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A Science Strategy for Space Physics: Chapter 4 A Science Strategy for Space Physics 4 The Middle and Upper Atmospheres and Their Coupling to Regions Above and Below SCIENTIFIC BACKGROUND Above the atmospheric region where "weather" takes place lies the middle and upper atmosphere, which provides a complex interface between the lower atmosphere and the space environment. Figure 12 gives a global view of the chemical, dynamical, and energetic processes that are thought to determine the properties of this transition region. Influences from above (solar radiation, energetic particles, magnetospheric electric fields and currents, meteors) and from below (trace gases, atmospheric waves, infrared radiation, thunderstorm electric fields) drive many interacting physical and chemical processes in the middle and upper atmosphere, including global-scale winds, gravity waves and turbulence, photochemical reactions, the global electrical circuit, and radiative, REPORT MENU dynamical, and chemical transports of energy. The interplay between these inputs and the NOTICE processes that redistribute energy from the individual source regions throughout the layer MEMBERSHIP is not well understood. Figure 13 illustrates the different layers of this atmospheric region SUMMARY and some of the important processes that occur there. The middle atmosphere is defined PART I PART II as the region between the tropopause (10 to 15 km altitude) and about 100 km, while the CHAPTER 1 upper atmosphere in the context of the present report extends roughly from 100 km to CHAPTER 2 1000 km. The ions and free electrons that are present above 60 km at day and 100 km at CHAPTER 3 night form the ionosphere. An important aspect of the magnetosphere-ionosphere- CHAPTER 4 atmosphere system is the presence of strong and interactive coupling processes that CHAPTER 5 influence the time-evolution and dynamical properties of each element. This chapter PART III considers those processes internal to the middle and upper atmospheres, as well as the APPENDIX influences of upper atmospheric processes on the nature of magnetosphere-ionosphere interactions. Magnetospheric influences on the ionospheric plasma and electrodynamic feedbacks to the magnetosphere are treated in Chapter 3. file:///C|/SSB_old_web/strach4.html (1 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 FIGURE 12 Global view of energetic, dynamic, and chemical processes that influence the state of the middle and upper atmosphere, along with energy and momentum inputs from above and below. The relative importance of, and interplay among, these processes are frontier areas in scientific research. (Courtesy of Janet Kozyra, University of Michigan.) FIGURE 13 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. (Courtesy of J.H. Yee and Associates, Applied Physics Laboratory, Johns Hopkins University.) In addition to contributing to understanding of how various physical and chemical processes operate and interact, studies of the middle and upper atmospheres have considerable practical value: 1. The middle atmosphere contains the ozone layer that shields the biosphere from solar ultraviolet radiation. Thus, changes in the amount of stratospheric ozone alter, the biosphere's exposure to this harmful radiation. Concern about stratospheric ozone depletion has motivated a concerted international effort to understand and assess the problem and to take appropriate actions. It is now well established that the Antarctic ozone hole is caused by anthropogenic emissions of halocarbons. This conclusion, based on ground-based measurements, four major airborne campaigns, and development and application of stratospheric models, together with increasingly strong evidence implicating halocarbons in global ozone depletion, underlies policy-including the 1990 London and 1992 Copenhagen amendments to the Montreal Protocol, and the U.S. Clean Air Act amendments-to accelerate the phaseout of halocarbons. Very recent measurements have shown that the rates of increase of chlorofluorocarbons (CFCs) and halons are beginning to decrease, while CFC substitutes are beginning to accumulate in the atmosphere. These results, documented through a worldwide measurement network, verify that control of the use and emission of halocarbons is beginning to have an impact on atmospheric halogen levels. file:///C|/SSB_old_web/strach4.html (2 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 Nevertheless, ozone depletion will likely worsen for at least another decade. Such depletion and the associated increase in ground-level ultraviolet radiation not only are significant environmental problems themselves, but are now known to be linked to climate change as well. Decision makers continue to turn to the world scientific community for advice. How well can we forecast (hence manage) our ozone-layer's future? The following issues must be addressed by the scientific community to provide the needed input for public policy: What will be the global ozone losses and surface ultraviolet increases in the next 20 years, during which atmospheric halogen levels are expected to peak? What ozone losses and surface ultraviolet radiation increases could occur in the Arctic (and the Antarctic) during the same time period? What is the quantitative relationship between global, lower-stratospheric ozone depletion, radiative forcing of the Earth-atmosphere system, and climate change? What are the global climate impacts of the Antarctic ozone hole (which is expected to persevere well into the next century)? What are the most "ozone-friendly" halogen substitutes? How well do we understand the role of methyl bromide in ozone depletion? When will the ozone layer begin to be "rehabilitated"? 2. Changes in the middle atmosphere may affect the troposphere and influence the Earth's climate. For instance, the CFCs are greenhouse gases. Thus, their increasing concentrations lead to climatic warming. The CFCs also lead to stratospheric ozone depletion. They may be responsible for observed ozone decreases in the lower stratosphere. Since lower-stratospheric ozone is itself a greenhouse gas, the combined radiative and ozone depletion effects of the CFCs lead to less greenhouse warming when their stratospheric ozone effects are considered. 3. Solar-driven disturbances in the upper atmosphere and ionosphere often affect space and terrestrial technological systems, leading to problems such as increased drag on satellites in low Earth orbit and disruption of radio communications. These and other effects such as radiation and material damage, satellite charging, single-event upsets, and induced ground currents all fall into a class now called space weather phenomena. In general, space weather is the response of the near-Earth space environment to solar variability on short time scales. The Sun's energetic radiation and variations in its extended atmosphere, the solar wind, directly affect the ionization and heating of the Earth's upper atmosphere and the energy variability and content of the file:///C|/SSB_old_web/strach4.html (3 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 magnetosphere. This solar variability results in changes in the atmosphere-ionosphere- magnetosphere system that lead to far-ranging and often detrimental effects, such as loss of communication satellites and damage to power distribution systems, on technological systems that we have come to rely on. The middle and upper atmospheres are strongly influenced by inputs of mass, momentum, and energy from above and below. The absorption of variable solar ultraviolet and x-radiation not only heats the atmosphere, but also initiates chains of photochemical reactions and ionizes the upper atmosphere to form the ionosphere. Additional ionization and chemical energy are provided by the precipitation of magnetospheric particles, galactic cosmic rays, and solar energetic particles, all of which respond to solar variability. Meteors produce ionized trails and are a source of metallic atoms and ions. Highly variable electric fields and currents of magnetospheric origin are a major source of energy and momentum to the high-latitude upper atmosphere, while tropospheric electrical storms can influence the electrical and chemical properties of the middle and upper atmosphere through upward conduction currents and discharges to the atmosphere. The electrical properties of the lower atmosphere in the region of tropospheric weather systems are strongly influenced by ionization due to cosmic ray impacts. Infrared radiation from the Earth's surface and lower atmosphere help determine the energy budget of the middle atmosphere. Many of the middle atmosphere constituents are introduced from below, either directly, or indirectly in the form of chemical precursors. Volcanic eruptions increase the aerosol loading of the stratosphere. Gravity, planetary, and tidal waves that originate partly from the lower atmosphere grow in amplitude as they propagate upward, where they contribute to the momentum and energy budgets of the middle and upper atmosphere and produce turbulence that influences mixing processes. There are important deficiencies in our knowledge of many of these inputs to the middle and upper atmosphere. The Sun's output of extreme ultraviolet and x-radiation is known to be highly variable, but only hard x-rays are regularly monitored, and the existing solar radiation measurements below 40 nm do not have calibrations adequate for many aeronomic applications. The accuracy of spectral irradiance measurements at longer wavelengths has improved in recent years, but the long-term variations are not well known, especially on the time scale of the solar cycle and longer. Average empirical models of particle precipitation from the magnetosphere and of high-latitude ionospheric electric fields and currents have been developed for a variety of geospace conditions, but space physicists often do not have complete knowledge of all key conditions in space coincident with a particular atmospheric event. Moreover, the distributions of particle precipitation, electric fields, and electric currents in the upper atmosphere at any given time can be very different from the average conditions. The flow of electric currents in the global circuit above the tropopause and through the middle atmosphere andionosphere is poorly understood. It is possible that present models of the global electric circuit, which emphasize quasi-DC current flow, have missed entirely one of the largest sources of electric charge movement illustrated by the recent observations of upward electrical discharges and gamma-ray bursts associated with thunderstorm activity. Figure 14 shows an impressive example of an upward electrical discharge from the top of a thundercloud to the ionosphere. file:///C|/SSB_old_web/strach4.html (4 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 FIGURE 14 The first true-color image of a sprite, an upward electrical discharge from the top of a thundercloud to the ionosphere. It was observed over a thunderstorm in the Midwest on the night of July 3, 1994, using a low-light-level color television camera in an airplane flying at 13 km. The top of the sprite is at an altitude in excess of 85 km; the blue root-like tendrils beneath the sprite are as low as 60 km. The white-blue area beneath the red sprite is an overexposure of normal lightning occurring at the thunderstorm's cloud top, which is some 17 km high. (Courtesy of Erin J. Parcher, Geophysical Institute, University of Alaska.) The sources of atmospheric waves entering the middle atmosphere from the troposphere are diverse and are not well quantified, except perhaps for atmospheric tides. The spectral characteristics, global distributions, and temporal variations of gravity waves and equatorial waves are poorly known. Complex interactions among the energetics, chemistry, dynamics, and electrodynamics of the middle and upper atmospheres determine their structure and variability. The absorption of solar ultraviolet radiation by ozone, and the absorption and emission of infrared radiation by carbon dioxide, ozone, water vapor, and other species, depend on the distributions of those components, which in turn depend on chemical processes and atmospheric circulation. In the upper mesosphere and lower thermosphere, heating and cooling are influenced by departures from conditions of local thermodynamic equilibrium and by exothermic chemical reactions. Heat conduction, viscous dissipation of sheared motions, and vertical air motions resulting from the effects of atmospheric waves also contribute to the heat budget. The net heating and cooling help determine the thermal structure of the atmosphere, which affects the emission of infrared radiation and the rates of chemical reactions. Theoretical studies predict that increasing anthropogenic carbon dioxide will lead to cooling of the upper atmosphere and significant reduction in high- altitude densities (Figure 15). Highly complex chains of chemical reactions can occur involving many different chemical compounds. Ozone in the middle atmosphere, and atomic oxygen in the thermosphere, are critical species that interact with many other compounds. The formation and the properties of aerosols depend strongly on atmospheric temperature and composition, while the presence of aerosols can significantly influence chemical and radiative processes, as well as plasma processes around the mesopause. The dynamics of the middle and upper atmospheres involve many types of interacting wave motions and the interaction of those waves with the mean circulation. For example, file:///C|/SSB_old_web/strach4.html (5 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 momentum transport by waves drives the tropical quasibiennial oscillation (QBO) and semiannual oscillation (SAO) in the middle atmosphere (Figure 16). FIGURE 15 (a) Calculated globally averaged neutral gas temperature change. (b) Calculated globally averaged neutral gas density change of major thermospheric specied from present day conditions for the case where carbon dioxide and methane are doubled (solid line) and halved (dashed line). How will changes in carbon dioxide and methan modify the mean structure of the mesosphere and thermosphere? (Reprinted from R.G. Roble and R.E. Dickinson, Geophys. Res. Lett. 16:1441-1444, 1989. Copyright 1989 by the American Geophysical Union.) file:///C|/SSB_old_web/strach4.html (6 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 FIGURE 16 Time-height section of the mean zonal wind component near 9° latitude with only the annual cycle removed. Shaded regions indicate winds from the west (w). Unshaded regions indicate winds from the east (e). Contours are drawn at 10 m s-1 intervals showing quasi-biennial oscillation below about 35 km and semiannual oscillation above. (After J.M. Wallace, Rev. Geophys. Space Phys. 11:191, 222, 1973. Copyright 1973 by the American Geophysical Union.) The maintenance and breakup of the winter polar vortex, of great importance to stratospheric ozone chemistry, is influenced by the strength of planetary waves. Small- scale wave processes in the mesosphere and lower thermosphere are believed to be the main source of turbulence at those heights. Turbulence contributes to atmospheric mixing and downward heat transport, and the dissipation of turbulence contributes to atmospheric heating. Tides are one of the main components of thermospheric dynamics. They display variability that appears to be related to their interactions with other wave motions in the middle atmosphere. The magnetosphere is coupled to the middle and upper atmospheres through the ionosphere, and this coupling is highly variable in space and time. Flows of thermal and energetic plasma, large-scale current systems, and imposed electric fields provide the basic magnetospheric inputs to the ionosphere. These can significantly affect the altitudinal and horizontal distributions of plasma density, temperature, ionic composition, and conductivity. Strong electric fields are not confined to the polar regions. Occasionally, high- latitude electric fields can penetrate to subauroral latitudes. These fields can produce rapid ion drifts, plasma outflows, ion temperature increases, deep ionization troughs, and chemical changes in the ionosphere. The neutral atmosphere is also affected by energetic inputs from the magnetosphere. Both chemical and dynamical changes lead to variations in the neutral density. The ionosphere carries the coupling current systems and provides feedback to the magnetosphere in the forms of ion outflow, conductivity changes, and dynamo electric fields. Tidal winds and propagating atmospheric waves produce electrodynamic influences that are mapped from the lower atmosphere upward. These coupling processes result in a temporally and spatially structured system. For example, discrete auroral forms can range in size from a few ion gyroradii to global dimensions and can exist over time scales from milliseconds to hours. There are many uncertainties in how all these processes operate and interact. An adequate quantification of the roles of turbulence and gravity waves in heating and cooling these regions has not yet been achieved. The distribution of net atmospheric heating throughout the middle atmosphere is not well known. Our knowledge of the relevant chemical reactions and their kinetic rates is incomplete. For example, one area of active research addresses the problem of why currently accepted photochemical models predict too little ozone in the upper stratosphere and mesosphere. There appears to be some "missing chemistry." Aerosol processes remain poorly understood, with respect to both their formation and evolution and their roles in heterogeneous chemistry, radiation, production of polar mesospheric clouds, and mesopause plasma irregularities that produce strong radar echoes. Space physicists' understanding of the characteristics and effects of interactions among gravity waves, tides, planetary waves, and the mean circulation is far from complete. Momentum transport by gravity waves is believed to be critical for the mesospheric circulation, but observations of the relevant waves and their interactions with the mean flow are very sparse, while quantitative models of those effects are in their infancy. Full identification of the waves that help drive the QBO and the SAO has not yet been achieved. file:///C|/SSB_old_web/strach4.html (7 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 The degree to which air inside the stratospheric winter polar vortices is exchanged with outside air is a subject of active debate. The manner in which gravity waves, tides, and planetary waves break, and the properties of turbulence thereby generated, are poorly understood, although some impressive modeling results have already been obtained (Figure 17). The quantitative effect of interactions between thermospheric winds and ionospheric-magnetospheric electric fields and currents on the structure and variability of the ionosphere and magnetosphere is similarly poorly understood. Researchers do not have useful predictive models of the complex system of generators, electrical charge storage systems, and both active and passive current paths that constitute the global circuit. Current research suggests that cosmic ray impacts in the lower atmosphere may produce paths of high electrical conductivity that result from cascading processes in electron production, but the effect of these spatially localized conductivity enhancements on the behavior of the global circuit is far from understood. The recent visual confirmation of the occurrence of lightning discharges upward to altitudes greater than 60 km, coupling thunderstorm energy directly into the middle atmosphere, has dramatically underscored the paucity of knowledge in this area. These electrical discharges and other interesting observations of x-ray and gamma-ray bursts associated with active thunderstorm cells indicate that the direct upward electrical energy input into the ionosphere and thermosphere is much more important than previously thought. Furthermore, development of a better understanding of electrodynamical energy coupling through the middle atmosphere may also help clarify the role of atmospheric electricity in any possible effects of solar variability on the weather. FIGURE 17 Left panel: A noctilucent cloud (NLC) display from Kustavi, Finland (61°N, 21°E), on July 22, 1989, file:///C|/SSB_old_web/strach4.html (8 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 showing characteristic band and streak features. Right panel: simulated NLC brightness pattern from the three- dimensional gravity wave breaking model of D.C. Fritts, J.R. isler, G. Thomas, and Ã. Andreassen, Geophys. Res. Lett. 20(19):2039-2042, 1993. (Copyright 1993 by the American Geophysical Union.) Many of the important physical and chemical processes that take place in the terrestrial upper atmosphere and ionosphere also occur in the environments of other planets and satellites. Other than Earth's, the upper atmosphere-ionosphere system that we have the most information about is that of Venus. This information came mostly from observations by the Pioneer Venus Orbiter. This unique but still skeletal database on the chemistry and energetics of the Venus atmosphere-ionosphere system has significantly advanced our understanding of those regions. There are some similarities between conditions at Venus and Earth, but also some very important and significant differences, mainly because of Venus's C02-dominated atmosphere and the direct contact of the solar wind with that planet's ionosphere. Examples of important differences at Venus are the low and relatively solarcycle-independent thermospheric temperature and the ionospheric plasma temperatures that are significantly higher than those that would result from extreme- ultraviolet heating alone. The most glaring lack of data concerns the dynamics of the upper atmosphere of Venus. Space physicists have no direct information on the thermospheric winds. Terrestrial thermospheric general circulation models that have been modified to represent conditions at Venus predict large global wind systems, but there are no data to confirm those models. The martian upper-atmosphere/ionosphere system is believed to be similar to that of Venus, but the possible presence of a small intrinsic magnetic field and a spin rate that is much faster than that of Venus may result in some potentially important differences. Despite all the missions to Mars, the only direct aeronomical measurements were those obtained by the neutral mass spectrometer and the retarding potential analyzer carried by the two Viking landers; the total database is equivalent to that which would be obtained from a single rocket flight. All the available information concerning the upper atmospheres and ionospheres of the outer planets and their moons come from the ultraviolet spectrometer and radio occultation observations obtained by the Pioneer and Voyager missions. This absolutely minimal database opened the door to a new and exciting world of mostly hydrogen- dominated atmospheres and ionospheres. Numerous theoretical models have been developed, but there are only weak observational constraints on these models. For example, in order to explain the relatively low electron densities observed in the ionosphere of Saturn, it has been proposed that a steady inflow of water from the ring plane has a significant impact on the atmospheric ion chemistry, changing the dominant ion species and thus the loss rates. This suggestion appears feasible but can be neither confirmed nor eliminated given the existing database. A major challenge to scientists studying the middle and upper atmospheres is to gain a comprehensive understanding of the relevant inputs and internal processes so that reliable models of these atmospheric regions can be developed that have useful operational and predictive capabilities. The following are some of the principal questions: What is the absolute solar spectral irradiance below 300 run, especially in the poorly determined region below 40 nm, and how does the irradiance vary on time scales of minutes to decades? How does the radiation vary as a function of altitude through the Earth's atmosphere? file:///C|/SSB_old_web/strach4.html (9 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 What are the spatial and temporal distributions of planetary waves, tides, gravity waves, and turbulence in the middle and upper atmospheres, and what are the sources of their variability? How do they affect atmospheric structure, and which wave motions drive the QBO and the SAO? What causes the apparent coupling between the QBO and extratropical dynamics? What are the global distributions of electrodynamic energy and momentum inputs to the middle and upper atmosphere, viewed as a time sequence rather than in a highly averaged or climatological sense? What corresponding electrical, chemical, and dynamical changes do these inputs force in the neutral atmosphere on comparable time scales? How do the coupled neutral and ionized components of the atmosphere evolve as a disturbance proceeds? How do long-lasting and large-scale changes in the coupled system manifest themselves in altered global patterns of conductivity, joule heating, ion convection velocities, and current systems and, ultimately, feed back on and alter the interaction with the magnetosphere? What is the importance of lightning (including all forms of transient discharge) to middle-atmospheric chemistry and dynamics and to ionospheric electrical phenomena? What are the generators, electric charge storage systems, and active and passive current paths that constitute the middle and upper atmospheric portions of the global electric circuit? How do coupled radiative, chemical, electrical, and dynamical effects determine middle and upper atmospheric structure and variability? What are the temporal and spatial distributions of aerosols in the stratosphere and mesosphere? What physical processes control their distribution, and what are their radiative and chemical effects? What are the contributions to climatological changes in the middle and upper atmosphere from solar influences, volcanic eruptions, anthropogenic effects, and changes in the lower atmosphere? What are the dominant aeronomic processes in the upper atmospheres and ionospheres of other planets or satellites? How do they differ from terrestrial processes, and why? Can models of terrestrial processes be improved or constrained by data obtained at other planets? CURRENT PROGRAM The first comprehensive global observations of temperature, winds, and some minor chemical species in the stratosphere and into the mesosphere are currently being file:///C|/SSB_old_web/strach4.html (10 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 made by the Upper Atmosphere Research Satellite (UARS) launched in September 1991. The Midcourse Space Experiment (MISX) spacecraft, scheduled for launch in 1995, will add further valuable data on the composition and temperature of the middle and upper atmospheres, but only for a limited number of event-oriented campaigns. Solar ultraviolet radiation and energetic particles are also being measured by UARS and SAMPEX as well as by polar-orbiting NOAA and DMSP satellites. Periodic shuttle missions are also being flown that make middle and upper atmosphere measurements. Figure 18 shows some early UARS results illustrating that significant effects on stratospheric composition may result from downward transport from the mesosphere. FIGURE 18 Two recent measurements from the Halogen Occultation Experiment (HAlOE) on UARS. Top: HALOE measurement of methane (CH4) as a function of longitude and pressure altitude at 76°S on October 22, 1991. Note the signature of descent (blue region at a longitude of approximately 300°) from the mesosphere (above 1 mb) to the lower stratosphere (about 30 mb). Bottom: HALOE measurements of reactive nitrogen oxides (NO + NO2) as a function of latitude and altitude for the period from July 11 to August 30, 1992. Note the signature of descent from the thermosphereic source of NO at high southern latitudes (yellow and green regions on left-hand side) and its connection to the stratospheric distribution of reactive nitrogen oxides (yellow regions across center of image). (Courtesy of J.M. Russell III, NASA/Langley.) NSF's CEDAR program, together with other ground-based observations (radar, lidar, spectrometers, interferometers, imagers, and photometers), continues to provide information on winds, chemical composition, temperature, radiative emissions, and ionospheric densities and drifts in fixed geographic locations, on scales and in locations not accessible to satellite observation. Much of the current knowledge about Antarctic ozone depletion has been obtained from intensive ground-based remote sensing, balloon, and aircraft campaigns. Similarly, current knowledge of gravity wave fluxes, mesospheric tides and planetary waves, the temperature structure of the summer mesopause, and the structure and variability of the ionosphere and thermosphere is being advanced by application of ground-based techniques. Suborbital experiments are a key component of the current program. For example, rockets and balloons can measure neutral and ion composition, turbulence, electric and magnetic fields, and energetic particles and can obtain altitude profiles of physical properties to determine, for example, the vertical structure of auroral features. A series of polar-orbiting satellites (DMSP, HILAT, Polar BEAR, Viking, Freja, Akebono) have obtained largescale images that define the global morphology of the aurora and measure the energy spectra of particles precipitating from the magnetosphere. ISTP will add to our understanding of the magnetospheric input to the upper atmosphere by placing in situ file:///C|/SSB_old_web/strach4.html (11 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 measurements of precipitating particles and fields in the context of the global magnetospheric configuration. Freja is making high-time-resolution measurements of auroral zone particles and fields, providing a key link in our understanding of the magnetosphere-ionosphere-atmosphere coupling. FAST will extend the measurements of electrodynamic parameters and energetic particles to higher resolution in three dimensions. Research into the role of the middle and upper atmospheres in global change requires well-integrated efforts in long-term and well-calibrated monitoring of selected variables in the climate system. Some of these long-term monitoring programs currently exist for the stratosphere. Total ozone has been monitored from the ground-based Dobson network for almost 40 years, and there have been continuous measurements of total ozone from space for over 15 years by NIMBUS 7, TIROS, and Russia's Meteor-3 spacecraft. New algorithms are being developed that might allow consistent, well- calibrated ozone profiles to be recovered from the NIMBUS and TIROS data, but those measurements will not give ozone profile information in the lower stratosphere. The SAGE instrument has given ozone profile measurements in this region, and the ILAS instrument on the Japanese ADEOS spacecraft should give similar information. Long-term measurements of ultraviolet-B radiation at the Earth's surface have not been sufficient to give reliable trends related to changes in stratospheric ozone because of inadequate calibration and spectral resolution as well as lack of simultaneous measurements of important accompanying atmospheric variations such as clouds and aerosols. Temperature profiles that extend up into the lower ' stratosphere are being measured by the worldwide radio-sonde system, but there are some problems obtaining a long-term, well-calibrated temperature record from the radiosonde network because it is limited to land-based launch facilities and there have been changes in instrumentation, procedures, time of launch, and station locations. The situation is much worse for satellite measurements of stratospheric temperatures because changes in satellite instrumentation and in the retrieval algorithm make it necessary to calibrate these data against independent measurements, which is no longer possible because of the declining number of rocketsonde launches. Future plans involve calibration against lidar temperature measurements, but that system has not yet been implemented. Thus, researchers are left with an unsatisfactory monitoring system for stratospheric temperatures. U.S. agencies have been working with international organizations to monitor a number of stratospheric variables through the Network for Detection of Stratospheric Change (NDSC). The aim of the network is to obtain a continuous, well-calibrated data record for the stratosphere that can be used to calibrate satellite measurements. The measurements will be very valuable, but they will exist at a very limited number of stations around the world. The World Climate Research Program has recently implemented a program on stratospheric processes and their role in climate to address some of the issues of the middle atmosphere and climate. The centerpiece of the space-based Earth observation program is the EOS mission, which was originally planned to provide comprehensive, well-calibrated measurements over a 15-year period. Budget problems have led to its substantial descoping, so that now it does not include measurements of particles and fields, and its observations of the middle atmosphere are limited to several key measurements of stratospheric composition and temperature. These EOS middle atmosphere measurements are not anticipated to begin for another 8 to 10 years. Thus, only the NOAA operational and other climate-monitoring systems such as NDSC can be relied on for file:///C|/SSB_old_web/strach4.html (12 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 longterm U.S. measurements. Other ground-based programs, such as NSF's CEDAR program, have very useful capabilities for those purposes, but that program is not configured for long-term measurements. Measurements from the Japanese ADEOS and the European Envisat platform may also make important contributions. Computer models have been developed for studying the coupled dynamics, energetics, and chemistry of the middle and upper atmospheres. The models provide a valuable beginning to the study of the fully coupled problem, but they all have limitations with regard to the level of detail included, the coupling with lower and/or upper levels of the atmosphere, the number of spatial dimensions, and/or the computational time required. The most ambitious planetary aeronomy observations in the cur-rent program are those planned for Saturn's satellite Titan as part of the Cassini mission. The European- provided Huygens probe will obtain atmospheric profiles from the middle atmosphere down to the surface, while the main Cassini spacecraft will carry a mass spectrometer for the analysis of ions and neutral species each time the spacecraft dips through the upper atmosphere for orbit-changing gravity assists. A magnetometer is also expected to be placed in orbit about Mars as part of the Mars Surveyor program. FUTURE DIRECTIONS In the atmospheric sciences, the application of the scientific method usually proceeds by continual interaction between theory and observations in the following manner. Highly simplified models, having few degrees of freedom, are advanced to explain specific observations. Typically, there are areas of disagreement between theory and observations that motivate improvements in the theory and the need for more observations. The mechanisms contained in these simplified models are then imbedded in more comprehensive models that contain a variety of interacting physical processes, and analyses of the results from those models are then compared to similar analyses of atmospheric observations. Thus, all through this process, it is the mismatch between theory and observations that motivates the need for more observations and the formulation of improved models. It is this process that motivates researchers' future needs. Global observations of the temperature, winds, composition (including the characterization of the physical and chemical properties of aerosols), and radiative emissions should be obtained and continually compared against models in order to understand middle and upper atmosphere processes and the coupling to regions above and below. It is important that the UARS satellite program (which is making important advances in stratospheric observations, and some inroads into the mesosphere) and the CEDAR ground-based program maximize the scientific return from the sizable investments already made. Furthermore, our current understanding of the electrodynamics of mesoscale and small-scale structures is poor because of the lack of high-resolution, magnetically conjugate measurements in the ionosphere, and because of the lack of detailed measurements of electrical, chemical, and optical phenomena above thunderclouds in the middle atmosphere. The latter observations might also provide a key for understanding the origin of atmospheric gamma-ray bursts. As discussed above, the feedback processes involving the magnetosphere and ionosphere are temporally and spatially scale-size dependent. The coordination of high-resolution FAST in situ measurements with ground-based and rocket observations could contribute to assessing file:///C|/SSB_old_web/strach4.html (13 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 the effectiveness of the feedback processes as a function of temporal and spatial scale size. Future programs will build on the results of these current programs. There are good prospects for developments in lidar techniques to enable high- resolution, daytime and nighttime measurements of middle-atmospheric winds and temperatures, as well as the concentrations of certain minor constituents. Scanning ground- based and airborne lidars enable measurement of the horizontal as well as vertical structures of atmospheric features. The global network of relatively inexpensive wind- measurement radars should be expanded, and the data should be used jointly with satellite observations to allow clear definition of the longitudinal and latitudinal variations of mesospheric tides, planetary waves, and gravity waves. These important waves not only give rise to wind oscillations like the QBO and SAO, but also give rise to stratospheric warming and determine the nature of the winter polar vortices. Passive optical and microwave techniques for middleatmospheric observations also show great promise for contributing to better understanding of minor constituents like ozone and water vapor. For understanding the complex behavior of the mesosphere/lower-thermosphere region, atmospheric measurements must be accompanied by well-calibrated measurements of solar ultraviolet, extreme ultraviolet and soft x-radiation, energetic particles, and electrodynamic parameters. Atmospheric wave processes including tides and gravity waves and their interactions are particularly important for this region. Our understanding of the lower thermosphere and mesosphere is currently based on an observational database that consists of ground-based and rocket-borne observations as well as campaign-oriented measurements from the space shuttle. These sources of data, though extremely useful for improving our understanding of this region, are restricted to fixed geographic locations and thus are insufficient to allow us to characterize the complex global behavior that involves distributed energy sources along with large-scale dynamics and both horizontal and vertical transport of energy. UARS has recently expanded this database by providing global observations of winds, temperatures, and some minor constituents over altitudes extending into the mesosphere. Even with the global characterization of a limited number of parameters through the UARS mission, fundamental questions still exist about basic parameters and their variability, and about important radiative, chemical, dynamical, and electrodynamical phenomena. One way to address the need for global data is through a spacecraft mission that uses remote sensing techniques to sample the required altitude range. However, a single orbiting spacecraft has a limited ability to resolve tidal variations necessary to understand the mean state as well as variations about the mean state. For example, UARS, given its orbital inclination and altitude, takes about 36 days to make measurements over all local times if both daytime and nighttime measurements are included. If better resolution of phenomena with large local time variations (tides, for example) is desired, it is necessary to use multiple spacecraft or to supplement the space observations with groundbased observations that have good local time resolution. CEDAR observations are particularly valuable for those purposes. The understanding of the mesosphere/lower-thermosphere region requires coordinated measurement of atmospheric dynamics, especially wave motions, and energy inputs from space. The study of climatological changes in the middle and upper atmosphere requires well-calibrated satellite and ground-based monitoring of the composition, dynamics, and temperature and of the inputs to this atmospheric region from above and below. The measurements need to extend at least over one solar cycle in order to adequately separate solar-induced variations from anthropogenic variations. Further development of models of these regions is needed not only to understand the complex workings of these file:///C|/SSB_old_web/strach4.html (14 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 regions but also to predict their future changes. Ideally, these models should be three- dimensional, should include realistic photochemistry and radiation, and should have the capability of fully resolving phenomena with very disparate scale sizes and of representing nonlinear coupling processes that combine to determine the behavior of the global system. Work with such models has proceeded slowly. One reason is that their computational requirements often exceed available resources. Global three-dimensional models must keep pace with instrument measurement capabilities, take advantage of new hardware architectures and software developments (e.g., parallel processing, high-performance computing), and be an integral part of future plans for measurement programs. Although there is much ongoing research on middle and upper atmosphere problems in ozone research (within NASA and NOAA, for example) and on ground-based aeronomic measurements and their interpretation (within NSF), there has been inadequate contact between these two communities. Research on the coupled middle-upper atmosphere system could be accelerated by the establishment of a coordinated research program that takes a more holistic view of research in this area. Understanding the time-dependent, dynamical coupling processes within the global electrodynamic system requires large-scale maps of key energy and momentum inputs, as well as ionospheric responses, with significantly better spatial and temporal resolution than has been available in the past. Past work has relied on broadly averaged parameter fields. The capability already exists to invert large-scale photometric images of the auroral regions into maps of estimated ionospheric conductivity and estimated energy inputs from particle precipitation. Improvements in imaging detector technology and electronics have enabled the development of a new generation of ground- and space-based optical instruments with greatly increased sensitivity and associated improvements in temporal and spatial resolution. Field-widened versions of Fabry-Perot and Michelson interferometers allow large, nightside regions to be imaged over short time scales. For dayside measurements, highly improved spaceborne limb-scanning systems are available. Improvements in detector technology also give us the ability to image emissions in the near-infrared, providing the capability of mapping ion convection and neutral wind systems from both the ground and space. New techniques are needed to determine global patterns of ionospheric currents, field-aligned currents, Joule heating, and neutral composition. One method of achieving this goal would be the use of a swarm of microsatellites in polar orbits spaced in local time to obtain nearly instantaneous global patterns of key parameters. Another approach would be to expand the network of ground-based incoherent scatter and other radars. The radars at Millstone Hill, Sondre Stromfjord, Goose Bay, and EISCAT have made great contributions to mapping the electric fields impressed into the auroral E region from the magnetosphere (Figure 19). The Europeans are building a radar on Svalbard (near the magnetic latitude of Sondre Stromfjord but at a different local time). Construction of additional radars at strategic sites in the polar cap and around the auroral oval would yield a network capable of resolving the flow pattern of magnetic flux from the dayside of the Earth, across the pole, to the nightside, and of determining how this flux returns to the dayside at lower latitudes. file:///C|/SSB_old_web/strach4.html (15 of 17) [6/18/2004 2:20:09 PM]
A Science Strategy for Space Physics: Chapter 4 FIGURE 19 The input of energy to the upper atmosphere in northern polar regions in winter under quiet geomagnetic conditions. The coordinates are magnetic latitutde and longitude. Ground-based radar facilities for studying the polar ionosphere are identified. (Courtesy of James Vickrey, SRI.) Available observational data in the lower-altitude regions of the ionosphere, where electrodynamic coupling is most effective, are exceedingly sparse. The reason for this lack of information is the inaccessibility of this altitude regime to conventional orbiting spacecraft. Current information from rockets and radars indicates that this region is highly structured. Again, the coupling processes between the magnetosphere, ionosphere, and atmosphere are strongly influenced by this region and its structure. Current ideas for a comprehensive survey of this region include spacecraft with highly eccentric orbits and a low-altitude perigee and instruments tethered to higher-altitude spacecraft. Whatever observational method is adopted, high-resolution measurements of electrodynamic parameters at these altitudes coordinated with higher-altitude, magnetically conjugate observations are essential to further understanding of this topic. Important gaps in our knowledge of the dynamics of the upper atmospheres of Venus and Mars Could be filled by observations of upper atmospheric winds using instruments such as a Fabry-Perot or a Michelson interferometer on spacecraft orbiting the planet. The chemistry and physics of the upper martian atmosphere could be addressed by orbiting instruments such as neutral and ion mass spectrometers and Langmuir probes. Theories about the ionospheres of the outer planets could be tested if ion composition measurements were made from any missions to those planets. file:///C|/SSB_old_web/strach4.html (16 of 17) [6/18/2004 2:20:09 PM]
