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4
Upper-Atmosphere and Near-Earth Space Research Entering the
Twenty-First Century1
Summary
This Disciplinary Assessment identifies those research
essentials with the strongest societal and environmental impacts
that derive from the scientific disciplines covered by the National
Research Council's (NRC's) Committee on Solar-Terrestrial Research
(CSTR) and Committee on Solar and Space Physics (CSSP). These
committees are concerned with the areas of solar and heliospheric
physics, magnetospheric physics, ionospheric physics, middle- and
upper-atmospheric physics, and cosmic-ray physics.
1 Report of
the Committee on Solar-Terrestrial Research and the Committee on
Solar and Space Physics. Committee on Solar-Terrestrial Research:
M.A. Geller (Chair), State University of New York, Stony Brook;
G.P. Brasseur, National Center for Atmospheric Research; J.V.
Evans, COMSAT Laboratories; P.A. Evenson, Bartol Research
Institute, University of Delaware: J.F. Fennell, The Aerospace
Corporation; J.T. Gosling, Los Alamos National Laboratory; S.R.
Habbal, Harvard-Smithsonian Center for Astrophysics; M. Hagan,
National Center for Atmospheric Research; M.K. Hudson, Dartmouth
College; G. Hurford; California Institute of Technology; M.C.
Kelley, Cornell University; J.U. Kozyra, University of Michigan;
N.F. Ness, Bartol Research Institute, University of Delaware; A.D.
Richmond, National Center for Atmospheric Research: T.F. Tascione,
Sterling Software; and R.K. Ulrich, University of California, Los
Angeles. Committee on Solar and Space Physics: J.G. Luhmann
(Chair), University of California, Berkeley; S.K. Antiochos, Naval
Research Laboratory; T.I. Gombosi, University of Michigan, Ann
Arbor; R.A. Greenwald, Applied Physics Laboratory; R.P. Lin;
University of California, Berkeley; M.A. Shea. Air Force Phillips
Laboratory: H.E. Spence. Boston University; K.T. Strong, Lockheed
Palo Alto Research Center; and M.F. Thomsen, Los Alamos National
Laboratory.
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Major Scientific Goals and
Challenges
When societal and environmental impacts are considered, the
dominant scientific and technical goals in upper-atmosphere and
near-Earth space can be identified as the following:
• to understand the physical, chemical, and dynamical
processes that determine the interactions between the stratosphere,
climate, and the biosphere;
• to develop the infrastructure that will permit
operational forecasting of "space weather";
• to understand the relationships between changes in the
middle and upper atmosphere and the Earth's surface and
lower-atmospheric climate; and
• to study solar variability and its influence on the
middle and upper atmosphere.
Key Components of the Scientific
Strategy
The components of the strategy to address the major scientific
issues in upper-atmosphere and near-Earth space science are
developed on the basis of four national goals:
1. To study atmospheric processes using observations, laboratory
research, theory, and modeling.
2. To have the necessary observations, understanding, modeling
capability, and transfer to operations to permit skillful forecasts
of "space weather."
3. To document middle- and upper-atmospheric change and produce
models that consistently simulate these changes along with those of
the lower-atmosphere-surface system.
4. To document changes in solar output, determine how these
affect lower-atmosphere and surface climate, and compare these with
the climate record.
Scientific Requirements for the Coming
Decade(S)
Role of the Stratosphere in Climate,
Weather Prediction, and Tropospheric Chemistry
The stratosphere plays two roles in the climate system. The
first involves the impact of stratospheric trace gases and
aerosols, including those of anthropogenic origin, on radiative
fluxes through the tropopause. The second role of the stratosphere
in the climate system is through the dynamic coupling between the
troposphere and the stratosphere. Considerations of the
stratospheric role in various aspects of climate and weather
include the following:
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• modeling and observational studies of how the
stratosphere determines various aspects of climate,
• determinations of how the stratosphere should be
correctly represented in numerical forecast models,
• analysis to determine if present and anticipated
stratospheric data are sufficient for climate and weather
forecasting purposes, and
• analysis to determine how stratospheric change will
affect tropospheric chemistry.
Space Weather
In order to "nowcast" and ultimately forecast key aspects of the
near-Earth space environment with the goal of mitigating the
negative effects of space weather on human life and technology,
progress must be made on the following fronts:
• achieving a basic understanding of the relevant physical
phenomena and processes so that physical models of the near-Earth
system can be developed;
• putting in place the infrastructure to convert research
models into operational models;
• obtaining the necessary data to assimilate into and test
numerical models of space weather;
• improving on existing statistical models that specify
"space climate"; and
• producing nowcasting and numerical forecasting
capabilities and using them to develop mitigation strategies.
Global Change in the Middle-Upper
Atmosphere
It is critical to understand the effects of natural variability
and anthropogenic effects on the ozone layer, the influences of the
stratosphere on tropospheric climate, and the impact of
upper-atmospheric changes on space-based systems and
telecommunications. Scientific requirements in this area
include
• analysis of historical data from systems operating from
the 1940s to the 1960s;
• monitoring sensitive parameters in the middle and upper
atmosphere;
• monitoring inputs to the middle and upper atmosphere from
space above and the lower atmosphere below;
• understanding atmospheric phenomena that are now poorly
understood, such as "sprites"; and
• developing models that correctly treat disparate and
interacting processes important for coupling the middle and upper
atmosphere with regions above and below.
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Long-Term Solar Variability and Global
Change
The Sun undergoes a variety of small changes in its radiant and
corpuscular energy output. Long-term, well-calibrated measurements
of the outputs and understanding solar variations and the
atmospheric response to them are the focus of studies in this area.
Scientific requirement for this topic include
• measurement of the solar energy output continuously over
at least one full solar cycle,
• investigations of the Earth's temperature and middle- and
upper-atmo-sphere chemical responses to changes in the Sun's energy
output, and
• studies comparing solar variations to those of similar
stars.
Expected Benefits and Contributions to
the National Well-Being
A successful program of research on the upper atmosphere and
near-Earth space, with implications for the long-term stability of
the ozone layer, will provide insight into issues of the biological
effects of increased ultraviolet radiation and the effects of
changes in the middle and upper atmosphere on spacecraft operating
practices and radio communication.
Upper-Atmosphere and Near-Earth Space
Research Tasks
Recommended Stratospheric
Research
Stratospheric Ozone
• Deploy manned and unmanned aircraft to make
high-precision, high-data-rate measurements.
• Use stratospheric satellite measurements from the Earth
Observing System (EOS) to make comprehensive upper-air chemistry
measurements.
• Develop three-dimensional, stratospheric models to assess
the response of stratospheric ozone to various atmospheric emission
scenarios.
Volcanic Effects
• Improve characterization and modeling of volcanic
aerosols in the stratosphere for studies of stratospheric
heterogeneous chemistry and radiation transfer.
• Improve microphysical models for characterizing
stratospheric aerosols to improve atmospheric models.
• Improve currently crude treatments of heterogeneous
chemistry in atmospheric models as a fundamental requirement for
making atmospheric chemistry models more realistic.
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Atmospheric Effects of Aircraft
• Develop three-dimensional stratospheric models including
heterogeneous chemistry and microphysics to make these models more
realistic.
• Improve characterization of stratosphere-troposphere
exchange to correctly treat the exchange of chemical constituents
near the tropopause.
Stratospheric Role in Climate and
Weather Prediction
• Test effects of including a more realistic stratosphere
in numerical predictions to determine how this will affect forecast
skill.
• Develop better understanding of upper-troposphere and
lower-stratosphere water vapor measurements for the improvement of
models.
Recommended Space Weather
Research
• Develop a basic understanding of the physical phenomena
and processes to provide the basic knowledge required for space
weather models.
• Develop better statistical space climate models to
provide useful forecasts of space climate.
• Develop nowcasting and numerical forecasting capability
to provide more skillful space weather forecasts.
• Evaluate mitigation strategies to ensure the best use of
state-of-the-art space weather forecasts.
Recommended Research on Global Change
in the Middle and Upper Atmosphere
• Analyze historical data to extend the data record for
identifying changes in the middle and upper atmosphere.
• Monitor sensitive parameters in the middle and upper
atmosphere to identify parameters that show unexpected
variability.
• Model inputs to the middle and upper atmosphere to
identify the drivers for middle- and upper-atmosphere change.
• Pursue research on poorly understood processes to
determine the significance of global change.
• Understand and model chemical and physical interacting
processes to permit the development of comprehensive general
circulation models.
• Distinguish between natural and anthropogenic effects to
determine their relative importance in middle- and upper-atmosphere
global change.
• Investigate the consequences of middle- and
upper-atmosphere changes on biotic systems, tropospheric chemistry,
and climate.
Recommended Research on Solar
Variability and Global Change
• Measure the solar energy output continuously over at
least a full solar cycle to establish the range of variation of
solar radiant and corpuscular energy.
• Establish the Earth's temperature sensitivity to
variations in the solar
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output to separate anthropogenically produced changes in
temperature from solar-induced changes.
• Determine the atmospheric effects of changes in solar
x-ray and ultraviolet emissions to determine the response of
middle- and upper-atmosphere chemistry and ionization to such
changes.
• Determine the Sun's interior dynamics to develop a model
of the solar dynamo.
• Explore the variability of solar-type stars to develop
statistical estimates of the likelihood of solar variations.
Introduction
This Disciplinary Assessment identifies those research
priorities with the strongest societal impacts that derive from the
scientific disciplines covered by the NRC's Committee on
Solar-Terrestrial Research and Committee on Solar and Space
Physics. These committees cover the scientific areas of solar and
helio-spheric physics, magnetospheric physics, ionospheric physics,
middle- and up-per-atmospheric physics, and cosmic-ray physics. A
brief description of the coupled Sun-Earth system that is the
subject of these research areas is given below. The four research
priorities identified are then discussed.
The Sun
The most obvious solar output reaching the Earth is the steady
5,700 K black-body photon emission from the Sun's visible
photosphere. Other, more subtle solar-terrestrial connections that
originate from different regions of the Sun and through other forms
of energy emissions also exist. Above the photo-sphere, the
temperature of the solar atmosphere first falls slowly in the
chromo-sphere and then rises rapidly up to 106 K in the solar
corona at a few thousand kilometers above the photosphere. The
solar corona is heated from below by mechanisms that are still not
well understood. In regions where the solar magnetic field cannot
constrain it, the hot ionized gas expands outward to form the solar
wind and reaches supersonic speeds at a distance of a few solar
radii.
Interplanetary Space
Interplanetary space is permeated by the dilute, yet hot and
fast-flowing, solar wind plasma (see Figure II.4.1). Owing to the
high electrical conductivity of the plasma, remnants of the solar
magnetic field are "frozen" into the solar wind flow. Rooted at one
end in the Sun, the interplanetary magnetic field (IMF) is twisted
into an Archimedean spiral by the combined effects of solar
rotation and the outward solar wind flow. Energetic particles
produced in solar outbursts and in interplanetary space are guided
by the IMF.
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Figure II.4.1
Solar connections.
The Magnetosphere
The geomagnetic field presents an obstacle to the solar wind.
Its interaction with the solar wind produces a large cavity in the
flow, called the magnetosphere (see Figure II.4.1), that surrounds
the Earth. This cavity is compressed on the sunward side by the ram
pressure of the solar wind, but it is elongated on the night side
into a very long magnetic "tail." While shielding the Earth from
the incident solar wind, the geomagnetic field also acts as a
magnetic bottle that traps and holds plasma that leaks in from the
solar wind and escapes from the Earth's ionosphere. These plasmas
are heated, accelerated, and transported within the magnetosphere
by a variety of processes that are only partially understood.
Of particular concern in this Disciplinary Assessment are
dramatic changes to the Earth' s magnetosphere occurring as a
result of propagating structures in the solar wind. A southward
turning of the interplanetary magnetic field increases the transfer
of energy from the solar wind into the magnetosphere, resulting in
increases in the trapped (Van Allen) radiation, auroras, changes in
the surface magnetic field, and heating of the upper atmosphere
that creates high-speed winds and composition changes.
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The Ionosphere-Upper Atmosphere
Earth's upper atmosphere extends out to several hundreds of
kilometers and is partially ionized by extreme ultraviolet
radiation from the Sun, creating what is known as the ionosphere.
Cosmic-ray and solar energetic particle bombardment and
magnetospheric particle precipitation augment solar-produced
ionization, especially at auroral latitudes. Daily changes in solar
ionizing radiation and heating drive large-scale motions in the
upper atmosphere and ionosphere. Electrodynamic coupling between
the ionosphere and magnetosphere allows magnetospheric currents and
fields to influence ionospheric structure. Collisional, frictional,
and chemical changes in the neutral atmosphere also force changes
in ionospheric structure and dynamics.
The Middle Atmosphere
The atmosphere is divided into a number of layers associated
with obvious changes in temperature structure. This structure (see
Figure II.4.2) is determined primarily by the absorption of solar
radiation. The atmosphere is mostly transparent to the bulk of
solar radiation, which is in visible wavelengths (400 to 700 nm).
This leads to most of the solar radiation being absorbed at the
Earth's surface. Sensible and latent heat transport from the
Earth's surface is responsible for heating the lower atmosphere,
and this accounts for the fall-off of temperature with increasing
altitude in the troposphere. Ultraviolet (UV) solar radiation
(wavelengths less than 242 nm) dissociates molecular oxygen and
leads to stratospheric ozone formation. Ozone, in turn, absorbs
ultraviolet radiation at slightly longer wavelengths (less than
about 300 nm). These processes account for the temperature
increasing with height throughout the stratosphere. Above this, the
temperature decreases with height in the mesosphere until
extreme-ultraviolet (EUV) radiation (wavelengths less than 180 nm)
dissociates and ionizes the atmospheric gases, leading again to a
temperature increase with altitude in the thermosphere.
Ozone losses occur as a consequence of chemical reactions in
which catalytic reactions with hydrogen, nitrogen, and halogen
oxides are crucial. Any process that increases the concentration of
such reactive species will lead to stratospheric ozone decreases.
For instance, industrial chlorofluorocarbon (CFC) emissions have
increased the concentrations of reactive chlorine in the
stratosphere and led to observable ozone losses that are of
societal concern.
Atmospheric waves, on various spatial and temporal scales, are
forced mainly in the troposphere. As these waves propagate upward
and grow in amplitude, they become very important and comprise a
major component of the circulation at higher altitudes. These waves
affect the dynamics of the middle atmosphere, which in turn gives
rise to transports of many minor atmospheric constituents,
including ozone. Since ozone absorbs solar UV radiation but is
affected by
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Figure II.4.2
Schematic illustration of the atmospheric thermal structure and electron (ion) content. Various processes
of interest are depicted, showing the altitude range over which they are observed. These include nacreous and
noctilucent clouds, the aurora, and the attenuation of selected solar wavelength ranges that produce the
ionospheric layer and excite and dissociate atmospheric species. Source: J.H. Yee and associates, Applied
Physics Laboratory, Johns Hopkins University.
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chemical and transport processes, middle-atmospheric behavior is
determined by rather complex radiation-chemistry-dynamics
interactions.
Cosmic Rays
In addition to the neutral gas, plasma, and field environments
described above, the Earth is immersed in an extremely tenuous rain
of highly energetic charged particles called cosmic rays. Such
particles are produced both in our galaxy and in other galaxies.
Processes occurring at the Sun and in interplanetary space also
occasionally accelerate particles to cosmic-ray energies. The
Earth's magnetic field acts as a barrier to this cosmic-ray
bombardment, but the shielding effect is imperfect, especially in
the magnetically open polar regions and at high altitudes. Owing to
their great energy, cosmic rays can be especially dangerous to man
and machine throughout space.
Research Priorities
A recent report (NRC, 1995b) of the CSSP-CSTR identified
research priorities for overall scientific progress in these areas.
Here, we identify imperatives for research, selected from the
broader priorities in the earlier report (NRC, 1995b), that
specifically address the national goals of
• protecting life and property,
• maintaining environmental quality,
• enhancing fundamental understanding, and
• enhancing economic vitality.
The topics selected with these criteria are, in priority
order,
1. stratospheric processes important for climate and the
biosphere,
2. space weather,
3. middle- and upper-atmospheric global change, and
4. solar influences.
This prioritization reflects not only national priorities but
also other considerations such as timeliness of the research and
relevance to other areas of atmospheric research.
Research into the topic ''stratospheric processes important for
climate and the biosphere'' is vital to our understanding of the
Earth's atmosphere. Anthropogenically produced substances have been
shown to be altering the stratospheric ozone layer; studies in this
area are important for maintaining environmental quality.
Regulations have been promulgated internationally that ban the
future
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use of chlorofluorocarbons, and decisions concerning future
supersonic transport aircraft may hinge on predictions of their
effects on stratospheric ozone. Thus, research in this area has
important considerations for national economic vitality. Finally,
given the fact that stratospheric ozone affects the biosphere's
exposure to harmful UV radiation, which can have consequences for
human health, and that changes in ozone can affect the Earth's
climate, research into this area clearly can have an effect on life
and property. Hence, studies of stratospheric processes are
relevant to all four of the criteria enumerated above.
"Space weather" encompasses all of the effects associated with
the variable release of energy from the Sun in the form of x-rays,
energetically charged particles, and streams of plasma with
embedded magnetic fields. Through a complex sequence of events, the
plasma streams interact with the Earth's magnetosphere giving rise
to auroral events, the Van Allen radiation belts, and other
geophysical phenomena grouped under the heading "magnetic storms."
Such events have caused failures in power transmission grids at
high latitudes and the loss of control over communication
satellites. Outbursts of x-rays (from flares) or protons pose
threats to humans in space. Even the occupants of high-flying
aircraft on polar routes are exposed to much higher levels of
radiation at certain times.
The space weather imperative is particularly strong because of
its timeliness. The reliance on space systems in the civilian
sector for meteorological weather forecasting, navigation, and
communications is increasing at a rapid rate. This enormous
investment of resources is at considerable risk until a coordinated
approach to space weather forecasting and improved models of
radiation hazards are developed. As a consequence of space research
conducted during the past several decades, our understanding of the
linkages between the Earth and the Sun has reached a level at which
the development of numerical models can now be attempted and
efforts undertaken toward using them for prediction. Our inability
to relate this topic to "maintaining environmental quality" is the
reason this topic has been given second priority, but its
importance for manned space flight and for prediction of near-Earth
space conditions that affect military communications adds strategic
elements that have not been fully explored in this report.
The "middle and upper atmosphere" is the region of our
atmosphere extending roughly from 10 km altitude out to several
hundred kilometers. This region is subject to long-term changes due
to both man-made and solar variability effects. Man-made changes in
the ozone layer and in the concentrations of other trace gases are
expected to cause major changes in the temperature structure within
these regions, which will be most noticeable at high altitudes.
These changes will influence the atmospheric circulation, including
possible effects on tropospheric climate, and may influence space
operations and radio-wave communications. Although much remains to
be learned about this region, models are being constructed that
include many of the important effects related to dynamics,
chemistry, and energetics. This topic was judged to relate less
strongly to the stated societal imperatives than those listed
previously, so that it was given third priority.
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Figure II.4.18
Reconstruction of historical record of sunspot number including times of near
absence of sunspots during the seventeenth century. Source: Hoyt et al., 1994.
Reprinted with permission of Springer-Verlag New York.
Figure II.4.19
The 11-year running mean of the sunspot number and global average sea surface
temperature anomalies. In a one-dimensional model of the thermal structure of the
ocean, consisting of a 100 m mixed layer coupled to a deep ocean and including a
thermohaline circulation, a change of 0.6 percent in total solar irradiance is needed to
reproduce the observed variation of 0.4°C in sea surface temperature anomalies.
Source: Reid, 1991. Reprinted with permission of the American Geophysical Union.
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trations since each gas component interacts with the radiation
flow through the Earth's atmosphere in a unique manner.
Our current knowledge of the sensitivity of climate to both
solar and anthropogenic effects is limited by the difficulty in
isolating the effect of a small-amplitude total irradiance
variation from the intrinsic short-term variability of the weather
system. Any atmospheric response to forcing function variations
will become evident only after the weather (or natural climate)
variability has been averaged out. Moreover a large amount of the
energy in the climate system is stored in the Earth's oceans, and
this will tend to smooth out the effects of small changes. Many of
the records of long temporal duration refer to a limited
geographical region and thus are especially vulnerable to local
effects. The fact that the effects, if present, have to be
extracted from a highly variable background has made it difficult
to detect a signal identifiable with solar influences. However,
longer-term effects may, in fact, be present in the system at a
significant level.
Another critical problem in understanding the effects of solar
variability comes from the greater variability in the Sun's
ultraviolet (UV) output than of visible radiation. This problem is
magnified because in those parts of the Earth' s atmosphere where
UV radiation is absorbed, its effects are dominant. The solar UV
output is strongly dependent on the phase of the solar cycle and is
often strongly modulated by solar rotation. These separate natural
time scales for solar variations can be used as a tool for the
identification of terrestrial responses although neither time scale
is ideally suited for study. The shorter periods are well suited to
solar observations but easily masked by terrestrial variability.
The longer periods have less reliable records for solar output and
terrestrial response, but the amplitude of response should be
largest. In addition, solar UV variability is not well described by
a single parameter since it is affected by the details of active
region size, strength, and position on the solar disk, as well as
by the strength of the widely distributed magnetic network. Thus, a
multivariate analysis is required in principle even though the
existing single-variate analyses have produced only tentative
correlations.
Progress toward the goal of separately measuring climate
sensitivity and its response to both solar and anthropogenic
forcing variations can be made by fully utilizing the opportunity
provided by UARS data, which measure simultaneously the solar
inputs and the atmospheric response. This UARS opportunity is of
limited duration and has sampled only the declining phase of the
solar cycle. The largest and strongest sunspots typically appear
during the rising phase of the sunspot cycle, and the effect of
sunspots on UV and EUV fluxes has not been monitored and studied in
detail yet. An extended UARS mission or future space-based
observations could provide the missing observations during the
rising phase of the next sunspot cycle.
Progress in the study of longer-term variability requires a
separate approach. Current space-based measurements provide
high-quality data that address shorter time-scale variations but
cannot help with the study of longer time-scale prob-
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Representative terms from entire chapter:
upper atmosphere
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lems. Historical reconstructions of the atmospheric and ocean
thermodynamic state, atmospheric composition, and details of the
solar output are required to disentangle the interrelationships on
longer time scales. These reconstructions will build on the base
provided by the verification from UARS and future spacecraft
observations. Although such reconstructions based on proxy models
are critical to the analysis of historical data, they will require
verification by future direct observations and cannot be used as
replacements for such observations.
Solar Influences on the Earth's Upper
and Middle Atmosphere
The concentration of chemical constituents in the atmosphere is
affected by photodissociation and photoionization processes, and
hence by the solar flux that penetrates the Earth's atmosphere.
Since the shortest wavelengths, which are most affected by solar
variability, are absorbed in the highest layers of the atmosphere,
the chemical response of the atmosphere is expected to be greatest
at high altitudes (i.e., in the thermosphere and mesosphere).
However, species such as ozone in the stratosphere, which is
produced from the photolysis of molecular oxygen, are also
sensitive to changes in extraterrestrial solar flux.
Solar activity has a direct impact on the Earth's ionosphere.
Substantial increases in solar EUV radiation and x-rays, associated
with enhanced solar activity, lead to substantial increases in the
concentrations of ions and electrons in the D-, E-, and F-regions
of the ionosphere. In the stratosphere, the largest ionization
source results from the penetration of galactic cosmic rays, which
is modulated by the solar cycle; hence, the stratospheric
ionization rate is reduced during periods of high solar
activity.
The abundance of neutral species is also affected by solar
activity in the middle and upper atmosphere. For example, the
concentration of nitric oxide, a constituent produced by ionic and
photolytic processes in the thermosphere, is significantly enhanced
in this region of the atmosphere during high solar activity. In the
mesosphere, significant variations associated with solar
variability affect the concentration of water vapor, a molecule
that is photolyzed by shortwave ultraviolet radiation. Finally, in
the case of ozone, the response is significant and results from the
combination of several processes. In the stratosphere and the
thermosphere, the ozone concentration increases with solar activity
as a result of enhanced photolysis of molecular oxygen. In the
mesosphere, the ozone response is dominated by the enhanced ozone
loss caused by hydroxyl (OH) and hydroperoxyl (HO2) radicals produced by a more vigorous
photolysis of water vapor during high solar activity. The resulting
change in the vertically integrated ozone concentration (column
abundance) over a solar cycle is not greater than 1 or 2
percent.
As middle-atmosphere heating results primarily from the
absorption of solar ultraviolet radiation by ozone, the
temperatures of the stratosphere and mesosphere are also affected
by solar activity. Amplitudes of the temperature varia-
Page 264
tion on the 11-year time scale have been inferred from
ground-based lidar and from satellite observations. Temperature
amplitudes derived from atmospheric models are not in agreement
with values deduced from observations. A major scientific question
that remains unsolved is the potential dynamical response of the
atmosphere to the 11-year solar cycle. Although substantial changes
in dynamical patterns within a period of 11 years have been
reported in the stratosphere and even in the troposphere on the
basis of statistical analyses, no mechanism has yet been identified
to explain these variations, which cannot be reproduced by
atmospheric models.
Although evidence for the response of chemical compounds such as
ozone to the 11-year solar cycle is provided by long-term
observations, definitive quantitative response has not yet been
established experimentally, because of insufficient precision in
the data and the limited lifetime of the instrumentation. The
observational evidence is much better established for the ozone and
temperature response on the 27-day time scale, where analyses of
satellite observations and model calculations are in fairly good
agreement.
The direct measurements of the solar UV and x-ray irradiance
provided by current satellites allow study of the current
atmospheric composition, but comparable observations are not
available for other time periods and may not be available in the
near future. Consequently, it is important to be able to model and
reproduce these irradiances based on observations of other solar
parameters. These quantities are mapped on a regular basis so their
positions on the solar disk can be used in the models. Other
integrated measurements (e.g., the 10.7 cm flux), which indicate
the strength of various integrated UV, EUV, and x-ray fluxes, are
needed for middle- and upper-atmospheric chemistry studies but
cannot provide definitive measurements with the necessary
precision.
The need for detailed knowledge of the distribution and strength
of the activity comes from the fact that solar ultraviolet
radiation is produced in regions on the solar surface where
magnetic fields are concentrated. Often these have sunspot groups
at their center, but sometimes the area of higher-than-average
magnetic field is a remnant of previous sunspots. An individual
sunspot typically is identifiable for a period of up to 30 to 60
days. However, there are regions of enhanced activity that can
persist for one to two years. It is common for the solar surface to
be covered very unevenly by sunspots so that one hemisphere will
emit a high level of ultraviolet radiation while the other
hemisphere is very quiet. This configuration produces a strong
rotational modulation to the solar UV flux.
The data bases of adequate direct measurements for both the
total solar irradiance and the solar UV irradiance are limited to
the last 10 to 20 years. Prior to this, the state of the solar
output had to be deduced from proxy information. The most readily
available proxy is the one shown in Figure II.4.18—the sunspot
number. This index is based on the visible distribution of sunspot
area and position and does not take into account the more widely
distributed magnetic field, which is typically associated with
sunspot groups. Other regions of en-
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hanced fields are sometimes found unassociated with sunspots and
indeed can be at higher latitudes than sunspots. They are also
present during times of sunspot minimum. To fully evaluate the
state of the Sun, more than one proxy parameter is required.
Current models of the solar UV irradiance include components due to
the quiet Sun, sunspots, active regions, and a fourth widely
distributed component that comes from linear boundary regions
between large convection cells.
Physical Basis of the Solar Activity
Cycle
Fundamental to the question of solar influences on the Earth's
environment is the occurrence on the Sun of an 11-year cycle of
magnetically driven activity in the form of sunspots, solar
atmosphere temperature changes, and unstable eruptions. Because of
terrestrial responses to solar activity, the functioning of the
solar cycle can impact society. The occurrence of periods of low
solar activity, such as those shown in Figure II.4.18, indicates
that the solar cycle must involve some complex nonlinear processes
that affect solar irradiance with or without magnetic activity. If
the periods of low activity coincide with periods of low total
solar irradiance, then entry of the Sun into a new quiet period
could produce global cooling. Indeed, the previous low period of
the Maunder Minimum coincided with a time of unusually low
temperatures in Europe, sometimes referred to as the Little Ice
Age. Figure II.4.20 illustrates this relationship. Without a
fundamental understanding of the solar cycle, neither the
probability of such future behavior nor the occurrence of possible
precursors can be recognized. Should there be a change in the
apparent behavior of the solar cycle at some time in the future, we
would want to know if this signaled the onset of a Maunder Minimum
period or some less significant statistical variation.
The most prominent indicators of solar activity are sunspots.
Within these spots the temperature is much lower than that of the
surrounding atmosphere, and the emergent flux of visible radiation
is substantially reduced. Evidently the convective motions that
bring energy from the Sun's interior to the surface elsewhere are
suppressed in the spots, and the reduced spot temperature results
from the absence of an efficient process to replace the radiation
emitted into space. Sunspots typically occur in pairs of opposing
polarity. Each spot pair in the Sun's Northern Hemisphere has one
east-west orientation and those in the Sun's Southern Hemisphere
have the opposite orientation. This configuration is naturally
interpreted in terms of a source toroidal magnetic field, with each
sunspot pair being an arch that breaks the solar surface. The
direction of the field within each torus is opposite in the Sun's
Northern and Southern Hemispheres. In addition, there is a weak
background solar polar field. The directions of the two toroidal
fields and the weak polar field all reverse every 11 years. In
addition, the Sun rotates differentially, with an inertial period
of 24 days at its equator and 35 days in the polar regions.
Although the above sequence of observed changes through the
solar cycle has
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Figure II.4.20
Relationship between severity of winter in Paris and London (top curve) and long-term solar activity variations (bottom
curve). Shaded portions of curve denote times of Spörer and Maunder Minima in sunspot activity. Dark circles indicate
sunspot observations by the naked eye. Details of solar activity variation since 1700 are indicated in the bottom curve
by sunspot number data. Winter severity index has been shifted 40 years to the right to allow for cosmic-ray-produced
14C assimilation into tree rings. Source: Daddy, 1976. Reprinted with permission of the American Association for the
Advancement of Science.
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been known for many years, there is as yet no successful theory
of the process. The principal components of the solar activity
cycle must come from the interaction of convection and rotation.
The pattern of differential rotation should be derived from
fundamental hydrodynamic theory but usually is just postulated as
part of the input to a model. More importantly, the pattern of
internal rotation has only recently become known, at least in a
preliminary fashion. This was a critical free parameter, and it was
expected that the Sun rotated more rapidly below the surface than
at the surface. Such a pattern made it possible to reproduce the
solar cycle, but that pattern is now known to be incorrect.
Speculation about the driving region for the solar cycle has
shifted from the zone just below the solar surface to the interface
between the convection zone and the radiative deep interior at a
distance of about 70 percent of the way from the Sun's center
toward the solar surface. These theoretical ideas are at a very
primitive stage of development and have not even been able to
reproduce such essential aspects of solar activity as the 11-year
period, the direction of sunspot migration, or sunspot size.
Reproduction of the most basic features of the solar cycle must
be the first objective of any modeling effort, but this is only a
step toward a more urgent goal: understanding the mechanisms or
indicators of changes in the Sun's overall level of activity as
measured by the strength of the solar cycle. Two historical changes
in the cycle strength—the absence of activity during the
Maunder Minimum (and similar earlier minima) and the growth of the
cycle strength during the past century—are even further from
being understood than the cycle itself but have potentially
substantial climatic implications. Because these changes have a
very long time scale, high-quality data from the most recent
space-based era provide little guidance. Hints in the historical
record from the late Maunder Minimum period indicate that there
were changes in the Sun's rotation pattern or radius associated
with the low level of output. Perhaps most intriguing in guessing
the nature of the activity cycle during this low period was the
chaotic nature of the first few cycles as the Sun recovered. The
11-year period did not manifest itself until nearly 50 years after
the first moderate number of spots were seen. There also seems to
be a gradual shortening of the cycle length to 9.5-10 years instead
of the nominal 11.
The new tool of helioseismology represents the best hope of
making fundamental progress in understanding the processes that
govern the solar cycle. With this tool, it has become possible for
the first time to measure velocities below the solar surface. Both
the largest-scale motions involving flows over the entire
convective envelope and the smaller-scale flows associated with
active regions are accessible with this tool. By making such
measurements over a full solar cycle, it should be possible to
obtain clues as to the origin and nature of the solar dynamo. Two
major experiments in helioseismology have recently begun with the
deployment of GONG (Global Oscillation Network Group) instruments
at six sites around the surface of the Earth and with the launch of
three helioseismology experiments on the SOHO (Solar and
Heliospheric Observatory) spacecraft. The
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GONG instruments already are beginning to return data of high
quality and maps of internal rotation.
In addition to providing a tool for understanding solar interior
dynamics, helioseismology may be able to assist with long-range
projections of space weather. Active regions and magnetism
ultimately depend on the dynamics of the deep solar interior. The
oscillation frequencies and their shifts are dependent on the
interior velocity field and on the interior structure, including
possible strong magnetic field effects. Thus, helioseismology data
have the recently demonstrated capability to detect magnetized
regions before they appear at the solar surface. On at least one
occasion, precursor changes in the solar acoustic spectrum were
measured prior to the arrival of a sunspot group on the solar
surface. Similar changes are not seen in control regions where
sunspots did not appear. This sequence is shown in Figure II.4.21.
This type of observation may provide a means of long-range solar
activity forecasting. Additionally, there may be relationships
between the Sun's acoustic spectrum and the coronal magnetic
configuration. This area is completely unexplored at present. Both
the GONG and the SOHO experiments have planned durations of two
years with possible extensions to longer periods.
Long-Term Changes in Solar Behavior:
Solar-Type Stars
The study of solar variability through observations of the Sun
is limited in two ways: there is no easy way to extend the time
base beyond the current era, and there is no way to change
parameters such as the rotation rate that govern solar dynamics.
The study of Sunlike stars can ease these problems by sampling a
range of states not exhibited by the Sun because of the natural
range in stellar rotation rates. We pay two prices for these
benefits: the properties of the stars are not fully and accurately
known, and there is no way to obtain spatially resolved information
about the distribution of activity over the stellar surface
(although Doppler imaging can provide some information of this
type). Estimates of stellar age are most difficult to obtain and
represent the greatest uncertainty in this technique, with rotation
rate being adopted as the best available indicator. Stellar
observations consist of two parts: (1) a regular measurement of the
strength of the ionized calcium emission (at the H and K
wavelengths) and (2) a regular measurement of broadband stellar
brightness. The longitudinal asymmetry in the distribution of
active regions is found for stars as well as for the Sun, so that
it is routinely possible to measure the rotation rate for stars
from the pattern repeat rate in brightening of the ionized calcium
features. More stars have been followed by using calcium emission
features than broadband photometry. The set with extensive enough
data in both ionized calcium and broadband contains just 10 stars.
By adding the Sun to this set, there are 11 stars.
An important question that can be addressed with this stellar
sample is whether the amplitudes of the solar chromospheric and
total irradiance variation
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Figure II.4.21
Time evolution of magnetic flux and p-mode scattering for the emerging sunspot
group NOAA 5247. Top panel shows magnetic flux as measured from KPNO
magnetograms versus time. Middle and bottom panels show l- and v-averaged
values of scattering phase shifts
d and absorption coefficient a, respectively. Time
of appearance of the spot is indicated by vertical dashed line. Negative phase shifts
in middle panel indicate a signature of p-mode scattering prior to emergence of the
sunspot group. Positive phase shifts observed in the emerged spot are consistent
with previous measurements of phase shifts in other (mature) sunspots.
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are typical for stars of its type. Figure II.4.22 illustrates
the relationship between the amplitude of variation in these two
quantities. This figure shows that
• the Sun is near the low range of variability in its
ionized calcium index, and
• the correlation between brightness variation and
variation in the ionized calcium index is somewhat atypical for the
Sun in the sense that the broadband variation is less than the
ionized calcium index variation for the Sun. Interpreted literally,
this implies that the Sun could in fact have had a much higher
level of change in its energy output than has been observed
recently.
Data of the type plotted in Figure II.4.22 can be obtained only
through long-term studies. At present, the number of stars for
which an adequate set of observations has been obtained is very
small and does not permit any statistically significant
conclusions. One difficulty in the use of stellar data is the
estimation of stellar age. A larger sample of stars would permit
better determination of the age through statistical use of the
rotation rate, spectroscopic characteristics, and stellar motion
indicators.
Key Initiatives
Solar influences on the Earth's environment are subtle and
require careful measurement to be detected reliably. Nonetheless,
as its ultimate energy source, the solar input is fundamental to
the Earth's atmosphere and climate system, and understanding solar
influences on the Earth's environment requires that we do the
following, which has been discussed earlier in more detail.
• Measure the solar energy output with space-based monitors
continuously over at least a full solar cycle.
• Investigate the sensitivity of the Earth's temperature to
variations in the solar energy output.
• Determine the response of the Earth's middle- and
upper-atmosphere chemistry and state of ionization to variations in
the Sun's UV and x-ray emissions.
• Measure the Sun's interior dynamics, and develop a model
of the solar dynamo that both agrees with the Sun's observed
internal dynamical state and reproduces the pattern of solar
magnetic activity.
• Study possible long-term changes in solar behavior
through the observation of solar-type stars.
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Figure II.4.22
Possible relationship between the total brightness variability of
the Sun and solar-typestars as measured by
DR'HK, which depends on
chromospheric emission of Ca II H and Kspectroscopic lines.
Chromospheric emission is sensitive to magnetic activity, whereas
the totalbrightness variation includes sunspot blocking,
chromospheric brightening, and other less wellunderstood effects.
Quantities shown are root-mean-square variations; peak-to-peak
variationsare roughly three times larger. Position of the Sun as
indicated by 
is taken from SSMmeasurements and the solar
DR'HK.
Dashed line defines the solar brightnesschromospheric activity
change ratio based on yearly averaged data of SSM and NSOfrom 1980
to 1988. Inverted triangle (
) is longer-term upper bound of
thesolar total irradiance variation from 1967 to 1984 taken from
rocket and balloon measurements;corresponding value of DR'HK is
estimated from combined solar measurements of MWO (1967-1978)and
NSO (1976-1984). Solid line is linear regression using all data
except the upper limit for the Sun.Most noteworthy in this figure
is the fact that the variability of solar total output seems to be
less than that for other solar-type stars with similar
chromospheric activity. Source: Soon et al., 1994.Reprinted with
permission of Springer-Verlag New York.
Contributions to the Solution of
Societal Problems
The program of research described above will lead to greater
understanding of the nature of solar variability, and will improve
our ability to predict future states of the Sun. Understanding of
the way in which solar variability affects the Earth and its
climate will be enhanced, and we will have more confidence in our
ability to distinguish anthropogenic effects from effects caused by
solar influences.