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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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Page 199 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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Page 200 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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Page 201 • 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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Page 202 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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Page 203 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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Page 204 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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Page 205 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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Page 206 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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Page 207 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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Page 208 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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Page 209 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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Page 261 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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Page 262 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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Page 263 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-

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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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Page 265 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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Page 266 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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Page 267 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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Page 268 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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Page 269 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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Page 270 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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Page 271 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 image 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 (image) 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.