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 chromospheres 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 Sunspot 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 Photosphere 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 Electron Scattering on the Sill 1816 Line in the Solar Chromosphere By K. E. Rangarajan and D.M. Rao (With I Figure) . .. 63 SllvIURIS: a High Resolution Solar Physics Interferometric Mission in Answer to the Chromospheric and Coronal Heating Problem By L. Dame (With 2 Figures) ....................................... 66 The Solar Ultraviolet Network (SUN) 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 Plage 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 Solar Wind 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)