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- 5 - 8. What is the nature of the turbopause? What are the characteristics of turbulent diffusion below the turbopause? What is the nature of the transition to molecular diffusion? Does it result from a sudden damping of turbulence, or from the exponential increase with altitude in molecular difussion (in inverse proportion to atmospheric density)? 9. What is the composition of the mesosphere, particularly with respect to minor but important species like NO and Na and excited states such as the al Ag state of 02? l0. What is the distribution of ozone and other important, chemically active species in the mesosphere as function of time of day, season, and position? What is the relative importance of reactions with constituents other than oxygen allotropes? ll. What is the role of eddy diffusion in determining .the atmoic oxygen content in the thermosphere? l2. What is the vertical distribution of major and minor constituents as a function of time? l3. To what extent do the troposphere waves penetrate the upper atmo- sphere? l4. Can planetary waves be observed in the stratosphere and mesosphere? The Role of Sounding Rockets in Scientific Research A number of techniques are available to study the Earth's atmosphere and space. Among them are artificial satellites and space probes, ground-based instruments employing, for example, optical, radio, and radar techniques, scientifically instrumented aircraft, balloons, and sounding rockets. Each has its role and its advantages; a well designed experimental program exploits the techniques best suited to its particular objectives. The greatest single advantage of sounding rockets for studying the upper atmosphere is their unique ability to obtain direct, vertical profiles in the altitude range of about 40 to 200 km. There is no other means of obtaining true vertical profiles by direct measurement, although some parameters can be measured indirectly by ground-based observations such as Thomson backscatter. This region of the atmosphere is of extreme interest in atmospheric and space research. Here are the D, E, and part of the F regions of the ionosphere, characterized by complex chemical reactions in response to solar energy, the global electric current system, and the electrojets. Complicated physical processes in the magnetosphere focus energy into the auroral zone, where pre- cipitating energetic particles, auroras, strong electric fields, electrojet currents, and enhanced ionization all give clues about the total magnetic storm and substorm process. This altitude range spans the region where mixing ceases to be the dominant process governing atmospheric composition and dif- fusive separation begins. Here, too, are found airglow, some aerosols, and noctilucent clouds. Most of the ultraviolet solar energy entering the
- 6 - atmosphere is absorbed here, and neutral particles, ions, and electrons are present simultaneously. Large changes in composition, temperature, and winds occur with height; much of what happens at higher altitudes is determined by the composition and structure in this region. Study of this region from 40 to 200 km by means other than sounding rockets is constrained by the fact that balloons cannot attain altitudes greater than about 40 km and that satellites are not effective below about 200 km because of lifetime limitations. The advent of variable orbit satel- lites able to dip down to about l40 km, possible in the early l970's, would ameliorate the situation, but they will not be capable of instantaneous vertical profiles free of latitudinal and local time variations which are so important in the study of many parameters of interest in this region of space. Ground-based instruments can, and do, probe this region by indirect means, and for some investigations are more effective than space-based instruments. For other investigations, such as detailed analyses of atmqspheric composition at various altitudes and times, in situ measurements are essential. DC electric fields and D-region ionic composition, for example, are impossible to infer from any type of ground-based technique. Similarly, only rocket- based instruments can measure the local rates of optical excitation and emission in the upper atmosphere. In addition to the unique contributions that sounding rockets make to the study of the 40-to-200 km region, they have several attributes that are used to great advantage in experimental programs. Their high velocity, which is usually large compared with the vertical transport processes occurring at altitude, combined with the short flight times (about six min- utes) and speed of data gathering, make them particularly suited to the study of rapidly occurring transient phenomena or brief events such as eclipses, and measurement of physical parameters at a given moment of time. These same characteristics make them less suitable than ground-based or satellite tech- niques for long-term monitoring or global coverage -- except, of course, in the 40-to-200 km region where they are the only method available for ^n situ observations. The logistic flexibility of sounding rockets is a substantial added benefit to experimental programs and increases the usefulness of the tech- nique in the study of special, transient events. By choosing the appropriate rocket type, trajectory, and hardware, almost any desired velocity, altitude, and spin rate may be attained. The ability to launch a sounding rocket at a specific time and place (consistent, of course, with range locations and usage) is often indispensable to a particular scientific objective. The vehicle can be held in readiness for a long period of time and then be launched at pre- cisely the most favorable moment. Ground-based installations are often quite difficult to move to other locations, particularly if they require large antenna systems (e.g., Thomson-scatter radar, partial reflection sounders). It is frequently more efficient to set up a temporary rocket launching facility for a special purpose. This is well illustrated by several recent eclipse expeditions and by the shipboard firings made in the l965 NASA Mobile Launch Expedition. To compare rocket and Thomson-scatter measurements of electron density and temperature, it has been easier to take the rockets to
- 7 - the sounders rather than the reverse. Similarly, rockets lend themselves more easily than satellites to coordinated work with ground-based instruments, Scientists who have attempted to compare satellite results with ionosonde and Thomson-scatter data appreciate that it is difficult to find suitable times when a satellite passes overhead at a specific location, particularly if certain ionospheric conditions are desired. It is not nearly so difficult to schedule rocket firings at the proper time and location. The relative mobility of a rocket launch site permits global studies which would not be feasible with ground-based installations alone. Sounding rockets are an excellent means of studying fine structure and detail owing to the sensitivity of their instruments and high rate of data sampling. For example, the sensitivity of rocket measurements of electron density is about l0*0 greater than that of ground-based radio. Given the greater confidence one can usually place in in situ measurements, it appears likely that rockets will play an increasingly major role in calibrating indirect ground-based techniques. By comparison, these ground-based tech- niques provide poorer height determinations and vertical resolution. Scien- tifically instrumented balloons and aircraft in particular can play a similar role vis a vis rockets by providing data below levels sounded by rockets and overlapping, comparative data. The ability to recover payloads by parachute descent is an enormous advantage since it permits use of photographic film (which provides a partic- ularly high rate of information accumulation), use of nuclear emulsions, examination of instrumentation, and post-flight calibration. The recovered instruments can normally be re-used, thus reducing costs and facilitating readjustment and redesign of experiments on the basis of the new information obtained. Rockets also permit the use of short-lived, perishable detection systems such as the liquid-helium-cooled telescope for observations in the far infrared. Because of the logistic simplicity of rocket firings, liquid helium can be added almost up to the moment of launch, and the supply carried is then adequate for the six minutes of flight. The ability of rockets to rise above the obscuring layers of the Earth's atmosphere is largely responsible for the revolutionary development of astro- nomy and solar physics in recent years and for the existence of x-ray, ultra- violet, and gamma-ray astronomy. The spinning rocket is ideally suited to exploratory surveys and, at its high altitudes, extends astronomical investi- gations to wavelengths that cannot be observed from the ground. Satellites also have these capabilities, with the added ability of obtaining many months of data rather than only six minutes of data. It would seem clearly prefer- able to conduct space astronomy studies from satellite bases when substantial observing time is needed; yet the fact remains that almost all the important space astronomy results obtained so far have been obtained from rockets, and rocket-borne studies still contribute dominantly to space astronomy knowledge. Among the reasons are the following. Six minutes of rocket data are a very great deal, anough to saturate the data analysis and interpretation efforts of a small group for more than a year. (In one case the high resolution echelle spectra of the Sun obtained on a single flight have required more than five years of analysis, and much information of scientific value still