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z o en 523. 01 BAN *MJ ENCYCLOPEDIA OF PHYSICS EDITED BY FLUGGE S. VOLUME LII ASTROPHYSICS THE SOLAR SYSTEM III: WITH 225 FIGURES SPRINGER-VERLAG BERLIN GDTTINGEN HEIDELBERG • • 1959 HANDBUCH DER PHYSIK HERAUSGEGEBEN VON FLUGGE S. BAND LII ASTROPHYSIKIII: DAS SONNENSYSTEM MIT 225 FIGUREN SPRINGER-VERLAG BERLIN GOTTINGEN HEIDELBERG • • 1959 *Wi a%;«*.-. :r*** £"'~ ^ S^^fo a \S ISBN 3-540-02416-6 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-02416-6 Springer-Verlag New York Heidelberg Berlin Das Werk ist urheberrechtlichgeschiitzt. Diedadurch begriindeten Rechte, insbesonderedie der Ubersetzung, des Nachdruckes, der Entnahme von Abbildungen, der Funksendung, der Wiedergabe aufphotomechanischem oder ahnlichem Wege und der Speicherung in Daten- verarbeitungsanlagen bleiben, auch bei nur auszugsweiserVerwertung, vorbehalten. BeiVer- vielfaltigungen furgewerbliche Zwecke ist gemafi § 54 UrhG eine Vergiitung an denVerlag zu zahlen, deren Hone mit dem Verlag zu vereinbaren ist. © by Springer-Verlag Berlin Gottingen Heidelbetg 1959. Printed in Germany. Offsetdruck: OffsetdruckereiJulius Beltz, Hemsbach/Bergstr. DieWiedergabevonGebrauchsnamen, Handelsnamen, Warenbezeichnungenusw. indiesem Werk berechtigt auch ohne besondere Kennzeichnung nicht zu der Annahme, dafi solche Namen im Sinne derWarenzeichen- und Markenschutz-Gesetzgebung als frei zu betrachten waren und daher von jedermann benutzt werden diirften. . Inhaltsverzeichnis Seite The Photosphere of the Sun. By Professor Dr. Leo Goldberg, Director, University of Michigan Observatory, Ann Arbor/Michigan (USA), and Dr. A. Keith Pierce, Associate ProfessorofAstronomy, University of Michigan, Pontiac/Michigan (USA). (With 27 Figures) 1 I. The continuous spectrum 1 a) Solar constant 2 b) Observed limb darkening 5 c) Observed solar energy distribution 13 d) Observational models ofthe photosphere 18 II. The Fraunhofer spectrum 33 a) Observational data 36 b) The formation of Fraunhoferlines 42 c) The analysis of Fraunhoferlines 58 General references 79 Structure and Dynamics of the Solar Atmosphere. By Dr. Cornelis de Jager, Assistant Professor of Astrophysics, Utrecht University, Sterrewacht Sonnenborgh, Utrecht(Netherlands). (With 136Figures) 80 A. The undisturbed photosphere and chromosphere 80 I. The undisturbed photosphere 80 a) Granulation 80 b) Temperature and densityinhomogeneitiesin thephotosphere 86 c) Micro- and macro-turbulence inthe photosphere 93 d) Theory ofconvection and turbulencein the solar atmosphere 98 II. The chromosphere 106 a) The chromosphere at the limb 106 b) The chromosphere on the disk 124 c) Structure and dynamics of the chromosphere; transition to the corona 135 B. Thedisturbed parts ofthephotosphere and chromosphere 151 I. Sunspots 151 a) The individual spots 151 b) Group of spots 166 II. Photospheric and chromospheric faculae 173 a) The faculae proper 173 b) The "centre of activity" 183 III. Flares and associated phenomena 191 a) Monochromaticobservations 191 b) Spectrographs observations 199 c) Dynamical phenomenaassociated with flares 210 IV. Filaments and prominences 224 a) The quiescentprominences 226 b) Movingprominences 237 VI Inhaltsverzeichnis. Seite C. Thecorona 248 I. Optical observations 248 a) The quiet minimum corona; photometry, ionization and excitation . . . 250 b) The structure of the quiet minimum corona; temperatures and densities 265 c) The active parts ofthe corona and the "maximum corona" 271 II. Radio emission from the Sun 283 a) The quiet Sun 283 b) Thermal radiation from centres of activity 296 c) Non-thermal radiation: type III andtype U bursts 301 .... d) Other non-thermal radiophenomena, often connected with flares 308 D. Solar rotation and the solar cycle 322 a) The solar cycle 322 b) Solar rotation and the Sun's general magnetic field 337 E. Solar and terrestrial relationship 344 General references 362 The Atmospheres of the Planets. By Dr. HaroldClayton Urey, Profes.so.r.o.f Chemistry, University of Chicago, Chicago/Illinois (USA). (With 3 Figures) 363 I. Introduction 363 II. The Earth's atmosphere 366 III. The atmosphere of Venus 383 IV. The atmosphere of Mars 393 V. The atmospheres ofMercury and the Moon 404 VI. The atmospheres of major planets and their satellites 406 VII. Conclusions 414 Acknowledgment 414 References 415 PlanetaryInteriors. ByDr.WendellC.deMarcus, ProfessorofPhysics,Universityof Kentucky, Lexington/Kentucky (USA). (With 3 Figures) 419 I. The mass-radius relation of cold bodies 420 II. Empirical features of the planets 431 III. Structure and composition ofthe planets 432 a) The constitution of the terrestrial planets 432 b) The constitution ofthejovian planets 440 Radio Echoes from Sun, Moon and Planets. By Frank J. Kerr, Principal Research Officer, C.S.I.R.O. Radiophysics Laboratory, Sidney/NSW (Australia). (With 8 Fi- gures) 449 I. Strength and characteristics ofechoes 449 II. Moon echoes 452 III. Futurepossibilities 460 IV. The Earth's immediate neighbourhood 462 General references 464 . Inhaltsverzeichnis. VII Seite Die Kometen. Von Professor Dr. Karl Wurm, Hauptobservator an der Sternwarte Hamburg-Bergedorf (Deutschland). (Mit 31 Figuren) 465 I. Einleitung 465 II. Kometenbahnen 469 III. Spektren und Physik der Kometen 477 IV. Zur Deutung der Kometenformen 496 V. Kosmogonie der Kometen 509 Meteors. By Dr. Fred L. Whipple, Director, Smithsonian Astrophysical Observatory, and Professor of Astronomy, Harvard University, and Prof. Dr. Gerald S. Haw- kins, Director, Boston University Observatory and Research Associate, Harvard College Observatory, Cambridge/Mass. (USA). (With 17 Figures) 518 1. Introduction 519 2. Basictheory of the meteoric processes 520 3. Meteor spectra , 524 4. Meteordynamics and orbits of comets, asteroids, and meteors . . . 527 5. Meteoritic dust 530 6. Visual meteor studies 534 7. Radio meteortechniques 536 8. Photographic meteor techniques 538 9- The masses and densities of meteoroids 542 10. Shower meteors 544 11 Sporadic meteors and the total influx 548 12. Origin of meteors, fireballs and meteorites 554 13- Meteorites 559 Acknowledgments 564 Sachverzeichnis (Deutsch/Englisch) 565 Subject Index (English/German) 583 The Photosphere ofthe Sun. By Leo Goldberg and A. Keith Pierce1 . With 27 Figures. The continuous spectrum. I. 1. Introduction. An empirical model of the photosphere rests on observations of: (1) thesolarconstant—whichfixesthe temperature scale, (2) limb darkening— from which the variation of temperature with depth throughout the atmosphere is determined, and (3) the energydistribution—which, togetherwith limb darkening, reveals the opacity at each depth and for each wavelength. All the basic observational data for a satisfactoryinterpretation ofthe physics of the solar atmosphere were available shortly after the turn of the century. By 1905 Abbot had clarified the causes of the disagreement among different observers in the determination of the value of the solar constant—values which varied all the way from 1.75 to 4.0 calories cm"2 min-1 He showed that the . diversity of solar constant values was not due to the use of inadequate instruments but was caused by a lack of an international scale of pyrheliometry and most particularly by failure of the extrapolation by empirical formula to zero air mass. Abbot's early value of 2.1 cal cm-2 min-1 is within a few percent of today's accepted figure. The first monochromatic limb darkening measures were obtained by Vogel (1873) and by Very (1902). As so often happens in astronomy, the construction ofnew and much more sensitive detectors permitted Abbot, Fowle andAldrich (1900 to 1922) to extend and improve greatly upon Vogel and Very's results. The Smithsonian observers, in an attempt to detect a variation oflimb darkening with time, obtained thousands of center-to-limb traces at 22 wavelengths, distri- buted between XX 323O—20970 A. Schuster in 1903 and Schwarzschild in 1906 were the first to provide an explanation of the limb darkening observations, based on the concept of a solar atmosphere absorbing and reemitting radiation. Parenthetically, the existence of this atmosphere was known at a very early date from its Fraunhofer spectrum (Kirchhoff and Bunsen, 1859 to 1862), but it was not considered to affect the continuum. An accurate determination of the energy distribution in the Sun's spectrum resulted from the work started by Wilsing (1905) at the Astrophysical Observa- tory in Potsdam, and by Langley (I883) at the Allegheny Observatory, and at the Smithsonian Institution, Washington. Wilsing compared the ratio of the Sun's intensity to that of a black body in the interval XX 4500—21000A. In the visible portion of the Sun's spectrum he found that the energy distribution corresponded to a color temperature of 6300°. The Smithsonian work followed as a byproduct of the solar constant determinations. To carry out Langley's program Abbot, Aldrich and Fowle (1908) obtained holograms (XX 3400— 1 Chap. I: The continuous spectrum was written by Pierce; Chap. II: The Fraunhofer spectrum by Goldberg. HandbucbderPhysik,Bd.LII. 1 2 Leo Goldberg and A. Keith Pierce: The Photosphere of the Sun. Sect. 2. 24000 A) at different air masses, and from these computed the extraterrestrial energy curve of the Sun. It is the purpose ofthis chapter to outline in a rather briefmanner the present status of the observational work on photospheric radiation and to discuss a few of the observational models of the solar atmosphere. For the reader interested in the theoretical models, Refs. [1] to [8] and the paper by D. Barbier in Vol. L of this Encyclopedia may be consulted. a) Solar constant1. 2. Langley'smethod; Smithsonianresults. Langley consideredthe observation of the amount of heat the Sun sends to the Earth as one of the most important and difficult problems in astronomical physics, and also the fundamental problem of meteorology. The measurement of the solar constant requires special instruments: one that measures the total radiation in absolute units and one that measures the atmospheric losses2 In 1838 Pouillet devised a radiation calori- . meter which he called a pyrheliometer. In its modern version by Abbot, it con- mm mm sistsofablackenedsilverdisk28 indiameterand 7 thick, drilledtoreceive a sensitive thermometer. The temperature rise during a 2-minute interval is taken as a measure of the incident energy. While the silver disk pyrheliometer is one of the most accurate and reliable of available instruments, it is not in itself an absolute instrument. Each instrument, however, is calibrated and is supplied with a constant that converts the observations to the absolute Smith- sonian scale of 1913. The latter was established by use of the standard water- flow pyrheliometer of Abbot and Aldrich. Radau and Langley showed that Bouguer's law of absorption applied to observations of high and low Sun, and that linear extrapolation to zero air mass was possible if limited only to a single wavelength. Thus it became evident that the solar constant could not be determined from pyrheliometer observations alone; separate measurements at each wavelength are required in order to eva- luate the atmospheric losses. The equation, lnI, = k,H,secZ+lnI (2.1) Xfi, relates the incident intensity 7^ to the observed intensity Ix. The optical path length is the equivalent height of the atmosphere H, times the secant of the Sun's zenith distance—corrected for refraction and curvature of the atmosphere at low solar altitudes (<10°). The coefficient of secz is variable with time, but at excellent observing locations the use of Eq. (2.1) for the reduction of a day's observations can give Ixo with an error of less than J of 1%. Langley employed the spectrobolometer to observe I Since the spectro- x. bolometer was not sufficiently stable, and could not be calibrated with sufficient accuracy, Langley normalized each bologram through concurrent pyrheliometer observations. Thus, the spectrobolometer has served only to give the relative energy at each wavelength, to perform the extrapolation to zero air mass, and to define the areas ofthewatervapor, ozone, and C0 absorptions. The Smithsonian 2 holograms extend from O.346 to 2.4 [x while the pyrheliometer observations cover the region receivable through the atmosphere: 0.295 to about 25 [x. Hence, in 1 Treated also by P.Moller, Vol. XLVIII, p. 182 of this Encyclopedia. 2 E.g. the detailed accounts and references of B. Stromgren: Handbuch der Experi- mentalphysik. Vol. 26. Leipzig: Akademische Verlagsgesellschaft 1937- — C.W. Allen: The Sun, ed. Kuiper. Chicago: Univ.ofChicagoPress 1953. — AdiscussionbyR.R.McMath and O.C. Mohler of solar instruments and accessories is to be found in Vol. LIV of this Encyclopedia. Sects. 3, 4. Rocket and balloon results. 3 order to equate the integrated spectrobolograms obtained at each solar altitude to the pyrheliometer values, small corrections are added to the integrated curves to account for the unobserved ultraviolet and infrared regions. These corrections depend upon the solar altitude and the water vapor content of the atmosphere at the time of observation. The band absorption and the effects of dust and Rayleigh extinction are removed by the extrapolation of the normalized holograms to zero air mass. To the integrated zero air mass holograms the Smith- sonian observers add theoretical estimates of the energy radiated by the Sun to the violet of 0-346 [i and the red of 2.4\i. These estimates amount to 3.1 and 2.0% of the value of the total solar constant for the ultraviolet and infrared respectively. The Smithsonian observers1 conclude from their extensive observations and intercomparisons between instruments that the most probable absolute value of the mean solar constant is: 1.940cal cm"2 min-1 . 3. Solar constant based on new solar spectral irradiance data. A critique of the Smithsonian reductions has been given by Johnson2 From the ground, . radiation from the Sun at wavelengths less than 0.295 fx cannot be observed, andwe are unable to predict the energyin this region of the spectrum from astrophysical theory. Only direct measures from high altitude rockets can supply the necessary data. By use of V-2 rockets, N.L.Wilson etal.3 obtained calibrated ultraviolet spectra which, on a relative scale, were considered accurate to i 5%• A relative spectral irradiance curve above the atmosphere and for the region 0.22 to 0.60 [x was obtained by fitting the rocket data to the spectral irradiance curve at 0.318fx of Dunkelman and Scolnik4. Finally, Moon's6 data were usedto extend the curve to 2.4(i. The resultingrelative energy distribution, integrated over the restricted intervalO.346to2.4 was then put on an absolute fx, scale by using the Smithsonian results. Johnson's values for the solar energy distribution are given in Table 4. The first column gives the wavelength in microns; the second lists H^, the mean zero air mass spectral irradiance in watts cm-2 micron'1; P is the percentage of the solar constant short of wavelength ju. In the ultraviolet, the U.S. Naval Research Laboratory curve led to a 1.24% larger zero air mass correction than that used by the Smithsonian Institution. In the infrared, Johnson concluded that the Smithsonian observers failed to recognize the great absorbing power of small amounts of water, which would produce a sharp upturn in the extrapolation to zero air mass. Further he also assumed that the Sun radiates as a gray body in the region beyond 2.4 \i, with temperature 6000° K, emissivity 0.990. In all, a value O.O38 cal cm-2 min-1 greater than the Smithsonian Institution zero air mass correction was derived. Johnson's final value is: (2.00± 0.04) cal cm"2min"1 . 4. Rocket and balloon results. Observations from high altitudes offer the greatest possibility of determining the precise value of the solar constant. A 1 L.B. Aldrich and W.H. Hoover: Ann. Astrophys. Obs. Smithsonian Inst. 7, 176 (1954). — 2 F.S.Johnson: J.Meteorology11, 431 (1954). Rapport de laCommission pour l'etude des Relations entre les Phenomenes Solaires et Terrestres. Paris: J.&R. Sennac 1957. 3 N.L.Wilson, R. Tousey, J.D. Purcell, F.S. Johnson and C.E. Moore: Astrophys. Journ. 119, 590 (1954). 4 L. Dunkelman and R. Scolnik: J. Opt. Soc. Amer. 42, 876 (1952). 5 P. Moon: J. Franklin Inst. 230, 583 (1940).