JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106,NO. A8, PAGES 15,797-15,801,
AUGUST 1, 2001
A historyof early work on the heliosphericmagneticfield
E. N. Parker
EnricoFermiInstitute,andDepartments
of PhysicsandAstronomy
Universityof Chicago,Chicago,Illinois
Abstract. The ideaof a magnetic
fieldin spacearoundEarthbegan400 yearsagowithGilbert's
recognition
thatthemagneticfieldof Earthextendsoutwardintospaceto formwhatwe nowcall
themagnetosphere.
Theconcept
of thesolarwindandtheheliosphere
haditsfirstglimmerings
withtherecognition,
about270 yearsago,thatgeomagnetic
activityis correlated
with solar
activity.It wassuggested
abouta 100yearsagothattheconnection
is throughsolarcorpuscular
radiationwith velocitiesof the orderof 103km/s. The observedaccelerationof comettails
indicated
theuniversalnatureof solarcorpuscular
radiation,givingriseto thefirstspeculation
on
theexistence
of theheliosphere.
Thehydrodynamic
expansion
of themilliondegreesolarcorona
wasthenshownto providethesolarwind (solarcorpuscular
radiation),stretchingoutthe
magneticfields of the Sunto fill the heliosphere
with a spiralmagneticfield. The adventof the
spaceagesoonverified the solarwind andmagneticfield with directmeasurements.
1. Early Concepts
period of about4 months,whereasthe magneticfluctuationAB
occurs in an hour. Hence Kelvin concluded that a solar cause is
The scientific pursuit of magnetic fields in interplanetary
impossible. He ignoredthe suggestion
that the geomagnetic
space(the heliosphere)beganabout400 years ago with research
variationsmight be the result of a beam of solarcorpuscular
on the form of the geomagneticfield. The crucialstepwas made radiation.
by Gilbert [1600] who first recognizedthat Earth possessesa
The concept of isolated collimated (cold) beams of solar
dipole magneticfield extendingfar out into spaceand declining
in intensityas 1/r3with distancer from the centerof Earth.
Geomagnetic fluctuations were discovered and associatedwith
auroral activity throughthe coordinatedwork of Graham [ 1724]
and Celsius[ 1741]. De Mairan [ 1754] suggestedthat Earth orbits
through the extended corona of the Sun, and it is the entry of
solarcoronalparticlesinto the geomagneticfield that causesthe
fluctuationsand the aurora. This prescientconceptwas basedon
the mistakenidea that the zodiacal light representsthe outward
extension of the corona observedclose to the Sun during an
eclipse of the Sun. The zodiacal light extendsfar beyond the
orbit of Earth. As the reader will soon see, the association of the
corpuscularemission of protons and electrons (103 krn/s)
developedthroughthe last decadeof the nineteenthcenturyand
the first half of the twentiethcentury[cf Chapmanand Ferraro,
1933, 1940] as the causeof geomagneticstormsand enhanced
aurora. The ideasbeganto take form with the discoveryof the
electronand the superficialresemblance
of the auroralrays to
cathoderay streamersin the laboratory Crooke's tube. The
velocity(103km/s)of the corpuscular
radiationwasestimated
from the delay of a couple of days betweena flare event at the
Sun and the associated
geomagneticactivity. The ultimatestep
in the concept of solar corpuscularradiation was Biermann's
[1951, 1957] realization that the solar corpuscularradiation is
responsiblefor the observedstrongantisolaraccelerationof the
zodiacallight with interplanetaryfree electrons(a concepttotally
unknown to de Mairan) persisteduntil about 1962 when direct
gaseouscomet tails. Since all comet tails show this acceleration,
measurementsin spaceshowedotherwise. Brief historiesof the
Biermann pointed out that the Sun emits solar corpuscular
developmentof geomagneticactivity [Chapman,1967] and space
radiation in all directions at all times. That is to say, the
science[Parker, 2000] are available in the literature.
corpuscular
radiation
is notconfined
tobeams
created
by special
It must be appreciatedthat interplanetaryspacewas regarded
events on the Sun, but rather is a universal and continuing
as a hard vacuum by many physicists, the zodiacal light
property of the Sun. Assumingthat the principal interactionof
notwithstanding. Space was consideredto be so empty that
the corpuscularradiation with the comet tail gas is through
electrostatic
potentialdifferences
of 109voltsor morecouldbe
charge exchange, Biermann estimated that the corpuscular
postulated without fear of ridicule. To cite an extreme case,
radiation has a density of the order of 500-1000 protons and
Kelvin asserted that solar activity could not be the cause of
electrons/cm
3at the orbit of Earth. This agreedwith the 500
geomagnetic fluctuations AB in spite of the overwhelming
electrons/cm
3
estimated
fromthebrightness
of thezodiacallight
evidenceof a connectionwith a 2-day delay. Kelvin considered
on the assumptionthat the light is scatteredsunlight from free
AB at Earth as the superpositionof a AB in a solar dipole
magnetic
field. Extrapolating
backto theSunwith 1/r3implieda electrons. In fact, the zodiacal light is principally sunlight
AB at theSunsome107timeslargerthanthemodestAB - 3x10'3 scatteredby dust grains. What is more, the solar corpuscular
gaussobserved
at Earth,thatis, 3x104gaussat theSun.Overthe radiationinteractswith the comet ions and electronsprincipally
1033
cm3volumeof theSun,thisrepresents
a magnetic
energyof through the magnetic field carried with corpuscularradiation.
4x104øergs,equivalent
to the solaroutput(4x1033
ergs/s)overa However these quantitative points were not understooduntil
later. The essentialstepwas the recognitionthat the corpuscular
radiationfrom the Sun is an everydayphenomenonand doesnot
Copyrl.gbt 2000 by the AmericanGeophysicalUnion.
dependupon specialflare eventsat the Sun.
Now there are two obviousplacesto look for the sourceof the
Papernumber2000JA000100
interplanetarymagneticfield. One is the interstellaror galactic
0148-0227/00/2000JA000100509.00
15,797
15,798
PARKER:
A HISTORY
OF EARLY
WORK ON THE HELlOSPHERIC
MAGNETIC
FIELD
magneticfield, whichmay extendthroughthe solarsystem,and
the other is the Sun itself. Hilther [1949] and Hall [1949]
observedthe systematicplanepolarizationof starlightthat has
beenreddenedby passagethroughinterstellardust,indicating
thatnonspherical
dustgrainsarepreferentially
alignedby some
large-scalefield. Spitzer and Tukey [1951] and Davis and
Greenstein[ 1951]madethepointthattheonlyplausiblefieldis a
galacticmagneticfield orientedmoreor lessalongthe galactic
arm. They proposedferromagneticgrainsor paramagnetic
grains, respectively. It was not possibleto deducethe field
strength from the degree of polarization without precise
knowledge
of thecomposition
andshapeof theindividual
grains,
but numbersof the orderof 10'5 gausswereoftenmentioned.
Fermi [1949, 1954] appealedto the galacticmagneticfield to
confinethe galacticcosmicrays, and, in fact, suggested
that
cosmicray particlesare acceleratedby bouncingoff Alfv6n
wavesand other movingdisturbances
in the galacticmagnet
.Simpson[1951, 1954; Simpson,Fonger, and Treiman, 1953]
developed the neutron monitor to measure the cosmic ray
intensity at energies in the range 1 - 10 GeV, where the
variationsAI are much larger. Simpsonestablishedfive cosmic
ray neutronmonitorsfrom Huancayo,Peru, on the geomagnetic
equator,wherethe geomagneticfield admitsparticlesonly above
about10 GeV, to Chicago,wherethe geomagnetic
cutoffis about
2 GeV. In this way he used the geomagneticdipole field as a
magneticspectrometer.By intercalibratingand intercomparing
AI at thesestations,he was ableto showthe energydependence
of the Forbushdecreaseand of the 11-yearvariationof the mean
cosmicray intensitywith the magneticsunspotcycle. He found
that the energy dependencecould be explainedby neither an
electrostaticpotentialin interplanetaryspacenor by an increase
in the geomagneticcutoff energy. He noted,then, that the only
remainingpossibilityis manipulationof interplanetarymagnetic
field by solar corpuscularradiationin sucha way as to partially
field.
excludethe lower energyparticles.
However, it soon became clear that the interstellarmagnetic
A number of magnetic effects were proposed. Alfvdn [1956,
field doesnot penetrateinto interplanetaryspace,becauseof the
1958] pointed out that the abrupt expansionof Hale's 40-gauss
powerful outwardsweepof the solarcorpuscularradiation. With
dipole field of the Sun would adiabaticallydecelerateand dilute
thenumbersavailableat thetime(500 protons
andelectrons/cm
3 the cosmicrays, producinga reductionin measuredintensitynot
and 1000 kin/s) Davis [1955] estimated that the interstellar
unlike the Forbush decrease. A particularly naYveidea was
magneticfield (10'5 gauss)is pushedaway to a distanceof proposedby Parker [1956], who suggestedthat interplanetary
several hundredAU. Hence, it would appearthat the Sun must plasmacloudscontainingmagneticfields might be capturedby
be the sourceof any interplanetarymagneticfields.
the gravitational field of Earth, thereby forming a temporary
The magnetic fields of the Sun were first detected and magneticbarrier around the geomagneticdipole and inhibiting
measured by Hale [1908] through their Zeeman effect in
the arrival of the lower energy cosmicrays. Fortunately,it was
sunspots,where the field strengthsare commonly 2000-3000
soon recognized that the feeble gravitational field of Earth is
gauss. Hale [1913] also thoughtthat he detectedpolar fields of
grosslyinadequatefor the taskandthe idea wasabandoned.
the general order of 40 gauss, indicating that the Sun has a
Morrison [1959] suggestedthat a plasma cloud containinga
general dipole magnetic field. In fact, when Babcock and
closed tangled internal magnetic field is emitted by the Sun.
Babcock [1955] began systematic mapping of the solar Expanding out through interplanetary space, the cosmic rays
photosphericmagnetic fields, the polar fields of the Sun proved within the cloud are adiabaticallydeceleratedand diluted, so that
to be only 10 gauss or less, reversing every 11 years near the
a Forbushdecreaseis observedas the cloudengulfsEarth. Gold
peak of the sunspotcycle. Re-examinationof Hale's old plates [1959] suggestedthat a plasmacloud is emitted from a bipolar
indicateda noiselevel of the orderof 40 gauss.
active region on the Sun, stretchingthe bipolar field out through
interplanetaryspaceto provide an expandingmagnetictongue,
2. Indirect
Evidence
whose
interior
would
contain
a decelerated
and diminished
cosmic ray intensity. The Forbush decreaseoccurswhen the
Now 50 years ago the only direct probes of interplanetary tongueengulfsEarth.
conditions were the solar corpuscularradiation impacting the
The great cosmicray flare of February23, 1956 providedan
geomagneticfield and the observedvariationsAI [Forbush,1937, unanticipated
and effectiveopportunityfor probinginterplanetary
1954] of the galactic cosmic ray intensity. Unfortunately, the magnetic conditions. Meyer, et al. [1956] used the time
corpuscular radiation gave no clue as to the interplanetary variationsrecordedby the five neutronmonitor stations,plus a
magnetic field, whereas the cosmic ray variations were clearly
sixth on shipboard(on the way to Antarctica) in the harbor at
the result of someform of interplanetaryelectromagneticeffect Wellington,New Zealandto determinethe energyspectrum.The
on an otherwise steady external field of galactic cosmic rays. promptarrival of solarcosmicraysat preciselydeterminedtimes
Forbushmade his historic ground-basedmeasurements
with ion
at different terrestriallongitudesand latitudesshowedthat the
chambers,which respondprimarily to the muonsproducedby
first arrivals came straightfrom the Sun. The intensitypeaked,
collisionsof the incoming protonsat the top of the atmosphere. and the directionality disappeared, after about 15 min,
Thus the ion chamberstrack the intensityof incomingcosmicray
subsequently
decliningast'3/2for a few hoursbeforegoingover
protonswith energiesof the orderof 10-15 GeV andhigher. Of
into an exponential decline exp(-t/'c). It was clear that any
particularinterestwas the occasionalabruptintensitydrop of a
interplanetarymagneticfield betweenEarth and Sun was more or
few percentover a periodof hours,graduallyrecoveringover the less radial. The simplestmodel for the declining phasewas a
next several days, usually simultaneouslywith a geomagnetic uniform diffusive magnetic barrier beginning at about 1.5 AU
storm, generally known as a Forbush decreasein honor of its
andextendingout to perhaps5 AU.
Collectively, the cosmicray variationssuggesteda variety of
discoverer. The similarity and close tracking of the two
phenomena suggestedto Forbush that the geomagnetic storm interplanetarymagneticfield scenariosat different times. The
may somehow increase the geomagneticcutoff energy at each indirect inferences from cosmic rays, comet tails, and
latitude, thereby reducing the intensity of the incoming cosmic geomagneticvariationswere insufficientto pin down the precise
rays. But whatever the mechanismfor the manipulationof the form and behavior of the interplanetary magnetic field.
cosmic rays, the Forbushdecreaseclearly indicatedthe heavy Presumably, the magnetic field is dominated by the solar
handof solarcorpuscularradiation.
corpuscularradiation,but the origin of the corpuscularradiation
PARKER' A HISTORY OF EARLY WORK ON THE HELlOSPHERIC
MAGNETIC
FIELD
15,799
was unclear. There were vague thoughts about corpuscular idealized case of a precisely radial solar wind with uniform
emissionfrom sunspots,
M regions,flares,etc., all confounded
by velocity v beyondsomeradiusr = a near the Sun. A smokepot
Biermann's revelation that the Sun emits solar corpuscular placedat somepoint (00, tp0
) on the spherer = a rotatingwith the
radiation at all times and at all heliocentric latitudes.
angularvelocity fl of the Sun at the polar angle 00 producesa
line of smokein the wind with the spiralconicalform 0=00, r- a
= (v/•)(qo- q00).The magneticline of forcethroughthe smokepot
3. Solar Wind
The next advance was made by Chapman [1959], who
calculatedthe extremely high thermal conductivity and small
thermalemissionof the million degreecoronalgas. He showed,
then, that the corona, strongly bound by gravity near the Sun,
extendsfar beyond the orbit of Earth, with the temperatureT
lies everywherein the line of smoke,so that the field line has the
same Archimedian spiral form as the line of smoke. The
magneticfield intensityis determinedby the field at r = a by
decliningoutwardonly as 1/r2/7.The ghostof de Mairanmust
havebeenpleasedwith the result.
Parker [ 1958] noted two fundamentaldifficulties with the idea
that Chapman'scoronacould remain static. The first difficulty
arosefrom integrationof theequationfor staticequilibrium,
dp/dr+ GMoM/r
2 = O,
(1)
wherep = 2NkT representsthe pressureof ionized hydrogenat a
temperatureT and numberdensityN, M 0 is the massof the Sun,
and M is the mass of a hydrogen atom. The result is
nonvanishingpressurep at r =oofor the simple reasonthat far
from the Sun the slow decline of the temperatureleads to the
thermal energy per ion exceeding the gravitational binding
energyof the Sun.
The seconddifficulty was that a tenuousstatic coronawould
not permit the passageof Biermann'suniversalsolarcorpuscular
radiation. Even if there were no transversemagnetic fields in
space,the two-streamplasmainstabilitywould quickly arrestthe
flow of one plasma through the other. It was evident that,
somehow,the stronglybound and seeminglystatic coronanear
the Sun must becomethe corpuscularradiationat large distance.
If the coronacannotbe static,suppose,then, that them is a steady
radial velocity v(r), so that, insteadof the static equilibrium
equation,we havethe momentumequation
NMvdv/dr+ dp/dr+ GMoM/r2 = 0,
Br(r,O,qo)
= Br[a,O,qo
- •(r-a)/v](a/r)2,
(3)
B0(r,0,q0)= 0,
(4)
B,(r,O,qo)
= B•[r,0,q0- •(r-a)/v]a2• sinO/vr.
(5)
In the real heliospherethe solar wind velocity is not uniform
around the Sun, nor is it always constantin time at any given
heliographic longitude of the rotating Sun. So the magnetic
structurein interplanetaryspaceis complicatedby divergingflow
(at high latitude), convergingflow (at low latitude), fast spiral
streams overtaking slow spiral streams, and blast waves
(nowadays recognized as coronal mass ejections). These
possibilitieswere sketchedat some length [cf. Parker, 1963,
Chaps.9, 10, 11 andreferences
therein;Sarahbai,1963]. One of
the mostinterestingaspectshasturnedout to be the overtakingof
slow regions of wind by fast regions, providing forward and
backwardpropagatingshocksalongthe spiralzonesof collision.
Burlaga [1997] has followed these effects over the years as
spacecrafthave venturedfarther out throughthe solar system,
finding that the phenomenonprovidesa sawtoothstructureof the
solarwind and magneticfield at the orbit of Jupiterand beyond.
The large fluctuationsin the otherwisespiralmagneticfield were
investigatedby Belcher and Davis [1971] and the dynamicsof
the deflection of the solar wind was treatedby Carovillano and
Siscoe [ 1969].
4. Space Observations
(2)
Observationalstudiesof the solar wind and its magnetic field
got underwaywith Explorer 10, on an orbit that carriedit out to
together
withthecondition
thatNvr2= constantfor conservation about15 Re (Re is the radiusof Earth)in the generaldirectionof
solarwinddensityof 500 H ions/cm
3at
of particles. With the million degreetemperatureextendingfar theSun. The presumed
out into space,the only solutionstartingat small r with small v velocities of 500-1000 km/s suggested that the sunward
and strong gravitational binding and extending smoothly and boundaryof the geomagneticfield was pushedin to about5 Re,
continuouslyto p = 0 at r = oo is the supersonicexpansion so Explorer 10 would be well out in the wind for most of the
solutionprovidingthe solarwind [cf. Parker, 1958, 1963, 1965, time. It was baffling, therefore,to find only an intermittentwind
1969; Hundhausen, 1972; Roberts and Soward, 1972].
of 4 - 8 H atoms/cm
3 and 200 - 400 km/s alternatingwith
With the generalhydrodynamicexpansionof the solarcorona
as the origin of the solarcorpuscularradiationit was immediately
clearthat the interplanetarymagneticfield is madeup of the solar
magneticfieldswhereverthey are not strongenoughat the Sunto
preventthe free expansionof the corona[Parker, 1958]. That is
relativelysteadymagneticfieldsof 10-4gauss[Heppneret al.,
1963] and no wind for irregularperiodsof the orderof an hour.
At first sight it suggesteda solar wind loosely filled with
magneticfilamentsfrom the Sun. However,it was soonpointed
to say,theactivecorona
is forciblyconfined
by thestrong
(-102
4-8 H atoms/cm
•, thenthe sunwardmagnetopause
lies at 10-15
gauss)bipolarmagneticfields of the activeregions,whereasthe
coronais able to pushits way out throughthe relatively weak
(-10 gauss) fields of the broad active regions. So the
interplanetarymagneticfield arisesprimarily from the forced
extensionof the quiet regionfields. Coronalexpansionand the
resulting solar wind create the heliosphereand fill it with
magneticfield drawn out from the Sun and extendedto the outer
boundaryat some 100- 200 AU.
The basic character of the interplanetary or heliospheric
magnetic field is easily illustrated [Parker, 1958] for the
Re rather than 5 Re. Thus Explorer 10 was moving along the
bouncing sunward magnetopause,which alternatelyretreated,
exposingthe spacecraftto shockedsolar wind, and advanced,
placingthe spacecraft
within the shelterof the geomagnetic
field.
out [Bonetti et al., 1963)] that if the solar wind density is only
Bonetti et al. [1963] sketched the configuration, with the
upstream bow shock and the comet shaped magnetosphere
extendingin the antisolardirection.
The next spacecraftto explore the interplanetarymagnetic
field was the Mariner 2 mission to Venus. It provided the first
long-term quantitativerecord of the solar wind velocity and
15,800
PARKER: A HISTORY OF EARLY WORK ON THE-HELlOSPHERIC
MAGNETIC
FIELD
Subsequentstudiesshowedthat the signof the interplanetary
density[Snyderand Neugebauer,1964; Neugebauerand Snyder,
1966, 1967]. Unfortunately, the Mariner spacecraftwas not magnetic field correlates best with the fields of the Sun at
magnetically clean, there being active electric currentloops as latitudes of about +30 ø and-30 ø, showing how the corona is
well as magneticstructuralmaterial, so that the magnetometer confined by the strongmagnetic fields of the active regions at
was able to recordrapidmagneticfluctuations
AB -0.6x10 -4 low latitude so that the wind at Earth comesmainly from the free
gaussin the interplanetarymagneticfield carriedpastthe space coronalexpansionat middlelatitudes[Wilcox, 1968; Wilcoxand
craft in the 500 km/s wind [Davis et al., 1964], but there was no
Howard, 1968; Wilcox and Colburn, 1969].
This brief history traces the investigationand thinking on
conditionsin space,especiallythe magneticfieldsin space,from
the early daysof indirectprobingand inferringto the later direct
studiesthat becamepossiblewith the dawningof the spaceage.
The hydrodynamicexpansionof the solarcoronaprovesto be the
of the field. Thus it was that Ness et al. [1964] succeededin controlling factor, stretchingout the weak regions of solar
making the first measurementsof the mean interplanetary magneticfield all the way out throughthe heliosphere,through
magneticfield at the orbit of Earth.The measuredfield fluctuated the presumed termination shock, and into the subsonic
rapidly, with AB- B. From their detailed measurements, downstreamwake in the local interstellarwind. The following
however,theyfounda meanfieldB- 0.6x10-4gauss
inclinedto paper by N. F. Ness picks up where the presenthistorical
theradialdirection
by theexpected
spiralangle(•t = tan4(rf•/v)) narrative leaves off, carrying the space exploration of the
of 300-45ø dependingupon the prevailingwind velocityat the interplanetary
magneticfield into the fascinating
detailsomitted
time. Thus the interplanetarymagneticfield is nearly radial in the grosspictureoutlinehere.
inside the orbit of Earth and more nearly azimuthalbeyondthe
Acknowledgments.JanetG. Luhmannthanksboth of the
orbit of Mars. One couldbegin to seethe basisfor the behavior
refereesfor their assistance
in evaluatingthispaper.
of the solarcosmicraysof February23, 1956.
The long-termmonitoringof the interplanetary
magneticfield
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