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3 Atmospheric and Space Physics I. INTRODUCTION Three fields of research are represented by the Working Group in Atmospheric and Space Physics: the atmosphere of the earth, the magnetosphere of the earth, and plasma physics in space. These fields will be able to make effective use of the Shuttle and Spacelab when they become available. Optical remote sensing of the mesosphere and stratosphere to study chemical composition, structure and dynamics, active plasma experiments, and controlled study of magnetospheric phenomena requires the payload capacity and flexibility offered by the Shuttle system. We foresee a need to make use of the sortie mode as well as free-flyers. Because many experiments require manned control in real time based on real-time analysis of observational data, the presence of scientists in a small pressurized module in the Spacelab is a preferred mode of operation. Because of the require- ment for data acquisition and control in polar regions for some experiments, the pallet-only Spacelab mode will fulfill all of our needs only if an adequate data analysis and tracking system is provided. II. ATMOSPHERIC SCIENCE A. Our Understanding of the Upper Atmosphere by 1980 By 1980, the neutral upper atmosphere will have been the subject of over 20 years of extensive investigation using space vehicles. Much of this investigation has concentrated on the region above 250-km altitude, where satellites can remain for prolonged periods in earth orbit. Reference atmospheres have been compiled that successfully describe the gross features of the structure, composition, and 14

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Scientific Uses of the Space Shuttle \ 5 variabilities of this upper part of the atmosphere, although a number of discrepancies remain. Further measurements are planned during the next several years using the Atmosphere Explorer AE-C, -D, and -E spacecraft, which will be equipped to measure simultaneously the neutral constituents, the incident photon flux, and the ionized constituents, down to an altitude of 120 km. The region of 120-200 km altitude, where most of the extreme-ultraviolet solar photons are absorbed, has not been studied in situ by satellites before and is the region of greatest uncertainty in existing upper-atmosphere models. With the completion of these missions, we expect that by 1980 the structure and photochemistry of the atmosphere above 120 km will be well understood. On the other hand, the relationship among the circulation of the thermosphere, the dynamo current system, and the magnetospheric electric field will not have been adequately studied. A follow-on Electrodynamics Explorer mission to investigate the magnetospheric electric field and its relationship to the large- scale dynamics of the thermosphere has been suggested but not yet approved; such a mission would contribute substantially to this study. By 1980, the most important regions of the atmosphere still to be explored and understood will be the stratosphere, mesosphere, and lower thermosphere up to 120 km. Circulation and the effects of transport on chemical composition will be of particular interest. Recent concern about the effect of emissions from supersonic transport engines on the ozone layer demonstrates the importance of understanding this region of the atmosphere. At present, the lack of suitable sampling techniques and sensors limits us to only scattered and fragmentary data concerning the concentration of such im- portant constituents as O3, NOX, H2O, H2, and HNO3. B. Need for the Shuttle Our understanding of the upper stratosphere and mesosphere will be limited in the next few years because of the inherent difficulties associated with direct in situ rocketborne measurements, which provide only partial information on spatial and temporal variations. Optical remote sensing from the ground has limitations, and the complexity of the instrumentation required for comprehensive active and passive topside soundings is formidable.

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16 ATMOSPHERIC AND SPACE PHYSICS The meteorological satellites planned for the next few years have remote-sensing capabilities in the spectral region from ultraviolet through microwave to provide information on temperature and ozone and water-vapor distributions in the troposphere and strato- sphere, but there are limitations in the capacity of these systems to probe higher altitudes and other constituents. The instruments needed for active and passive remote sensing include sensitive, high-resolution scanning instruments and large tunable monostatic and bistatic laser radar (lidar) systems. The Shuttle will be able to accommodate the long focal lengths and large apertures of such instruments. The Shuttle will also be able to accommodate cryogenic systems to cool detectors and give vastly improved signal-to-noise ratios in some of the instruments. The Shuttle will provide for the first time the means to study adequately the distribution of the chemically active constituents of the middle atmosphere in space and time by remote sensing. The interplay of optical radiations and chemical effects shown in Figure 2 is reasonably well understood, although the number of significant neutral and ion species involved is large. However, the dynamical properties and the ways in which they interact with the structure and chemistry are understood only in very general terms. OPTICAL RADIATIONS CHEMICAL EFFECTS STRUCTURAL PROPERTIES DYNAMICAL PROPERTIES FIGURE 2 Interactive scheme of the atmosphere below 120 km (excludes particle precipitation, cosmic radiation, and meteor influx).

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Scientific Uses of the Space Shuttle 17 C. Thermal Structure and Dynamics The global thermal structure of the stratosphere and mesosphere is important because of its role in chemical, dynamical, and radiative processes. For example, many of the reaction rates involved in the ozone chemistry are strongly temperature-dependent, and the pres- sure systems arising from heating form the driving force for horizontal motions in the atmosphere. There are several energy inputs into the atmospheric region, 30-120 km, that are significant from the thermal viewpoint, including solar ultraviolet and x rays, auroral particles, Joule heating from electric currents, and the breaking of internal gravity waves. Energy losses are by infrared radiation. In addition, there is a massive transfer of energy within this atmospheric region by transport by large- and small-scale motions. The amount and distribution of ozone are very important in controlling the energy input, and knowledge of ozone concentrations near 30 km (and perhaps down to 15 km) will be of value in making clear the nature of the mixing processes. The temperature distribution up to 80 km should be measurable by remote sensing using the edge of the 15-Mm absorption band of CO2, from 80 to 120 km by laser probing of the sodium layer and from 100 to 120 km from observations of the profiles of airglow lines. Temperature profiles in the 80-120 km region provide considerable information about atmospheric structure. Motions in the stratosphere and mesosphere are important because of their role in transporting long-lived chemical and other trace constituents. The motions significant for transport occur on different space and time scales and may be categorized as follows: 1. Zonal-wind and meridional-circulation systems, 2. Planetary waves, 3. Synoptic scale (~ 100- 1000 km) and mesoscale motions, 4. Gravity waves, 5. Small-scale turbulent motions. The following are some of the questions concerning thermal structure and dynamics that can be answered by remote sensing from above in conjunction with data from ground-based and rocket sensors: 1. What are the propagation characteristics of the diurnal and semidiurnal tides in the transition region between the mesosphere

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18 ATMOSPHERIC AND SPACE PHYSICS and thermosphere where the wind field changes from a near geostrophic pattern (controlled by Coriolis force) to an ageostrophic pattern (influenced by molecular and eddy viscosity, ion drag, and electric fields)? Can these tides be seen in measurements of neutral temperatures and concentrations by topside remote sensing? 2. What are the turbulent transport properties of the mesosphere, and how do they affect the wind field, the composition, and the energy budget of the atmosphere? 3. How significant in the mesosphere and lower thermosphere are gravity waves excited in the lower atmosphere: what is their energy deposition, and what disturbances of the ionosphere do they produce? Are these gravity waves related to thunderstorm activity and earthquakes? Can they be detected by remote sensing from above? 4. How is the large-scale circulation related to topography? What is the global effect of this circulation on chemically active species? 5. What are the mesopheric chemical processes that determine the distribution of O and H? What is the global pattern of their distribution? 6. How important is the transport of chemical energy associated with the global wind circulation to the energy budget of the mesosphere? What correlations can be seen between the winter anomaly and horizontal thermal structure? 7. What is the cause of the semiannual variations in the lower thermosphere? How is magnetic storm activity associated with Joule heating and particle precipitation, or how is nonlinear coupling from the annual circulation involved? What is the relationship between the mesospheric and thermospheric variations, and what are the coupling mechanisms? 8. What is the relative importance of transport induced by wind circulation above the turbopause and variations in the eddy diffusion at lower altitudes for producing the observed winter bulges in the O and He and other atmospheric anomalies? Are similar effects observed in H, and what are the implications for the structure of the geocorona? D. Neutral-Atmosphere Chemistry below 120 km Compared with studies of other regions of the atmosphere, research on the chemistry of the stratosphere and mesosphere has been limited and fragmentary. In the past two years, interest in these

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Scien tific Uses of the Space Shuttle 19 regions of the atmosphere has increased markedly because of the possible deleterious effects of the effluents from supersonic trans- ports on the ozone column density. Before detailed predictions of the effects of these added gases and particulates can be made, the natural atmosphere must first be understood. For example, the isotopic abundances of nitrogen in nitrous oxide and other nitrogen oxides are different depending on whether the source is the upper atmosphere or the earth's surface, because of isotopic separation by biological processes in the surface source. The possibility of remote sensing of the abundances and isotopic ratios (14N/1SN) for N2O should illuminate the nature of the vertical transport processes. It appears that the ozone distribution is controlled chemically by the reaction O + O2 +M-*O3 +M, followed by the catalytic cycle NO+O3-»NO2 +O2 NO2 + (O or /»>)->• NO + (O2 or O). A complex chemistry involving water and related radicals or dissociation products such as OH, HO2, H, and H2 also plays a significant role. An excited state of oxygen, OC'D), present in 1 part in 1021 at the surface of the earth may be of controlling importance. However, it is also quite clear that vertical and horizontal transport of ozone and the atmospheric constituents in its life cycle are important in controlling the distribution. In turn, changes in ozone abundance by as little as 2 or 3 percent may produce biologically significant changes in the ultraviolet irradiation on earth. It will be necessary to understand the circulation system as well as the chemistry before we understand this vital part of the atmosphere. There is little prospect that a net of rocket, balloon, aircraft, and ground-based sounding stations sufficiently extensive to define the circulation will be set up by 1980. On the other hand, there are remote-sensing techniques available for use in orbit that offer the possibility of obtaining three-dimensional data on the distribution of a significant number of the important species that play a role in the mesosphere and stratosphere. Specifically these are O, O3, NO, OH, CO2, CH4, and possibly H2; we discuss these techniques in Section V.A.I. However, optical techniques cannot measure all constituents through the entire height range, and vertical profiles by in situ sensors carried on rockets will be needed at a limited number of locations.

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20 ATMOSPHERIC AND SPACE PHYSICS Thus, the major questions on the chemistry of the stratosphere and the mesosphere that must be answered are the following: 1. What is the chemistry of the natural stratosphere? How can it be studied by remote sensing of the structure, composition, and temperature? 2. What are the relative roles of chemistry, dynamics, and radiative processes in the stratosphere and mesosphere? Can these be revealed by long-lived constituents used as tracers? 3. What are the vertical distributions of the minor atmospheric species, what processes control these distributions, and what role do these species play? How do they vary diurnally, seasonally, and globally? 4. How are the aerosols distributed with height, and how are they formed? What effect do they have on the chemistry of the stratosphere? Can they be explored by topside remote sensing? E. Ion Chemistry of the D-Region and Lower E-Region The circulation, waves, and mixing in the neutral atmosphere between 60- and 120-km altitude affect the ionization distribution in three ways: 1. By changing the concentration of minor neutral constituents that produce or modify the ions; 2. By interacting with the earth's magnetic field to move the ionization vertically, sometimes compressing it into thin layers (sporadic E); 3. By changing vertical gradients of ionization by mixing to give random fluctuations in electron density that can scatter radio waves. These interactions are illustrated in Figure 3. The sensitivity of the ion concentration to motion fields below 120 km is further illustrated by the lifetimes against recombination of the various species at 103 cmt3 concentration: hydrated positive ions 100 sec NO+, O2 +ions 0.5h hydrated negative ions 6 h meteoric ions 1 week

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Scientific Uses of the Space Shuttle 21 NEUTRAL-CONSTITUENT SOURCE IONS PRODUCED CIRCULATION AND TURBULENCE EFFECTS WAVE-FIELD EFFECTS — »• Nonbtanketing Es 3 1 . D-layer ©2t, 03 Other minor mesospher D layer hvdrated negative ions FIGURE 3 Effects of neutral-atmosphere motions on D- and E-region ionization. As is evident from Figure 3, the ionization distribution is a sensitive indicator of changes in neutral chemistry and transport. Ground-based and rocket measurements have already demonstrated great variabilities in the ionization structure in the D-region. Among the questions that arise are the following: 1. What is the cause of the steep ionization gradient at 80-85 km altitude and its variability in height? Is the variation associated with temperature or composition changes in the mesosphere? 2. What effects produce the variability of the ion concentration during winter of an order of magnitude from day to day at heights of 70-90 km? Are they related to local changes in the thermal or dynamic structure or to gross changes in thermospheric circulation? 3. What produces the changes in the D-region in the aftermath of a magnetic storm? Are long-lived mesospheric constituents observed to move equatorward from the polar regions at this time? III. MAGNETOSPHERIC DYNAMICS A. Background In space physics, prime emphasis is placed on understanding the key physical processes associated with the energy, mass, and momentum

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22 ATMOSPHERIC AND SPACE PHYSICS transfer from the solar wind to the magnetosphere and atmosphere. Available evidence indicates clearly that most of the magnetosphere and the high-latitude ionosphere act as a single closely coupled system. The entire system may be quiescent or disturbed, with the disturbances (called substorms) having characteristic spatial and temporal forms. The fraction of the incident solar-wind energy captured by the magnetosphere may vary more than an order of magnitude, and there exist a variety of paths through which this energy is conducted to the atmosphere, involving direct particle in- jection, magnetic energy storage, particle acceleration (electrons, pro- tons, and heavier ions), electromagnetic waves, and Joule heating of the ionosphere. Within the closely coupled solar wind-magnetosphere system a number of well-defined boundaries exist between different plasma regimes. Two additional key elements are motion of the medium and associated electric fields and the wide variety of plasma instabilities that are involved in major magnetospheric processes. For example, it is believed likely that these plasma instabilities give rise to anomalous resistivity that restricts current flow parallel to the magnetic field. This resistivity in turn gives rise to field-aligned electric fields, which contribute to particle acceleration and can enhance or modulate their precipitation into the atmosphere. This precipitation produces auroras, and also ionization, which change the ionospheric conduc- tivity and hence in turn change the general electric field and current distribution. Other instabilities may lead to a self-limitation of the fluxes of stably trapped energetic particles in certain energy ranges, and a very large-scale explosive instability appears to be responsible for the expansion phase of magnetospheric substorms. Since 1958, a large community of space scientists from many countries has participated in an intensive and successful research effort to study the processes that control the magnetosphere. The result is a morphological description of the magnetospheric field, the particle population embedded in it, and its interface with the solar wind. Space physicists have also identified, and are beginning to understand, many of the physical processes involved. A number of key steps are required to further the understanding of the dynamics of the magnetosphere. In order to distinguish spatial and temporal variations and to study detailed configuration of boundaries, pairs of satellites are required. A coordinated program of measurements in widely separated regions is required to understand

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Scientific Uses of the Space Shuttle 23 the time sequence of events and medium changes in different regions associated with substorm disturbances. This is the main objective of the International Magnetospheric Study (IMS), a coordinated pro- gram to be conducted during 1976-1978. If successful, the IMS program should answer many questions concerning the timing of dynamical changes during substorms and identify the spatial locations of these changes. Progress should also be made on many other questions concerning the solar-wind input to the magnetosphere, the nature of magnetospheric boundaries, and the location of particle acceleration regions. The energy budget in the magnetosphere and ionosphere and the phenomena that control magnetosphere-ionosphere coupling would be the main target of research with the proposed, but not yet approved, Electrodynamic Explorer missions, to be carried out during the period between the IMS and the time at which high-inclination Shuttle orbits become available. These programs will provide important information on relatively small regions in space and time of processes occurring over very large regions of space, but it is not to be expected that these samples can be fitted in all cases by unambiguous interpretations. Many basic questions will necessarily remain unanswered, many new questions will emerge, and many theories will remain untested after the IMS. To some extent this must be expected because a characteristic feature of the magnetosphere-ionosphere system is that several individual physical mechanisms govern its behavior as parts of complex closed chains of cause-and-effect relationships. A characteristic example is shown schematically in Figure 4. The dynamics of these strong feed- back systems will be difficult to understand as long as the dynamic response is studied exclusively through observation of natural per- turbations that are highly unpredictable and whose initial conditions are difficult to establish. For this reason, after the IMS a new stage of research will become important in which the major emphasis is shifted from "passive" information gathering to a program involving controlled geophysical experiments. These experiments will be designed to break into some of the chains of cause-and-effect relationships by introducing, at appropriate points, man-made perturbations with various well- defined initial conditions and by determining the ensuing responses. For example, in the diagram shown in Figure 4, several forms of break in are possible:

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24 ATMOSPHERIC AND SPACE PHYSICS ANOMALOUS PLASMA RESISTIVITY PRECIPITATING PARTICLES FIGURE 4 Schematic example of a closed chain of cause-and-effect relation- ships in the magnetosphere. 1. Injecting energetic particles along a field line downward into the lower ionosphere to simulate a beam of precipitating particles of known characteristics; 2. Injecting an intense beam of plasma up a field line or releasing, at large distances, ionized gas from a rocket-launched canister to modify the magnetospheric plasma population under controlled conditions; 3. Modifying the ionospheric conductivity by release of chemi- cals; 4. Injecting waves of appropriate frequency and intensity to stimulate precipitation via wave-particle interaction; 5. Injecting plasma and/or waves to cause changes in resistivity by enhancing wave- particle scattering. Systematic man-made perturbations will also serve to stimulate individual mechanisms that are inaccessible to quantitative deter- mination when occurring under natural conditions; in addition they can be used to probe distant regions of the magnetosphere from low-altitude, high-inclination orbits. For example: 1. Release of ions in the solar wind near the magnetopause to

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Scientific Uses of the Space Shuttle 29 (d) Can one take advantage of zero-g conditions to conduct new types of experiments such as those involving convection-free plasma arcs, steady MHD flows, and certain long-term magnetic confinement devices? (e) How effectively does an antenna radiate electromagnetic waves at low frequencies in a plasma? What is the radiation pattern, and how does the radiation pattern depend on the plasma density and magnetic-field strength? (0 How does a plasma beam interact with a neutral gas, and what is the role of the gas ionization potential in determining the plasma temperature? What information can be obtained in this way about the interaction between the solar wind and a comet? 2. WAKE AND SHEATH STUDIES The Shuttle will move in the ionospheric plasma with a velocity that is intermediate between the ion and the electron thermal velocities; a complex sheath and wake will form, whose investigation is of interest in itself and to better understand the interaction of the solar wind with planets and satellites. The following are some outstanding questions in this area: (a) How stable are the wake and sheath regions? How does this stability change when the target is biased? Do Cerenkov cones develop in the wake region? (b) What is the structure of the shock in front of an obstacle moving at supersonic speed at low-earth-orbit altitudes? (c) How are phenomena such as antenna impedance and the response of plasma probes affected by the wake and sheath? (d) Can large-scale terrella experiments be conducted in space using strong deployed magnets? How does the interaction depend on the orientation of the dipole? 3. DIAGNOSTIC AND PROPULSION DEVICES STUDIES Long-standing questions involving the operation of various conven- tional probes can be attacked with the Shuttle sortie laboratory, making use of precisely known plasma conditions. Moreover, one would like to know the following, for example: (a) Can one use electron beams to measure longitudinal electric fields over long distances? Can one short-circuit the v x B electric field with an electron beam so that energy can be extracted?

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30 ATMOSPHERIC AND SPACE PHYSICS (b) Can one develop new diagnostic techniques based on plasma resonances suitable for space applications? (c) Can new propulsion devices, such as the magnetoplasmady- namic arc, be tested and perfected in the unbounded space plasma without the distortions due to finite chamber size and gravity? V. INSTRUMENTS AND TECHNIQUES A. Atmospheric Science 1. CORE INSTRUMENTS Most experiments envisaged would involve remote sensing of the atmosphere below either the Spacelab or a free-flyer to determine the spatial distribution—vertical and horizontal—of the constituents of the stratosphere and mesosphere, with sufficient global coverage to enable us to understand the dynamics and chemistry of these regions. The following modes of study are envisaged: (a) Horizon scanning of selected airglow features by high spec- tral and spatial resolution photometers and interferometers. A field of view of the order of 3 min of arc vertical x 3 deg of arc horizontal is required to achieve a spatial resolution of 1 km in the vertical. There is a need for corresponding accuracy in pointing. Such spectral features as the NOy bands, the OH electronic transition at 3100 A, the atomic oxygen forbidden transitions at 5577 and 6300 A, and the Herzberg O2 band system can be studied to give vertical distributions of NO, OH, O, O2, and O3. In the infrared, similar information can be obtained for CH4 and H2O. Fabry-Perot interferometers will be required as filters to achieve discrimination against Rayleigh scattered sunlight. Input apertures of the order of 50-cm diameter will be required for adequate throughput, and photometers will have to be about 2 m in length to provide adequate baffling and focal length. A battery of eight such instruments would be required, which would be mounted on a gimbal system about 2 m long by 2 m in diameter. (b) Vertical passive probing by infrared interferometry to deter- mine the vertical distribution of constituents such as CO2 and O3. This requires three instruments: one covering the range 1-5 mm, the other two from 5 to 150 mm; cooled optics and detectors will be required. (c) Lidar probing of the lower atmosphere. This is a powerful

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Scientific Uses of the Space Shuttle 31 technique that should be exploited in the 1980's from spacecraft launched by the Shuttle. Pulses in the ultraviolet (2200-3000 A), generated by tuned lasers, will be returned by Rayleigh scattering at altitudes measured by the time delay. By selecting wavelengths in absorption bands of interesting species and at nearby wavelengths and noting ratios of returned radiation, the abundance of the absorbing species can be determined as a function of the altitude. Alternately, at wavelengths where interesting constituents scatter resonantly the distribution of these constituents may be measured. A third mode of operation involves detection of Raman components of the transmitted pulse scattered by molecular species. It appears that these techniques will permit the measurement of the concentration profiles of the following atmospheric constituents: O3,02, NO, OH, H2O, NO2, N2O, CH4 , HNO3, H2, CO2, CO, and possibly others. The intensity of the backscattered signal will probably be sufficient to allow the measurement of Doppler shifts produced by horizontal winds. Noctilucent clouds can also be studied. The lidar system would consist of the laser and a receiver (~1 m2 area). Currently tunable lasers of the power envisaged measure 2 m x 1 m x 1m, but much of this space is occupied by power supplies that could be mounted elsewhere. (d) Measurement of the absorption of light in selected spectral regions between the Spacelab and a steerable subsatellite. The subsatellite might be a corner reflector located from 1 to 2 x 103 km away from the Spacelab in order to return light originating from a laser-telescope system on the Spacelab. Absorption in the ultraviolet would yield the concentration of such species as O, H2, and N in the upper atmosphere, while in the infrared the concentration of such species as O3, H2 O, NO, and O2 in the lower atmosphere could be measured. At this time, we foresee a clear need for only a few experiments in the thermosphere above 120 km after 1980. Among these would be the release of a chemical such as barium and the observation of the light scattered from it with high spectral resolution to obtain the wind velocity from the Doppler shift of the spectral line and the electric field*from the separation of neutral and ionized portions. Natural airglow emissions may also be observed for this purpose. Flexibility of approach will be maintained if provision is made to launch small free-flying subsatellites from the Shuttle for in situ sampling experiments or to allow continued use of other techniques such as sounding rockets.

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32 ATMOSPHERIC AND SPACE PHYSICS 2. MODES We foresee using combinations of techniques (a) through (d) to follow the vertical and horizontal motions of atmospheric species as a function of position on the globe and time of day and year. From our knowledge of the behavior of the atmosphere near 100 km, we have reason to believe that time scales as short as one day and as long as three months are involved. Thus, useful information concerning atmospheric chemistry and dynamics could be obtained during seven-day missions from a downward-pointed Spacelab carrying all four experiments. For example, in a high-inclination orbit flown near the solstice and again near the equinox the extreme season-lati- tudinal variations might be displayed. Universal time and longitudinal effects can also be studied. Alternately, orbits at low inclination would highlight diurnal variations. Experience suggests that such occasional short samples may miss important long-term effects. Hence we foresee the need to package some of these experiments—selected portions of (a), (b), and (c) for example—into free-flyers. These satellites could be recovered after a suitable period—six months to one year—and reflown after a lapse of one or two years. Both polar and equatorial orbits would be useful. In comparison with the sortie missions, however, these experiments would be comparatively low-data-rate missions and would con- centrate on a few key species, O3, H2O, NO, OH, and O, for example. B. Space Plasma and Magnetospheric Physics 1. CORE INSTRUMENTS AND EQUIPMENT The following are considered to be the core instruments and equipment required to perform space-plasma and magnetospheric- physics experiments from the Shuttle during the 1980's: (a) Electron and Ion Accelerators. Electron and ion accelerators are required to inject energetic beams into the ionosphere and magnetosphere. These accelerators must be capable of providing a wide range of energies (up to 50 keV) and currents (up to 1 A); they must be able to operate in a pulsed mode with pulse durations as short as a few milliseconds. (b) Plasma Gun. A plasma gun capable of producing intense fluxes of electrically neutral plasma is required to provide high local plasma densities for plasma-physics studies and for injecting high-energy density plasmas into the magnetosphere. A suitable device for generating these plasmas could be a magnetic-plasma dynamic arc of

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Scientific Uses of the Space Shuttle 33 the type currently being investigated for plasma-propulsion applica- tions. A typical beam produced by this arc would consist of 10,000 A of 200-eV argon ions. Operation on hydrogen, all noble gases, and N2 would be possible. (c) Remote Chemical and Gaseous Injection Devices. Several ejection devices are required to launch canisters containing chemicals and gases from the Shuttle. In most cases, we expect that these devices will be spring ejectors; however, in some instances (e.g., when separations from the sortie lab ranging from hundreds of kilometers to several earth radii are required), rocket launchers will be needed. The canisters to be employed will contain chemicals (such as barium oxide or SF6) for release into the magnetosphere, gases for release into the upper atmosphere, and metals to produce meteorlike trails in the lower ionosphere and atmosphere. It may prove preferable in some cases to use coordinated sounding-rocket launches for these releases instead of launching them from the Shuttle. (d) Transmitters and Antennas. A high-powered radio transmitter and long antennas (up to 1000 ft per element, as on the radio-astronomy Explorer satellites) are required to generate low- frequency radio waves for probing the ionosphere and magne- tosphere. Very large vlf and If voltages, up to 20 kV, will be required to produce high-power radiation from this antenna. The major present uncertainty in this area concerns the operation and perfor- mance of an antenna immersed in the ionospheric plasma. A separate transmitter and antenna will also be used, covering in frequency range the plasma and the electron cyclotron frequencies. At frequencies well below the local electron plasma and gyrofre- quencies, some problems involving tuning, sheath effects, and dipole unbalance arise. (e) Local Diagnostics. A wide variety of diagnostic instrumenta- tion is required to provide the measurements of the local environ- ment necessary for properly identifying the basic magnetospheric phenomena of interest. These instruments include magnetometers for magnetic-field measurements, electrostatic charged-particle energy analyzers for energy-spectrum measurements, Langmuir probes for electron-density and temperature measurements, mass spectrometers for local composition measurements, double probes for electric-field measurements, and loop or search-coil antennas for ac magnetic-field measurements. (f) Long Boom Assemblies. To carry out many experiments, especially those involving low-energy plasmas and small-amplitude waves, it will be required that active perturbing and sensing

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34 ATMOSPHERIC AND SPACE PHYSICS equipment be remotely located from the large Shuttle system. It is anticipated that two retractable 50-m booms, mounted on swivel platforms so that the relative locations of the elements on the ends of the booms can be varied over a wide range, will be needed as a basic element of the sortie laboratory. (g) Subsatellites. Controllable subsatellites released from the Shuttle are required to perform coordinated observations remote from the sortie lab. With subsatellites, it will be possible to study characteristics of long-wavelength plasma waves and to perform remote studies such as magnetic conjugate-point investigations, which could not be performed otherwise. This subsatellite may be similar in size and capability to the current Atmospheric Explorer spacecraft. The Atmospheric Explorer spacecraft, with its propulsion and spin-up and spin-down capability, is well suited to perform a variety of experiments currently envisaged. 2. MODE Because of the variability and unpredictability of conditions in the magnetosphere, a maximum of flexibility of the experimental setup is required. For instance, for elliptical, high-inclination orbits, the ambient plasma density may change from 106 cmt3 to as low as 102 cmt3 (at high altitudes and high latitudes). The low-altitude range would be best suited for some ionospheric wave-propagation and plasma resonance experiments, while the apogee portions of the orbit would be optimum for collisionless or long-time-scale plasma- confinement studies. During highly active periods at auroral lati- tudes, low-energy electron beams could be directed along field lines to detect and measure magnetic-field-aligned electric fields, while during quiet times, high-energy electron pulses could be used to study magnetic-field configuration. Wave injection to stimulate emission of whistler-mode waves will be required for a range of frequencies that generally varies in a known way with latitude but that will also depend to some extent on the conditions at large distances from the Shuttle. Some specific measurements (not included in all missions) will require the use of small subsatellites in similar orbits for optical and radio transmission (and absorption) studies, and a few flights will require rockets for distant plasma-injection studies. A well-coordinated, ground-based observational program will be required to complement the active experiments and passive diag-

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Scientific Uses of the Space Shuttle 35 TABLE 3 Mission Model for Atmospheric and Space Physics Free- Seven-Day Sortie Piggy- Flyers Missions back Inclination Low High Low High Low High - Altitude Low Low Low Low Low High 6.6 RE Pointing Down Down Down Down Up Up — High-power laser and detector system X X X X — — — Laser reflector subsatellites — - X X - - — Passive optical remote sensing X X X X X X — Solar instruments X X X X — — — Plasma accelerators, electron and ion guns — — X X X X — Multipurpose transmitter and antennas - - X X X X — Plasma diagnostic instruments On booms — — X X X X On pointed pallet platforms - - X X X X Maneuverable subsatellite — — X X X X — Local releases and deployment - — X X - X — Remote releases and deployment — — — - X — — Frequency of flights 0.5/yr 0.5 1/yr 1/yr 0.3/yr 2/yr 0.2/yr nostic measurements conducted from the sortie laboratory. In addition, taking into account the long-proven value of the synchro- nous orbit for magnetospheric research, small packages should be available in such orbit for coordination with the magnetospheric experiments on the Shuttle. VI. MISSION MODEL FOR ATMOSPHERIC AND SPACE PHYSICS We summarize here a tentative model in which seven different mission types are listed, based on the use of three different instrumentation groups (Table 3): a single sortie laboratory facility, a single free-flyer, and a single piggyback package. Each sortie laboratory mission carries a maneuverable subsatellite that will generally be deployed and recovered. However, in some cases, it can be anticipated that the subsatellite will remain as a free-flyer for a long-duration investigation. (A) and (B) Structure, Composition, and Dynamics of the Atmosphere Objectives: To understand the factors that control the dis- tribution of stratospheric and mesospheric constituents—such as ozone—by determining their diurnal, seasonal, and spatial variations.

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36 ATMOSPHERIC AND SPACE PHYSICS Free-flyers with weight less than 5000 kg, to be flown in nearly circular orbits. The (A) missions are in low-inclination orbits to study diurnal effects, and the (B) missions are in high-inclination orbits to analyze global and seasonal variations. One pallet is needed to carry other auxiliary optical instruments. (C) Remote Sensing of the Atmosphere; Local Plasma Physics Objectives: To measure vertical and horizontal distribution of atmospheric constitutents of the mesosphere and stratosphere and the roles of chemistry and transport and to conduct basic plasma- physics experiments in the low-altitude, near-equatorial orbits. The instruments can be mounted on three pallet elements. The energy required is of the order of 500 kWh, if the control functions are all carried on board. Since the plasma-physics investigations are to be experiments that must be operated in real time, a man must control the instrumentation based on a real-time evaluation of the data being obtained. He may be in a pressurized module or on the ground, but the latter choice would clearly require a very extensive telemetry and command link to the pallet facility, with continuous orbital coverage. (D) Remote Sensing of the Atmosphere; lonosphere-Magne- tosphere Coupling Objectives: To measure vertical and horizontal distribution of atmospheric constitutents of the mesosphere and stratosphere to elucidate the role of chemistry and transport and to carry out controlled study of magnetospheric phenomena. The instruments can be mounted on three pallet elements with energy requirements of the order of 500 kWh [see comments on man's role in (C) above; for this mission without a pressurized module, continuous telemetry in the polar region is vital]. (E) Solar-Wind Entry and Plasma Injection Objectives: To investigate motion, stability, and energization of injected plasma clouds and their influence on radiation-belt pre- cipitation. Rockets would be launched from the Space Shuttle "to inject plasma clouds at radial distances 3-15 earth radii with optical observations of the plasma clouds from the Space Shuttle. The total weight is not well defined at present [see comments on man's role in (C) above].

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Scientific Uses of the Space Shuttle 37 (F) Controlled Magnetospheric Experiments; Horizon Scanning of the Atmosphere Objectives: To study by active techniques the stability of the radiation belts, the phenomena that precipitate the ring current and the trapped particles, and the configuration of the magnetosphere. Laser horizon scanning techniques will be used to study the upper atmosphere. Three pallet elements to be used, with an energy requirement of 500 kWh [see man's role in (C) above]. (G) Geosynchronous Diagnostic Package Objectives: From a remote platform, a 50-kg experimental package is required to study directly the perturbations produced from the Shuttle. The atmospheric and space physics (A&SP) sortie mode experi- ments require manned control, in real time, based on real-time analy- sis of observational data. This control and analysis can be carried out very efficiently if one to three scientists (of whom the payload spe- cialist may be one) are on board, along with a small pressurized module. All A&SP sortie missions require about 10,000-12,000 Ib of instrumentation placed on three 3-m pallet sections, and inclusion of the pressurized module will not raise the total weight above the allowable Shuttle limits. It is possible to perform all A&SP sortie missions in a pallet-only mode, if an adequate real-time tracking and control network is set up to give continuous coverage for all contemplated orbits, including those of high inclination. It should be noted, however, that some of the highest priority A&SP experiments must be conducted over the polar regions. The proposed Tracking and Data Relay Satellite (TDRS) system will apparently not provide sufficient coverage over the poles. In low-inclination orbits, the pallet-only option appears to be a viable one for many experiments provided the proposed TDRS uplink and downlink systems together with real-time data-handling facilities are established. VII. RECOMMENDATIONS 1. Considering the need to conduct experiments in space con- trolled by man in real time, without interruption during several complete orbits, and taking into account the potential availability of different modes of operation of the sortie lab, we recommend that

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38 ATMOSPHERIC AND SPACE PHYSICS the National Aeronautics and Space Administration and European Space Research Organization conduct a study of the relative advantages in scientific payload weight, cost, available data rate, and coverage of systems using active experiment control based on real-time data evaluation including subsatellite operations, and repair of control instrumentation, of the following modes of operation: (a) a sortie lab configuration consisting of a pressurized module with three pallet modules and zero to four men in addition to the four-man Shuttle crew, using active control from the pressurized module; (b) a sortie lab configuration consisting of three or four pallet modules monitored by a payload specialist with active control from the ground. 2. Noting the penalty in payload weight and distribution imposed by the inclusion of the docking module in the sortie lab configura- tions, we recommend a study by the National Aeronautics and Space Administration and the European Space Research Organization as alternate means of rescue. 3. The scientific observation and experimental program planned in the field of atmospheric and space physics will require certain key instruments or facilities, which will form the nucleus of almost all missions. Consideration of present-day technology and anticipated improvements indicate that suitable instrumentation can be available if a program is started now in certain crucial areas. We recommend that supporting research and technology funds be used to (a) work on the development of the appropriate lidar tech- nology; (b) improve general and multiplex detector technology in the optical, infrared, and ultraviolet regions; (c) develop new in situ sensors for measurement of chemically active minor constituent concentrations in the stratosphere and mesosphere; (d) develop antenna-transmitter systems for efficient high- power, low-frequency wave generation from within the magne- tospheric plasma; (e) develop, design, and test high-power electron and ion accelerators and plasma beam devices with appropriate energy, density, and beam optics requirements. It is also important that laboratory studies of reaction rates and theoretical atmospheric modeling studies be continued to allow

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Scientific Uses of the Space Shuttle 39 optimal use to be made of the atmospheric data to be gathered by the Shuttle mission. 4. We strongly recommend that an Announcement of Planning Opportunity be issued as soon as possible to select scientists to participate in the detailed scientific definition of the planned programs and the development of the planned instrumentation.