Contains reports on three research projects.National Aeronautics and Space Administration (Grant NsG-419)National Aeronautics and Space Administration (Grant NsG-250-62)U. S. Air Force under Contract AF 19(628)-500Lincoln Laboratory, Purchase Order DDL BB-10

Barrett, Alan H.

Rogers, A. E. E.

Staelin, David H.

DSpace@MIT

III. RADIO ASTRONOMY*Prof. A. H. BarrettProf. J. W. GrahamProf. R. P. RafuseProf. W. C. SchwabR. J. AllenR. K. BreonE. GautI. GlaserW. KuiperB. LenoirLindesM. Moran, Jr.M. A.A. E.T. S.J. H.D. H.D. H.PalfyE. RogersSlaterSpoorStaelinSteinbrecherA. MEASUREMENTS OF THE WATER-VAPOR LINE IN THE TERRESTIALATMOSPHEREMeasurements of atmospheric absorption have been made recently in the spectralregion 21. 0-32. 4 Gc/sec. These were made at Lincoln Laboratory, M. I. T., with the28 ft paraboloid and the Research Laboratory of Electronics 5-channel k-band micro-wave radiometer. The absorption was measured by observing the atmospheric18 20 22 24 26 28FREQUENCY (GC/SEC)30 32 34 36Fig. III-1. Experimental and theoretical atmosphericspectra in the 18-33 Gc/sec regions.absorptionextinction of the sun during several sunsets.Figure III-i shows one theoretical and two experimental spectra. At the beginningThis work was supported in part by the National Aeronautics and Space Adminis-tration (Grants NsG-250-62 and NsG-419); and in part by Purchase Order DDL BB-107with Lincoln Laboratory, a center for research operated by Massachusetts Institute ofTechnology, with the support of the U. S. Air Force under Contract AF 19(628)-500.QPR No. 74(III. RADIO ASTRONOMY)of the experiment, on April 21, 1964, the temperature was 500 F and the relative humi-dity was 35%; on May 6, 1964, the temperature was 660 F, and the relative humiditywas 43%. These results are preliminary, and the measurements will be continued.D. H. StaelinB. THEORETICAL WATER VAPOR SPECTRA FROM AN EARTH-ORBITINGSATELLITETheoretical atmospheric emission spectra have been computed for the range 5-40 Gc.These computations indicate that satellite-based measurements of the atmosphericwater-vapor content may be quite feasible if microwave radiometers are used. Thesecalculations include the effects of water vapor and oxygen, 2 and also the radiationreflected and emitted from the earth's surface and the effects of the earth's curvature.8= 40E PARALLEL SURFACEEARTH10 20 30 4FREQUENCY (GC/SEC)Atmospheric spectra as seen fromis over a body of water.space when the satelliteIn the computations an atmosphere containing a typical exponential distribution ofwater vapor was assumed: HZ = 10 e - a / 2 . g/m , where a represents altitude in kilo-meters. The temperature and pressure distribution were approximately those of theARDC standard atmosphere. 3Figure III-2 shows spectra that would be observed by a satellite receivingQPR No. 74180 r--150 --120 -Fig. 111-2.(III. RADIO ASTRONOMY)horizontally polarized radiation from a smooth body of water. These spectra show thatthe effect of water vapor is quite large, producing a microwave brightness temperaturedifference of 30-100 ° K between the center of the emission line and the wings. Sincemicrowave radiometers can be built with noise fluctuations less than 2Z K for 1-secondaveraging, such an emission line could readily be measured. Although more typicalwater-vapor densities are perhaps half the values assumed here, the signal is stilllarge.2608 = 40230200WITH CLOUDO 170 -STHNO CLOUD0E PARALLEL SURFACE140 10 10 20 30 40FREQUENCY (GC/SEC)Fig. 111-3. Atmospheric spectra as seen from space when the satelliteis over a smooth surface of dielectric constant E = 5. Thecloud layer is assumed to be uniform at 2-3 km with 1 g/m 3condensed water.Over land such measurements would be more difficult because the ground is oftenapproximately a black body with almost the same microwave brightness temperatureas the air above, and thus yields almost no spectral line. This fact is shown in Fig. III-3,which presents spectra for a smooth planetary surface with dielectric constant E = 5.The spectral line is large at O = 64* because of the assumption of a smooth surface -which is rarely true.Also shown in Fig. 11-3 is the effect of cloud on the spectral line. Thecloud was assumed to be 1 km thick and to contain 1 g/m 3 condensed water,which is a moderately large amount. Although the cloud reduces the line inten-sity, it does not conceal it. Over bodies of water the line could be even moredistinct.Of course, the exact amounts of atmospheric water vapor and cloud cover are notQPR No. 74(III. RADIO ASTRONOMY)simply related to the spectra. The spectra are also affected by temperature, pressure,and water-vapor distributions in the atmosphere, and these have to be carefully consid-ered in accurately interpreting any data.D. H. StaelinReferences1. M. L. Meeks, Atmospheric emission and opacity at millimeter wavelengths dueto oxygen, J. Geophys. Res. 66, 3749-3757 (1961).2. A. H. Barrett and V. K. Chung, A method for the determination of high altitudewater-vapor abundance from ground-based microwave observations, J. Geophys.Res. 67, 4259-4266 (1962).3. U. S. Air Force, Handbook of Geophysics (Macmillan Company, New York,revised edition, 1960).C. EXCITATION TEMPERATURE OF INTERSTELLAR OH LINESThe recent detection of the 18-cm absorption lines of OH in the radio spectrum ofthe interstellar mediuml has forced consideration of the excitation temperature of theOH A-doublet energy levels to provide proper interpretation of the observations in termsof OH abundance. If n1 and no are the densities of OH molecules in the upper and lowerenergy levels of the transition, respectively, the excitation temperature Ts is definedbyn1 g 1 -hv/kTs=-e , ()no gowhere gi is the statistical weight of the level, and v is the frequency of the transition.The excitation temperature is related to the optical depth, or total absorption, T , ashc 2 AN 1gr= (2)V 8rkTs v(Av) E giwhere A is the transition probability for the line, N is the total integrated number ofOH radicals per unit area, and Av is the linewidth.At the present time, OH has been observed only in absorption. The observationsallow a determination of the optical depth T , but this does not enable a determinationof N without a knowledge of the excitation temperature T s. Observations of OH emis-sion and absorption in adjacent regions of the sky would greatly aid our knowledge ofboth N and T .sIt is apparent from Eq. 1 that a theoretical estimate of Ts can be made by consid-ering the various physical mechanisms by which the OH energy levels can be populated.QPR No. 74(III. RADIO ASTRONOMY)908070t 60 -504030201010-1 1 10 100NH (CM- 3Fig. 111-4. Excitation temperature T s of the 18-cm linesof OH, under the assumption that TR = 50 K.Two general processes must be considered: collisional and radiative. Collisional proc-esses involve the interaction of an OH molecule with another particle whose kineticenergy is distributed according to the Maxwell-Boltzmann law; radiative processes arethose in which the OH molecule interacts with the interstellar radiation field. Radi-ative processes can be conveniently subdivided into two types: those involving directtransitions between the A-doublet levels and governed by the interstellar radiation fieldat 18-cm wavelength, and those involving transitions to higher OH levels and subsequentdecay to other A-doublet levels of the ground state. In the steady-state case, the exci-tation temperature 2 is given byT R +k Tk + YLTLTs 1 +yk +yLwhere T R is the characteristic radiation temperature of the interstellar 18-cm radi-ation field, Tk is the kinetic temperature of the colliding particles, and T L is the char-acteristic temperature of the particular radiation required for an indirect transition.The yi are the "efficiencies" of the various processes for establishing the steady-statepopulation distribution.Calculations of the efficiencies from present knowledge of the environment of theinterstellar medium 3 show that the collisions of OH with the heavy ions, especiallyC , is the most important process. This process has a large collision cross-sectionbecause the heavy ions produce a large 18-cm component in the frequency spectrum ofthe pulse of electric field experienced by the OH molecule during the passage of an ion.If the relative abundance of ions in relation to hydrogen atoms is assumed to be constant,QPR No. 74(III. RADIO ASTRONOMY)typically ~10 -4 , then the excitation temperature is obtained as a function of hydrogendensity as shown in Fig. I1-4. Taking the average hydrogen cloud density to be-310 atoms cm-3 the excitation temperature is estimated to be -30' K. On the other hand,observations set an upper limit of 100 K on the excitation temperature. The apparentdisagreement with the upper limits set by observations is being investigated and maybe due to a high abundance of OH in regions of low hydrogen density. Another possibleexplanation is the existence of a radiation pumping effect tending to reduce the excitationtemperature due to a larger radiation density of appropriate profile in the infra-redregion than that used in estimating the light temperature and "efficiencies." Furtherobservation will help to resolve the difficulty and lead the way to other possible expla-nations.A. H. Barrett, A. E. E. RogersReferences1. S. Weinreb, A. H. Barrett, M. L. Meeks, and J. C. Henry, Nature 200, 829(1963).2. G. B. Field, Proc. IRE 46, 240 (1958).3. M. J. Seaton, Ann. Astrophysik 18, 188 (1955).QPR No. 74

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