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Solar and Stellar Magnetic Activity SOLAR AND STELLAR MAGNETIC ACTIVITY C . J . SCHRIJVER C . ZWAAN CAMBRIDGE UNIVERSITY PRESS This timely volume provides the first comprehensive review and synthesis of the current understanding of the origin, evolution, and effects of magnetic fields in the Sun and other cool stars. Magnetic activity results in a wealth of phenomena – including starspots, nonradiatively heated outer atmospheres, activity cycles, deceleration of rotation rates, and even, in close binaries, stellar cannibalism – all of which are covered clearly and authoritatively. This book brings together for the first time recent results in solar studies, with their wealth of observational detail, and stellar studies, which allow the study of how activity evolves and depends on the mass, age, and chemical composition of stars. The result is an illuminating and comprehensive view of stellar magnetic activity. Ob- servational data are interpreted by using the latest models in convective simulations, dynamo theory, outer-atmospheric heating, stellar winds, and angular momentum loss. Researchers are provided with a state-of-the-art review of this exciting field, and the pedagogical style and introductory material make the book an ideal and welcome introduction for graduate students. Cambridge astrophysics series Series editors Andrew King, Douglas Lin, Stephen Maran, Jim Pringle and Martin Ward Titles available in this series 7. Spectroscopy of Astrophysical Plasmas by A. Dalgarno and D. Layzer 10. Quasar Astronomy by D. W. Weedman 17. Molecular Collisions in the Interstellar Medium by D. Flower 18. Plasma Loops in the Solar Corona by R. J. Bray, L. E. Cram, C. J. Durrant and R. E. Loughhead 19. Beams and Jets in Astrophysics edited by P. A. Hughes 20. The Observation and Analysis of Stellar Photospheres by David F. Gray 21. Accretion Power in Astrophysics 2nd Edition by J. Frank, A. R. King and D. J. Raine 22. Gamma-ray Astronomy 2nd Edition by P. V. Ramana Murthy and A. W. Wolfendale 23. The Solar Transition Region by J. T. Mariska 24. Solar and Stellar Activity Cycles by Peter R. Wilson 25. 3K: The Cosmic Microwave Background Radiation by R. B. Partridge 26. X-ray Binaries by Walter H. G. Lewin, Jan van Paradijs and Edward P. J. van den Heuvel 27. RR Lyrae Stars by Horace A. Smith 28. Cataclysmic Variable Stars by Brian Warner 29. The Magellanic Clouds by Bengt E. Westerlund 30. Globular Cluster Systems by Keith M. Ashman and Stephen E. Zepf 31. Pulsar Astronomy 2nd Edition by Andrew G. Lyne and Francis Graham-Smith 32. Accretion Processes in Star Formation by Lee W. Hartmann 33. The Origin and Evolution of Planetary Nebulae by Sun Kwok 34. Solar and Stellar Magnetic Activity by Carolus J. Schrijver and Cornelis Zwaan SOLAR AND STELLAR MAGNETIC ACTIVITY C. J. SCHRIJVER Stanford-Lockhead Institute for Space Research, Palo Alto C. ZWAAN Astronomical Institute, University of Utrecht PUBLISHED BY CAMBRIDGE UNIVERSITY PRESS (VIRTUAL PUBLISHING) FOR AND ON BEHALF OF THE PRESS SYNDICATE OF THE UNIVERSITY OF CAMBRIDGE The Pitt Building, Trumpington Street, Cambridge CB2 IRP 40 West 20th Street, New York, NY 10011-4211, USA 477 Williamstown Road, Port Melbourne, VIC 3207, Australia http://www.cambridge.org © Cambridge University Press 2000 This edition © Cambridge University Press (Vitrtual Publishing) 2003 First published in printed format 2000 A catalogue record for the original printed book is available from the British Library and from the Library of Congress Original ISBN 0 521 58286 5 hardback ISBN 0 511 00960 7 virtual (netLibrary Edition) Die Sonne t¨ontnach alter Weise In Brudersph¨arenWettgesang, Und ihre vorgeschriebene Reise Vollendet sie mit Donnergang. Ihr Anblick gibt den Engeln St¨arke, Wenn Keiner Sie ergr¨undenmag. Die unbegreiflich hohen Werke Sind herrlich wie am ersten Tag. Johann Wolfgang von Goethe Contents Preface page xiii 1 Introduction: solar features and terminology 1 2 Stellar structure 10 2.1 Global stellar structure 10 2.2 Convective envelopes: classical concepts 14 2.3 Radiative transfer and diagnostics 19 2.4 Stellar classification and evolution 38 2.5 Convection in stellar envelopes 45 2.6 Acoustic waves in stars 60 2.7 Basal radiative losses 65 2.8 Atmospheric structure not affected by magnetic fields 70 3 Solar differential rotation and meridional flow 73 3.1 Surface rotation and torsional patterns 74 3.2 Meridional and other large-scale flows 77 3.3 Rotation with depth 79 4 Solar magnetic structure 82 4.1 Magnetohydrodynamics in convective envelopes 83 4.2 Concentrations of strong magnetic field 92 4.3 Magnetohydrostatic models 98 4.4 Emergence of magnetic field and convective collapse 105 4.5 Omega loops and toroidal flux bundles 108 4.6 Weak field and the magnetic dichotomy 110 5 Solar magnetic configurations 115 5.1 Active regions 115 5.2 The sequence of magnetoconvective configurations 126 5.3 Flux positioning and dynamics on small scales 126 5.4 The plage state 132 5.5 Heat transfer and magnetic concentrations 137 ix x Contents 6 Global properties of the solar magnetic field 138 6.1 The solar activity cycle 138 6.2 Large-scale patterns in flux emergence 143 6.3 Distribution of surface magnetic field 155 6.4 Removal of magnetic flux from the photosphere 167 7 The solar dynamo 173 7.1 Mean-field dynamo theory 174 7.2 Conceptual models of the solar cycle 178 7.3 Small-scale magnetic fields 182 7.4 Dynamos in deep convective envelopes 184 8 The solar outer atmosphere 186 8.1 Topology of the solar outer atmosphere 186 8.2 The filament-prominence configuration 197 8.3 Transients 199 8.4 Radiative and magnetic flux densities 209 8.5 Chromospheric modeling 217 8.6 Solar coronal structure 220 8.7 Coronal holes 227 8.8 The chromosphere–corona transition region 229 8.9 The solar wind and the magnetic brake 231 9 Stellar outer atmospheres 238 9.1 Historical sketch of the study of stellar activity 238 9.2 Stellar magnetic fields 238 9.3 The Mt. Wilson Ca II HK project 242 9.4 Relationships between stellar activity diagnostics 246 9.5 The power-law nature of stellar flux–flux relationships 252 9.6 Stellar coronal structure 258 10 Mechanisms of atmospheric heating 266 11 Activity and stellar properties 277 11.1 Activity throughout the H–R diagram 277 11.2 Measures of atmospheric activity 281 11.3 Dynamo, rotation rate, and stellar parameters 283 11.4 Activity in stars with shallow convective envelopes 291 11.5 Activity in very cool main-sequence stars 294 11.6 Magnetic activity in T Tauri objects 296 11.7 Long-term variability of stellar activity 299 12 Stellar magnetic phenomena 303 12.1 Outer-atmospheric imaging 303 12.2 Stellar plages, starspots, and prominences 305 Contents xi 12.3 The extent of stellar coronae 310 12.4 Stellar flares 312 12.5 Direct evidence for stellar winds 314 12.6 Large-scale patterns in surface activity 318 12.7 Stellar differential rotation 319 13 Activity and rotation on evolutionary time scales 324 13.1 The evolution of the stellar moment of inertia 324 13.2 Observed rotational evolution of stars 326 13.3 Magnetic braking and stellar evolution 329 14 Activity in binary stars 336 14.1 Tidal interaction and magnetic braking 336 14.2 Properties of active binaries 340 14.3 Types of particularly active stars and binary systems 342 15 Propositions on stellar dynamos 344 Appendix I: Unit conversions 351 Bibliography 353 Index 375 Image taken with TRACE in its 171-A˚ passband on 26 July 1998, at 15:50:23 UT of Active Region 8,272 at the southwest limb, rotated over −90◦. High-arching loops are filled with plasma at ∼1 MK up to the top. Most of the material is concentrated near the lower ends under the influence of gravity. Hotter 3–5 MK loops, at which the bulk of the radiative losses from the corona occur, do not show up at this wavelength. Their existence can be inferred from the emission from the top of the conductively heated transition region, however, where the temperature transits the 1-MK range, as seen in the low-lying bright patches of “moss.” A filament-prominence configuration causes extinction of the extreme-ultraviolet radiation. Preface This book is the first comprehensive review and synthesis of our understanding of the origin, evolution, and effects of magnetic fields in stars that, like the Sun, have convec- tive envelopes immediately below their photospheres. The resulting magnetic activity includes a variety of phenomena that include starspots, nonradiatively heated outer at- mospheres, activity cycles, the deceleration of rotation rates, and – in close binaries – even stellar cannibalism. Our aim is to relate the magnetohydrodynamic processes in the various domains of stellar atmospheres to processes in the interior. We do so by ex- ploiting the complementarity of solar studies, with their wealth of observational detail, and stellar studies, which allow us to study the evolutionary history of activity and the dependence of activity on fundamental parameters such as stellar mass, age, and chem- ical composition. We focus on observational studies and their immediate interpretation, in which results from theoretical studies and numerical simulations are included. We do not dwell on instrumentation and details in the data analysis, although we do try to bring out the scope and limitations of key observational methods. This book is intended for astrophysicists who are seeking an introduction to the physics of magnetic activity of the Sun and of other cool stars, and for students at the graduate level. The topics include a variety of specialties, such as radiative transfer, convective simulations, dynamo theory, outer-atmospheric heating, stellar winds, and angular mo- mentum loss, which are all discussed in the context of observational data on the Sun and on cool stars throughout the cool part of the Hertzsprung–Russell diagram.
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