The Atmospheric Sciences Entering the Twenty-First Century (1998) / Chapter Skim
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4 Upper Atmosphere and Near-Earth Space Research Entering the Twenty-First Century
4 Upper Atmosphere and Near-Earth Space Research Entering the Twenty-First Century
Pages 199-271
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and Committee on Solar and Space Physics (CSSP)
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Scientific Requirements for the Coming Decadents) Role of the Stratosphere in Climate, Weather Prediction, and Tropospheric Chemistry The stratosphere plays two roles in the climate system.
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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 1960s; · analysis of historical data from systems operating from the 1940s to the · 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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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-atmosphere 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.
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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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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.
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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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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.
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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.
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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.
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Although the subject of ozone depletion was already receiving a lot of attention in the middle 1980s and satellite data were being used to look for stratospheric ozone depletions, it was a small group of British researchers, who had been making ground-based observations of total ozone at Halley Bay in Antarctica since the 1950s, that first noticed the precipitous decrease in springtime stratospheric ozone there. They had, moreover, suggested that its cause might be found in industrial chlorofluorocarbons released into the atmosphere.
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On the one hand, ozone decreases result in less absorption of solar UV in the atmosphere, thereby allowing more solar radiation to heat the surface. On the other hand, reduced stratospheric ozone leads to a cooler stratosphere that, in turn, radiates less infrared energy downward into the troposphere, resulting in tropospheric cooling.
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Ground-based, aircraft, and satellite observations have clearly shown that the Antarctic ozone hole is caused by increased chlorine and bromine radical concentrations in the lower stratosphere and that these are a result of the anthropogenic emission of huge amounts of halogen-containing compounds in combination with the unique meteorological conditions of the Antarctic lower stratosphere. Similar enhancements in chlorine and bromine radicals are also observed in the Arctic winter lower stratosphere for periods of time, and smaller ozone decreases are seen in this region and at lower Northern Hemispheric latitudes (due to the different meteorological conditions there)
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· What is the atmospheric impact of possible future fleets of stratospheric aircraft? Below, the four specific areas in which research on stratospheric effects is critical (stratospheric ozone, volcanic effects, atmospheric effects of aircraft, and the stratosphere's role in climate and weather prediction)
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214 THE ATMOSPHERIC SCIENCES ENTERING THE TWENTY-FIRST CENTURY 90 60 30 .~ O ct -30 -60 90 60 30 o -30 -60 -90 DNA daily dose, J m~2 days (1979 - 1989 average)
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UPPER-ATMOSPHERE AND NEAR-EARTH RESEARCH 500 400 ° 300co O 200 100 11 Nov.
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· What is the quantitative relationship among global, lower-stratospheric ozone depletion; radiative forcing of the Earth's atmosphere system; and climate change? · What are the global impacts of the Antarctic ozone hole (which is expected to continue well into the next century)
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Both field measurements and laboratory work are going on to understand anthropogenic chemicals that may pose threats to stratospheric ozone, such as methyl bromide and the CFC substitutes whose atmospheric concentrations are increasing rapidly. Lastly, the Upper Atmosphere Research Satellite (WARS)
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Volcanic Effects Discovery of the Antarctic ozone hole in the austral spring spurred intense effort to learn the reason for this unanticipated decrease in total ozone column. Models were developed suggesting that heterogeneous chemical reactions were responsible for converting relatively chemically inactive chlorine species (usually referred to as reservoir species)
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More realistic treatment of heterogeneous chemistry in atmospheric models is needed and should be coupled to detailed microphysical models. Solar Effects The abundances of ozone and other chemical species in the stratosphere are affected by photodissociation processes and hence by the level of solar ultraviolet radiation penetrating the atmosphere.
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Atmospheric Effects of Aircraft Much of the impetus for modern stratospheric ozone research has its roots in the supersonic transport research of the 1970s. At the time, the concern was that such aircraft would emit large amounts of water vapor and nitrogen oxides into the stratosphere where they could initiate catalytic ozone destruction.
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Experimental facilities have been built to measure the exhaust products of present aircraft and proposed aircraft engines. Laboratory investigations are proceeding to measure the rates of chemical reactions that transform the aircraft exhaust products and influence atmospheric composition.
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For instance, CFCs are greenhouse gases, so their increasing concentrations lead to climatic warming. CFCs also lead to stratospheric ozone depletion and are probably responsible for the observed decreases in ozone in the lower stratosphere.
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In all cases, a strategy that combines observations, laboratory studies, and modeling is needed. Stratospheric Ozone · Deployment and Utilization of Unmanned Aircraft: Aircraft observations have shown their great value in unraveling the physics and chemistry of polar ozone.
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Yet these characteristics are crucial in considerations of heterogeneous chemistry and radiative transfer. · Improved Microphysical Models: In recent years, several microphysical models have been developed for characterizing stratospheric aerosols.
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It will reduce some key uncertainties affecting the behavior of ozone and other chemical constituents in the middle atmosphere, and specifically in the lower stratosphere, where large ozone depletions have been observed. A successful program should also provide the information needed to determine whether it is possible to take measures that would enhance the long-term stability of the ozone layer and the climate of the Earth.
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The complex, time-dependent variations of particles and electric and magnetic fields in the solar wind, the magnetosphere, and the ionosphere produced by solar variability are known collectively as "space weather." The domain of space weather is distinct from that traditionally associated with better-known lower-atmospheric weather (tropospheric meteorology)
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These aspects are described below in the context of solar variability and the corresponding changes that occur in the solar wind and in the Earth's magnetosphere, ionosphere, and upper atmosphere.
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that comprise the solarterrestrial system were outlined. Space climate models are especially important at present, given that our ability to provide accurate and specific space weather forecasts is rather limited at the present time.
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These temporal variations in solar wind are usually organized into alternating streams of high- and low-speed flows, with the density and field strength generally being strongest on the leading edges of the high-speed streams as a result of compression that occurs in interplanetary space. When the magnetic field within a compression region on the leading edge of a high-speed stream is directed southward, the solar wind is particularly effective in stimulating geomagnetic activity.
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Because the magnetic field embedded in the solar wind often contains a southward-directed component even when the solar wind is not disturbed, magnetospheric substorms occur at a typical rate of one to a few per day. As noted above, particularly strong geomagnetic responses are triggered by the strong southward-directed fields often contained within compression regions on the leading edges of high-speed streams and by interplanetary shock disturbances driven by fast coronal mass ejections.
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The most severe geomagnetic storms usually are associated with interplanetary disturbances driven by fast CMEs and thus are most common near the maximum of solar activity. Geomagnetic storms associated with high-speed stream compression regions are generally less severe, but tend to recur at the 27-day rotation period of the Sun, particularly on the declining phase of the solar activity cycle.
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The ionosphere-upper atmosphere also responds to rapid changes in solar ionizing radiation and energetic particle precipitation that accompany transient solar events. These events, including solar flares and CMEs, produce significant changes in electron density at lower altitudes (80 to 90 km)
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region of the Earth's ionosphere. Longer-lived disruptions occur as a result of heating of the upper atmosphere at auroral latitudes.
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Some are climatological for example, the degradation of electronics, solar cells, and materials that results from long-term radiation exposure or the erosion of materials via oxygen bombardment when traversing the oxygen-rich upper atmosphere. Similarly, polymerization and embrittlement of some materials by UV exposure or the single-event effects induced in electronics by galactic cosmic rays (see Figure II.4.10A)
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These disturbances, which are called Appleton anomalies after their discoverer, were first photographed during the Apollo mission. Created by a fountain effect due to the geometry of the Earth's magnetic field, the anomaly zone often becomes extremely disturbed
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In addition, there are effects driven by the quasi-stationary solar wind. Because the disturbances evolve as they propagate
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· What is the most crucial factor in determining the overall effect of a CME on the space weather system: speed, mass, energy content or magnetic field? In addition to understanding the causes and properties of space weather, a crucial element in progress toward quantitative and accurate space environment
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Consequently, the element that is crucial for progress toward quantitative space environment predictions of sufficient accuracy and specificity is a comprehensive physical understanding of the internal magnetospheric response to external variations. The dynamism of the magnetospheric space environment is governed by physical transfer mechanisms operative largely within the magnetospheric boundary layers, which separate interplanetary and magnetospheric regimes.
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. What is the role of the ionosphere-upper atmosphere in magnetosphere ionosphere coupling?
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~. As a result, the first numerical models are now being developed to specify, nowcast, and forecast the space environment.
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It is important to implement existing space weather models quickly so that deficiencies can be identified rapidly and remedied. Concurrently, a vigorous research program should continue to explore the basic physics of the comprehensive space environment, and new advances should be included in improved operational numerical models through the NSWP.
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. Additionally, key measurements are required to image coronal mass ejections as they leave the Sun, and a secure, reliable source of real-time, continuous, upstream solar wind data must be established and maintained.
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2. Societal: The general measure of societal success is assessment of the degree to which increased basic understanding of the solar-terrestrial connection and the space environment is used to modify and improve applications that are of benefit to society at large, specifically: · Have science-based applications or products been developed that are used to provide accurate determinations of the space environment?
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Finally, the program outlined here will result in an increased and broader intellectual understanding of our environment, an environment whose boundaries have expanded and continue to expand upward from the ground, through the lower atmosphere early in this century, to the upper atmosphere, and into the near-Earth space environment. This program will provide a driving motivation to unify the diverse fields of solar physics, space physics, magnetospheric physics, and atmospheric physics using the tangible test of prediction as a metric to judge the success of our understanding.
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The upper atmosphere responds strongly to cyclic changes in solar ultraviolet and x-radiation. Weaker changes in upper-stratospheric temperatures and ozone concentrations are also related to the solar cycle.
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The sensitivity of stratospheric ozone concentration to the presence of chlorine compounds, to stratospheric temperature changes, and to the presence of aerosols is one example. Another example is the greater cooling of the middle and upper atmosphere than heating of the troposphere, due to increased concentrations of carbon dioxide.
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Thermal balance in the upper atmosphere is strongly affected by nonlocal thermodynamic-equilibrium radiative processes that have been difficult to quantify in models. Critical Science Questions What physical processes determine the state of the middle and upper atmosphere?
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? How are climatological changes in the state of the middle and upper atmosphere related to variability in the forcing of this region from above and below due to solar variability; the diffusion of trace gases; changing patterns of tides, gravity waves, and planetary waves propagating upward from the lower atmosphere; and electrodynamic coupling to higher altitudes?
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· MonitorInputs to the Middle and UpperAtmosphere: To understand the causes of any climatological changes in the middle and upper atmosphere, we must know how the forcing has changed. Thus, it is critical to establish and/or maintain long-term programs to make stable, accurate measurements of parameters that influence the middle and upper atmosphere, including solar ultraviolet
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Areas in which understanding is particularly deficient include middleatmospheric electrodynamics, heterogeneous chemistry, polar mesospheric clouds, wave-mean-flow interactions, and turbulence. · Understand and Model Interacting Processes: Even though the nature of many atmospheric processes is reasonably well understood (e.g., basic dynamics, chemistry, radiation, and ionospheric electrodynamics)
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For example, solar radiation influences have a strong 11-year cyclical component that often helps identify these influences in the atmospheric response. Similarly, observed changes in the middle and upper atmosphere that are found to be most marked in recent decades may be associated with the rapid increase in certain anthropogenic gases.
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The middle atmosphere is a particularly sensitive indicator of perturbations to trace gases originating in the lower atmosphere that diffuse upward and disturb the sensitive balance in this region. Measures of Success An aggressive and successful research program will enable us to do the following: · Identify changes in the state of the middle and upper atmosphere that have already occurred through careful analysis of well-calibrated historically available and targeted observations of atmospheric parameters and external inputs to the region.
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Within the past few decades it has become possible to monitor solar variability through space-based observations, and most recently, large-scale observations of the Earth's upper and middle atmosphere have provided evidence of a terrestrial atmospheric response to the solar output. In building on this new data base, several activities that are poised for significant progress: · Measure the solar energy output with space-based monitors continuously over at least a full solar cycle.
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Observations of solar-type stars can provide a statistical estimate of the future likelihood of such behavior through the large number of realizations available at any given time. Solar Energy Output over a Solar Cycle The total solar irradiance is the energy flux crossing a surface at the average distance between the Sun and the Earth.
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The overall range of variation for the total solar irradiance from minimum to maximum solar activity has thus far not been determined with complete reliability. To firmly establish the range of variation of the total solar irradiance, it is imperative that space-based monitors be deployed with adequate regularity to ensure overlap in their periods of observation.
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Over longer periods, where increasing greenhouse gas concentrations should have a greater effect, there are no measurements of total solar irradiance. Over shorter periods, the rate of change in the climate forcing function during either the rising or the falling phase of the solar cycle is comparable to the rate of change in the forcing function due to changes in the greenhouse gas concentration.
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; net anthropogenic forcing from greenhouse gases, aerosols, clouds, and ozone changes (dotted line) ; and solar irradiance variations associated with the 11-year solar activity cycle alone (lowest line)
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0.4 O 0.2 _ O -0.2 -0.4 - SUNSPOT NUMBER A \, 1 1 1 1 1 1 1 1 100 ~_ 90 ~ 80 70 z 60 o 50 ~ 40 <~, 30 ,, \ 1 / _ GLOBAl7SEA SURFACE TEMPERATURE Al JOMALIES / 1 1 1 1 1 1 1 1860 1880 1900 1920 1940 1960 1980 YEAR 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.
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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.
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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.
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, 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.
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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.
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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.
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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.
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UPPER-ATMOSPHERE AND NEAR-EARTH RESEARCH 4 3 _ 2 21ux o 40 20 (by)
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· 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.
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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.
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