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- 2 - The existence of three new branches of astronomy - ultraviolet, x-ray, and gamma-ray astronomy - is also attributable to the ability of sounding rockets to place instruments above the obscuring atmosphere. Rockets have recorded the ultraviolet spectra of stars and planets, and measurements of the hydrogen Lyman -a absorption line in the stellar spectra have permitted determination of hydrogen atom concentrations in interstellar space. Other lines have provided evidence of high-velocity mass ejection from hot super- giant stars. X-ray sources in the galaxy were discovered by rocket-borne detectors; later flights measured their spectra, variability, angular size, and location, permitting the identification of some sources with optical objects. The main features of the structure of the Earth's atmosphere above bal- loon altitudes have been identified and partially mapped by rockets. Of particular note are the temperature maximum near 50 km, the temperature minimum near 80 km, and the higher temperatures at greater altitudes. Satel- lites have contributed a great deal of information on density variations above 200 km, but rockets have provided the basic framework for interpretation of the satellite data in terms of variations of temperature and composition. However, the picture of atmospheric structure is not complete in a geographic sense and more rocket data are needed. The geocorona, which constitutes the outermost portion of the atmosphere, was also first recognized and described on the basis of rocket data. An understanding of the relevant physics was developed with the guidance provided by the rocket data. Sounding rocket investigations changed the concept of ionospheric layers to ionospheric regions, showing that what had been regarded as separate layers were generally just ledges on an electron-concentration distribution that increases fairly steadily up to the F£ maximum. Almost all our knowledge of ionospheric chemistry, critically important to an understanding of upper atmo- spheric processes, stems from rocket soundings. Also of major significance to this understanding are the precise data obtained by rocket-borne instruments on the heights of airglow emissions; prior to the availability of sounding rockets very little could be learned on this subject. Rocket measurements have given the most precise description available of particle fluxes in auroras. Although such data should in principle be obtainable by satellites as well, a satellite's high velocity as it passes through narrow auroral features requires an extremely high rate of data acquisition, and this factor has increased the relative contributions pro- vided by rockets in auroral measurements. Finally, the existence of electrical current systems in the ionosphere and the polar and equatorial electrojets was detected by rockets and the altitudes of the current systems were measured. Major Scientific Questions Likely to be Answered by Rocket-borne Experiments Some of the outstanding fundamental questions on the nature and processes of the atmosphere and space are more accessible to sounding rocket investiga- tions than to other techniques. Data from satellites and from ground based
- 3 - observations are also important, and these also are problems that are most amenable to attack by these techniques, or by some combination of techniques It is likely that the following questions, at least, will be resolved by rocket-borne experiments or a combination of rocket results with observations from other sources. Astronomy and Solar Physics Far Infrared - 20 to l000 urn: l. Is the spectrum of cosmic background-radiation at 400 to l000 p.m like that of a 3 K blackbody? 2. Do interstellar grains emit radiation and does this radiation yield important information on their composition? 3. What is the nature of Seyfert galaxies, quasi-stellar sources, and protostars in our galaxy? Do other kinds of objects which also emit infra- red radiation exist as well? Far Ultraviolet - 900 to 3000 A: 4. What is the nature of interstellar grains? 5. What is the ultraviolet flux of hot stars? This information is needed to determine the accuracy of model atmospheres and temperature scales which will, in turn, aid in determining elemental abundances and processes of stellar evolution. 6. What are the effects of pressure broadening on the strong resonance lines, and what are the elemental abundances, in the photospheres of hot stars? These measurements can be made in the ultraviolet without the large corrections for excitation and unobserved ion states necessary in the visible. 7. What is the rate of mass loss in stars with expanding envelopes? 8. What are the abundances of elements in the interstellar medium? 9. What is the thermal and kinetic structure of the Sun's chromo- sphere and corona? l0. What kinds of stars have chromospheres and coronas? X Rays - l to 60A ll. Does a hot intergalactic plasma exist and does it have enough mass to close the universe gravitationally? l2. What is the nature of x-ray emission lines in various sources? If the x rays originate in a hot gas surrounding a neutron star or white dwarf, such emission lines should be present in Scorpius XR-l.
- 4 - l3. Is there a variation with galactic latitude due to galactic neutral hydrogen in the absorption of soft x rays? Magnetosphere. Aurora, and Airglow l. What is the nature of auroral and magnetic substorms and what is their relation to the physics of the magnetosphere? 2. What is the nature of the ionospheric current systems, especially in the auroral ionosphere? 3. What are the processes that accelerate auroral particles? 4. What wave-particle interactions are associated with aurora and what is their role in magnetospheric dynamics? Ionosphere l. What is the influence on the ionosphere of motions and energy balance in the upper atmosphere? 2. Are stratospheric warmings a major influence in increases in D-region ionization? 3. What is the level and source of ionization in the nighttime E region? 4. What energy degradation processes occur associated with the absorp- tion of solar ultraviolet radiation and what is the flux and spectrum of the resulting photoelectrons? Aeronomy l. What is the momentum source for the semiannual oscillation in the equatorial mesophere? 2. What is the vertical extent and general morphology of sudden stratospheric warmings and quasi-biennial oscillations (26-month periodicity)? 3. What process is responsible for the high temperature of the winter mesopause? 4. What is the origin and composition of noctilucent clouds? 5. What is the nature of wave structures in the 70-to-l20 km region? 6. What is the magnitude of tidal wind and temperature variations? To what extent are they affected by ambient winds, longitude, nonlinearity, instability, viscosity, and ion drag. 7. To what extent do wave disturbances associated with tropospheric weather systems penetrate the upper atmosphere?
