SOLAR SYSTEM MAGNETIC FIELDS

Transcription

1 SOLAR SYSTEM MAGNETIC FIELDS

2 GEOPHYSICS AND ASTROPHYSICS MONOGRAPHS Editor B. M. McCORMAC, Lockheed Palo Alto Research Laboratory, Palo Alto, CaUf, U.S.A. Editorial Board R. GRANT ATHA Y, High Altitude Observatory, Boulder, Colo., U.S.A. W. S. BROECKER, Lamont-Doherty Geological Obsen'atory, Palisades, New York, U.S.A. P. J. COLEMAN, JR., University of California, Los Angeles, Calif, U.S.A. G. T. CSANADY, Woods Hole Oceanographic Institution, Woods Hole, Mass., U.S.A. D. M. HUNTEN, Unh,ersity of Arizona, Tucson, Ariz., U.S.A. C. DE JAGER, The Astronomical Institute, Utrecht, The Netherlands J. KLECZEK, Czechoslovak Academy of Science, O n d l ~ eczechoslovakia j o v, R. LUST, Pre;'ident Max-Planck Gesellschaft fur Forderung del' Wissenschaften, Miinchen, FR. G. R. E. MUNN, University of Toronto, Toronto, Ont., Canada Z. SVESTKA, The Astronomical Institute, Utrecht, The Netherlands G. WEILL, Service d'aeronomie, Verrieres-le-Buisson, France

3 SOLAR SYSTEM MAGNETIC FIELDS Edited by E. R. PRIEST St. Andrews University, Stland D. REIDEL PUB ISHING COMPANY MEMBER OFTHE KL WER ADEM I P BLI HER GRO P DO RDRECHT I BOSTO I L C T I:R I TOK YO

4 library of Congress Cataloging in Publication Dala Main entry under title: Solar system magnetic fields. (Geophysics and astrophysics monographs) Based on lectures presented at Summer School on Solar System Plasmas, held in Sept at Imperial College wilh the support of the Science and Engineering R~ s re cacouncil: h aimed at Ph. D. students. Includes bibliographies and index. I. Space plasmas. 2. Magnetosphere. 3. Magnetohydrodynamics. 4. Magnetic fields (Cosmic physics) I. Priest. E. R. (Eric Ronald), Summer School on Solar System Plasmas (1984 : Imperial College) 111. S ~ r i e s. QC809.P5S ISBN 13:971: OOU o. t 001t.971:1-94..oo9-So182 3 Published by D. Reidel Publishing Company, P.O. Box 17,3300 AA Dordrccht, Holland. Sold and distributed in the U.S.A. and Canada by KJuwer Academic Publishers, 190 Old Dcrby Street, Hingham, MA 02043, U.S.A. In all other untries, sold and distributed by Kluwer Academic Publishers Group, P.O. Box 322, 3300 AH Dordrecht, Holland. All Rights Reserved 1985 by D. Rcidel Publishing Company, Dordrecht, Holland Soflver reprint 01 the hardver 1 st edition 1985 No pait of the material protected by this pyright notice may be reproduced or utilized in any fo rm or by any means, electronic or mechanical including photopying, rerding or by any information stonge and retrieval system, without written permission from the pyright owner

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6 CONTENTS PREFACE ACKNOWLEDGEMENTS xi xii CHAPTER 1 INTRODUCTION TO SOLAR ACTIVITY (E R Priest) Some Basic Properties of the Sun 1.2 Basic Equations of Magnetohydrodynamics Magnetohydrostatics Waves Instabilities 1.3 Sunspots 1.4 Prominences Prominence Formation Magnetostatic Support 1.5 The Corona Models of the Corona Coronal Heating 1.6 Solar Flares 1.7 Conclusion CHAPTER 2 AN INTRODUCTION TO MAGNETOSPHERIC MHD (D J Southwood) Introduction 2.2 Why is There a Magnetosphere? 2.3 The Open Magnetosphere Morphology 2.4 Momentum Transfer 2.5 Magnetospheric Substorms 2.6 Magnetohydrodynamic Waves CHAPTER 3 MAGNETOHYDRODYNAMIC WAVES (B Roberts) Structuring and Stratification 3.2 Waves in a Magnetically Structured Atmosphel'e 3.3 Waves in a Uniform Medium The Alfven Wave Magnetoaustic Waves 3.4 Waves in Discretely Structured Media Inmpressible Medium Compressible Medium 3.5 Oscillations in a Low S-Gas Slab Inhomogeneities Cylindrical Inhomogeneities Impulsively Generated Fast Waves 3.6 Damped Alfven Waves

7 viii CONTENTS 3.7 Waves in Stratified Atmospheres Sound Waves The Influence of a Horizontal Magnetic Field Slender Flux Tubes The Slender Flux Tube Equations: Sausage Modes Pulse Propagation Kink Modes Instabilities in Tubes CHAPTER 4 MHO INSTABILITIES (A W Hood) Equilibrium Solutions Introduction Energetics The Lorentz Force Magnetohydrostatic (MHS) Equilibria Cylindrically Symmetric Magnetic Fields Dimensional Magnetic Fields 4.2 Physical Description of MHO Instabilities 4.3 Linearised MHO Equations 4.4 Normal Modes Method 4.5 Energy (or Variational) Method 4.6 The Rayleigh-Taylor Instability Normal Modes - Two Fluids Normal Modes - Continuous Fluid Simple Energy Method - Two Fluids Energy Method - Continuous Fluid MHO Inmpressible Rayleigh-Taylor Instability 4.7 The Sharp Pinch - Normal Modes Inner Solution r < a Outer Solution r > a Matching Conditions at r = a 4.8 General Cylindrical Pinch - Energy Method Minimisation of ozw Suydam's Criterion - A Necessary Condition 4.9 Necessary and Sufficient Conditions - Newmb's Analysis 4.10 Resistive Instabilities - Tearing Modes Introduction The Analysis of FKR 4.11 Applications of MHO Instabilities Introduction Ideal Kink Instability of Coronal Loops Two-Ribbon Flares CHAPTER 5 MAGNETIC RECONNECT ION (S W H Cowley) Introduction 121

8 CONTENTS ix 5.2 Rennection: What It Is and What It Does 5.3 Fluid (MHD) Models of Rennection 5.4 The Single-Particle Approach in a Collision-Free Plasma CHAPTER 6 MAGNETOCONVECTION (N 0 Weiss) 6.1 Small Flux Tubes 6.2 Convection in a Strong Magnetjr. Field 6.3 Structure of the Large-Scale Magnetic Field CHAPTER 7 ASPECTS OF DYNAMO THEORY (H K Moffatt) 7.1 The Homopolar Disc Dynamo The Stretch-Twist-Fold Dynamo Behaviour of the Dipole Moment in a Confined S y s t e ~ The Pros and Cons of Dynamo Action Flux Expulsion and Topological Pumping Mean-Field Electrodynamics Some Properties of the Pseudo-Tensors a.. and S.. k The Solar Dynamo lj lj Magnetic Buoyancy as an Equilibration Mechanism CHAPTER 8 SOLAR WIND AND THE EARTH'S BOW SHOCK (S J Schwartz) The Solar Wind as a Fluid Fluid Models of the Solar Wind Solar Wind Magnetic Fields Mass and Angular Momentum Loss Refinements of Fluid Models The Solar Wind as a Plasma Why a Plasma Description is Needed Solar Wind Protons Minor Ions in the Solar Wind Waves in the Solar Wind The Earth's Bow Shock Why a Shock is Needed General Shock Considerations Macrospic Fields at Collisionless Shocks Particle Dynamics at Collisionless Shocks Conclusion - Electrons Particle Dynamics at Collisionless Shocks - Ions The Global Structure of the Earth's Bow Shock and Foreshock

9 x CHAPTER 9 CONTENTS PLANETARY MAGNETOSPHERES (F Bagenal) Comparative Theory of Magnetospheres Obstacles in a Flowing Plasma Plasma Sources Magnetospheric Flows 9.2 Planetary Magnetospheres Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune and Pluto 9.3 Conclusions CHAPTER 10 COMETS (A 0 Johnstone) Introduction to Comet Structure 10.2 Interaction between the Solar Wind and the Comet 10.3 Production of Neutral Gas Vaporisation Neutral Gas Density 10.4 Ionisation Ionisation Processes Size of the Coma 10.5 Ion Pick-Up Ion Pick-Up Trajectories Stability of the Distribution 10.6 Principal Plasma Regimes Main Regions The Contact Surface Bow Shock 10.7 Magnetohydrodynamic Flow at a Comet Numerical Solution of the MHO Equations Validity of the MHO Approach 10.8 Special Features of the Morphology Rays, Tail Streamers Disnnection Events Dusty Plasmas 10.9 Conclusion INDEX 285

10 PREFACE In September 1984 a Summer School on Solar System Plasmas was held at Imperial College with the support of the Science and Engineering Research Council. An excellent group of lecturers was assembled to give a series of basic talks on the various aspects of the subject, aimed at Ph.D. students or researchers from related areas wanting to learn about the plasma physics of the solar system. The students were so appreciative of the lectures that it was decided to write them up as the present book. Traditionally, different areas of solar system science, such as solar and magnetospheric physics, have been studied by separate mmunities with little ntact. However, it has beme clear that many mmon themes cut right across these distinct topics, such as magnetohydrodynamic instabilities and waves, magnetic rennect ion, nvection, dynamo activity and particle acceleration. The plasma parameters may well be quite different in the Sun's atmosphere, a metary tailor Jupiter's magnetosphere, but many of the basic processes are similar and it is by studying them in different environments that we me to understand them more deeply. Furthermore, direct in situ measurements of plasma properties at one point in the solar wind or the magnetosphere mplement the more global view by remote sensing of a similar phenomenon at the Sun. Clearly, much can be gained in the future from cross-fertilization between the different branches of solar system science, and it is hoped that this glimpse at a few of them can help in some small way. E R Priest St Andrews, May 1985

11 ACKNOWLEDGEMENTS The authors gratefully acknowledge permission to reproduce the following pyright figures: Fig. 1.2 (S Martin), Fig. 1.3 (G Newkirk), Fig. 1.4 (J Harvey, National Solar Observatory), Fig. 1.5 (D Webb, American Science and Engineering), Fig. 1.6 (H Zirin, Big Bear Solar Observatory), Fig. 1.9 (R Tousey, Naval Research Laboratory), Fig (D Rust, American Science and Engineering), Figs. 8.3 and 8.4 (E Marsch), Fig. 8.6 (J Belcher), Fig (M Thomsen), Fig. 9.7 (J Phillips, C Russell), Fig. 9.9 (J Spreiter), Fig (V Vasyliunas), Fig (Hale Observatories), Fig (W Ip), Fig (H Alfven), Fig (A Delsemme), Fig (J Brandt), Fig (H Keller), Figs (H Schmidt). The editor is also most grateful to Shiela Wilson for her efficiency and good humour. xii