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Zirker J.B. the Magnetic Universe (JHUP, 2009)(ISBN 080189302X

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Zirker J.B. the Magnetic Universe (JHUP, 2009)(ISBN 080189302X THE MAGNETIC UNIVERSE This page intentionally left blank J. B. ZIRKER THE MAGNETIC THE ELUSIVE TRACES OF AN INVISIBLE FORCE UNIVERSE THE JOHNS HOPKINS UNIVERSITY PRESS BALTIMORE © 2009 The Johns Hopkins University Press All rights reserved. Published 2009 Printed in the United States of America on acid- free paper 2 4 6 8 9 7 5 3 1 The Johns Hopkins University Press 2715 North Charles Street Baltimore, Mary land 21218- 4363 www .press .jhu .edu Library of Congress Cataloging- in- Publication Data Zirker, Jack B. The magnetic universe : the elusive traces of an invisible force / J.B. Zirker. p. cm. Includes bibliographical references and index. ISBN- 13: 978- 0- 8018- 9301- 8 (hardcover : alk. paper) ISBN- 10: 0- 8018- 9301- 1 (hardcover : alk. paper) ISBN- 13: 978- 0- 8018- 9302- 5 (pbk. : alk. paper) ISBN- 10: 0- 8018- 9302- X (pbk. : alk. paper) 1. Magnetic fi elds. 2. Cosmic magnetic fi elds. 3. Magnetism. 4. Magnetosphere. 5. Heliosphere (Ionosphere) 6. Gravity. I. Title. QC754.2.M3Z57 2009 538—dc22 2008054593 A cata log record for this book is available from the British Library. The last printed pages of the book are an extension of this copyright page. Special discounts are available for bulk purchases of this book. For more information, please contact Special Sales at 410- 516- 6936 or [email protected]. The Johns Hopkins University Press uses environmentally friendly book materials, including recycled text paper that is composed of at least 30 percent post- consumer waste, whenever possible. All of our book papers are acid- free, and our jackets and covers are printed on paper with recycled content. CONTENTS Preface vii 1 GETTING REACQUAINTED WITH MAGNETISM 1 2 THE EARTH 9 3 SUNSPOTS AND THE SOLAR CYCLE 31 4 THE VIOLENT SUN 59 5 THE HELIOSPHERE: WINDS, WAVES, AND FIELDS 84 6 THE EARTH’S MAGNETOSPHERE AND SPACE WEATHER 113 7 THE PLANETS 141 8 MAGNETIC FIELDS AND THE BIRTH OF STARS 174 9 ABNORMAL STARS 198 10 COMPACT OBJECTS 212 11 THE GALAXIES 244 12 SOMETHING FROM NOTHING: SEED FIELDS 265 Notes 281 Index 291 This page intentionally left blank PREFACE Imagine that Federal Express delivers a package to you. There is no return address on the package, so you open it warily. Inside, you fi nd a pair of dark eyeglasses. You put them on, and immediately you are in total darkness. But as your eyes adapt, you see that you are immersed in a thicket of shimmering threads. Everywhere you turn you see them. As you turn and move about the room, they part to let you pass, but you feel nothing. You notice that the bulb in your fl oor lamp is sheathed in fl ickering coils of light. When you touch the bulb gingerly, all the threads in the room vibrate. Now you go outdoors and look up at the night sky. The whole dome of the sky is covered with a veil of parallel shimmering threads that line up north to south. When you walk back indoors, an idea occurs to you. You pull out your key chain, from which hangs a toy compass. Sure enough, the north and south ends of the compass needle are linked by faint loops. Eureka! Now you understand what’s happening. These strange eyeglasses allow you to see magnetic fi elds. They are everywhere, unnoticed but detect- able with proper equipment. Indeed, the entire universe is permeated with magnetic fi elds. We can’t see them, because unfortunately no magical eyeglasses exist. But astrophysi- cists can detect them by the eff ects they have on visible light, radio waves, and x-rays. In stars like our Sun, the fi elds are thousands of times stronger than Earth’s. In pulsars and white dwarfs, the fi elds are millions of times stronger. Between the arms of our Milky Way Galaxy they are millions of times weaker. The weakest fi elds of all connect galaxies within a cluster and stretch across im mense voids to reach distant clusters. Magnetic fi elds work in many wondrous ways during the evolution of the universe. They store and release energy, sometimes catastrophically, as in solar and stellar fl ares. They channel fl ows of ionized gases, as in pulsar jets and stellar winds. They assist in forming stars and planetary systems from the interstellar gas. They may even help to form galaxies. And incidentally, vii viii Preface they keep us Earthlings safe from the bombardment of cosmic rays. One could argue that magnetism is second in importance only to gravity as an agent of change in the universe. Just to whet your appetite, take a look ahead at fi gure 10.4. It shows a double radio source (Cygnus A) that consists of two gigantic lobes connected by a faint thread to a tiny star- like object at the center. This dumbbell- shaped object is im mense; the lobes are separated by about 10 million light- years, or a hundred times the diameter of the Milky Way. At the center lies an active galaxy that is thought to contain a black hole with the mass of a billion suns. As the hole sucks in the surrounding gas, the gas is forced to spin into a fl at “accretion disk.” The disk ejects two jets of relativistic electrons and protons in opposite directions— the long threads we see. The actual structure of the jets is controversial, but they must con- tain strong ordered magnetic fi elds that keep the jets aligned. The jets collide with the tenuous intergalactic gas and infl ate the huge lobes with energetic charged particles. The whole object glows in radio radiation. This object, the largest of many like it, contains several of the key mag- netic structures we’ll be discussing, namely, accretion disks around black holes and protostars, jets, and magnetized bubbles, or “magnetospheres.” Astronomers are currently learning more about cosmic magnetism than ever before. They are using the latest Earth- based optical and radio tele- scopes and infrared satellites to explore how the fi elds operate in diff erent settings. Computer simulations of basic magnetic pro cesses have come a long way, too. But this is only the beginning. In the near future we will be able to observe with even more sensitive millimeter-wave interferometers and larger space telescopes. This is an exciting time in the unfolding story of cosmic magnetic fi elds. I invite you to join me on this voyage of discovery. We’ll begin by recalling how several famous experimenters unveiled the basic properties of magnetism. Then we’ll talk about the Earth’s fi eld and how studies of the Sun helped to clarify how many astronomical bodies gen- erate magnetic fi elds. We’ll also see how far we’ve come in simulating these pro cesses with computers. Aft er that, we’ll consider the impressive magnetospheres of Jupiter and Saturn. Star formation is our next topic; we’ll see how magnetic fi elds become critical in the birth of stars and, perhaps, galaxies. We’ll take a big jump then to learn how astronomers are able to detect and mea sure fi elds in the Milky Preface ix Way and in distant galaxies. We’ll also examine the enormously powerful fi elds in some of the densest objects in the universe. Moving out even far- ther, we’ll see how astronomers are mapping the fi elds between the galaxies and theorizing on how they were formed. Finally, we’ll hear the speculations on how the very fi rst magnetic fi elds in the universe were created. Hop on board! We have a long trip ahead! This page intentionally left blank THE MAGNETIC UNIVERSE This page intentionally left blank 1 GETTING REACQUAINTED WITH MAGNETISM Before we plunge into the cosmos, let’s review some of the essen- tial properties of magnetism that scientists have discovered in the laboratory over the past two centuries. You’re probably familiar with some of them, while other aspects, having to do with lines of force, may be unfamiliar. Nowadays, a child fi rst encounters magnetism in school while handling a permanent magnet. She sprinkles iron fi lings on a paper held over the magnet and smiles at the lovely pattern she’s created (fi g. 1.1). She’s taught that these are “lines of force,” and she can see that they connect the “poles” of the mag- net. With two magnets in hand, she can actually feel the force of repulsion of similar poles and the attraction of opposites. She learns that the Earth has such lines of force and is itself a big magnet. Until the early nineteenth century, the most advanced scientists knew no more about magnetism than does this modern child. As we’ll see in the next chapter, Sir Edmund Halley, of comet fame, had previously mapped the Earth’s magnetism as an aid to navigation, but even he had no understand- ing of the origin of its force. Hans Christian Oersted, a Danish natural phi los o pher, made the fi rst criti- cal discovery in 1820. He was born in Denmark in 1777 of poor parents, and as a young man he studied to become a pharmacist. His strong interests in phi- losophy and science led him to abandon pharmacy, however, and to pursue an academic career. He became intrigued by the possibility that all the forces of 1 2 THE MAGNETIC UNIVERSE Fig. 1.1. Iron fi lings around a bar magnet outline lines of force. nature must originate from a common source. (He was correct! See note 1.1.) In par tic u lar, he suspected that the forces of magnetism and electricity were closely related, which was contrary to the current opinion of experts.
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