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Upper Atmosphere Investigations Require Coordinated Approach Sidney Teweles upper atmosphere investigations Environmental Science Services Administration require coordinated approach Silver Spring, Md. Abstract trating the mesopause; and 2) the brief lifetime of satel- The atmospheric layers from 60 to 200 km above the lites with perigee below 200 km. Most of the presently Earth's surface can be sounded only by expensive rocket- insufficient information comes from the photography of launched experiments, problematical indirect methods, chemiluminescent rocket trails and from rocket-borne in- or satellites in rapid orbital decay. Measurements of struments such as pressure gages and mass spectrometers. large-scale phenomena are masked by large amplitude The use of falling spheres to deduce the density from perturbations, both aperiodic and tidal. Still the needs drag deceleration data has been achieved to increasingly of the rapidly increasing traffic of aerospace vehicles higher altitudes, but relatively few reliable observations through this region must be served. There is a growing have been obtained above 100 km. Indirect information requirement for meteorologists to apply their special has accrued from ground-based ionosonde measurements, skills to the analysis of the costly data now accumulating photographic and radar tracking of meteor trails, and in substantial amounts from the mesosphere and lower few specialized photometric and spectrographs measure- thermosphere. ments of radiation absorbed or emitted in the meso- sphere and lower thermosphere. 1. Introduction The collection of data in the region under discussion is a lengthy and expensive process. Aerospace environ- A pressing need exists for a coordinated scientific effort mental consultants familiar with the operation of aero- to probe and define the nature of the atmosphere at space vehicles can foresee some of the problems which altitudes from 50 to 200 km. Further knowledge of the are bound to arise in the future operation of these ve- physical structure, composition, and circulation of this hicles. But they find it difficult to convince the engineer region, comprising the mesosphere and lower thermo- of the importance of a potential problem until his ve- sphere, is essential to the economical design and effec- hicle encounters the problem in flight. By then it is too tive operation of aerospace vehicles. late to start collecting atmospheric data. Lower layers of the atmosphere are sounded daily up to about 30 km by balloon-borne radiosondes released 2. Environmental effects on aerospace vehicles from hundreds of weather stations. Between 30 and Important problems are encountered which cannot be 60 km, increasingly frequent measurements are being satisfactorily resolved with the present grossly inade- made by means of instruments ejected from the rela- quate knowledge of atmospheric phenomena in the ma- tively inexpensive rockets fired at more than a dozen jor layers above and below the mesopause. For example, sites of the Meteorological Rocket Network. Thus lack of any sort of synoptic climatology for this region weather maps can now be drawn for levels as high as is one source of error in predictions of aerospace vehicle the 0.4-mb surface (at about 55 km, or 180,000 ft). Far responses, since conditions may deviate significantly from beyond, in the high atmosphere above 200 km, many those in the Standard Atmosphere. Costly overdesign of of the satellites orbited since 1957 continue to supply hypersonic and re-entry vehicles is a potential result of vast amounts of physical information. the current inadequacy of information on horizontal In the great intervening layer, however, relatively few and vertical distributions of density and wind. atmospheric observations have been made, owing pri- Various general examples of the importance of density marily to: 1) the high cost of rockets capable of pene- in this region can be given. When the perigee of a satel- i The text of this article includes various statements and lite drops below about 180 km, the rate of orbital decay was amended in accordance with suggestions made by many becomes critical, and the prediction of final descent and specialists in aerospace science, including Bernard Charles, re-entry is extremely sensitive to the atmospheric density. Arnold Court, Roderick S. Quiroz, Norman Sissenwine, and The range of ballistic missiles can be significantly af- William W. Vaughan. One result of their consideration of this text was the decision to bring together many prominent fected by density variations in the 60-90 km region aerospace scientists at Miami Beach, May 31-June 2, 1967. (after cut-off and before full re-entry); the effect on the (See meeting program in the BULLETIN, 48, pg. 210.) range depends also, of course, on other factors such as 262 Vol 48, No. 4, April 1967 Unauthenticated | Downloaded 10/07/21 03:07 AM UTC Bulletin American Meteorological Society mass/area ratio and initial climb angle. Extreme density ture varies markedly from day to night, and even more gradients, particularly in the altitude range 40-70 km, through the 11-year cycle in solar activity. must be taken into account for evaluations of aerody- The mesopause separates regions which in the past namic heating of re-entering boost-glide vehicles. have received major emphasis from different disciplines. The possible effects of gravity waves and noctilucent Hence, it has suffered the usual fate of a boundary cloud particles on hypersonic vehicles traversing their region—conflicting terminology, incompatible measure- domain are of special interest. It appears necessary to ments, irreconcilable theories, and perhaps most impor- investigate whether effects analogous to those of turbu- tant, lack of communication between investigators. Even lence and hail on conventional aircraft might not occur directions of motion of the air are reported differently, in association with these phenomena. Wind tunnel stud- according to the direction from which it comes by ies at hypersonic velocities indicate that very-small-scale meteorologists and according to the direction toward perturbations in the natural environment could trigger which it goes by many physicists. Ionospheric physicists off turbulent aerodynamic flow at a point much farther prefer to speak of the D, E, and F regions of the forward on an airfoil than under "quiet" ambient con- ionosphere, at 60-90, 90-140, and above 140 km, re- ditions; this situation could lead to unprogrammed heat- spectively; while meteorologists generally utilize the ing and loss of lift. Aerodynamicists thus need to know thermally-based terms, mesosphere and thermosphere wave length and amplitudes at the short end of the (separated by the mesopause). spectrum of turbulent motions. Other physical phe- Noctilucent clouds, observed near the mesopause, are nomena of significance to aeronomers and aerospace a most perplexing feature of the upper atmosphere. designers are also of rapidly increasing interest to me- They are seen most commonly in summer in the sub- teorologists. These include the inter-relations between polar regions. The question of whether they are pre- ionospheric disturbances, proton showers, electron densi- dominantly ice-crystal or dust clouds remains to be ties, extreme ultraviolet radiation, X-rays, solar disturb- settled, despite evidence from recent rocket cloud sam- ances, etc., and their effects upon materials, communi- pling experiments which suggests the presence of ice cations, and personnel exposed at altitudes above 50 km. particles. The turbopause is another feature that appears to 3. Atmospheric phenomena have some relationship to the mesopause, although it There are many important phenomena in the 80-120 is usually found at a level about 20 km higher. Below km altitude region, most of which are only partly under- the turbopause, rocket trails bulge and twist apparently stood. Molecular dissociation is known to occur to an in response to turbulent eddy motions, while above it appreciable degree above about 100 km. The turbo- these trails are smooth and cylindrical, dissipating only pause, a physically complex boundary region in the through molecular diffusion. In the so-called "homo- vicinity of 105 km, marks the top of a turbulent regime sphere" below the turbopause, composition and hence and separates it from smooth flow above in the region molecular weight are nearly constant. In the "hetero- where diffusive separation becomes a dominant process. sphere" above this boundary, the molecular weight of Substantial enhancements of ionization (Sporadic-E) ap- air decreases with height due to the upward diffusion pear most often at about 100-110 km. In the auroral of the lighter atomic gases that are formed by the dis- high-latitude zones, occasional penetrations of solar radi- sociation of the molecular constituents. ations down to a height of 60-80 km greatly increase Near and above the mesopause, radar observations of D-region ionization and are suspected of having a pro- meteor trails indicate air motions and, under certain found effect on the neutral atmosphere as well. conditions and with less accuracy, the atmospheric den- The mesopause itself is a boundary region of very low sity. Even though the east-west and north-south motions temperatures observed in the vicinity of 85 km above are observed at points 100 to 200 km apart, time-smooth- which only meager information has been provided. De- ing of the wind data can provide information on tidal spite the low density and pressure of the mesopause— and large-scale motions at altitudes
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