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Solar Radiation and the Lower Atmosphere by Sigmund Fritz

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Solar Radiation and the Lower Atmosphere by Sigmund Fritz VOL. 43, 1957 GEOPIIY SICS: S. FRITZ 95 than 40, and on three rocket flights in that year Lyman-alpha was 4. 5.7, and 9.2 ergs/cm2/sec. The general conclusion at present is that the intensity of Lyman- alpha increases and decreases erratically with the sunspot number. Summary.-From the observed changes in the ionosphere over two solar cycles from 1932 to 1954 the solar intensity in the wave lengths which cause the E-, Fl-, and F2-regions was calculated to increase by a factor of about 2.3 from sunspot minimum to maximum. The cause of E is attributed probably to X-rays of wave lengths 10-20 A and of F2 to short ultraviolet wave lengths in the region 100-600 A. 1 F. S. Johnson, J. D. Purcell, R. Tousey, and N. Wilson, "The Ultraviolet Spectrum of the Sun," in Rocket Exploration of the Upper Atmosphere, ed. R. L. F. Boyd and M. J. Seaton (New York: Interscience Publishers, 1954), pp. 279-288. (A survey paper with bibliography.) 2 H. Friedman, "The Solar Spectrum below 2000 Angstroms," Ann. Geophys., 11, 174-180, 1955. (A survey paper with bibliography.) 3 J. C. Seddon, A. D. Pickar, and J. E. Jackson, "Continuous Electron Density Measurements up to 200 Km," J. Geophys. Research, 59, 513-524, 1954. 4R. J. Havens, H. Friedman, and E. O. Hulburt, "The Ionospheric F2 Region," in Proceed- ings of Conference on the Physics of the Ionosphere, Cambridge, England, September 1954 (London: Physical Society, 1955), pp. 237-244. 6 E. 0. Hulburt, "The E Region of the Ionosphere," Phys. Rev., 55, 639-645, 1939. 6 Ionosphere Data (Central Radio Propagation Laboratory, U.S. National Bureau of Standards). 7 "Wolf's Sunspot Numbers, Annual Mean," in Smithsonian Physical Tables (9th rev. ed., 1954), p. 727, Table 824. 8 D. F. Martyn, "Geomagnetic Anomalies of the F2 Region and Their Interpretation," in Proceedings of Conference on the Physics of the Ionosphere, Cambridge, September 1954 (London: Physical Society, 1955) pp. 260-264. 9J. A. Ratcliffe, "Some Regularities in the F2 Region of the Ionosphere," J. Geophys. Research, 56, 487-507, 1951. SOLAR RADIATION AND THE LOWER ATMOSPHERE BY SIGMUND FRITZ U.S. WEATHER BUREAU, WASHINGTON, D.C. Introduction.-The sun may conceivably affect the major circulation of the lower atmosphere in several ways. One way is through the influence of variable solar emissions; meteorologists have long speculated about the possible effects of irregular solar emissions upon the general circulation. "Abnormal" wave or particle emis- sions from the sun, absorbed either high in the atmosphere or even at the ground, have been studied in many ways in connection with meteorological variables of the lower atmosphere or troposphere. A less glamorous solar influence is that of the nonvariable sun. In this case the excess net heating in the tropics and summer hemisphere, on the one hand, and the net radiative cooling in much of the winter hemisphere, on the other hand, coupled with the earth's rotation, produce the basic general circulation pattern. On this meteorologists agree; regarding the details of the general circulation and how they come into existence, there is much less agreement. The Variable Sun (Direct Heating).-Let us consider first the basis for the so- called "solar-weather relations" which involve the variable sun. Interesting Downloaded by guest on September 27, 2021 96 GEOPHYSICS: S. FRITZ PROC. N. A. S. relations have been suggested from time to time, of which some recent ones are those of Duell and DuellI Wexler,2 and Farthing3; many others exist. In general, each of these studies has used some parameter which supposedly represented an undefined wave or particle emission from the sun. This solar parameter was then supposed to be related to some meteorological feature of the lower atmosphere. For example, the Duells related magnetic character figure C to atmospheric sea- level pressure near Iceland. Wexler suggested a relation between sunspots and world-wide January surface temperature and pressure distribution. Farthing used coronal measurements to forecast temperature and precipitation at Kansas City. These studies have all been statistical in nature, and it was usually not clear whether the results were statistically significant or not. No plausible physical explanation has yet been accepted. Before one can determine the physical basis for claims about "solar-weather" relations, it will be necessary to examine the spectral emission of both the quiet and the disturbed sun. If we take the energy of the quiet sun in a 1 A wave- length interval near 5000 A as unity, we can express the energy at other wave lengths as a fraction of this unit. The solid line in Figure 1 shows such smoothed data from Johnson4 for X > 2200 A, from Newell5 for X < 2400 A, and from Hulburt6 Angstrom') 5 _0 20 50 100 200 500 1000 2000 5000 104 \ ( 977--293- - 955 .9-34 SOLAR RADIATION / lo-.05 891 10-° 794 * 631 / .501I lo-5 316 l 200 > 102 7 / -'_.;n!, (;e nerg- io-2 /crcr.nt+ rocf Sc 3 - Const a , 10-3 / Disftu rbed Sun Observation 0 A 8-20 / i ~~~~~~~~~/ I0-20 / / ! ~~~~~~~~/ I ~~~~~/;. '!-0005002 2; 52 CC SY '00 5000 ° \ MA-gboOn 2 33: 0 FIG. 1.-Solid curve: solar radiation (smoothed) from quiet sun. Dashed curve: radiation from a black body at 6,000° K. Stippled curve: cumulative solar energy, P(X), below wave length, X. Symbol 0: cumulative energy from disturbed sun. Downloaded by guest on September 27, 2021 VOL. 43, 1957 GEOPHYSICS: S. FRITZ 97 for X < 100 A. For comparison, the energy in similar units for a black body of 6,000° K. is shown by the dashed curve. We note that the solar energy is about equal to that of a 6,0000 K. black body near 5000 A. The solar energy falls below the black-body curve until about 1000 A is reached, although this may not be so for individual spectral lines. Then apparently the hot corona becomes more important, and by the time the X-ray region near 100 A is reached, the sun's energy is larger than that of the 6,0000 K. black body by a factor of about 101°°. Never- theless, the solar energy in a band of the X-ray region is still about 10-6 of the energy in a similar band in the visible spectrum. The curves just mentioned give the relative spectral energy. The remaining, broadened curve in Figure 1 shows the amounts of energy, P(X), in the solar spec- trum below any given wave length, X, on the assumption that little or no energy exists in the solar spectrum below 6 A (Newell5). This energy has been expressed in units of the solar constant, taken to be 2 ly/min or 1.35 X 106 ergs/cm2/sec, and is summarized approximately in Table 1. TABLE 1 FRACTION OF SOLAR ENERGY, P(X), BELOW WAVE LENGTH X (Unit Is Solar Constant) X (A) 10 100 1000 2000 3000 4000 P (X) 10-10 10-8 10-6 10-4 10-2 10-1 Thus we see that rather small amounts of energy reach the outer limits of our atmosphere in the X-ray and far-ultraviolet regions of the spectrum. A few rocket measurements (Hulburt6) indicate that, in the X-ray region, increases of a hundred fold have occurred during disturbed sun conditions. Such an increase is indicated in Figure 1. Thus the total energy below 20 A is still only 10-8 of the solar constant even during disturbed sun conditions, although, of course, additional measurements may later indicate some further increase in this value. Aside from the rocket X-ray measurements, direct spectral measurements of the emission of the disturbed sun have apparently not been made for X < 3200 A. At X = 3200 A, the measurements of Pettit7 suggest a variation during the sunspot cycle. Pettit's measurements indicated that the solar intensity at 3200 A was about 1.5 times greater at sunspot maximum than at sunspot minimum. He himself discounted the variations as being too large but apparently felt that some variation at 3200 A existed in relation to the sunspot cycle. What the magnitude of the variations is in the spectral regions between 20 A and 3200 A can at present only be estimated from indirect evidence. Lyman- alpha is expected to vary because of the observed H(a) variations in red light. Mitra8 suggests that the variations during solar flares in L(a) cause sudden iono- spheric disturbances (S.I.D.). During an intense flare, Mitra estimates an in- creased emission by a factor of 90 at X = 1216 A. But Wulf and Deming9 have suggested that emissions in the region between 2300 and 2800 A might be an important cause of S.I.D. How much of an increase, if any, might occur in these wave lengths from a disturbed sun is not known. Of importance from the meteorological point of view is the region in the atmos- phere where solar energy is absorbed. The heights at which absorption occurs Downloaded by guest on September 27, 2021 98 GEOPHY SICS: S. FRITZ PROC. N. A. S. are shown in Figure 2, in which the spectral alsorptions are taken from De Jager'0; the temperature, pressure, and density data are summarized from HulburtA Just under the wave-length scale have been added the logarithms of P(X) taken from Figure 1. Thus in the region above 120 km., where the mass of the earth's atmos- phere is about 10-1 of its total mass, less than 10-5 of the solar constant is absorbed.
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