Substorm initiation is associated with a disrup-
tion of the magnetotail neutral sheet and a diver-
sion of neutral-sheet current into the ionosphere.
The location of the disruption, the role of mag-
netic reconnection, and the formation of plas-
moids that propagate down the tail are topics
that are presently under active investigation.
Northward IMF components lead to a strik-
ingly different disturbance state of the magneto-
sphere. The auroral oval shrinks and moves
poleward and the polar cap itself becomes the
site of increasing auroral activity in the form of
polar cap arcs and, at times, the theta aurora.
The field-aligned currents, which drive magneto-
spheric convection, change their global character-
istics substantially, leading to sunward convec-
tion in the polar cap. If anything, auroral activity
_G PAGE BLANK NOT FILMED
25
https://ntrs.nasa.gov/search.jsp?R=19900009018 2020-03-20T00:04:18+00:00Z
at dayside high latitudes, near the cusp,
becomes more active as nightside auroral activity
subsides.
The transfer of solar-wind energy into the
magnetosphere for both northward and south-
ward IMF appears to proceed, at least in part,
through a process involving magnetic reeonnec-
tion. For southward IMF, this process results in
the formation of flux transfer events, by which
solar wind and magnetospheric plasmas inter-
mingle and a convection electric field is imparted
to the high-latitude magnetosphere. Flux transfer
events appear to form at low latitudes along the
dayside magnetopause. Multipoint observations
in that region will be required for an improved
understanding of this very important magneto-
spheric phenomenon.
One of the most fundamental problems of
magnetospheric physics is the acceleration of
auroral particles. It is widely held that electric-
field components parallel to the magnetic field
are responsible for the downward acceleration of
auroral electrons and the simultaneous upward
acceleration of ionospheric ions. The parallel E-
fields seem to occur preferentially in regions of
strong field-aligned currents associated with con-
vective shear reversals in the magnetosphere and
ionosphere. Whether the parallel electric fields
are localized in altitude as double layers or are
greatly extended in altitude is not known. Defini-
tive new information on the auroral potential
structure will require the use of multipoint mea-
surements at altitudes of several thousand
kilometers above the auroral oval. Active prob-
ing of the potential structures with test particles
will be necessary for the unambiguous measure-
ment of the parallel E-fields because they tend to
occur in regions of strong perpendicular
components.
Much has been learned recently about the
sources of magnetospheric plasmas. The iono-
sphere is, of course, the source of thermal plasma
throughout the magnetosphere. The ionosphere
and the solar wind are both important sources of
the more energetic plasma. Recent data has
shown that the cusp region is not only the prime
site of entry of solar-wind plasma into the
magnetosphere but it also is possibly the strong-
est source of ionospheric plasma to the
magnetospheric regions beyond the plasma-
sphere. Turbulence in the cusp ionosphere,
coupled with strong antisunward convection
emanating from the cusp region, leads to the
formation of a cusp ion fountain, which feeds
accelerated ionospheric plasma to the entire
outer nightside magnetosphere. We need to know
whether or not this ionospheric plasma is trans-
ported to the neutral-sheet region of the magne-
totail where it would become an important con-
stituent of the plasma sheet and ultimately the
ring current. An answer to this question will
require the measurement of thermal plasma
composition in the equatorial region of the mag-
netotail, particularly in the region within about
20 Earth radii down the tail.
Typical magnetospheric plasma distributions
contain significant amounts of free energy in the
form of beams, loss cones, and other types of
velocity-space density gradients. This free energy
leads to wave growth and significant wave-
particle interactions, which result in the accelera-
tion of electrons and ions and in their precipita-
tion into the atmosphere. Significant new data
has been obtained recently on the processes
involved in the generation of auroral kilometric
radiation (AKR) and auroral hiss as a result of
spacecraft that have probed the auroral accelera-
tion regions directly. AKR appears to be gener-
ated within plasma density cavities with energy
derived from electron loss-cone distributions. A
relativistic effect known as the cyclotron maser
instability is genex_ally accepted as the generation
mechanism. As yet there is no general agreement
on the source of auroral hiss emissions except
that they also could be generated by electron
velocity-space density gradients, possibly asso-
ciated with the electron "hole" distributions,
which are produced by a loss-cone distribution in
the presence of a parallel electric field. Other
important wave modes, such as ion cyclotron and
lower hybrid waves, with frequencies near the ion
gyrofrequency have been invoked for the trans-
verse acceleration of ionospheric ions (ion conics)
and the parallel acceleration of electrons
(suprathermal bursts).
5.3 Science Goals
The ISTP program will include the follow-
ing major Spacecraft missions:
26
• Wind,a phasedmissionusingdouble-
lunar-swingbyandLI haloorbitsto sample
theinterplanetarymediumupstreamof the
magnetosphere.
• SOHO,asolarobservatoryat Li;
• Polar,a phasedmissionusingelliptical
polarorbitswith apogeesof 6and9Earth
radii. Thespacecraftwill carryvisibleand
ultravioletauroralimagingandacompre-
hensiveplasmaandfieldspayload.
• Geotail,aJapanesespacecraftwith plasma
andfieldsinstrumentationfor thein situ
samplingof thenearanddistantgeo-
magnetictail usingdouble-lunar-swingby
orbitsextendingto distancesbeyond200
Earthradii.
• Cluster,four ESA spacecraft in a tight
array for study of turbulent phenomena in
a 2 by 20 Earth polar orbit with the initial
apogee at the equator.
The Soviet Union and possibly other coun-
tries will also be contributing spacecraft to the
ISTP program.
The overall science objective of the ISTP
program is the study of large-scale energy trans-
port in geospace with contributions to smaller-
scale turbulent phenomena from Cluster. Aside
from the auroral imaging on Polar, all the ISTP
spacecraft will use in-situ measurements.
The next significant step in magnetospheric
physics beyond ISTP will require the use of glo-
bal imaging of magnetospheric plasma and ener-
getic particle populations and further use of
spacecraft arrays for studies of the spatial struc-
ture and temporal evolution of major particle
acceleration and energy dissipation processes.
The imaging techniques and the use of spacecraft
arrays should be employed simultaneously for
greater effectiveness.
Because of the lack of global plasma and
particle imaging capability on ISTP and the sin-
gle orbit of Cluster, which will not allow for
multi-point measurements near the subsolar
magnetopause, in the auroral acceleration
regions, or in the neutral sheet within and
beyond -- 20 Earth radii, the following science
questions are not expected to be answered satis-
factorily by ISTP:
• What processes are responsible for energiz-
ing the magnetosphere in the subsolar
region of the magnetopause and for phe-
nomena such as flux transfer events, which
result from these interactions?
• What is the three-dimensional topology of
the magnetosphere and its various plasma
boundaries, current systems, and convec-
tion patterns, and how do these configura-
tions evolve during magnetospheric
substorms?
• What is the three-dimensional structure of
the auroral potential structure and how
does it evolve in time?
• How is the neutral sheet within and beyond
20 Earth radii disrupted during substorms?
5.4 Magnetospheric Science Missions
Two types of missions to the Earth's mag-
netosphere will be required after ISTP. The first
of these is the global imaging mission, which was
advocated in the recently published National
Academy of Sciences publication on space
science in the twenty-first century. Recently deve-
loped approaches to the imaging of plasma popu-
lations using photons (e.g., helium 304 A emis-
sions) and energetic neutral particles (produced
in charge-exchange interactions), along with
established auroral-imaging techniques, hold the
promise of global imaging of the magnetosphere.
In addition to auroral imaging from elliptical
polar orbits (as with the Polar spacecraft) there is
also the possibility of imaging from lunar-based
observatories or spacecraft in orbit about the
Lagrangian points, particularly L4 and L5. Fig-
ure 5-1 illustrates some possible imaging
missions.
The second class of missions is the multi-
point mission involving arrays of identically
instrumented spacecraft with either large or rela-
tively small spacing. An example of a large array
would be a set of"current probes" deployed to
map the current systems throughout the magneto-
sphere. Such a network, which was also des-
cribed in the NASA report on space science in
the twenty-first century, would have to be aug-
mented by closely spaced arrays moving through
the large-scale network to measure the spatial
structure of the electric and magnetic fields,
27
Figure 5-1. Imaging of Auroras, Plasmas, and Energetic Particles
which are the primary observables associated
with the current systems.
Indeed, relatively closely spaced arrays (as in
Cluster) are needed for the detailed probing of
the important plasma phenomena in the magneto-
sphere. Primary targets for spacecraft arrays with
identical plasma and field instrumentation are
the low-latitude dayside magnetopause, the
auroral acceleration regions at altitudes of 1 to 2
Earth radii along field lines, and the neutral sheet
within and beyond 20 Earth radii (the Cluster
apogee). Figure 5-2 shows the current probes and
various spacecraft arrays in critical regions of the
magnetosphere.
Other types of platforms, as well as ground-
based observatories, are also applicable to the
future magnetospheric science goals. Table 5-1 is
a matrix showing the various platforms and the
four outstanding science questions from Section
5.3. The double Xs indicate a primary contribu-
tion and the single Xs a valuable contribution.
The use of manned platforms at low altitudes
and inclinations is envisioned as involving active
plasma experimentation as discussed further in
Chapter 7 (Space Plasma Science).
5.5 Data Analysis, Modeling, and Theory
Data analysis, modeling, and theory need to
be increasingly integrated to make the most effec-
tive use of the flight opportunities, to identify
unresolved issues and thereby assist in planning
future research requirements. On the time scale
of the turn of the century, following implementa-
tion of ISTP and other programs, we expect to
be in a position to attempt a fully quantitative
treatment of the phenomena observed in the
Earth's magnetosphere.
We assume that by 1995, an internationally
based program will have been put in place, with
the objective of integrating the scientific output
of the US, European, Japanese, and Soviet space-
craft which will explore the different regions of
the magnetosphere. In addition to the extended
operations and data analysis phases for each of
these missions, we recommend a worldwide
coordinated effort to combine the global data
28
Figure5-2.Arraysof SpacecraftNeededfor Studyof Major
AccelerationandEnergyDissipationProcesses
Table5-1.MagnetosphericScienceMissions
Science Goals
Energy Transfer at Neutral Sheet
Platform Subsolar Magnetopause 3D Structure Disruption Particle Acceleration
Imaging X
Observatories X
Multipoint X X X
Spacecraft X X X
Manned X
Station X
Polar
Platform
Space X
Shuttle X
Sounding
Rockets
X
X
X
X
Ground X
Based X
XX denotes primary mission, X denotes secondary mission
Imaging observatory would view Hell 304 A. and energetic neutral atoms
Chemical releases in crucial regions could enhance imaging
Injection of test particles is needed for probing of acceleration regions
Continuous monitoring of the interplanetary environment is needed for all missions
29
setsandcarryout thescientificanalysisof the
dataacrossall themissions.Thisrequiresa new
initiative,anewwayto organizeandmanagethe
scientificoptimizationof multi-spacecraftinter-
nationalflight programs.Oneapproachwouldbe
to haveanofficialdataanalysismission,which
wouldutilizeessentiallythesamemanagement
andmethodologiesasaflight program.
Theknowledgethusacquiredwouldbe
unprecedentedin qualityandquantity,and
wouldprovidethebasisfor:
• A truly quantitativethree-dimensional
modelingcapability
• Thefull-scalemergerof dataanalysisand
theorysystems.
Paralleltheoryandmodelingefforts,in close
approachwith thegoalof loweringcost
andshorteningdeliverytime.
• Fundcompetitivestudiesof standardized
low,medium,andcomplexspacecraft.
Implementoneof eachbasedoncompeti-
tiveselectionto establishaccuratecostand
interfacemodels.
• Fundparallelsystemsresearch(to above
items)of presentsystemof implementing
missions.Thesynergisticgoalwouldbeto
reducecostsby identifyingcost/benefit
payoffof presentmethodologieswith elim-
inationof non-beneficialelements.
• Baseselectionson relevanceto mission
plansthrough2015.
• Specificrecommendationswouldbeto (1)
emphasizeremotesensingof magneto-
cooperationwith thedevelopmentof dataanaly_ spherefor globalphysics,and(2)developlow cost,standardclusterspacecraftto
sistechniques,shouldleadto: examinerelevantmicrophysics.
• Developmentof a magnetosphericglobal
circulationmodel
• Simulationof physicalprocessesat all
scales
• Useof local,high-resolutionparticle-code
simulationsto analyzeandinterpretdata
from clustersof spacecraft
• Ability to "fly" spacecraftthroughMHD
(magnetohydrodynamic)andsingle-particle
computermodelsandgenerateasimulated
dataset.
5.6 Advanced Instrument Development
Program
The lead times and hence the costs of future
magnetospheric missions could be measurably
decreased through an advanced instrument and
spacecraft development program. An effective
program would include the following elements:
• Fund as an identified line item in NASA
budget to stress its fundamental
importance.
• Develop instruments to engineering model
level. Define refined costs through this
• Corre]ate with theory and modeilng pro-
grams to establish key regions and parame-
ters to be targeted for verification and
refinement of models and theory.
5.7 Crucial Elements of the Future
Magnetospheric Physics Program
Accomplishment of the science goals of the
future in magnetospheric physics will require the
development of several crucial programs and
capabilities, including:
• Photon and/or neutral particle imaging of
the aurora and of all the plasma popula-
tions of the magnetosphere
• Arrays of low-cost spacecraft measuring
critical parameters in carefully selected
regions
• Active probing and stimulation of magneto-
spheric plasmas
• A vigorous theory and modeling program
• Missions to data bases
• An advanced instrument and spacecraft
development program
• Continuous solar-wind monitoring.
30


Report from magnetospheric science

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server