Most of the understanding of the thermosphere resulted from the analysis of data accrued through the Atmosphere Explorer satellites, the Dynamics Explorer 2 satellite, and observations from rockets, balloons, and ground based instruments. However, new questions were posed by the data that have not yet been answered. The mesosphere and lower thermosphere have been less thoroughly studied because of the difficulty of accessibility on a global scale, and many rather fundamental characteristics of these regions are not well understood. A wide variety of measurement platforms can be used to implement various parts of a measurement strategy, but the major thrusts of the International Solar Terrestrial Physics Program would require Explorer-class missions. A remote sensing mission to explore the mesosphere and lower thermosphere and one and two Explorer-type spacecraft to enable a mission into the thermosphere itself would provide the essential components of a productive program of exploration of this important region of the upper atomsphere. Theoretical mission options are explored

Carignan, G. R.

Hudson, R. D.

Mende, S. B.

Nagy, A. F.

Roble, R. G.

English

NASA Technical Reports Server

N90-18332
Chapter 3
Report from Upper Atmospheric Science
Panel: G.R. Carignan, Chair; R.G. Roble, S.B. Mende, A.F. Nagy and R.D. Hudson
3.1 Introduction
The neutral thermosphere and the ionos-
phere are physically inseparable, although they
are treated separately for the purpose of this
report. Advancing scientific understanding of
these regions demands an increased understand-
ing of the relationships between the two as well
as the couplings from the regions below and
above. For the purposes of this report, the upper
atmosphere is taken to embrace the region from
the base of the mesosphere to the exobase, nomi-
nally 50 to 500 km altitude. During the 1970s,
our understanding of the mid- and low-latitude
thermospheric photochemistry was expanded
greatly, largely through the measurements of the
Atmosphere Explorers. Thermospheric dynamics
have been studied subsequently from the Dynam-
ics Explorer 2 satellite and from ground-based
optical and radar observations. The information
obtained from these experimental programs has
been amplified through the use of large scale
models, both physical and empirical. The com-
bined approach enabled the most effective use of
observations in deriving information on global
dynamic atmospheric processes. Despite consid-
erable progress in understanding the thermo-
sphere, significant gaps remain in knowledge of
its behavior. The lower thermosphere, because of
a paucity of measurements, is still in need of
exploratory study.
Progress in understanding the mesosphere
has been much slower largely because of the rela-
tive difficulty of direct access and the difficulty
thus far in remotely observing the region on a
global basis. Our overall knowledge of the region
has been pieced together from diverse and iso-
lated measurements. The Upper Atmosphere
Research Satellite (UARS) (scheduled for launch
in 1991) will concentrate on stratospheric and
lower mesospheric measurements. The upper
mesosphere, like the lower thermosphere, is still
basically unexplored. There is, however, consid-
erable scientific interest in these regions because
of nonlocal thermodynamic radiational pro-
cesses, complex photochemistry, and gravity
wave/mean flow interactions that dominates this
region. The extent of the downward penetration
of solar and auroral variability, that so dominate
the thermosphere, into the mesosphere also needs
to be determined.
3.2 Background
The thermosphere absorbs extreme ultravi-
olet (EUV) and UV radiation (wavelength,
_<200 nm) and, because of the absence of effec-
tive IR cooling mechanisms, is heated, giving rise
to its name. EUV radiation and precipitating par-
ticles ionize the thermosphere and form the iono-
sphere. Energy and momentum are coupled into
the upper atmosphere from the magnetosphere
and waves and other disturbances propagating
from below deposit their energy as they dissipate
over wide ranges of altitudes. The ionospheric
plasma is controlled through complex chemical
and electrodynamic processes that operate
throughout the region with rates dependent on
plasma temperature and composition. Finally,
the thermosphere becomes the energy sink for
plasma. Neutral winds are driven by pressure
gradients and ionospheric winds by electric fields
of magnetospheric and dynamo origins, and
these wind systems interact as they flow within
each other. Joule dissipation from the differential
motions of plasma and neutral gas is frequently
the dominant heat source in some regions. Ther-
mospheric winds redistribute the plasma directly
through collisions and indirectly through dyna-
moelectric fields. In the mesosphere, exotic chem-
ical processes operating in a complex dynamical
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field produce a highly variable state that begs for
elucidation. Most of the operative processes
appear to have been identified, but the sensitivity
of the resultant state to the details of the interac-
tions causes the models to yield descriptions that
are far from reality. The upper atmospheric dis-
cipline stands in great need of additional obser-
vations, most of them well defined and within
measurement capability, to enable the next
increment in understanding. The complex suite
of processes that operate in the thermosphere-
mesosphere system is illustrated in Figure 3-1.
3.3 Outstanding Scientific Problems
The important advances in understanding of
the upper atmosphere in the 1970s and 1980s,
including recent results from modeling, have also
identified a number of outstanding scientific
problems. This report identifies four important
problem areas. The scientific issues embedded in
these areas are related and could have been posed
as a greater number of problems, but the four-
problem taxonomy seems to cover the main
areas.
3.3.1 Mesosphere/Lower Thermosphere
Interaction
Because this region is virtually unexplored on
a global basis, fundamental questions exist about
basic parameters and their variability as well as
questions of important chemical, radiative, and
dynamical phenomena. These include:
ENERGETIC
ELECTRONS
PHOTOELECTRON
HEATING
SOLAR
PROTONS SOLAREUV AND UV
PHOTOIONIZATION
AURORA
02 +hv--O ;-O
IONOSPHERE MAJOR MINOR
ELECTRON WITH O SPECIES SPECIES
AND ION DIFFUSION; DIFFUSION DIFFUSION
ENERGY EQ. 02 , N2 , NO, N 02 , O AND N2 N (2D), N (4S)
IN P.C.E. NO
JOULE
HEATING
HEATING
IO_/NEUTRAL
CHEMISTRY
HEATING e/i/n COLLISIONS
NEUTRAL
GAS
ENERGY
EO.
D-REGION [ Znn [ 03"MINOR..SPECIESoH.
IONOSPHERE < Zn i _ EQUILIBRIUMPHOTOCHEMICAL
ZN i . _-Ni, no O(tD), H202
H02, NO2
HEATING
0
RECOMBINATION
HEATING NEUTRAL/NEUTRAL
CHEMISTRY
MINOR
SPECIES
DIFFUSION
H20, H, H2, CH4
CO, CO2
_::_: Figure 3-1. Diagram of Processes Operating in the Thermosphere and Mesosphere
10
• Basic structure of wind, temperature and
composition
• Role of breaking gravity waves on energy
and momentum budgets and as a mecha-
nism for species transport
• Dissipation of upward propagating tides by
molecular viscosity, thermal conductivity
and ion drag
• OX, HOX and NOX chemical cycles and
their response to solar radiative outputs,
particle precipitation and disturbances from
the lower atmosphere
• Role of nonlocal thermodynamic equili-
brium infrared radiation from CO 2 and NO
as cooling mechanisms.
The broad question of the chemical, radia-
tive, and dynamic response of the mesosphere/
lower thermosphere to solar and auroral variabil-
ity is not well understood and experimental and
theoretical studies should be made. The neutral
winds in the region need to be measured in a
global sense to guide modeling efforts with better
measurement constraints. The basic structure of
the temperature, composition and dynamics
fields need to be defined to provide a baseline
against which modifications from the increasing
CO 2 burden can be evaluated.
3.3.2 Thermosphere/Ionosphere/
Magnetosphere Interactions (_200 kin)
A series of spacecraft programs, including
OGO-6, AE-C, AE-D, AE-E, and the Dynamics
Explorers, together with ground-based and other
related observations have provided the data
necessary to define most of the large scale pro-
cesses operating in the upper atmosphere. Theo-
retical and numerical models have also been
developed to provide a quantitative description
of the region and its variability. The advanced
state of our understanding notwithstanding, there
are still a number of critical aeronomic processes
that are highly parameterized because of a lack
of the fundamental measurements that are
needed to adequately describe them. Solutions to
these problems could come from advanced mea-
surement capabilities and increased insight into
the nature of the region. It would be possible to
significantly advance the state of knowledge of
the interactions of the magnetosphere with the
ionosphere and atmosphere while simultaneously
complementing the International Solar Terres-
trial Physics Program (ISTP). An advanced
Explorer class aeronomy satellite will provide the
observations necessary to describe the region that
is the sink for most of the processes being inves-
tigated in the ISTP. This is a mission that begs to
be undertaken.
Such a mission could be designed to answer
several fundamental aeronomic problems, e.g.,
N(2D) + O quenching, O + CO 2 vibration excita-
tion, and IR cooling. New instruments developed
since the last flight opportunities should provide
increased capability to resolve the fundamental
aeronomic problems. Refinements in our under-
standing the chemical, radiative, and dynamical
processes and their dependence on solar variabil-
ity are needed and could be obtained by sophisti-
cated instruments applied in effective measure-
ment strategies. Altogether, a substantial
scientific return would accrue from a modest
mission that returns to the thermosphere during
the epoch of ISTP.
3.3.3 Global Electrodynamics
The global electric circuit consists of three
separate but interacting generators: (1) global
thunderstorms maintain an Earth/ionosphere
potential of 105 volts with a current of 103
amperes; (2) the ionospheric wind dynamo
generates a potential of 104 volts with a current
of 105 amperes; and (3) the solar wind/magneto-
sphere dynamo generates a potential of 10_volts
and a current of 106 amperes. Understanding this
global electric circuit is fundamental to atmos-
pheric science and an integrated study of the
total process is indicated. The study must include
investigation of the relationship of the thunder-
storm and lightning to upper atmospheric phen-
omena and include mapping of electric fields
from thunderstorms to the ionosphere, from the
ionosphere to the surface of the Earth, and
between magnetosphere and ionosphere. The
interaction of the neutral gas with the plasma
and the resultant electrodynamic processes need
to be quantified; new mapping procedures that
incorporate disparate data sets can be of particu-
lar value in defining the morphology of the elec-
trodynamic process which, through the use of
1]
models,canadvanceunderstandingof this inter-
estingandcomplexphenomenon.Finally,the
dependenceof theglobalelectrodynamicspro-
cesseson solarvariabilitymustbeunderstood.
3.3.4 Fundamental Process Measurements in
the Space Environment
During the evolution of a scientific disci-
pline, it inevitably becomes possible to pose ques-
tions of increasing detail and sophistication and
it becomes necessary to attempt measurements of
greater precision. In the recent past, exploration
of the upper atmosphere has lagged behind the
theoretical and associated modeling activities and
has not exploited the latest technology available
for space application. Atmospheric models
require a large number of input parameters, and
at the pioneering stage only a few of these are
known. As the field matures, more parameters
are known with better precision, but inevitably
some are more poorly known than others. The
experiment becomes an exercise in determining
those least well known. This of course does not
provide an adequate test for the model--what is
needed is an overdetermined set of parameters,
with sufficient accuracy. Whereas in the past,
measurements with 50% accuracy could advance
our knowledge, now the requirements may be for
accuracies of 10% or better. It is, thus, possible
to envision experimental programs that would
advance our state of understanding through more
precise quantification of states and processes
through application of advanced remote and in-
situ techniques. Some outstanding examples are:
° Solar, airglow and infrared radiation fluxes
• Excited and metastable species
concentrations
• Ion/neutral and neutral/neutral rate
constants
• Absorption and collisional cross sections.
Technological developments now make pos-
sible these more comprehensive and accurate
measurements. In the optical field, array detec-
tors provide vastly more efficient collection of
spectral and spatial information. In terms of
sources, fluorescence lamps, particle beams, and
lasers provide excellent potential for active mea-
surements. The optical field can be used in the
local or remote sensing mode. The latter is prac-
tically the only way the mesosphere can be
explored, but novel platforms such as the
tethered satellite could be used to great advan-
tage. A concerted effort to better define these and
other fundamental attributes of the upper atmos-
phere in the actual thermosphere and ionosphere
environment by in situ and remote measurements
should be an important part of future explora-
tion and could effectively exploit new technology
and advanced spacecraft and missions.
3.4 Missions and Platforms
The outstanding scientific problems in the
upper atmosphere can and must be studied by a
wide range of remote and in-situ measurements,
by missions to existing data bases, and through
continued theoretical and modeling studies. The
measurements could be provided by a variety of
spacecraft--indeed, there is application for the
full arsenal of planned platforms including space
station, the polar platform, and tethered
satellites--but the platform of choice tends to be
Explorer-type free flyers Where the science can
fully drive the mission scenarios and where the
perturbation environment is in the hands of the
investigator. Table 3-1 shows, in a very simplistic
way, how various platforms satisfy the needs of
the experiments that would be undertaken to
solve the scientific questions that have been
posed. The spacecraft of choice is indicated by
the double X.
Table 3-1. Applicability of Platforms to
Experiment
Science
Goal
Platform
Spare Station
Attached Payloads
Polar Platform
Co-orbit Platform
Tethered
Man-Tended
ISTS
Space Lab
Tethered
Explore_
Rockets
*Balloons
Ground-Based
Lower
Thermosphere
& Mesosphere
X
XX
X
Global
Electro-
dynamics
X
XX
X
X
X
Detailed Fundamental
Understanding of Aeronomi¢
Thermosphere Processes
X
X
X
X X
XX
X
XX XX
X
X X
12
The optimal missions for studying the identi-
fied problems need to be (and are being) defined
by the scientific community. Two missions that
emerge clearly from even a superficial analysis of
the state of understanding of the upper atmo-
sphere are (1) a mission that would remotely
sense the mesosphere and lower thermosphere,
and (2) a mission of one or two spacecraft that
would combine in-situ and remote sensing in the
thermosphere that would be in orbit during the
ISTP program. These latter spacecraft would
employ on-board propulsion, as did the Atmos-
phere Explorers, to give them access to a wide
range of altitudes and a long lifetime. These two
missions, supplemented by the application of att-
ached payloads, tethered satellite measurements,
and ground based measurements, would fuel a
vital scientific enterprise in the upper atmos-
pheric discipline.
3.5 Summary
During the 1970s and 1980s the thermo-
sphere was explored extensively with the Atmo-
sphere Explorer satellites, with the Dynamics
Explorer 2 satellite, and through supporting
observations from rockets, balloons and ground-
based instruments. A major increment in our
understanding of the thermosphere resulted from
the analysis of the data accrued through these
observations, but, as in any such complex endea-
vor, new questions were posed by the data that
have not yet been answered. The mesosphere and
lower thermosphere have been less thoroughly
studied because of the difficulty of accessibility
on a global scale, and many rather fundamental
characteristics of these regions are not well
understood. Given these circumstances, together
with the great power of today's models and the
availability of new measuring technology, sub-
stantial gains in our understanding of the upper
atmosphere can be envisioned as a product of a
relatively modest investment in experimental
investigations and associated theoretical and
modeling studies. Moreover, the ISTP program
shows a compelling need for simultaneous mea-
surements in these regions to define the state of
the ultimate sink for many of the energetic pro-
cesses that will be studied in the ISTP. Through
careful planning, a major defect in the ISTP
measurement suite can be eliminated by a mis-
sion that would also address most of the out-
standing problems in thermosphere and mesos-
phere science.
A wide variety of measurement platforms
can be used to implement various parts of the
measurement strategy, but the major thrusts of
the program would require Explorer-class mis-
sions. A remote sensing mission to explore the
mesosphere and lower thermosphere and one or
two Explorer-type spacecraft to enable a mission
into the thermosphere itself would provide the
essential components of a productive program of
exploration of this important region of the upper
atmosphere.
13



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