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A Brief History of Magnetospheric Physics Before the Spaceflight Era

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A Brief History of Magnetospheric Physics Before the Spaceflight Era A BRIEF HISTORY OF MAGNETOSPHERIC PHYSICS BEFORE THE SPACEFLIGHT ERA David P. Stern Laboratoryfor ExtraterrestrialPhysics NASAGoddard Space Flight Center Greenbelt,Maryland Abstract.This review traces early resea/ch on the Earth's aurora, plasma cloud particles required some way of magneticenvironment, covering the period when only penetratingthe "Chapman-Ferrarocavity": Alfv•n (1939) ground:based0bservationswerepossible. Observations of invoked an eleCtric field, but his ideas met resistance. The magneticstorms (1724) and of perturbationsassociated picture grew more complicated with observationsof with the aurora (1741) suggestedthat those phenomena comets(1943, 1951) which suggesteda fast "solarwind" originatedoutside the Earth; correlationof the solarcycle emanatingfrom the Sun's coronaat all times. This flow (1851)with magnetic activity (1852) pointed to theSun's was explainedby Parker's theory (1958), and the perma- involvement.The discovei-yof •solarflares (1859) and nent cavity which it producedaround the Earth was later growingevidence for their associationwith large storms named the "magnetosphere"(1959). As early as 1905, led Birkeland (1900) to proposesolar electronstreams as Birkeland had proposedthat the large magneticperturba- thecause. Though laboratory experiments provided some tions of the polar aurora refleCteda "polar" type of support;the idea ran into theoreticaldifficulties and was magneticstorm whose electric currents descended into the replacedby Chapmanand Ferraro's notion of solarplasma upper atmosphere;that idea, however, was resisted for clouds (1930). Magnetic storms were first attributed more than 50 years. By the time of the International (1911)to a "ringcurrent" of high-energyparticles circling GeophysicalYear (1957-1958), when the first artificial the Earth, but later work (1957) reCOgnizedthat low- satelliteswere launched, most of the importantfeatures of energy particlesundergoing guiding center drifts could the magnetospherehad been glimpsed, but detailed have the same effect. To producethe ring current and understandinghad to wait for in situobservations. INTRODUCTION EARLYWORK ON GEOMAGNETISM This is an account of early researchon the Earth's The historyof geomagnetismbegins with the magnetic distant magnetic environment,work that led to mag- compass,invented in China around the year 1000 and netosphericphysics and to space plasma physics. It tellsc•f quickly adoptedby Arabsand Europeans[Mitchell, 1932]. a sciencein its earliest,most primitive stage, when Gradually,it wasrealized that the magneticneedle did not explanationswere qualitativeand full of speculation.The point to truenorth; Columbus observed during his crossing early stagelasted here a long time, becauseremote sensing of the Atlantic that it shifted from one side of true north to of the spaceenvironment from the grounddid not tell the other [Mitchell, 1937]. enoughfor a full understanding.Researchers relied mainly Magnetism was the avocationof William Gilbert, on global magneticdata with some help from solar and Queen Elizabeth I's personalphysician. Gilbert gave a auroralobservations. Their prime tools were insight and convincingexplanation of the action of the compass:the imagination,and their mathematicalskills could only Earth was a greatmagnet. He reachedhis conclusionwith occasionallybe broughtto bear. With all thoselimitations the help of a sphericalmagnet, a modelof the Earth which it is remarkablehow many of our fundamentalideas on he named the "terrella," or "little Earth." Moving a spacephysics were glimpsedduring those early years. compassover the surfaceof the terrella, he observedthat Note thatfor early work, recentand relatively accessible its needlepointed toward the magneticpoles, and he also publicationsare sometimesgiven and readersseeking the demonstratedthis before the queen. Gilbert's book De originalpapers will find themcited there. The Journal of Magneteappeared in 1600 and describedall that was then GeophysicalResearch was known prior to 1957 as knownabout magnetism and electricity[Gilbert, 1958]. It TerrestrialMagnetism and Atmospheric Electricity. was one of the importantscientific books of the age of Thispaper is not subjectto U.S. copyright. Reviewsof Geophysics,27, 1 / February1989 pages 103-114 Publishedin 1989 by the AmericanGeophysical Union. Paper number 89RG00097 ß 103ß 104 . REVIEWS OF GEOPHYSICS / 27,1 Stern: HISTORY OF PRE-SPACE AGE MAGNETOSPHERE Galileo and amongother thingscontained the first use of With such tools it was observed that the Earth's field the term "electric force" which led to the later term was occasionallydisturbed for a day or so: theseevents "electricity." were termed "magneticstorms," but no one knew their Important advancesin geomagnetismfollowed in the cause. Celsiusfound that the large magneticdisturbance next two centuries[Chapman and Bartels, 1940, volume2, of April 5, 1741, was detectedsimultaneously by him in chapter26; Nelsonet al., 1962]: Uppsalaand by Grahamin London[Chapman and Bartels, 1. The discoveryby Gellibrandin 1635 of the slow 1940, section26.10], demonstratingthe nonlocalnature of variation of the Earth's field [Malin and B ullard, 1981; magneticstorms. The magneticnetwork started by Gauss Brushand Banerjee, 1988]. and Weber later showed the storms to be a worldwide 2. The discoveryby Graham[1724] (seeChapman and phenomenon. Bartels [1940, section26.9]) of "magneticstorms" (later term),large irregular disturbances of the compassneedle. 3. The first magneticsurvey of the AtlanticOcean by THE SUNSPOT CYCLE Halley, in 1699 [Bullard, 1956; Ronan, 1969; Evans, 1988]. Enter the Sun. In the first half of the nineteenthcentury 4. The discoveryby Oerstedin 1820 that electric there lived in the German town of Dessaua pharmacist currentsproduced magnetic forces [Shamos, 1959; Dibner, named Samuel Heinrich Schwabe whose hobby was 1962]. astronomy[Newton, 1958]. Every day when the Sun was 5. The laws of electromagnetism,by Ampbrein 1821 not obscured,Schwabe observedit, paying attention to [Williams, 1965, 1989]. sunspots,noting their numbers,and keeping a tally of days 6. Electromagneticinduction, by Faraday in 1831 when they were absent [Meadows, 1970]. He started [Faraday,1952; Williams,1963, 1965]. observingin 1826 and 10 yearslater publisheda reportof In 1839 Carl Friedrich Gauss [Gauss, 1839, 1877; his results: no one seemedto pay attention. In 1843 he Dunnington,1955] publisheda methodfor mathematically publisheda more completeaccount, suggesting a 10-year describingthe Earth's field B by means of a scalar cycle: at first, again,no response.Eventually, however, potential¾, Schwabe'swork caughtthe eye of Alexanderyon Hum- (1) boldt,naturalist and promoter of the sciences,who in 1851 included Schwabe's results in his third volume of Kosmos, expandedat anypoint (r, O,•) in sphericalharmonics: an encyclopaediccompilation of information about the physicalworld [Schwabe,1851]. Suddenly,sunspots and 7= a 2;(a/r)n+l P7 (0) [g7 sinm•) + h• cosm•)] their cycle became a hot topic: astronomersbegan countingsunspots and studyingthem, earlier cycleswere + a 2;(r/a)'• P7 (0) [G7 sinm•) + H• cosm•)] (2) reconstructedfrom old observations,and searchesbegan for terrestrialeffects which correlatedwith the sunspot The first sumrepresents sources inside the Earth, and the cycle. second one external sources. Gauss and his associate Very soon such a correlation was found. Edward Wilhelm Weber then went on to found a network of Sabine, a British scientist and the main architect of a observatories,greatly expandedby British and Russian worldwidenetwork of magneticobservatories (an expan- help [Cawood, 1979; Malin, 1969]. From data thus sion of an earlier effort by Gaussand Weber), announced obtained, the coefficients due to sourcesinside the Earth in 1852 that the frequencyof magneticstorms rose and fell were derived[see Barraclough, 1978]; as for the external with the numberof sunspots[Sabine, 1852; Meadows and coefficients,the calculationgradually continned what had Kennedy,1982] (seealso Lamont [1852]). beensuspected, that better than 99% of the field originated Evidencewas soonalso found that the polaraurora was inside the Earth. more frequentlyseen (at relatively low latitudes)near the However,as Graham'swork suggested,some magnetic peakof the sunspotcycle. Here too a magneticconnection effectsdid originateon the outside. Observationsof such existed: as early as 1741 the Swedishscientist Celsius effectswere advancedby the work of CharlesCoulomb, reportedthat during auroraldisplays the magneticneedle who in 1777 greatlyincreased the sensitivityof magnetic was disturbed [Stoermer, 1955, section 6; Eather, 1980]. measurementsby suspendinga magneticneedle from a The actual discoverymay have been due to Hiorter, a fine string[Gillmor, 1971; Shamos,1959]. Suchinstru- studentof Celsiuswho later wrote that when he reported mentscould be made even more sensitiveby attachinga the magneticeffect of the aurora to his mentor, Celsius small mirror which moved a spot of light, and this type saidthat he too had observedthe phenomenonbut had not dominatedgeomagnetism for closeto 200 years[Nelson et mentioned it in order to see whether his student would find al., 1962;Multhauf and Good, 1987]. it independently. Stern: HISTORY OF PRE-SPACEAGE MAGNETOSPHERE 27,1 / REVIEWSOF GEOPHYSICSß 105 SOLAR FLARES electrically charged particles whose properties were measuredby J. J. Thomsonand which were eventually How did sunspotsexert their influence?The first clue named electrons [Thomson, 1967;
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