cambridge university press 0521582865 - solar and stellar...

This timely volume provides the first comprehensive review and synthesis of thecurrent understanding of the origin, evolution, and effects of magnetic fields in the Sunand other cool stars. Magnetic activity results in a wealth of phenomena – includingstarspots, nonradiatively heated outer atmospheres, activity cycles, deceleration ofrotation rates, and even, in close binaries, stellar cannibalism – all of which are coveredclearly and authoritatively.

This book brings together for the first time recent results in solar studies, withtheir wealth of observational detail, and stellar studies, which allow the study of howactivity 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 momentumloss.

Researchers are provided with a state-of-the-art review of this exciting field, andthe pedagogical style and introductory material make the book an ideal and welcomeintroduction for graduate students.

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Cambridge University Press0521582865 - Solar and Stellar Magnetic ActivityC. J. Schrijver and C. ZwaanFrontmatterMore information

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SOLAR AND STELLAR MAGNETIC ACTIVITY






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 Plasmasby A. Dalgarno and D. Layzer

10. Quasar Astronomyby D. W. Weedman

17. Molecular Collisions in the Interstellar Mediumby D. Flower

18. Plasma Loops in the Solar Coronaby R. J. Bray, L. E. Cram, C. J. Durrant and R. E. Loughhead

19. Beams and Jets in Astrophysicsedited by P. A. Hughes

20. The Observation and Analysis of Stellar Photospheresby David F. Gray

21. Accretion Power in Astrophysics 2nd Editionby J. Frank, A. R. King and D. J. Raine

22. Gamma-ray Astronomy 2nd Editionby P. V. Ramana Murthy and A. W. Wolfendale

23. The Solar Transition Regionby J. T. Mariska

24. Solar and Stellar Activity Cyclesby Peter R. Wilson

25. 3K: The Cosmic Microwave Background Radiationby R. B. Partridge

26. X-ray Binariesby Walter H. G. Lewin, Jan van Paradijs and Edward P. J. van den Heuvel

27. RR Lyrae Starsby Horace A. Smith

28. Cataclysmic Variable Starsby Brian Warner

29. The Magellanic Cloudsby Bengt E. Westerlund

30. Globular Cluster Systemsby Keith M. Ashman and Stephen E. Zepf

31. Pulsar Astronomy 2nd Editionby Andrew G. Lyne and Francis Graham-Smith

32. Accretion Processes in Star Formationby Lee W. Hartmann

33. The Origin and Evolution of Planetary Nebulaeby Sun Kwok

34. Solar and Stellar Magnetic Activityby Carolus J. Schrijver and Cornelis Zwaan






SOLAR AND STELLARMAGNETIC ACTIVITY

C . J . S C H R I J V E RStanford-Lockhead Institute for Space Research, Palo Alto

C . Z W A A NAstronomical Institute, University of Utrecht






PUBLISHED BY THE PRESS SYNDICATE OF THE UNIVERSITY OF CAMBRIDGE

The Pitt Building, Trumpington Street, Cambridge, United Kingdom

C A M B R I D G E U N I V E R S I T Y P R E S S

The Edinburgh Building, Cambridge CB2 2RU, UK http://www.cup.cam.ac.uk40 West 20th Street, New York, NY 10011-4211, USA http://www.cup.org10 Stamford Road, Oakleigh, Melbourne 3166, AustraliaRuiz de Alarcon 13, 28014 Madrid, Spain

c© Cambridge University Press 2000

This book is in copyright. Subject to statutory exceptionand to the provisions of relevant collective licensing agreements,no reproduction of any part may take place withoutthe written permission of Cambridge University Press.

First published 2000

Printed in the United States of America

Typeface Times Roman 10.5/12.5 pt. and Gill Sans System LATEX 2ε [TB]

A catalog record for this book is available from the British Library.

Library of Congress Cataloging in Publication Data

Schrijver, Carolus J.

Solar and stellar magnetic activity / Carolus J. Schrijver,Cornelis Zwaan.

p. cm. – (Cambridge astrophysics series ; 34)

ISBN 0-521-58286-5 (hc.)

1. Solar magnetic fields. 2. Stars – Magnetic fields. I. Zwaan,Cornelis. II. Title. III. Series.

QB539.M23S37 1999523.7′2 – dc21 99-21364

CIP

ISBN 0 521 58286 5 hardback






Die Sonne tont nach alter WeiseIn Bruderspharen Wettgesang,Und ihre vorgeschriebene ReiseVollendet sie mit Donnergang.

Ihr Anblick gibt den Engeln Starke,Wenn Keiner Sie ergrunden mag.Die unbegreiflich hohen WerkeSind herrlich wie am ersten Tag.

Johann Wolfgang von Goethe






Contents

Preface page xiii

1 Introduction: solar features and terminology 1

2 Stellar structure 102.1 Global stellar structure 102.2 Convective envelopes: classical concepts 142.3 Radiative transfer and diagnostics 192.4 Stellar classification and evolution 382.5 Convection in stellar envelopes 452.6 Acoustic waves in stars 602.7 Basal radiative losses 652.8 Atmospheric structure not affected by magnetic fields 70

3 Solar differential rotation and meridional flow 733.1 Surface rotation and torsional patterns 743.2 Meridional and other large-scale flows 773.3 Rotation with depth 79

4 Solar magnetic structure 824.1 Magnetohydrodynamics in convective envelopes 834.2 Concentrations of strong magnetic field 924.3 Magnetohydrostatic models 984.4 Emergence of magnetic field and convective collapse 1054.5 Omega loops and toroidal flux bundles 1084.6 Weak field and the magnetic dichotomy 110

5 Solar magnetic configurations 1155.1 Active regions 1155.2 The sequence of magnetoconvective configurations 1265.3 Flux positioning and dynamics on small scales 1265.4 The plage state 1325.5 Heat transfer and magnetic concentrations 137

ix






x Contents

6 Global properties of the solar magnetic field 1386.1 The solar activity cycle 1386.2 Large-scale patterns in flux emergence 1436.3 Distribution of surface magnetic field 1556.4 Removal of magnetic flux from the photosphere 167

7 The solar dynamo 1737.1 Mean-field dynamo theory 1747.2 Conceptual models of the solar cycle 1787.3 Small-scale magnetic fields 1827.4 Dynamos in deep convective envelopes 184

8 The solar outer atmosphere 1868.1 Topology of the solar outer atmosphere 1868.2 The filament-prominence configuration 1978.3 Transients 1998.4 Radiative and magnetic flux densities 2098.5 Chromospheric modeling 2178.6 Solar coronal structure 2208.7 Coronal holes 2278.8 The chromosphere–corona transition region 2298.9 The solar wind and the magnetic brake 231

9 Stellar outer atmospheres 2389.1 Historical sketch of the study of stellar activity 2389.2 Stellar magnetic fields 2389.3 The Mt. Wilson Ca II HK project 2429.4 Relationships between stellar activity diagnostics 2469.5 The power-law nature of stellar flux–flux

relationships 2529.6 Stellar coronal structure 258

10 Mechanisms of atmospheric heating 266

11 Activity and stellar properties 27711.1 Activity throughout the H–R diagram 27711.2 Measures of atmospheric activity 28111.3 Dynamo, rotation rate, and stellar parameters 28311.4 Activity in stars with shallow convective envelopes 29111.5 Activity in very cool main-sequence stars 29411.6 Magnetic activity in T Tauri objects 29611.7 Long-term variability of stellar activity 299

12 Stellar magnetic phenomena 30312.1 Outer-atmospheric imaging 30312.2 Stellar plages, starspots, and prominences 305






Contents xi

12.3 The extent of stellar coronae 31012.4 Stellar flares 31212.5 Direct evidence for stellar winds 31412.6 Large-scale patterns in surface activity 31812.7 Stellar differential rotation 319

13 Activity and rotation on evolutionary time scales 32413.1 The evolution of the stellar moment of inertia 32413.2 Observed rotational evolution of stars 32613.3 Magnetic braking and stellar evolution 329

14 Activity in binary stars 33614.1 Tidal interaction and magnetic braking 33614.2 Properties of active binaries 34014.3 Types of particularly active stars and binary systems 342

15 Propositions on stellar dynamos 344

Appendix I: Unit conversions 351Bibliography 353Index 375






Image taken with TRACE in its 171-A passband on 26 July 1998, at 15:50:23 UT ofActive Region 8,272 at the southwest limb, rotated over −90◦. High-arching loops arefilled with plasma at ∼1 MK up to the top. Most of the material is concentrated nearthe lower ends under the influence of gravity. Hotter 3–5 MK loops, at which the bulkof the radiative losses from the corona occur, do not show up at this wavelength. Theirexistence can be inferred from the emission from the top of the conductively heatedtransition region, however, where the temperature transits the 1-MK range, as seen inthe low-lying bright patches of “moss.” A filament-prominence configuration causesextinction of the extreme-ultraviolet radiation.






Preface

This book is the first comprehensive review and synthesis of our understanding of theorigin, evolution, and effects of magnetic fields in stars that, like the Sun, have convec-tive envelopes immediately below their photospheres. The resulting magnetic activityincludes 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 inthe 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 thedependence 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 donot dwell on instrumentation and details in the data analysis, although we do try to bringout the scope and limitations of key observational methods.

This book is intended for astrophysicists who are seeking an introduction to the physicsof magnetic activity of the Sun and of other cool stars, and for students at the graduatelevel. The topics include a variety of specialties, such as radiative transfer, convectivesimulations, 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 andon cool stars throughout the cool part of the Hertzsprung–Russell diagram. Althoughwe do assume a graduate level of knowledge of physics, we do not expect specializedknowledge of either solar physics or of stellar physics. Basic notions of astrophysicalterms and processes are introduced, ranging from the elementary fundamentals of ra-diative transfer and of magnetohydrodynamics to stellar evolution theory and dynamotheory.

The study of the magnetic activity of stars remains inspired by the phenomena of solarmagnetic activity. Consequently, we begin in Chapter 1 with a brief introduction of themain observational features of the Sun. The solar terminology is used throughout thisbook, as it is in stellar astrophysics in general.

Chapter 2 summarizes the internal and atmospheric structure of stars with convectiveenvelopes, as if magnetic fields were absent. It also summarizes standard stellar termi-nology and aspects of stellar evolution as far as needed in the context of this monograph.

The Sun forms the paradigm, touchstone, and source of inspiration for much ofstellar astrophysics, particularly in the field of stellar magnetic activity. Thus, having

xiii






xiv Preface

introduced the basics of nonmagnetic solar and stellar “classical” astrophysics in thefirst two chapters, we discuss solar properties in Chapters 3–8. This monograph is basedon the premise that the phenomena of magnetic activity and outer-atmospheric heatingare governed by processes in the convective envelope below the atmosphere and its in-terface with the atmosphere. Consequently, in the discussion of solar phenomena, muchattention is given to the deepest part of the atmosphere, the photosphere, where the mag-netic structure dominating the outer atmosphere is rooted. There we see the emergenceof magnetic flux, its transport across the photospheric surface, and its ultimate removalfrom the atmosphere. We concentrate on the systematic patterns in the dynamics of mag-netic structure, at the expense of very local phenomena (such as the dynamics in sunspotpenumbrae) or transient phenomena (such as solar flares), however fascinating these are.Page limitations do not permit a discussion of heliospheric physics and solar–terrestrialrelationships.

Chapter 3 discusses the solar rotation and large-scale flows in the Sun. Chapters 4–8cover solar magnetic structure and activity. Chapter 4 deals with fundamental aspects ofmagnetic structure in the solar envelope, which forms the foundation for our studies offields in stellar envelopes in general. Chapter 5 discusses time-dependent configurationsin magnetic structure, namely the active regions and the magnetic networks. Chapter 6addresses the global properties of the solar magnetic field, and Chapter 7 deals with thesolar dynamo and starts the discussion of dynamos in other stars. Chapter 8 discussesthe solar outer atmosphere.

Chapters 9 and 11–14 deal with magnetic activity in stars and binary systems. Thisset of chapters is self-contained, although there are many references to the chapters onsolar activity. Chapter 9 discusses observational magnetic-field parameters and variousradiative activity diagnostics, and their relationships; stellar and solar data are compared.Chapter 11 relates magnetic activity with other stellar properties. Chapter 12 reviewsspatial and temporal patterns in the magnetic structure on stars and Chapter 13 discussesthe dependence of magnetic activity on stellar age through the evolution of the stellarrotation rate. Chapter 14 addresses the magnetic activity of components in binary systemswith tidal interaction, and effects of magnetic activity on the evolution of such interactingbinaries.

Two integrating chapters, 10 and 15, are dedicated to the two great problems in mag-netic activity that still require concerted observational and theoretical studies of the Sunand the stars: the heating of stellar outer atmospheres, and the dynamo action in starswith convective envelopes.

We use Gaussian cgs units because these are (still) commonly used in astrophysics.Relevant conversions between cgs and SI units are given in Appendix I.

We limited the number of references in order not to overwhelm the reader seeking anintroduction to the field. Consequently, we tried to restrict ourselves to both historical,pioneering papers and recent reviews. In some domains this is not yet possible, so therewe refer to sets of recent research papers.

We would appreciate your comments on and corrections for this text, which we intendto collect and eventually post on a web site. Domain and computer names are, however,






Preface xv

notoriously unstable. Hence, instead of listing such a URL here, we ask that you sende-mail to kschrijver at solar.stanford.edu with either your remarks or a request to let youknow where corrections, notes, and additions will be posted.

In the process of selecting, describing, and integrating the data and notions presentedin this book, we have greatly profited from lively interactions with many colleagues byreading, correspondence, and discussions, from our student years, through collaborationwith then-Ph.D. students in Utrecht, until the present day. It is impossible to do justice tothese experiences here. We can explicitly thank the colleagues who critically commentedon specific chapters: V. Gaizauskas (Chapters 1, 3, 5, 6, and 8), H. C. Spruit (Chapters 2and 4), R. J. Rutten (Chapter 2), F. Moreno-Insertis (Chapters 4 and 5), J. W. Harvey(Chapter 5), A. M. Title (Chapters 5 and 6), N. R. Sheeley (Chapters 5 and 6), P. Hoyng(Chapters 7 and 15), B. R. Durney (Chapters 7 and 15), G. H. J. van den Oord (Chapters 8and 9), P. Charbonneau (Chapters 8 and 13), J. L. Linsky (Chapter 9), R. B. Noyes(Chapter 11), R. G. M. Rutten (Chapter 11), A. A. van Ballegooijen (Chapter 10), K.G. Strassmeier (Chapter 12), and F. Verbunt (Chapters 2 and 14). These reviewers haveprovided many comments and asked thought-provoking questions, which have greatlyhelped to improve the text. We also thank L. Strous and R. Nightingale for their help inproof reading the manuscript. It should be clear, however, that any remaining errors andomissions are the responsibility of the authors.

The origin of the figures is acknowledged in the captions; special thanks are given toT. E. Berger, L. Golub and K. L. Harvey for their efforts in providing some key figures.C. Zwaan thanks E. Landre and S. J. Hogeveen for their help with figure production andwith LaTeX problems.

Kees Zwaan died of cancer on 16 June 1999, shortly after the manuscript of this bookhad been finalized. Despite his illness in the final year of writing this book, he continuedto work on this topic that was so dear to him. Kees’ research initially focused on the Sun,but he reached out towards the stars already in 1977. During the past two decades heinvestigated solar as well as stellar magnetic activity, by exploiting the complementarityof the two fields. His interests ranged from sunspot models to stellar dynamos, and fromintrinsically weak magnetic fields in the solar photosphere to the merging of binary sys-tems caused by magnetic braking. His very careful observations, analyses, solar studies,and extrapolations of solar phenomena to stars have greatly advanced our understandingof the sun and of other cool stars: he was directly involved in the development of theflux-tube model for the solar magnetic field, he stimulated discussions of flux storage andemergence in a boundary-layer dynamo, lead the study of sunspot nests, and stimulatedthe study of stellar chromospheric activity. And Kees always loved to teach. That wasone of the main reasons for him to undertake the writing of this book.






xvi Preface

Kees Zwaan (24 July 1928–16 June 1999)






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