PHYSICAL PROCESSES IN SOLAR FLARES

PHYSICAL PROCESSES IN SOLAR FLARES

ASTROPHYSICS AND SPACE SCIENCE LIBRARY A SERIES OF BOOKS ON THE RECENT DEVELOPMENTS OF SPACE SCIENCE AND OF GENERAL GEOPHYSICS AND ASTROPHYSICS PUBLISHED IN CONNECTION WITH THE JOURNAL SPACE SCIENCE REVIEWS Editorial Board R. L. F. BOYD, University College, London, England W. B. BURTON, Sterrewacht, Leiden, The Netherlands C. DE JAGER, University of Utrecht, The Netherlands J. KLECZEK, Czechoslovak Academy of ScienCes, Ondfejov, Czechoslovakia Z. KOPAL, University of Manchester, England R. LOST, Max-Planck-Institutfiir Meteorolof{ie, Hamburf{, Germany CURRENT RESEARCH VOLUME 172

PHYSICAL PROCESSES IN SOLAR FLARES by BORIS V. SOMOV Sternberg Astrollomicallnstitute, Moscow State University SPRINGER SCIENCE+BUSINESS MEDIA, B.v.

Library of Congress Cataloging-in-Publication Data Somov, B. V. (Boris V.) Physical processes in sclar flares I by Boris V. Somov. ~. cm. Includes bibl iographical references (p. ) and index. ISBN 978-94-010-5056-2 ISBN 978-94-011-2396-9 (ebook) DOI 10.1007/978-94-011-2396-9 1. Solar flares. 2. High ter;perature plasmas. QB526.F6S65 1991 523.7'5--dc20 1. Title. 91-14565 ISBN 978-94-010-5056-2 ASTROPHYSICS AND SPACE SCIENCE LIBRARY, VOLUME 172 Printed on acid-free paper All Rights Reserved 1992 Springer Science+ Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1992 Softcover reprint of the hardcover 1 st edition 1992 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 photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner.

Table of Contents PREFACE vii CHAPTER 1 / THE PRESENT STATE OF THE SOLAR PROBLEM 1 1.1. Observed phenomena and the energetics of solar flares 1 1.2. Physical concepts of the nature of flares 13 1.3. Flare-induced physical processes in the solar atmosphere 35 1.4. The solar flare problem 44 CHAPTER 2 / CHROMO SPHERIC HEATING: THE MECHANISM PRODUCING HIGH-TEMPERATURE PLASMA IN SOLAR FLARES 49 2.1. Impulsive heating by accelerated electrons 49 2.2. Impulsive heating by intense heat flows 87 2.3. Chromo spheric heating by X-ray emission 109 CHAPTER 3 / MAGNETIC RECONNECTION AND ENERGY RELEASE IN SOLAR FLARES 115 3.1. Reconnection as a heating mechanism for X-ray coronal loops 115 3.2. The natural high-temperature turbulent current sheet 134 3.3. The high-temperature turbulent current sheet with a small transverse magnetic field 156 3.4. The current sheet structure under the fast and slow reconnection regimes 185 3.5. Stabilization oftearing instability in current sheets on the Sun 201 3.6. The effect of the longitudinal magnetic field in the current sheets on the Sun 206 CONCLUSION 219 REFERENCES 223

Preface During the last three decades a new field has been developing rapidly on the borderline between extra-atmospheric astronomy and high-temperature plasma physics; under the name the physics of solar flares this field has in effect acquired the status of a new branch of physics. The interest in the investigation of solar flares has been caused by two facts. First, the solar flare is a complex electromagnetic phenomenon of a cataclysmic nature in the cosmic plasma with particles being accelerated to high energy, which can be studied by a variety of methods both from the Earth and from space. Second, flares have a strong influence on interplanetary and near Earth space and on the Earth's atmosphere and biosphere. There are good reasons why flares are interesting to physicists and astronomers, biologists and physicians, specialists in energetics as well as in astronautics. The above two facts characterize the essence of the problem of solar flares as well as its significance for both pure and applied science. On the way to the solution of this problem we come closer to an understanding of a wide class of nonstationary phenomena in plasmas both in space and in the nuclear fusion laboratory, and to the establishment of a scientifically sound and reliable method of forecasting the radiation situation in near space. Also, the physics of solar flares now becomes an 'etalon' for contemporary astrophysics, in particular for gamma and X-ray astronomy. An essential step on the way to the solution of the problem is a study of the energetics and dynamics of high-temperature plasma in the flares. It is in the high-temperature plasma that the primary energy release takes place in the form of fast magnetohydrodynamic plasma flows, intense heat flows, accelerated particles and hard electromagnetic radiation. Constructing the theory of this process is the goal of those who investigate the flares. Achieving this goal is complicated considerably by the fact that the information about the primary flare process (called the flare mechanism) can be obtained only in a very indirect way - by studying various secondary effects in the solar atmosphere caused by the flows of energy released from the flare. These consequences of the primary energy release are what produce the complex observational signature of a flare. The structure of this book is derived from this fact. There are three chapters in the book, each representing a separate stage in the solution of the problem. VB

V111 Preface The first chapter is a kind of bridge between contemporary observational data and contemporary theoretical concepts. First the main observational characteristics of flares are described. We stress the necessity of investigating not only the flares themselves, but also the related nonstationary large-scale phenomena in the solar atmosphere. Based on a generalization of the observed facts a phenomenological systematization of energy release and transformation is proposed. Elementary physical concepts of the flare mechanism are then outlined - magnetic field reconnection in current sheets. We discuss the characteristics of the current sheet as the primary source of energy in various stages of flare development. Particular attention is paid to the prospects for the detection of pre-flare current sheets, which is crucial for short-time flare forecasting. The second chapter is devoted to constructing the theory of secondary physical processes, in particular to the study of the dynamics of hightemperature plasmas during impulsive heating of the solar atmosphere by accelerated electrons, intense flows of heat and radiation. The main problem here is to calculate the time-dependent physical characteristics of the flare, such as the distribution of the electron and ion temperatures and of plasma density and velocity, or the X-ray and UV radiation intensity. Comparing the calculated and observed characteristics imposes restrictions on models of the primary source of flare energy. In the third chapter we use the results of the preceding two chapters to choose a model and to construct a theory of non-neutral turbulent current sheets (having a small transverse and arbitrary longitudinal magnetic field component), which serve as the source of energy of the flares. In recent years great progress has been achieved in this part of the physics of solar flares. The reason is that a self-consistent theory of high-temperature turbulent current sheets has been formulated, which takes account of physical processes that are essential for the field reconnection in the solar active regions, where flares occur. In sum, the book aims at a consistent model of physical processes in high-temperature solar flare plasma, including the theory of the primary release of energy in turbulent current sheets and the theory of secondary transformations of this energy in the solar atmosphere that could account for the observed signature of the flare. The author hopes that this model will contribute to the solution of the global problem of solar flares in both its theoretical and applied aspect - the forecasting of flares and their effects in near space and on the Earth.