SOLAR PHOTOSPHERE: STRUCTIJRE, CONVECTION AND MAGNETIC FIELDS INTERNATIONAL ASTRONOMICAL UNION UNION ASTRONOMIQUE INTERNATIONALE SOLAR PHOTOSPHERE: STRUCTURE, CONVECTION AND MAGNETIC FIELDS PROCEEDINGS OF THE 138TH SYMPOSIUM OF THE INTERNATIONAL ASTRONOMICAL UNION, HE L DIN K I E V, U . S. S. R., MAY 1 5 - 2 0,1989 EDITED BY J. O. S TENFLO Institute ofA stronomy, Zurich, Switzerland KLUWER ACADEMIC PUBLISHERS DORDRECHT I BOSTON I LONDON Library of Congress Cataloging in Publication Data Solar photosphere : structure, convection, and magnetic fields: sy.pOSiUN no. 138 held in Kiev, USSR. May 15-20. 1989 / edited by J.O. Stenflo. p. c •. At head of title: International Astronomical Union, Union astono.ique internationale. ISBN-13:978-0-7923-0530-9 e-ISBN-13:978-94-009-1061-4 DOl: 10.1007/978-94-009-1061-4 1. Solar photosphere--Congresses. I. Stenflo. Jan Olof. II. ·International Astronomical Union. CB528.S65 1989 523.7'4--dc20 89-27781 ISBN-13:978-0-792 3-05 30-9 Published on behalf of the International Astronomical Union by Kluwer Academic Publishers, P.O. Box /7,3300 AA Dordrecht, The Netherlands. Kluwer Academic Publishers incorporates the publishing programmes of D. Reidel, Martinus NijhofJ, Dr W. Junk and MTP Press. Sold and distributed in the U.S.A. and Canada by Kluwer Academic Publishers, 101 Philip Drive, Norwell, MA 02061, U.S.A. In all other countries, sold and distributed by Kluwer Academic Publishers Group, P.O. Box 322,3300 AH Dordrecht, The Netherlands. Printed on acid-free paper All Rights Reserved © 1990 by the International Astronomical Union Softcover reprint of the hardcover 1st edition 1990 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical including photo copying, recording or by any information storage and retrieval system, without written permission from the publisher. TABLE OF CONTENTS FOREWORD Xl 1. Global Properties of the Photosphere MODELS OF THE SOLAR PHOTOSPHERE E.H. Avrett (Invited Review) 3 THERMAL BIFURCATION OF THE OUTER PHOTOSPHERE T.R. Ayres 23 TEMPERATURE DIAGNOSTICS OF THE UPPER PHOTOSPHERE N.G. Shchukina, T.G. Shcherbina, R.J. Rutten 29 SOLAR OSCILLATOR STRENGTHS AS A DIAGNOSTIC TOOL E.A. Gurtovenko, R.I. Kostik, R.J. Rutten 35 AVERAGE VARIATIONS OF PHOTOSPHERIC Fe I AND Fe II LINE PARAMETERS AS FUNCTION OF THE MAGNETIC FILLING FACTOR P.N. Brandt, M. Steinegger 41 II. Photospheric Fine Structure 47 HIGH RESOLUTION OBSERVATIONS OF THE PHOTOSPHERE A.M. Title, R.A. Shine, T.D. Tarbell, K.P. Topka, G.B. Scharmer (Invited Review) 49 PROPERTIES OF THE SOLAR GRANULATION V.N. Karpinsky (Invited Review) 67 ANALYSIS OF THE SOLAR GRANULATION IN THE OPACITY MINIMUM REGION S. Koutchmy 81 FINE STRUCTURE OF PHOTOSPHERIC FACULAE R. Muller (Invited Review) 85 BRIGHT FEATURES IN THE INTERGRANULAR REGION Z. Suemoto, E. Hiei 9i VI III. Small-scale Magnetic Fields 101 EMPIRICAL MODELS OF PHOTOSPHERIC FLUX TUBES S.K. Solanki (Invited Review) 103 EMPIRICAL PHOTOSPHERIC FLUXTUBE MODELS FROM INVERSION OF STOKES V DATA C.U. Keller 121 PROPERTIES OF PHOTOSPHERIC FLUXTUBES DERIVED FROM MAGNETOGRAPH OBSERVATIONS V.G. Lozitskij, T.T. Tsap 125 SMALL-SCALE MAGNETIC FEATURES OBSERVED IN THE PHOTOSPHERE S.F. Martin (Invited Review) 129 HIGH-RESOLUTION OBSERVATIONS OF EMERGING MAGNETIC FIELDS AND FLUX TUBES IN ACTIVE REGION PHOTOSPHERE T. Tarbell, S. Ferguson, Z. Frank, R. Shine, A. Title, K. Topka, G. Scharmer 147 SMALL SCALE MOTIONS OVER CONCENTRATED MAGNETIC FIELD REGIONS OF THE QUIET SUN H.C. Dara, C.E. Alissandrakis, S. Koutchmy 153 SMALL SCALE MAGNETIC STRUCTURES IN ACTIVE CENTERS A. Dollfus 157 THEORETICAL ASPECTS OF SMALL-SCALE PHOTOSPHERIC MAGNETIC FIELDS M. Schussler (Invited Review) 161 MODEL CALCULATIONS OF THE PHOTOSPHERIC LAYERS OF SOLAR MAGNETIC FLUXTUBES O. Steiner, J .0. Stenflo 181 WAVE HEATING IN MAGNETIC FLUX TUBES W. Kalkofen 185 IV. Magnetohydrodynamics of the Photosphere 189 SOLAR MAGNETOCONVECTION A. Nordlund, R.F. Stein (Invited Review) 191 RESULTS FROM 2-D NUMERICAL SIMULATIONS OF SOLAR GRANULES M. Steffen, D. Gigas, H. Holweger, A. Kruss, H.-G. Ludwig 213 WAVES AND OSCILLATIONS IN THE NON-MAGNETIC PHOTOSPHERE F.-L. Deubner (Invited Review) 217 WAVES AND OSCILLATIONS IN MAGNETIC FLUXTUBES M.P. Ryutova (Invited Review) 229 ON THE 5-MINUTE PHOTOSPHERIC OSCILLATION AND ITS MODELING C. Marmolino, G. Severino 251 vii CLASSIFICATION OF MAGNETOATMOSPHERIC MODES IN SUNSPOT UMBRAE S.S. Hasan, Y. Sobouti 255 THE OBSERVATIONAL SIGNATURE OF FLUX TUBE WAVES AND AN UPPER LIMIT ON THE ENERGY FLUX TRANSPORTED BY THEM S.K. Solanki, B. Roberts 259 MAGNETIC FLUX CONCENTRATION BY SIPHON FLOWS IN ISOLATED MAGNETIC FLUX TUBES J.H. Thomas, B. Montesinos 263 ELECTRIC CURRENTS IN THE ATMOSPHERE OF THE SUN V.I. Abramenko, S.I. Gopasyuk, M.B. Ogir 267 GENERATION OF MAGNETIC FIELDS AND ELECTRIC CURRENTS IN THE SOLAR PHOTOSPHERE J.C. Henoux, B.V. Somov 273 V. Large-scale Structure and Dynamics 279 GLOBAL EVOLUTION OF PHOTOSPHERIC MAGNETIC FIELDS V.I. Makarov, K.R. Sivaraman (Invited Review) 281 ORIGIN OF THE SUN'S DIFFERENTIAL ROTATION L.L. Kichatinov (Invited Review) 297 THE SUN'S ROTATION RATE AS INFERRED FROM MAGNETIC FIELD DATA J.O. Stenflo 309 GENERATORS OF SOLAR DIFFERENTIAL ROTATION AND IMPLICATIONS OF HELIOSEISMOLOGY G. Rudiger, I. Thominen 315 ASYMMETRY OF EMERGING FLUX LOOPS CAUSED BY RADIAL DIFFERENTIAL ROTATION M. Marik, K. Petrovay 321 CONVECTION AND ITS STABILITY IN THE EQUATORIAL REGIONS OF THE CONVECTION ZONE A.V. Klyachkin 325 INVERSE CASCADE IN HYDRODYNAMIC TURBULENCE AND ITS ROLE IN SOLAR GRANULATION V. Krishan 329 GENERATION OF TORSIONAL OSCILLATIONS IN THE SUN Y.V. Vandakurov (Invited Review) 333 viii VI. Generation of Solar Magnetic Fields 341 ORDER AND CHAOS IN THE SOLAR CYCLE A.A. Ruzmaikin (Invited Review) 343 SYMMETRY BREAKING IN THE SOLAR DYNAMO: NONLINEAR SOLUTIONS R.L. Jennings, N.O. Weiss 355 EXCITATION OF DYNAMO MODES P. Hoyng (Invited Review) 359 STUDY OF SUN'S "HYDROMAGNETIC" OSCILLATIONS USING SUNSPOT DATA M.H. Gokhale, J. Javaraiah, K.M. Hiremath 375 VARIATION OF EVEN AND ODD PARITY IN THE SOLAR DYNAMO A. Brandenburg, R. Meinel, D. Moss, I. Tuominen 379 SELF-ORDERING OF PHOTOSPHERIC MAGNETIC FIELDS J.K. Lawrence 383 TORSIONAL OSCILLATIONS AND THE SOLAR DYNAMO REGIME L Tuominen, G. Riidiger, A. Brandenburg 387 LARGE-SCALE INTERNAL MAGNETIC FIELD OF THE SUN A.E. Dudorov, V.N. Krivodubskij, A.A. Ruzmaikin, T.V. Ruzmaikina 391 VII. Convection and Magnetic Fields in Solar-type Stars 395 OBSERVING, MODELING, AND UNDERSTANDING STELLAR GRANULATION D. Dravins (Invited Review) 397 A MODEL FOR STELLAR CONVECTION AND SPECTRAL LINE ASYMMETRIES H.M. Antia 417 COMPARATIVE ANALYSIS OF PHYSICAL CONDITIONS IN THE SOLAR AND PROCYON ATMOSPHERES LN. Atroshchenko, A.S. Gadun, R.I. Kostik, K.N. Pikalov 421 MAGNETIC FIELDS ON SOLAR-LIKE STARS: THE FIRST DECADE S.H. Saar (Invited Review) 427 RESULTS OF COORDINATED MULTIWAVELENGTH OBSERVATIONS OF SOLAR-TYPE STARS J. Huovelin, S.H. Saar 443 NONLINEAR DYNAMO MODES AND TIMESCALES OF STELLAR ACTIVITY G. Belvedere, M.R.E. Proctor 447 DOES A COMMON DYNAMO MECHANISM EXIST FOR LOWER MAIN SEQUENCE STARS ? R.B. Teplitskaya, V.G. Skochilov 455 ix VIII. Future Directions 461 GROUND BASED AND SPACE FUTURE PROSPECTS IN SOLAR INTERFEROMETRY APPLIED TO THE PHOTOSPHERE J.P. Rozelot 463 NEW OBSERVATIONAL ASPECTS O. Engvold (Invited Review) 469 OUTSTANDING THEORETICAL PROBLEMS C.J. Durrant (Invited Review) 489 SUMMARY LECTURE R.J. Rutten (Invited Summary) 501 LIST 0F POSTER PAPERS 517 LETTER TO G. BUSH AND M. GORBACHEV 529 LIST OF PARTICIPANTS 531 AUTHOR INDEX 537 SUBJECT INDEX 541 FOREWORD Solar and stellar photospheres constitute the layers most accessible to observations, forming the interface between the interior and the outside of the stars. The solar atmosphere is a rich physics laboratory, in which the whole spectrum of radiative, dynamical, and magnetic processes that tranfer energy into space can be observed. As the fundamental processes take place on very small spatial scales, we need high· resolution observations to explore them. On the other hand the small-scale processes act together to form global properties of the sun, which have their origins in the solar interior. The rapid advances in observational techniques and theoreticallllodelling over the past decade made it very timely to bring together scientists from east and west to the first lAU Symposium on this topic. The physics of the photosphere involves complicated interactions between magnetic fields, convection, waves, and radiation. During the past decade our understanding of these gener ally small-scale structures and processes has been dramatically advanced. New instrumen tations, on ground and in space, have given us new means to study the granular convection. Diagnostic methods in Stokes polarimetry have allowed us to go beyond the limitations of spatial resolution to explore the structure and dynamics of the subarcsec magnetic struc tures. Extensive numerical simulations of the interaction between convection and magnetic fields using powerful supercomputers are providing deepened physical insight. Granulation, magnetic fields, and dynamo processes are being explored in the photospheres of other stars, guided by our improved understanding of the solar photosphere. Not only are we beginning to understand the relation between the small-scale processes and the large-scale structures, but it is also becoming increasingly clear that the sun cannot be properly sepa.rated into a "quiet" and an "active" part. The sun (and other stars for that matter) should rather be looked upon as a complicated but indivisible organism, whose numerous and interrela.ted properties exhibit both cyclic and secular variations, on short as well as long time scales. The symposium has considered the photosphere of the sun within this broader context, as an integral part of a larger system. The symposium took place in Kiev, USSR, May 15-20, 1989, and was attended by more than 200 participants from 24 countries. The presentations included 20 invited review papers, 37 orally contributed papers, and more than 100 poster papers. The meeting was sponsored by IAU Commission 12 and cosponsored by Commissions 10 and 36. We are grateful for the travel support provided by the IAU and by the Academy of Sciences of the Ukrainian SSR. The Scientific Organizing Conmlittee consisted of V. Bumba, C.J. Durrant, D.F. Gray, E.A. Gurtovenko, R. Howard, V.N. Karpinsky, R. Muller, A. Nordlund, R.W. Noyes, R.J. Rutten, H.C. Spruit, J.O. Stenflo (Chairman), H. Yoshimura, and C. Zwaan. I am grateful to the SOC members for their support. In particular I am indebted to Rob Rutten, who was one of the originators of the idea of having this type of a symposium in Kiev, and who has provided an important link between SOC and the organizers in Kiev. xi