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Mechanisms of Chromospheric and Coronal Heating A NIXT solar X-ray photo in the Fe XVI line at 63.5 A taken on a rocket flight on 11 Sept. 1989 by Leon Golub (see the article on p. 115 of this book) P. Ultnschneider E. R. Priest R. Rosner (Eds.) Mechanisms of Chromospheric and Coronal Heating

Proceedings of the International Conference, Heidelberg, 5-8 lune 1990

With 260 Figures

Springer-Verlag Berlin Heidelberg GmbH Professor Dr. Peter Ulmschneider Institut für Theoretische Astrophysik. Im Neuenheimer Feld 561. D-69oo Heidelberg. Fed. Rep. ofGermany Professor Dr. Eric R. Priest University of St. Andrews. The Mathematical Institute North Haugh. St. Andrews KY16 9SS. Great Britain Professor Dr. Robert Rosner E. Fermi Institute and Dept. of Astronomy and Astrophysics. 5640 S Ellis Ave .• Chicago. IL 60637. USA

Cover picture: In this scenario by Chitre and Davila (see the article on p.402 of this book) acoustic waves shake coronal magnetic loops and get resonantly absorbed in the loop.

ISBN 978-3-642-87457-4 ISBN 978-3-642-87455-0 (eBook) DOI 10.I007/978-3-642-87455-0

This work is subject to copyright. All rights are reserved. whether the whole or part of the material is concerned. specifically the rights of translation. reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. Duplication ofthis publication or parts thereofis only per­ mitted underthe provisions ofthe German Copyright Law ofSeptember9, 1965. in its current version, and a copy­ right fee must always be paid. Violations fall under the prosecution act ofthe German Copyright Law. © Springer-Verlag Berlin Heidelberg 1991 Originally published by Springer-Verlag Berlin Heidelberg New York in 1991 The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore for gener,i1 use. 56/3140-5432\0 - Printed on acid-free paper Preface

The plans for the Heidelberg Conference Mechanisms 0/ Chromospheric and Coro­ naI Heating, held from June 5 to 8, 1990, materialized during several discussions between Eric Priest, Roben Rosner, Peter Ulmschneider, and later Jeff Linsky at meetings in Titisee and Tenerife in 1987 and 1988, in whieh we all realized that, despite the extensive work in the field, our understanding of the physics of stellar and coronae, the sources of stellar radio, UV and X-ray emission, and stellar winds remains in a dismal state. The principal reason for this is that the fundamental physieal process - the mechanieal heating mechanism, which is the cause of these important stellar layers - is still largely unknown, 50 years after W. Grotrian and B. Edlen discovered that the solar corona is hot by identifying the source of the 6374A line as FeX, and 45 years after L. Biermann proposed the first coronal heating theory. We appreciatcd that the identification of the heating mechanism is a formidable observational problem because it involves very sm all spatial scales, possibly down to (for current sheets) thicknesses in the meter range, and far beyond even the most sophistieated present instrumentation. This observational difficulty is complemented by severe problems on the theo­ retieal side. The many very complicated heating theories, most of which derive from terrestrial plasma and MHD applications, cannot be adequately simulated at present even with the most advanced supercomputers. Moreover, it is not well understood whether these (primarily) fusion-physies-related applications are in fact suitable for stellar environments. Yet worse, there has been an apparent lack of contact between the many researchers in the field of stellar atmospheric heating, who often laboured exdusively on their own favoured heating mechanism and disregarded the work of their colleagues or of the observers. (This son of behaviour is of course not only characteristic of stellar astronomers!) These realizations led to the idea of organizing a meeting devoted entirely to the topic of atmospheric heating, where for the first time a comprehensive inventory of our knowledge of hearing mechanisms could be made, and where disparate workers could leam abaut the wide scope of the field and abaut the new observations. The present book is the result of this effort. We leave it to the reader to judge how well we have succeeded. But even if this book fails our expectations, it nevertheless reveals the exciting new plans to use large-scale numerical simulations and fascinating new instrumentation to attack the formidable heating problem. Our most sincere thanks go to the other members of the Scientific Organizing Committee, to Lawrence Cram, Joe Hollweg, Jeff Linsky, Dan Spieer, Giuseppe

v Vaiana, Yuzef Zhugzhda, and Comelis Zwaan for help in the selection of the speak:­ ers. We also very wannly thank the Local Organizing Committee for the smooth organization, particularly Bodo and Elke Baschek, Hans-Peter Gail, Darren KeHy, Peter Korevaar, Helgard and Katharina Ulmschneider, and Rainer Wehrse. The sup­ port by the Max-Planck-Institut through Hans Elsässer, which offered to house the conference, is gratefully acknowledged. These thanks also extend to the Institut für Theoretische Astrophysik, where Wemer Tscharnuter gave us all the support we needed. We are especially grateful to the Deutsche Forschungsgemeinschaft, to the State of Baden-Württemberg, and particularly to Dr. Theodor Mayer, Firma Bessey, for generous provision of funds.

Heidelberg, Peter U/mschneider, Eric Priest, Robert Rosner December 1990

VI Contents

Introduction ...... 1

Chapter 1: Solar Observations

1.1 Observations of OsciIIations Observations of Waves and Oscillations By F.-L. Deubner (With 2 Figures) ...... 6 Observations of Waves and Oscillations in Solar Magnetic Fluxtube Concentrations By B. Fleck and F.-L. Deubner (With 1 Figure) ...... 19 The Generation of Long Period Acoustic Waves by Solar Global Oscillations By B. Fleck and F. Schmitz (With 1 Figure) ...... 22 Measurements of I-Hz Coronal Oscillations at Total Eclipses and Their Implications for Coronal Heating By J.M. Pasachoff ...... 25 Short-Term Oscillations in Green and Red Coronal Lines By V. Rusin and M. Minarovjech (With 1 Figure) ...... 30 Height-Dependent Short-Period Oscillations in the Fe XIV (530.3 nm) Solar Corona Above a Group Crossing the Limb By V.N. Dermendjiev (With 2 Figures) ...... 33

1.2 Photospheric Dynamics and Heating On the Dynamies of Granulation in Active Regions and the Heating Problem By A. Nesis, A. Hanslmeier, R. Hammer, R. Komm, and W. Mattig (With 2 Figures) ...... 36 Generation of Electric Currents and Waves on Magnetic Flux Tubes by Horizontal Velocities in the By T.D. Tarbell, G.L. Slater, Z.A. Frank, R.A. Shine, and K.P. Topka (With 1 Figure) ...... 39 On the Magnetic Field Activity in Solar Active Regions By J. Linke ...... 42

VII 1.3 Chromospheric Observations and Heating The Bright Points in the Ca II K-Line and Their Relarion to the Inner Network Magneric Structures By K.R. Sivaraman (With 1 Figure) ...... 44

K2V Cell Grains and Chromospheric Hearing By R.J. Rutten and H. Uitenbroek (With 1 Figure) ...... 48 Wave Hearing in Chromospheric Bright Points By W. Kalkofen ...... 54 Do We Really Know What the Actual Chromospheric Heating Requirements Are? By 1. Trujillo Bueno (With 1 Figure) ...... 60 The Effects of Scattering on the Sill 1816 Line in the Solar By K. E. Rangarajan and D.M. Rao (With I Figure) ...... 63 SllvIURIS: a High Resolution Interferometric Mission in Answer to the Chromospheric and Coronal Heating Problem By L. Dame (With 2 Figures) ...... 66 The Solar Network () By L. Dame (With 3 Figures) ...... 73

1.4 Chromosphere-Transition Layer Observations and Modelling HRTS Time Series Observations: C;hromospheric and Coronal Heating By C.-C. Cheng (With 5 Figures) ...... 77 High Spatial Resolution Observations of the Solar Transition Region: Spicules and Microftares By 1. W. Cook (With 6 Figures) ...... 83 New Models of the Chromosphere and Transition Region By E.H. Avrett (With 2 Figures) ...... 97 The Role of Particle Diffusion in the Lower Transition Region: Revised Interpretation of Emission Measures By E.H. Avrett and I.M. Fontenla (With 2 Figures) ...... 100 Why Heating is Not Necessary in the Transition Region or Upper Chromosphere By P. S. Cally (With 3 Figures) ...... 103 Height-Dependent Solar Temperature Distribution By A. Kucera, Z. Scherbakova, and E. Baranovsky (With 3 Figures) 109 On the Temperature Inhomogeneity of the Lower Solar Corona By K./. Nikolskaya ...... 113

1.5 Coronal and Observations and Modelling Very High Resolution Solar X-ray Imaging By L. Golub (With 6 Figures) ...... 115

VIII Spectra of MHD Turbulence in Coronal Active Regions By D. Gomez, P. Martens, M. Herant, F. Pardo, and L. Golub (With 3 Figures) ...... 124 Observations of Coronal Bright Points and Implications for Coronal Heating Mechanisms By S.R. Habbai (With 3 Figures) ...... 127 Modelling Coronal Active Region Emission Patterns By S.F. Brown and C.J. Durrant (With 1 Figure) 132 The Fe Ionization Equilibrium in the Solar Corona With a Non-Maxwellian Distribution Function By E. DziJcakova (With 3 Figures) ...... 135 Energetics of Solar Coronal Holes By Y.-Q. Lou ...... 137 Coronal Radio Emission By A. O. Benz (With 7 Figures) 140 Implications of Microwaves for Heating and Particle Acceleration on the Sun By A. Krüger, J. Hildebrandt, and S. Urpo ...... 150 Properties of Impulsive Events in aPolar By S. Koutchmy and M.L. Loucif (With 6 Figures) ...... 152 Explosive Instability in Solar Coronal Loops By M. Ryutova ...... 159 Signatures of Coronal Structures and Turbulence in the Solar Wind By E. Marsch (With 1 Figure) ...... 162

Chapter 2: Stellar Observations

2.1 Chromospheric and Coronal Observations, UV What Can Solar and Stellar Ultraviolet Observations Tell About Chromospheric and Coronal Heating Mechanisms? By J.L. Linsky (With 1 Figure) ...... 166 He I 5876 A Line As an Indicator of Chromospheric Heating in Young F-Type MS By R.J. Garc(a LOpez, R. Rebolo, J. E. Beckman, and C.D. McKeith (With 1 Figure) ...... 179 Chromospheric Modelling of Active Regions on AU Mic By E.R. Houdebine (With 2 Figures) ...... 182

2.2 Chromospheres of Cool Giant Stars, New Clues to Atmospheric Heating Processes in Luminous Cool Stars By A.K. Dupree (With 7 Figures) ...... 185

IX Chromospheres of Cool Non-Mira Giant Stars By H.R. Johnson, U. G. JI/Jrgensen, and D. G. Luttemwser (With 2 Figures) ...... 200 What Can Observations of Giants and Supergiant Stars Tell Us About Chromospheric and Coronal Hearing? By M. Cuntz andR.E. Stencel ...... 206 Chromospheric Activity in Late-Type Giants and Supergiants: Constraints on Heating Theories By L. Pasquini, E. Brocato, and R. Pallavicini (With 1 Figure) 222 Coronal Heating and the Dividing Line By B. Haisch (With 2 Figures) ...... 225 An Infrared Perspective on Chromospheres By T.R. Ayres (With 3 Figures) ...... 228

2.3 Activity and Magnetic Fields Magnetic Activity Across the Henzsprung-Russell Diagram By C. Zwaan (With 8 Figures) ...... 241 Relations Between Activity and Magnetic Fields By c.J. Schrijver (With 6 Figures) ...... 257 Recent Measurements of Stellar Magnetic Fields By S. Saar (With 3 Figures) ...... 273 Chromospheric/Coronal Emission Correlarions in 'Quiescent' and Eruptive Phenomena in M-Dwarf Stars By M. Mathioudakis and J. G. Doyle (With 1 Figure) ...... 279 Global Electrodynamic Coupling in Stellar Atmospheres By L. Cram ...... 282

2.4 Coronal Heating Constraints, X-rays, Hot Stars, Accretion Disks Implications for Coronal Heating Theories from Stellar X-ray Observations By R. Rosner (With 5 Figures) ...... 287 Empirical Constraints on Coronal Heating Processes By C. Jordan (With 5 Figures) ...... 300 Minimum Coronal Energy Requirements: Constraints for Heating Mechanisms By R. Hammer (With 2 Figures) ...... 316 Sun-Hot Contrast in Chromospheric/Coronal Te(r). Nonradiative Heating vs. Outflow Enhanced Opacity By V. Doazan and R.N. Thomas ...... 319 Formation of Chromospheres and Coronae of Accrerion Disks by Viscous Dissipation By R. Wehrse, H. Störzer, and G. Shaviv (With 2 Figures) ...... 324 x Chapter 3: Wave Heating Mechanisms 3.1 Acoustic Waves, Pulsations Acoustic Heating By P. Ulmschneider (With 9 Figures) 328 On the Intrinsic Difficulty of Producing Stellar Coronae With Acoustic Waves By R. Hammer and P. Ulmschneider (With 1 Figure) ...... 344 The Effect of Waves on Optically Thin Transition Region Lines By V. Hansteen (With 2 Figures) ...... 347 Heating of the Solar Atmosphere by Spicules By Q. Q. Cheng, P. Ulmschneider, and P. Korevaar (With 4 Figures) 350 Nonlinear Pulse Propagation in a Stratified Atmosphere By G. Rodo, W. Kalkojen, S. Massaglia, and P. Rossi (With 1 Figure) 353 The Shock Wave Heating Mechanism of Pulsating Star Chromospheres By S.M. Andrievsky and G.A. Garbunov (With 2 Figures) ...... 356 Ionization Pumping By C. Lindsey (With 3 Figures) 359 Shock Amplification by Radiation By M. Carlsson and R. Stein (With 1 Figure) 366

3.2 Acoustic and MHD Wave Generation Recent Developments in Theories of Wave Generation By Z. E. Musielak (With 5 Figures) ...... 369 Generation of Acoustic Flux Derived from Numerical Simulations of the Solar Granular Convection By M. Steffen, A. Krüss, and H. Holweger (With 3 Figures) ...... 380

3.3 Magnetoacoustic Waves Magnetoacoustic Waves and Their Generation by Convection By R.F. Stein and A. Nordlund (With 15 Figures) ...... 386 Magnetoacoustic Heating of the Solar Chromosphere By S.M. Chitre and J.M. Davila (With 1 Figure) ...... 402 Effects of Line-Tying and Non-Uniformities on Thermal Instabilities and Slow MHD Modes By D. Hermans, A.W. Hood, L. Clifford, and A. Milne (With 2 Figures) 405 Heating in Intense Flux Tubes By S.S. Hasan (With 4 Figures) 408 Line Simulation of Solar Structures Permeated by Acoustic and MHD-Waves By W. Rammacher (With 6 Figures) ...... 414 Damping of Shocks in Magnetic Flux Tubes By A. Ferriz Mas and F. Moreno Insertis ...... 417

XI Heating of the Solar Chromosphere by MHD-Waves By R. Erdetyi and M. Marik (With 3 Figures) ...... 420

3.4 Alfven Waves Alfven Waves By I.v. Hol/weg 423 Refteetion of Alfven Waves and Heating in Solar Coronal Holes By R.L. Moore, Z.E. Musielak, S.T. Suess, and C.-H. An (With 1 Figure) 435 Alfven Wave Propagation in a Solar Magnetie Strueture By P.L. Similon and S. Zargham (With 3 Figures) ...... 438 On Propagation and Absorption of Alfven Waves in Coronal Loops By Y.D. Zhugzhda ...... 442 Magnetie Confinement, Alfven Wave Refteetion, and the Origin of X-ray and Mass Loss "Dividing Lines" By C.-H. An, R. Rosner, Z.E. Musielak, R.L. Moore, and S.T. Suess ...... 445 Heating in Stochastie Magnetie Fields By R.N. Sudan (With 10 Figures) ...... 448 Resonanee Absorption Heating By I.M. Davila (With 5 Figures) 464 Resonant Absorption of MHD Waves in Magnetie Loops in the Solar Corona By M. Goossens ...... 480 On the Time Seales and the Efficieney of Solar Heating by Resonant Absorption By S.M. Poedts (With 1 Figure) ...... 486 Line-Tying Effeets on Stability and Heating of Solar Coronal Loops By G. Halberstadt, I.P. Goedbloed, S.M. Poedts, and R.A.M. Van der Linden (With 2 Figures) ...... 489 Coronal Loop Heating by Discrete Alfven Waves By C.A. Azevedo, A.S. de Assis, H. Shigueoka, and P.H. Sakanaka 492

3.5 Magnetoacoustic and Alfven Surface Waves Magnetohydrodynamic Surfaee Waves By B. Roberts (With 2 Figures) ...... 494 Magnetoaeoustie-Gravity Surfaee Waves By A.J. Mi/es and B. Roberts (With 1 Figure) 508 Properties of Non-Parallel Magnetoaeoustic Surfaee Waves By R. Iain and B. Roberts (With 1 Figure) ...... 511 Viseous Damping of Magnetohydrodynamie Surfaee Waves By M. Ruderman ...... 514 Coronal Loop Heating by the Fast Surfaee Wave By A.S. deAssis andK.H. Tsui ...... 517

XII Chapter 4: Electrodynamic Heating Mechanisms 4.1 Current Sheet Formation and Heating The Fonnation of Current Sheets and Coronal Heating By E.R. Priest (With 12 Figures) ...... 520 Current Sheet Fonnation in Force-Ft-ee Magnetic Fields By G. Vekstein and E.R. Priest (With 2 Figures) ...... 536 Two-Dimensional Magnetic Neutral Points By N.R. Strachan and E.R. Priest (With 1 Figure) 539 The Significance of Magnetic Null Points By K. Galsgaard and A. Nordlund (With 1 Figure) 541 Effect of Coronal Heating on Coronal Arcades By C.D. C. Steele and E.R. Priest (With 3 Figures) 544 Heating by Field Aligned DC Joule Dissipation By D.S. Spicer ...... 547 Joule Heating in the Sun's Lower Transition Region By G. Rourneliotis ...... 562 Plasma Heating by Current Sheets in Solar Active Regions

By B. Kliem and N. Seehafer ... 0 •••• 0 • • • • • • • • • • • • • • • • • • • • • • • • • • • •• 564 Chromosphere Generation in Magnetic Flux-Tubes By J. C. Henoux and B. V. Somov (With 1 Figure) 567

4.2 Heating and Helicity Coronal Magnetic Structure: the Role of Ideal MHD Invariants

By M.A. Berger (With 4 Figures) 00' 0 0 0 0 0 00000000. 0 0 0 0 • 0 0 0 0 0 • 0 • 0 • o. 570 Current Helicity and the Generation of Magnetic Field Aligned Currents

By N. Seehafer and K.-H. Rädler 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 0 0 0 0 • 0 0 • 0 0 582 Nonlinear Evolution of a Force-Free Arcade Field Driven by Shear Flow

By N. Bekki, To Tajima, and J.Wo Van Dam (With 1 Figure) 000 •• 0 • 0 • 0 • 0 o' 585

4.3 Reconnection, Heating by Flux Emergence Two-Dimensional Magnetic Reconnection

By M. Jardine (With 7 Figures) 0 0 • 0 0 0 0 0 0 0 0 0 0 • 0 0 0 0 0 • 0 0 0 •• 0 0 0 0 0 • 0 ••• 0 588 Magnetic Field Annihilation Within a Stagnation Point Flow By M. Jardine, EoR. Priest, and HoR. Allen (With 2 Figures) 601 Three-Dimensional Magnetic Reconnection: Basic Concepts

By M. Hesse, K. Schindier, and J. Birn (With 3 Figures) 0 0 0 0 0 0 •• 0 • 0 • 0 0 0 0 604 Atmospheric Heating in Emerging Flux Regions By K. Shibata, S. Nozawa, R. Matsumoto, T. Tajima, and A.C. Sterling

(With 2 Figures) 0000000000000. 0 0 0 0 0 0 0 0 0 0 0 • 0 •• 0 0 0 0 0 0 0 000 0 0 • 0 0 0 • o' 609

XIII 4.4 MicrolNanoftare Coronal Heating MicrolNanoflare Coronal Heating By E.N. Parker (With 8 Figures) ...... '. 615 Numerical Simulation of Microflare Evolution in the Solar Transition Region and Corona By A.C. Sterling and J. T. Mariska (With 4 Figures) 630 Coronal Heating by Nanoftares: Plasma Dynamies of Elementary Events By R.A. Kopp and G. Poletto (With 2 Figures) ...... 634 Coronal Heating by Nanoftares: Possible Evidence of Plasmoids in Radio Occultation Data By D.J. Mullan ...... 637 Author Index 641 List of Participants 643

XIV International Heidelberg Conference Mechanisms ofChromosphcric ami Coronal Hearing, June 5-8, 1990, in front of the Max-Planck-Haus, Berlinerstr.lO