Plasma Wave Electric Fields in the Solar Wind&Colon
VOL. 82, NO. 4 JOURNAL OF GEOPHYSICAL RESEARCH FEBRUARY 1, 1977
Plasma Wave Electric Fields in the Solar Wind: Initial Results From Helios 1
DONALD A. GURNETT •
Max-Planck-lnstitutfiir extraterrestrischePhysik, 8046 Garching,West Germany
ROGER R. ANDERSON
Departmentof Physicsand Astronomy, University of Iowa, Iowa City, Iowa 52242
Plasmawave measurementsprovided by Helios 1 showthat the electricfield intensitiesin the solarwind are usuallyvery low, muchlower than those for comparablemeasurements near the earth, where particles moving upstreamfrom the bow shock often cause large disturbancesin the solar wind. The most commonlyoccurring plasma wave detected by Heliosis a sporadicemission at frequenciesfrom about1 to 10 kHz, betweenthe electronand ion plasmafrequencies. These waves are thoughtto be ion sound waveswhich are Doppler-shiftedupward in frequencyfrom belowthe ion plasmafrequency. The maximumelectric field intensityof thesewaves is a few hundredmicrovolts per meter.At higher frequencies,from about20 to 100kHz, electronplasma oscillations are detectedat frequenciesnear the localelectron plasma frequency. These electron plasma oscillations are more intense, with fieldstrengths sometimesas largeas a few millivoltsper meter,but occurvery infrequently. Both the ion soundwaves andthe electronplasma oscillations show a tendencyto occurat higherfrequencies closer to thesun but no pronouncedvariation in intensitywith radial distancefrom the sun.In four cases,electron plasma oscillationshave beenfound in associationwith type IlI radio bursts.
INTRODUCTION In the early solar wind models of Parker [1958] it was In this paper we surveyand analyze the electricfield mea- realized that microscopiccollisionless interactions must be surementsobtained from the Universityof Iowa plasmawave present to account for the fluidlike properties of the solar experimenton the Helios I spacecraft.Helios 1, which was wind. The importance of these collisionlessinteractions was launched on December 10, 1974, is in an eccentricsolar orbit later confirmedin more detail by the two-fluid calculationsof near the ecliptic plane with initial perihelion and aphelion Hartle and Sturrock [1968], which showedthat the solar wind radial distancesof 0.309 and 0.985 A U, respectively.The protons must be heated somewhereinside 1 AU to accountfor Universityof Iowa plasmawave experiment on Helios I pro- the observedproton temperature and velocity at the earth. videsmeasurements of electricfield intensities in the frequency Becauseof the low collision frequencyin the solar wind this rangefrom 31 Hz to 178 kHz. This frequencyrange includes heatingmust involvewave-particle interactions. Over the past most of the characteristicfrequencies of the plasma(the elec- 10 years,numerous theoretical studies have been performedto tron plasmafrequency f,-, the ion plasmafrequency f•+, and try to identify the most important and relevant plasmawave the electrongyrofrequency fg-) expectedin the solar wind instabilitieswhich occur in the solar wind. See, for example, from 0.3 to 1.0 AU. The data obtainedfrom this experiment the recent review by Hollweg [1975]. Most of these studies provide the first observationsof plasmawaves in the region have concentratedon various types of low-frequencyhydro- closeto the sun(R -• 0.3 AU) and measurementswith greatly magnetic(Alfv6n) waves and instabilitieswhich occur at fre- improvedsensitivity and frequencyrange in comparisonto quenciesbelow the ion gyrofrequency.The propertiesof these any previoussolar wind plasmawave investigations performed low-frequency hydromagnetic waves have now been exten- at 1.0 AU. sively studied by using satellite-bornemagnetometers on in- Plasmawaves play a crucialrole in most,if not all, plasma terplanetary spacecraft(see, for example, Coleman [1966], interactionswhich take place in low-densitycollisionless Belcher and Dat•is [1971], and Burlaga [1971]). At higher plasmas.When the mean free path of the particlesbecomes frequencies,in the rangeappropriate for comparisonwith the muchlarger than the basicscale length of the system,as occurs Helios plasma wave measurements,several types of elec- in the solarwind and the earth'smagnetosphere, collisions can trostaticand electromagneticinstabilities have been suggested no longer_providethe mechanismfor energyand momentum which may play an important role in controlling the macro- exchangebetween the particles.As the systemevolves, devia- scopicstructure of the solar wind. Fredricks [1969] suggested tions from thermal equilibrium occur which produceunstable that electrostaticion soundwaves, driven by currentsat dis- particle distributions.In the region of instability,waves de- continuitiesin the solar wind, could heat the protons in the velop which grow to suchlarge amplitudesthat they directly solar wind by resonantinteractions. Substantial proton heat- affect the distributionfunction of the interactingparticles. ing by wavesof this type is only expectedat radial distancesof These wave-particleinteractions usually tend to drive the dis- lessthan about 0.3 AU. Forslund[1970] consideredthe pos- tribution function toward thermal equilibrium, the waves sible types of two-stream instabilitieswhich could occur be- playinga role similarto collisionsin an ordinaryfluid. cause of the skewing of the electron distribution function causedby heat conductionin the solar wind. Four typesof instabilitieswere found which could be drivenby electronheat • Permanentaddress: Department of Physicsand Astronomy, Uni- conduction:electromagnetic ion cyclotron, electrostaticion versity of Iowa, Iowa City, Iowa 52242. cyclotron, magnetoacoustic,and ion acoustic.Any of these Copyright¸ 1977 by the American GeophysicalUnion. modescould in principlebe unstablein the solar wind at 1 AU.
Paper number 6A0740. 632 GURNETT AND ANDERSON: SOLAR WIND PLASMA WAVES 633
Schulz and Eviatar [1972] and Hollweg [1975] have suggested useda short telemetry antenna as an electricfield sensor),only that these heat conduction driven instabilities may signifi- the most intenseplasma wavescould be detected.The princi- cantly lower the heat conductionin the solar wind, with corre- pal results of the Pioneer 8 and 9 plasma wave experiments sponding modifications of the radial temperature gradient. were the occasionaldetection of stronglow-frequency (f -• 400 When streamsof energeticelectrons are ejectedinto the solar Hz) electric field turbulence in the solar wind. Several different wind by a solar flare, electronplasma oscillationsare expected types of turbulencewere identifieddepending on the detailed to be producedby two-stream instabilitiesat frequenciesnear temporal variations and correlationswith other parameters. the electron plasma frequency. Electron plasma oscillations Short-duration 'spikes' in the electric field intensity at •400 have receivedextensive theoretical study becauseof their pre- Hz tended to occur at discontinuitiesin the magneticfield or sumed relationshipto solar radio emissions,particularly type plasma parameters(such as at interplanetaryshocks) and were Ill solar radio bursts [Ginzburgand Zheleznyakov, 1958; Stur- closely associated with the occurrence of sudden com- rock, 1961; Tidman et al., 1966; Smith, 1974]. These waves are mencementsand geomagneticdisturbances at the earth [Siscoe thought to play an important role in controllingthe energy et al., 1971]. Detailed studies of the electric field intensities spectrum and temporal evolution of the energetic 1- to during selectedstorm periodsshowed a significantvariation in 100-keV solar flare electrons[Papadopoulos et al., 1974]. the broadband electricfield intensitywith radial distancefrom Although plasma waves are thought to be of considerable the sun, with maximum broadbandintensities of nearly 50 mV importance in the solar wind, relatively little is known about m -• (equivalent 100-Hz sinewave response)at 0.75 AU [Scarf the typesof waveswhich occurin the solar wind at frequencies et al., 1973]. Longer-duration events, sometimeslasting for above the ion gyrofrequency.Near the earth many measure- one to several days, were also occasionallyobserved in the ments of plasma wave electricand magneticfields have been broadband electric field measurementswith amplitudes of made in the solar wind with sensitive and reliable instruments. typically 1-10 mV m -• in the 400-Hz channel [Siscoe et al., However, in the absence of interplanetary shocks or other 1971]. These long-duration eventswere found to be correlated large-scaledisturbances the proper identificationof the waves with regions of enhanced density, such as occur ahead of intrinsic to the solar wind from measurements near the earth is high-speed streams. Near the earth, electron plasma os- greatly complicatedby the disturbingeffects of particlesarriv- cillations associated with the earth's bow shock were detected ing from the earth's bow shock and magnetosphere.By very by Pioneer 8 and 9 [Scarfet al., 1968]; however, comparable careful investigationof the particle distribution functions to plasma oscillationswere not observedin interplanetary space determinethe origin of the particlesproducing an instability,it away from the effectsof the earth [Scarfet al., 1972]. is possibleto identify waves intrinsic to the solar wind. An As will be shown, the plasma wave electric field measure- exampleof an investigationof this type is given by Gurnenand ments provided by Helios 1 add considerablyto the Pioneer 8 Frank [1975], in which electronplasma oscillationsassociated and 9 resultsby providing measurementsover a wider range of with electronsfrom a solar flare are identified by the Imp 8 frequenciesand radial distancesand with considerably better spdcecraftnear the earth. Because of thedifficulty of separa- sensitivity.For most of the typesof wavesdetected by Helios 1 ting out the effectscaused by the earth's interaction with the the electric field intensitiesare below the sensitivitythreshold solar wind, few measurementsof this type have been made of the Pioneer 8 and 9 instruments,so no direct comparisons near the earth. can be made. Waves with intensitiescomparable to the intense In interplanetaryspace, far away from the disturbingeffects low-frequency spikes and long-duration events observed by of the earth, the only previous measurementsof high-fre- Pioneer 8 and 9 have not yet been detectedby Helios 1. quencyplasma waves (f >> fg+) are from the Pioneer8 and 9 INSTRUMENT DESCRIPTION spacecraft[Scarf et al., 1968; Scarf and Siscoe, 1971], which provided electricfield measurementsin the solar wind at radial The plasmawave and radio astronomyexperiment of Helios distances between about 0.75 and 1.0 AU. Since the Pioneer 8 1 consists of a combination of three instruments: a 16-channel and 9 electricfield experimentswere relativelyinsensitive (they spectrumanalyzer from the University oflowa to provide high
HIGH GAIN I,•__.____._•MEDIUM GAIN ANTE ANTENNA
ELECTRIC FIELD ANTENNA, 32 METERS TIP-TO-TIP Fig. !. The geometryof the electricfield antennaused for the Heliosplasma wave measurements.The spacecraftis spin- stabilizedwith the medium gain telemetryantenna axis oriented perpendicularto the eclipticplane. 634 GURNETTAND ANDERSON: SOLAR WIND PLASMA WAVES
FILTERS LOG COMPRESSORS 16-METER (kHz) PEAK ELECTRIC FIELD AVERAGE ANTENNA (+Y) CHAN15 SHOCK
PREAMPLIFIERS CHN14L1 •
DATA : PROCESSING TO SPACECRAFT UNIT DATA SYSTEM DIFFERENTIAL : AMPLIFIER
16 -METER ELECTRIC FIELD CHAN0 LII ANTENNA (-Y) i
SHOCK TRIGGER SIGNAL ALARM TO SHOCK MEMORY
Fig.2. A blockdiagram of the University of Iowaplasma wave experiment on Helios. The shock alarm is used to initiatea veryrapid sampling rate mode using the spacecraft memory.
timeresolution plasma wave measurements in the frequency processing unit whichprovides analog-to-digital conversion rangefrom 31 Hz to 178kHz, a narrowbandwidth spectrum andformating for thespacecraft data system. A widerange of analyzerwith 168 frequency steps from the University of Min- samplingrates is possible,depending in detailon the space- nesotato providehigh frequency resolution plasma wave mea- craftbit rateand telemetry format. The fastest sampling rate in surementsin the frequencyrange from 10.4 Hz to 209 kHz, real-timeoperation provides a completeset of 16peak and 16 anda 16-channelradiometer from Goddard Space Flight Cen- averagesamples every 1.125 s. Evenfaster sampling rates, up terto providesolar radio wave measurements in the frequency to 14.2samples per second for eachchannel, are possible when rangefrom 26.5 kHz to 3.0 MHz. Only resultsfrom the a rapidread-in to thespacecraft memory is used.The memory Universityof Iowainstrument are presented in this paper. All read-inis triggeredby a 'shockalarm' circuit (see Figure 2) threeinstruments share a singleelectric dipole antenna with whichresponds to thefield strength in a preselectedchannel. A a nominaltip-to-tip lengthof 32 m, whichis extendedoutward shockalarm signal is generatedwhenever the fieldstrength perpendicularto the spacecraft spin axis, as is shown in Figure exceedsthe largestfield strengthdetected since the last mem- 1. The spacecraftspin axis is orientedperpendicular to the ory readout. The memory is organizedto store data both eclipticplane, and the nominalspin rate is 1.0 rps. immediatelybefore and immediatelyafter the shockalarm A blockdiagram of the Universityof Iowa plasmawave signal occurs. The overall function of the shock alarm and instrumentis shownin Figure2. Thisinstrument consists of 16 memoryread-in is to providevery rapid samplingof the continuouslyactive receiver channels with four frequencyelectric field spectrum,as well as other magneticfield and channelsper decade.The bandwidth of thesechannels is rela- plasmadata, around the time of maximum field strength. tively wide, +10% for the channelsfrom 31 Hz to 1.78 kHz and +8% for the channels from 3.11 to 178 kHz. The fre- IN-FLIGHT OPERATIONAND N EAR-EARTHOBSERVATIONS quencypassbands overlap to provideessentially continuous Shortly after the launch of Helios 1 it was discoveredthat coverageof all frequenciesfrom about 20 Hz to 200 kHz. The oneof the two antennaelements which make up the electric signalfrom each filter channel is processedby a logarithmic dipoleantenna did not extendproperly and was electrically compressorwhich provides a voltageoutput proportional to shortedto thespacecraft structure. The resulting antenna con- thelogarithm of theelectric field strength. The dynamic range figurationis thereforean electricmonopole, the spacecraft of the logarithmiccompressor is 100 dB. Two typesof field bodyand associatedbooms acting as a groundplane. Al- strengthmeasurements, called the average and the peak, are thoughthis is not the intendedantenna configuration, the madefor eachchannel. The averagefield strength is the ex- effectsof thisfailure are not particularlyserious. M onopole ponentiallyweighted average (a resistance-capacitancein- antennasof this typehave been successfully used on several tegrator)of the logarithmof the field strengthwith a time previousspacecraft plasma and radio wave experiments constantof 50 msfor all channelsexcept 56 and31 Hz, which [S!]'sh,1965; Benediktot; et at., 1968;Scarf et at., 1968].The havetime constants of 100and 200 ms, respectively. The peak primarydetrimental effects for theHelios 1 experimentare a fieldstrength isobtained from a peakdetector which gives the lossof 6 dB in the electricfield sensitivitybecause of the maximumfield strength since the preceding sample. The con- shorterantenna length and an increasein the noiselevel of the tinuouslyactive channels, with peak detection and overlapping 178-kHz channelby about 25 dB becauseof an interference frequencyresponse, assure that anywave which occurs within signalconducted into the experimentfrom the shortedan- the frequencyand sensitivityrange of the instrumentwill be tenna. It was also thought that the reducedcommon mode detected,even very short burstswith durationsmuch shorter rejectioncaused by the asymmetricalantenna configuration thanthe sampling period. The 16peak and 16 average field wouldresult in largerinterference levels, particularly from the strengthsare sampledessentially simultaneously by a data spacecraftsolar array. However, comparisons with the Helios GURNETTAND ANDERSON:SOLAR WIND PLASMAWAVES 635
MAGNETOSHEATH ELECTRONPLASMA AURORALKILOMETRIC ELECTROSTAT IC OSCIL L AT IONS RADIAT ION NOISE
178.0
100.0
56.2
31.1
17.8
10,0 N • 5.62
z LU 1.78 o w 1.00
.562
.311
.178
.100
.056
.031
U.T.(HRMN) I000 1200 1400 1600 1800 R (Re) 13.I 24.2 35.2 46.I 56.9 LT (HR) 17.8 18.0 18.0 18.1 18.1
HELLOS I, DAY 344, DECEMBER I0, 1974 Fig. 3. The electricfield intensities detected by Helios1 duringthe outboundpass through the bowshock shortly after launch.
2 spacecraft,which was recentlylaunched and for which both The ordinatefor eachchannel is proportionalto the logarithm antennasextended properly, show that the backgroundnoise of the electricfield strength,and the interval from the baseline voltages are essentiallythe same in all except the 178-kHz of one channel to the base line of the next higher channel channel. representsa rangeof electricfield strengthsfrom about 1 •V One disadvantageof a monopoleantenna is that the effec- m -• to 100 mV m -• for/err = 8.0 m. The peak electric field tive length cannot be estimatedas well as for a dipole antenna strengthsare indicatedby dots,and the averagefield strengths becauseof the complicatedgeometry of the spacecraftbody. are indicatedby verticalbars (the solid black areas)extending To calculateelectric field strengthsfrom the Helios 1 data, we upward from the baseline of each channel. assumethat the spacecraftbody actsas a perfectground plane, During the passthrough the bow shockone of the strongest in which casethe effectivelength is one half of the lengthof the and most prominent typesof noisedetected by Helios 1 is a monopoleelement, or./err = 8.0 m. This assumptionis justified high-frequency(50-500 kHz) radio emissiongenerated in the mainly on the groundsthat the spacecraftbody and associated earth'sauroral zones. This noise,which is calledterrestrial (or booms have a rather large capacitanceto the surrounding auroral) kilometric radiation [Gurnett, 1974; Kurth et al., plasma which should maintain the spacecraftpotential essen- 1975],is detectablefor severalweeks after launchand is clearly tially constantwith respectto the local plasma potential. At evidentin the frequencychannels above 50 kHz in Figure 3. higherfrequencies, above the electronplasma frequency where Auroral kilometricradiation provides a usefulsource for veri- the plasma effectsare not important, the effectivelength is fying the effectivelength of the monopoleantenna. During a probably somewhatsmaller, by about 15-20%, becauseof the period of maximum intensity,at a radial distanceof 31.1 RE finite size of the ground plane. Becauseof the large range of from the earth (1315 UT), the power flux of the auroral electricfield strengthsencountered in the solar wind, uncer- kilometric radiation is found to be 2.98 x 10 -•5 W m -2 at 178 tainties of this magnitude are not considered serious. The kHz for an effectivelength of 8.0 m. From a comparisonwith calculatedelectric field strengthsalso rely on the assumption Figure 5 of Gurnett [1974] it is seenthat this power flux is in that the wavelengthsare longer than the antenna length. In goodquantitative agreement With the typicalmaximum in- most cases,specific test• based on spin modulation measure- tensitiesof auroral kilometric radiation detectedby the Imp 6 mentsof the antennapattern and Doppler shift estimatescan spacecraftat comparableradial distancesfrom the earth. be performed to verify this assumption. Becauseof the data gap from 1128 to 1255 UT in Figure 3, To demonstratethe operation of the experimentin an envi- no plasmawave measurementsare availableat the time of the ronment which has already been extensivelyinvestigated, we actual bow shock crossing.Before the bow shock crossing, have analyzedthe electricfield intensitiesobserved as Helios 1 during the interval from about 1045 to 1128 UT, a broad passesthrough the region near the earth's bow shock shortly spectrumof electricfield noise is detected,extending from after launch. The electric field intensities in this region are frequenciesbelow 31 Hz to greaterthan 56.2 kHz. This noiseis shownin Figure 3 for all 16 channels,from 31 Hz to 178 kHz. characteristic of the intense electrostatic turbulence which is 636 GURNETTAND ANDERSON: SOLAR V•'IND PLASraA WAVES
164
165 J•eff(HELLOS I) = 8.0 METERS _ '•eff (HELLOS2) = 16.0 METERS 166 '•eff (IMP 6) : 46.25 METERS lef f (IMP8) = 60.9 METERS _ 'E I(•8 Jeff (PIONEER8,9), = I METER PIONEER8,9 '{eff (OGO5) = 0.6 METER • 169- o •//.-PIONEER 8
z - OGO 5
• 13
-. 1•14 HELLOSI • IMP 6 . HELIOS 2 -
"• I(• -
m6m6 _ IMP 8
•6• _
mOm9 ...... 1 ...... I , ,,,,,,,I , ...... I , i i I iiii, m 10 102 103 104 105 106 FREQUENCY, hz Fig.4. The thresholdelectric field sensitivity of Heliosi andHelios 2 comparedto severalother plasma wave experimentswhich have made electric field measurements in the solar wind. The rapidincrease in thenoise level at low frequenciesis caused by interferencefrom the spacecraft solar array. commonly observedthroughout the magnetosheathdown- coupledto the antennathrough the plasmasheath surround- streamof the bow shock[Rodriguez and Gurnett, 1975]. After ingthe spacecraft and produce strong interference over a very the bow shockcrossing, from about 1255to 1425 UT and in broadrange of frequencies.This sametype of interferencealso several intervals thereafter, an intense narrow band electric occursat low frequenciesin the Imp 6 and 8 electricfield field emission is evident in the 10.0- and 17.8-kHz channels. measurements but is more intense on Helios because of the This narrow band emissionconsists of electronplasma os- higherspin rate (I.0 rps) in comparisonto the spinrates of cillationsof thetype discussed by Scarfet al. [1971],excited by Imp 6 and 8 (0.083and 0.4 rps).The differencein sensitivity electronsstreaming into the solar wind from the bow shock. betweenHelios I and Helios2 is causedby the factorof 2 For an effectivelength of 8.0 m the electricfield strengthof differencein the antennalengths. The improvedsensitivity of theseplasma oscillations is foundto rangefrom about 100tzV the Imp 6 and 8 experimentsis mainlydue to the longer m-• to I mV m-•, with occasionalpeaks as large as 3 mV m-•. antennasused on thesespacecraft. Since comparisons are to be Thesefield strengthsare in goodquantitative agreement with made later with the Pioneer 8 and 9 electric field measure- similarmeasurements obtained from the Imp 6 and 8 space- ments,the noiselevels of the Pioneer8 and9 experimentsare craft [seeGurnett and Frank, 1975]. alsoshown in Figure4, basedon the instrumentparameters In Figure4 we showthe in-flight noiselevel of the Helios 1 givenby Scarfet al. [1968]and Scarfand Siscoe [1971]. The plasmawave experiment(and also Helios2) and compare noise levels of the Pioneer experimentsassume an ef- thesenoise levels with severalother similar experiments which fectivelength of 1.0m, which,as was pointed out by Scarf et al. have made electric field measurements in the solar wind. In [1968],is consideredsomewhat uncertain because of possible eachcase the noiselevels have been calculated by usingthe errorsintroduced by local sheatheffects. Also shownfor com- effectivelength determined from strictlygeometrical consid- parisonare the noiselevels of theOgo 5 electricfield experi- erations,without regard for errorscaused by sheatheffects or ment (F. Scarf, personalcommunication, 1976), which has wavelengthsshorter than the antenna.The rapid increasein madeelectric field measurements in the solar wind [Scarf'et al., the noiselevel of the Heliosexperiments at low frequencies, 1972].The noiselevels of the Ogo 5 7.3- and 14.0-kHzchan- belowabout 1.0 kHz, iscaused by interferencefrom the space- nelsare not shownbecause of the presenceof strongspace- craft solar array. Becauseof the spacecra•'trotation, large craft-generatedinterference in these channels. voltagetransients, as largeas 70 V, occuras the solarpanels A highlyunusual and unexpectedinterference problem was rotateinto andout of the sunlight.These voltage transients are encounteredon HeliosI whenthe S bandtelemetry signal was GURNETT AND ANDERSON: SOLAR WIND PLAS•IA WAVES 637
178.0
I00.0
56.2
31.1
17.8
I0.0
5.62
3.11
1.78
1.00
.562
.311
.178
.100 I I .056
.031
U.T.(HRMN) 1400 1600 1800 2000 2200
R (A.U.) .984 .984 .984 .984 .984
HELLOS I, DAY 346, DECEMBER 12, 1974 Fig. 5. An exampleof one of the many 'quiet' periodsobserved in the Helios 1 plasmawave data. switchedfrom the medium to high gain telemetry antenna bauer, personal communication, 1976) and a charging of the about 10 days•after launch. When the telemetry transmitter electricantenna elementto -30 to -40 V with respectto the was switchedto the high gain antenna, a very intensebroad- spacecraftstructure. Subsequentinvestigations of these effects band interference occurred, with an increase in noise level of have led to the conclusion that the interference and electron nearly 50 dB in some channels. This interference was also heating are causedby a multipacting breakdown of the high accompaniedby a number of other dramatic effectsindicating gain antenna feed. This breakdown is essentiallya transit time a major disturbancein the plasmaaround the spacecraft,the resonancefor electronsaccelerated across the gap in the an- most notable being a large increasein the E -• 100 eV electron tenna feed and occurswhen the secondaryemission, coefficient flux detectedby the solar wind plasma experiment(H. Rosen- is sufficientlylarge to causea rapid buildup of electrondensity in the gap [Hatch and Williams, 1954, 1958]. Since the reso-
-4 nance condition is highly sensitiveto the gap spacing, the I0 * [*'""1 ' '*'***'I ' '• •'"'1 ' *,' antenna feed on Helios 2 was redesignedwith a larger gap HELIOS I spacingto eliminate this problem. As a result of this modifica- SELECTED QUIET TIMES tion, no similar interferenceeffects have yet been observedon • DAY 346, 1600 UT - H elios 2. -+- DAY 3l, 0500 UT Because of the multipacting problem with the high gain _ • DAY 53, 0130 UT antenna, useful plasma wave data could initially be obtained - 4- - DAY 81, 0530 UT _ only by usingthe medium gain antenna. Since operation with the medium gain antennaresulted in a larger bit rate penaltyto _ the other experiments,this mode of operation was quite lim- ited, usually consistingof only a few 8-hour passesper week.
_
R '" 0.98 A.U. Fortunately, shortly after the first perihelion passagethe mul- tipacting interferencecompletely disappeared,and it has not '•••'•dr"" 0.76 A.U. - reappearedagain except for a short period during the second t%.•._•/// R-•0.52 A.U. _ perihelion passage.The disappearanceof the multipacting breakdown is not understoodin detail but is generallythought to be causedby modificationsof the surfaceproperties (sec- ondary electronemission coefficient) of the high gain antenna feeddue to the high temperaturesencountered near perihelion. _ Since someof the resultswhich will be presentedinvolve the detection of relatively rare plasma wave phenomena, it is im- I I tillIll I i Ifilill I I Illill] f J Iflfii[ I I 102 ,03 ,04 ,05 106 portant to comment on the total quantity of data which has FREQUENCY (Hz) been analyzed. For this study all of the real-time data obtained from Helios 1 during the first 16 months of in-flight operation, Fig. 6. The Helios 1 electric field noise levels at several selected consistingof approximately two and one-half orbits around quiet times betweenlaunch and first perihelion. These spectrumspro- vide upperlimits to the electricfield intensitiesin the solarwind during the sun, have been processedand analyzed in detail. In addi- quiet conditions.The enhancednoise levels near perihelion are be- tion, a survey has been made of the data currently available lieved to be causedby spacecraft-generatedinterference. from Helios 2 duringthe first 2.5 monthsof in-flight operation. 638 GURNETT AND ANDERSON:SOLAR WIND PLAStaAWAVES
I 1 I I I 178.0 i I00.0
56.2
31.1 , , 17.8
I0.0 ENHANCED NOISE i LEVELS 5.62
5.11
1.78
1.00
.562
.511 17-..... 7- :•-...... :'•"'...... - ' ' - .... I i- .178
.100 _1, _ .I r II I .056
--
U.T.(HRMN) 0000 0200 0400 0600 0800 R (A.U.) .764 .765 .762 .761 .761
HELLOS I, DAY 31, JANUARY 31, 1975 Fig. ?. Another exampleoF a quiet period in which slightlyenhanced noise icyels arc clearlyevident in the 562-Hz to 3.1 l- kHz channels.
Becauseof the multipacting interferenceearly in the flight of ingradial distance, reaching maximum intensity near peri- Helios I and the reduced telemetry coverageafter the first helion. Although the origin of the increasednoise level near perihelion the overall fraction of the time for which usable perihelion is not known, it is reasonablycertain that this noise plasmawave data are availableis only about 30%. This limita- is not causedby plasmawaves. The reasonfor this conclusion tion in the available data significantlyreduces the detection is that the noiseintensity displays a pronouncedvariation with rate for relativelyinfrequent events such as type III solar radio the spacecraftrotation, with a singlemaximum when the an- bursts and interplanetary shocks. No interplanetary shocks tenna is pointing away from the sun. To be interpretedas an have yet been identified in the Helios plasma wave data, al- electric field, the spin modulation must have two maximums though suchevents may be identifiedlater when more detailed per rotation. The most likely explanation is that the noise is comparisonsare made with the plasma and magnetic field caused by some type of spacecraft-generatedinterference data. which increasesin intensitynear perihelion.Numerous param-
QUIESCENTCONDITIONS IN THE SOLARWIND A characteristicfeature of the Helios 1 plasmawave data is
the almost completeabsence of detectableelectric fields for a HELIOS I
substantial fraction of the time, 50% or more, and for long _ DAY 31, JANUARY 31, 1975 periods, sometimeslasting several days or longer. This is in --o--- o51o UT striking contrast to measurementsin the solar wind near the o 10 ----A-- 0530UT earth, where moderately intenseplasma waves, producedby particlesarriving from the bow shockand magnetosphere,arc presenta large fraction of the time. The electricfield intensities z I during one such quiescentperiod arc shown in Figure 5. Ex- • -I?_ i• R•0.76 A.U.- cept for somevery low intensitywaves between about 562 Hz <• I0 and 3.11 kHz, which will be discussedshortly, all of the electric !,.• ENHANCEDNOISE LEVEL n 13 field channels arc near their minimum noise level, as deter-
mined by the solar array noise at frequenciesbelow about 1.0 ..:.:.:....,. ,;- ic•14 kHz and by the prcamplificr noise at frequenciesabove 1.0 :•'":'"•'"':•';'•!•/":•'"'-/.'."'•;•iii:x.:..•:.,/QUIESCE'"":'"ß...::...::•..:•..': '.'f::.,, P NT NOISELEVEL kHz. Comparable quiescentperiods arc a common feature of the Helios data at all radial distances from 1.0 to 0.3 AU. One • •o-•5 notable effect, which was observed on both the first and the secondperihelion passage,is a gradual increasein the back- ground noise level with decreasingradial distance from the io2 ,o• ,g ,05 io•
sun. This radial dependenceis illustrated in Figure 6, which FREQUENCY, Hz shows electric field spectrumsselected at four representative Fig. 8. An electric field spectrumduring one of the periods of radial distancesduring quiet times. The quiescentnoise levels enhancednoise level evident in Figure 7. The broadband electricfield arc sccn to remain essentiallyconstant from 0.98 to 0.76 AU strengthof the waveswhich producethese enhancements is only about but then to increasegradually by about 10-15 dB with decreas- 10 #V m -• GURNETT AND ANDERSON:SOLAR WIND PLASMAWAVES 639
178.0 ß . I00.0 I I I I < f< f;- I I I • 56.2
31 .I
17.8
I0.0
5.62
3.11
1.78
1.00
.562
.311
.178 ß
.100
.056
.031
U.T. (HRMN) 0800 1000 1200 1400 1600
R (A.U.) .984 .983 .983 .983 .983
HELLOS I, DAY 348, DECEMBER 14, 1974 Fig. 9. A more disturbedperiod during which moderatelyintense electric fields arc detectedover a broad rangeof frequenciesbetween electron and ion plasmafrequencies fp- and fp+. The plasmafrequencies arc computedfrom the plasmadensities measured simultaneously by the solarwind plasmaexperiment on Helios.
eters of the spacecraftpower system,which could possibly low-intensity enhancementsare particularly evident is shown affect the interferencelevels at high frequencies,vary consid- in Figure 7. The electric field spectrum during one of the erably with radial distance from the sun. periods of enhancednoise intensity, at 0530 UT, is shown in During quiescentperiods such as the one in Figure 5 there is Figure 8 and is compared with the spectrum at a time of often a slight irregular enhancementof the noise level in the minimum noise level, at 0510 UT. Although it is very difficult 562-Hz to 3.11-kHz channels,suggesting that somevery weak to be certain that theseperiods of enhancedintensity are not plasma waves may be present in this frequencyrange. An causedby spacecraftinterference, most of the evidencesug- exampleof the otherwisequiescent interval during which these gests that these enhancementsare caused by plasma waves.
I i I I I I 178,0 I I II I II I I00.0 NOISE 56,2
17,8
I0.0
5.62
3,11
1.78
1.00
.562
.311
.178
.100
.056
.031
U.T.(HRMN) 0900 1000 I i00 1200 1300
R (A.U.) .479 .478 .478 .477 .476
HELLOS I, DAY 56, FEBRUARY 25, 1975 _
Fig. 10. Another exampleof the fv+ < f < fv- noise,similar to Figure 9 but closerto the sun. 640 GURNETTAND ANDERSON:SOLAR WIND PLASMAWAVES
The fact that the enhancementsappear in only a limited num- most commonlyoccurring type of plasmawave detectedby ber of channelsindicates that the noiseis distinctlydifferent Helios 1. When this noiseoccurs, it is usuallyobserved with i•romthe multipacting interference and from the interference sporadic intensities for periods rangingfrom a few hours to encounterednear perihelion. Also, this same noise is detected severaldays. Such periods of enhancedf,+ < f < f,- noise even more clearly by Helios 2, which has better electric field intensitiesusually occur a fewtimes per month.It seemslikely sensitivitythan Helios 1 and for which no multipactinginter- that this noise is associatedwith a corotatingfeature of the ference has been detected. solar wind which recursfor every solar rotation; however,at If the very weak irregular enhancementsin the 562-Hz to the presenttime the availabledata havenot beenanalyzed in 3.11-kHz channelsare due to plasmawaves, then the electric enoughdepth to establisha definite association.The noisehas field intensity of these waves is very small, the broadband beendetected at essentiallyall radial distancessurveyed by electricfield strengthbeing only about 10 uV m-•. Although Helios 1. thesewaves are very weak, they are apparentlypresent a large It is evidentin Figures9 and 10 that the peak intensitiesof fraction of the time, possible50% or more, becausethey are thefv+ < f < fv- noiseare largerthan the averageintensities almost alwaysdetectable, even during quiescentperiods such by a factorof approximately20-30 dB. This largeratio of the as appearin Figures5 and 7. Becauseof the difficultyof clearly peak to averagefield strengthsindicates that the noiseis very resolvingthese very low intensities,further investigationof impulsive,consisting of many brief burstsoccurring on time thesewaves will be delayeduntil more data are availablefrom scalesthat are short in comparisonto the averaginginterval H•lios2, whichhas better sensitivity than Helios 1. (which is about 1 min for Figure 9 and 30 s for Figure 10). These impulsivetemporal variationsclearly distinguishthis WAVES BETWEEN THE ELECTRON AND ION PLASMA noise from the quiescent noise enhancements,as shown in FREQUENCIES Figure 7, whi-chhave a more nearly constantamplitude. To Occasionally,intervals occur in the Helios 1 plasmawave illustratethe temporalstructure of the fv+ < f < fv- noise, data in which moderatelyintense electric field noiseis detected Figure 11 showsa greatly expandedtime scale,with 0.28-s in the frequencyrange from about 1 to 10 kHz. Two examples resolution, from a shock mode memory read-in which oc- of this noiseare shownin Figures9 and 10, at radial distances curredat about 1500 UT during the event in Figure 9. These of 0.98 and 0.47 A U, respectively.As will be shown, the hightime resolutionmeasurements show that the f•,+ < f < frequencyrange of this noiseconsistently lies between the local f,- noiseconsists of manyshort bursts lasting for onlya few electronand ion plasmafrequencies fo- andfo+. We therefore tenthsof a second.Even on this greatlyexpanded time scale refer to this noiseas f•,+ < f < f•,- noise.This terminologyis the temporal structure of the individual bursts is sometimes adoptedfrom a strictly observationalviewpoint. It shouldbe not clearly resolved.Often when an individual burst occurs,it noted that if sizableDoppler shiftsoccur, then the actualwave tends to occur simultaneously(within the time resolution frequenciesin the restframe of the plasmamay not, in fact, be available)across a broad rangeof frequencies. directly related to either of thesefrequencies. Typicalpeak and averagefrequency spectrums of thef,+ < Except for the extremely weak noise enhancementsdis- f < fo- noiseare shown in Figure12 at timesof relativelyhigh cussedin the previoussection, the f,+ < f < fp-' noiseis the intensityselected from the eventsin Figures 9 and 10. The
ic•4 f•' f < f•' NOISE 5.62kHz(+8%) 1(75 E I(j 6 [- 1(73 o .3.11kHz (+8%) i• 4 1(55 i• 6
1.78 kHz (_+8%) (54 (55 (56
1.00 kHz (_+8%) I(j 4 1(75 i(j 6
+0 +10 +20 +30 +40 +50 +60 +70 +80 +90 +100
TIME (SEC) HELIOS I, DAY 348, DEC. 14, 1974 START TIME 1500:40.53 U.T., R = 0.98 A.U. Fig. 11. High time resolutionmeasurements of the fp+ < f < f,- noiseshowing that the noiseconsists of manyshort burstslasting only a few tenthsof a second. GURNETT AND ANDERSON: SOLAR WIND PLASMA WAVES 641
plane of rotation of the electric antenna can be determined q•P -I f;< f< f•' from the spin modulation of the electric field intensities.Be- cause the noise bursts usually occur on a time scale com- '•,'• ...:....•p.Vm NOISE_ parable to the time for one rotation (1 s), a statisticalanalysis •i,,•,. f'i DAY348,DEC. 14, ,974- must be performed over many events,usually for severalhun- v 1429- 1432 UT - R = 0.98 A.U. - dred spacecraftrotations, to determinean averagespin modu- PEAK lation pattern. This procedure is simplified on Helios by the '•{• •- AVERAGE - fact that the average (50-ms time constant) field intensity measurementsare always sampledin 16 equally spacedangu- _ -- lar sectors. However, becauseof the very low average field LEVEL strengthof the fv+ < f < fv- noiseand the very impulsive • 235•v• I intensity variations, simple averaging of the intensity as a function of the sector number does not give a good indication of the spin modulation.The approachused is to computethe frequency of occurrence of field intensities above a given threshold as a function of the threshold and the sector num- ber. The spin modulation is then determined from the con- •. R= 0.47 A.U. tours of constant frequencyof occurrenceas a function of the sector number. This method of analysis is less sensitivethan simple averaging to errors introduced by a few exceptionally intense bursts. An exampleof this spinmodulation analysis for a fv + < f < i• i6 io• io• io• io5 io• fv- noiseevent detected at about 0.98 A U is shownin Figure 13. The accumulationinterval for this frequencyof occurrence FREQUENCY (Hz) determination is I hour. The antenna orientation angle½se A is Fig. 12. Selectedspectrums of the fv + < f < fv- noiseshowing the the spacecraft-centeredsolar ecliptic longitude of the + Y an- shift toward higher frequencieswith decreasingradial distancefrom tenna axis, including a correction of 18ø to account for the the sun. Also note the large ratio of the peak to averageelectric field strength, indicating very impulsive temporal variations. phase shift causedby the 50-ms receiver time constant. The solid and dashed lines give the electric field intensitiesat fre- large ratio betweenpeak and averageelectric field intensitiesis quenciesof occurrenceof 5 and 10%, respectively.Even with a clearlyevident in thesespectrums. Also shownare the electron l-hour accumulation interval, considerable scatter is still evi- and ion plasma frequenciesas computed from the plasma dent in the frequency of occurrencecontours. Nevertheless,a densitiesgiven by the Helios I solar wind plasmaexperiment: consistentmodulation pattern is clearly evident with the max- fv- is computed from the total density of all ions, charge imum intensities at about ½seA •- -30 ø and + 150ø. During neutralitybeing assumed, and fv+ - (melmv)•/'fv-, whereme and rnvare the electronand proton masses.The spectrumsare seen to consistof a single broad maximum between the elec- tron and ion plasma frequencies.By c9mparingthe spectrums t,,, f;< f
ELECTRON TYPE TIT PLASMA OSCILLATIONS RADIO BURST
178.0 II I I I I I I I II I II III -- I00.0 56.2 / / ...... $1 .I
17,8
I0.0 ...... -: ...... •: ...... _ _-,_..• .-/x,,,•--_-_ _ ._._ _.,, ..... • ...... •-_ . .. •_. 5.62
3.11
1.78
1.00
,562
.311
,178
,100
.056
.051
U.T. (HRMN) 1600 1800 2000 2::)00 0000 R (A. U.) .860 ,859 .858 ,858 ,857
HELLOS I, DAY 208, JULY 27, 1975 Fig. 14. Examplesof' electron plasma oscillations at f -• fp- detectedduring a periodof' enhancedsolar flare activity.
this interval the averagespacecraft-centered solar ecliptic lon- isolated interval lasting from a few minutes to about half an gitude and latitude of the magnetic field are approximately hour. The plasmaoscillation events which occur on day 208 (in ½sE• = -40 ø + 10ø and 0sE• = 0 ø + 20ø , respectively(F. Figure 14) and also continueto day 209 (not shown) represent Neubauer, personalcommunication, 1976). Becausethe mag- an exceptionallyactive period with respectto the occurrenceof netic field is oriented close to the spin plane of the electric electronplasma oscillations. This period correspondsto one of antenna, this event provides a good case for determining the the few periodsof substantialsolar flare activity detectedby, electric field orientation relative to the solar wind magnetic Helios 1 during the first year of in-flight operation. On the field direction. For comparison the magnetic field directions basis of the previous association of electron plasma os- +B and -B, projected into the plane of rotation of the an- cillations with low-energy (few keV) electrons ejected by a tenna, are shown in Figure 13. The electric field intensities solar flare [Gurnett and Frank, 1975] it seemslikely that the clearly have a symmetrical distribution with respect to the electron plasma oscillationsdetected during this period are solar wind magnetic field direction, with the maximum in- associatedwith electronsfrom this flare activity. Preliminary tensity when the antenna axis is aligned parallel to the mag- comparisonswith the data from the energetic particle and netic field. The sharp nulls in the electric field intensity, by plasma experimentson Helios 1 (E. Keppler and H. Rosen- about a factorof 10, indicatethat the electricfield of thefp+ < bauer, personal communication, 1976) confirm that low-en- f < f,- noiseis very closelyaligned along the directionof the ergy (1-20 keV) electronsof solar origin are presentduring solar wind magnetic field, probably to within lessthan 15ø . this period. The type III radio burst associatedwith one of theseflares is clearly evident in the 100-kHz channelof Figure ELECTRON PLASMA OSCILLATIONS 14, starting at about 2243 UT. This type III radio burst and Occasionally, narrow band electric field emissionsare de- others detected by Helios are discussedin the next section. tected in the frequencyrange from about 20 to 100 kHz which The electricfield spectrumof the electronplasma oscillation are almost certainly causedby electron plasma oscillationsat detectedon day 208, at a time of relatively high intensity,is the local electron plasma frequencyf,-. An example of a shown in the top panel of Figure 15. Also shown are the period during which electron plasma oscillationsare detected spectrums for two other examples of electron plasma os- by Helios 1 is shown in Figure 14. The identificationof these cillations detected at closer radial distances to the sun. For waves as electron plasma oscillations is based on the close comparison and confirmation of the identification of these similarity of the frequency spectrums and intensitiesto the waves as electron plasma oscillations, the local electron electron plasma oscillations observed upstream of the bow plasma frequenciesdetermined from the solar wind plasma shock, as seen in Figure 3. In contrast to the plasma os- experiment are also given in Figure 15. The closecorrespon- cillations detected near the earth, which are associated with dencebetween the emissionfrequency and the local electron electrons from the bow shock, the plasma oscillationsshown plasma frequencyis clearly evident. The spectrumsin Figure in Figure 14 must be generatedby particles of solar origin or 15 also show the expected increasein the electron plasma by nonthermal particle distributionsin the solar wind. frequencyas the solar wind densityincreases closer to the sun. Typically, only one or two brief periods occur per month in This tendency,for increasingfrequencies closer to the sun, is a which electron plasma oscillationsare detectedby Helios 1. In general characteristicof all of the electron plasma oscillations most casesthe plasma oscillationsare observedduring a single detectedby Helios 1. GURNETTAND ANDERSON:SOLAR WIND PLASMAWAVES 643
I f,g• JUlY27, ,975 iC•12I_ • 1918UT - R Au_
iff ¸
io-li _ 16,0/zVm -I -
ici2 _ AUG. 17, 1975 - 1411UT iC•I3 _ R = 0.68 A.U. _
i(•i4 _
iC15 ELECTRON PLASMA OSCILLATIONS
ic•II _ fl•i•.-.--780/a. Vm-I _ /:...'..':,,•MAR, 19, 1975 f':-:':..'..':....:..'• 1652 UT iC•12 - R : 0.52 A.U. -
iCI5 _ _
! ...... I ', ,, ..... I , ,,,,,,, io2 i05 i06
FREQUENCY, Hz Fig. 15. Selected spectrumsof electron plasma oscillations showing the tendency for increasing frequency with decreasingradial distancefrom the sun. The frequencyof the plasmaoscillations is in good agreementwith the electron plasmafrequency f.. determinedfrom the solar wind plasmaexperiment.
As can be seen from Figure 14, the peak electric field in- larger field strengthsdo occur, as will be discussedin the next tensitiesof the electron plasma oscillationsare considerably section. Electron plasma oscillationswith field strengthsex- larger than the averagefield intensities,which remain near the ceeding I mV m -• are extremely rare. Becauseof the limited receivernoise level during the entire event. The large ratio of number of events available and the large variation in the the peak to averagefield strengthsindicates that the electron intensity from event to event it is not known for certain plasmaoscillations, like the fj,+ < f < fj,- noise,consist of whether the electric field strength has a significant variation many brief burstswith durationsthat are short in comparison with radial distance from the sun. Some of the most intense to the averaginginterval. The temporal structureof a typical events have been observed at radial distances near the sun, as burst of electron plasma oscillationsis illustrated in Figure 16, is shown in Figure 15. which shows a greatly expanded time scale for a series of As is true for the fo+ < f < fo- noise,the electricfield plasma oscillation burstsdetected during a shock'modemem- orientation of electron plasma oscillationscan be determined ory read-in. The rise and decay times of the individual bursts from spin modulation measurements.Because long-duration are clearly resolvedin thesedata and are typically in the range bursts sometimesoccur with nearly constant (saturated) in- from about 0.25 to 1.0 s. In this case the durations of the bursts tensities,the spin modulation pattern can be determined very are only a few seconds.Occasionally, much longer burstsare easily in these cases. Figure 17 shows the spin modulation observedlasting as long as several minutes. When multiple observedduring a singlerotation for an unusuallylong (2 min) burstsoccur, a moderatelywell definedupper limit to the field burst of plasma oscillationsdetected by Helios 1 near perihe- strengthis often evident (as is true for the first three burstsin lion (R = 0.332 AU). During this event the solar wind mag- Figure 16) and is an indication that some nonlinear effect netic field is oriented near the spin plane of the antenna;4>sE • saturatesthe instability. The saturation level is usually in the =-15 ø + 3ø , and 0sE• = 19ø + 1ø (F. Neubauer, personal range from about 100 to 500 #V m-•, although occasionally, communication, 1976) in a direction favorable for determining 644 GURNETTAND ANDERSON:SOLAR WIND PLASMAWAVES
IC•3- ' ' ' ' I ' ' ' ' "'l ' ' " ' I " ' ' ' ] ' i i i • ELECTRON PLASMA I00 kHz(_+8%) iC•4 OSClLLAT IONS T -5 E
• I o i0-• / I 56.2 kHz (+ 8%) :Z ICF4 0 -5 z
• 10-$ 31.1 kHz (+8%)
_
o i• 5 _
o I "'-J I O_3_ 17.8kHz (+8%) -
•cF5_ -
0 +5 +10 +15 +20 +25
TIME (SEC)
HELIOS I, DAY 279, OCT. 6, 1975 START TIME 0414:50.75 UT, R =0.432 A.U.
Fig. 16. High time resolution measurementsshowing several bursts of electronplasma oscillationswith durationsof only a Fewseconds. Note the nearly constant saturation amplitude of the first three bursts. the electricfield orientation relativeto the magneticfield direc- trons as they propagateoutward from the sun. If the type III tion. As can be seen from Figure 17, the maximum electric radio emissionis causedby the conversionof electronplasma field intensityoccurs when the antenna axis is parallel to the oscillationsto electromagneticradiation at 2f,-, as is widely magnetic field. The sharp nulls when the antenna axis is per- believed,then the onsetof the plasma oscillationsat the local pendicular to the magneticfield indicate that the electricfield electronplasma frequency fp- shouldoccur essentially simulta- of the electronplasma oscillations is very closelyaligned along neouslywith the onsetof the type III radio emissionat 2fp- the direction of the solar wind magnetic field, probably to and should last for an interval comparableto the duration of within less than 20 ø.
TYPE Ill SOLAR RADIO BURSTS One of our primary objectivesin studying type III solar ELECTRONPLASMA '• SUN radio bursts is to determine whether electron plasma os- OSCILLATIONS A cillations are involved in the generation of these radio emis- f --IOO kHz •SE DAY 270, SEPT. 27, 1975 sions,as has been suggestedfor many years.In all of the data 0501'I0 UT ANTENNA "LIOS currently available a total of 18 type III solar radio burstshave R -- 0.3:32 A.U. been detectedby Helios I and 2. Of these 18 type III radio bursts, 4 show clear evidence of electron plasma oscillations which could possiblybe associatedwith the radio emission. i-
Two of these events, shown in Figures 18 and 19, occur in z associationwith the solar flare activity on days 208 and 209 (July 27 and 28), 1975, discussedin the previoussection, and the secondtwo events, shown in Figures 20 and 21, occur in associationwith the solar flare activity on day 92 (April 1), 1976. Before detailed discussion of these events it is useful to considerthe ideal 'signature'to be expectedif electronplasma • •64 oscillationsare involved in the generation of type III radio emissions.As is well known, type III radio burstsare produced by electronsejected into the solar wind by a solar flare. At low frequencies(•<1 MHz), considerableevidence exists showing that the type III radio emissionsoccur at the harmonic2f,- of -180 -90 0 +90 +180 the local electron plasma frequency [Fainberg et al., 1972; Haddock and Alvafez, 1973; Kaiser, 1975]. The characteristic •sAE, ANTENNA-SUN ANGLE frequency-timedispersion of a type III radio burst, decreasing Fig. 17. Spin modulation measurementsof electron plasma os- in frequencywith increasingtime, is causedby the decreasin.g cillations showing that the electric field of these waves is oriented electron plasma frequencyencountered by the solar flare dec- nearly parallel to the solar wind magneticfield. GURNETT AND ANDERSON: SOLAR WIND PLASMA WAVES 645
I00 kHz(+8%) TYPE'ITF RADIO BURST
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/ 0.3_17.8 kHz(+8%) /'"•
•-6 •i•idMmhii•iih["•iiii•iiiii•"ii"mi•M•"hii•M•iih"•"iii"hii•iiiiiii"i"h•hh"•iihii•M•iihiiiiii•iiiI,Ilil,II,,,,,,,,,,% U.T. (HRMN) 2100 2200 2300 0000 R (A.U.) .858 .858 .858 .857
HELLOS I, DAY 208, JULY 27, 1975 Fig. 18. A type III radio burst which is possiblyassociated with the electronplasma oscillationsdetected in the 17.8- kHz channelfrom about 2257 to 2335 UT. In this casethe onsetof the electronplasma oscillations at fp- -• 20 kHz is in good agreementwith the estimatedonset time of the type III radio emission(dashed line) at 2fp- •- 40 kHz. the type III burst at this frequency. Electron plasma os- onsettimes are indicatedby the smoothdashed curve in Figure cillations are not expected to be observed for all type III 18. Startingat about 2257 UT, a pronouncedburst of electron bursts,since the spacecraftmust be within the sourceregion to plasmaoscillations occurs in the 17.8-kHz channel,lasting for detect the locally generated plasma oscillations. about half an hour. At the time of maximum intensity the peak From the standpoint of the expectedplasma wave signature electricfield strengthof theseplasma oscillationsis about 223 the event on day 208 in Figure 18 representsan excellent #V m -•. Sincethe plasma oscillationsare detectedin the 31.1- exampleof the expectedrelationship between electron plasma kHz channel but not in the 10.0-kHz channel, the electron oscillationsand typeIII radioemission. The onsettime of the plasma frequency must be slightly greater than 17.8 kHz, X ray flare associatedwith this event is 2232 UT, as deter- probably about 20 kHz. From the dashedcurve in Figure 18 mined from the soft X ray detector on Helios 1 (J. Trainor, the onsettime of the type III radio emissionat 2f,- -• 40 kHz personal communication, 1976). The type III radio emission is estimated to be about 2254 UT, coincident within a few producedby the electronsfrom this flare is first detectedin the minutes with the onset time of the electron plasma oscillations 100-kHz channel at 2243 UT and subsequentlyin the 56.2- and at f,- -• 20 kHz. By usingthe 56.2-kHzchannel as a guidethe 31.1-kHz channels at 2251 and 2313 UT. The approximate durationof the typeIII radioemission at 2f,- -• 40 kHz is esti-
iC•3 I I00 kHz(+8%) I TYPE 'ITr RADIO BURST
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l•34 •'X 17'8kHz(•8ø/ø) F •5 • v I I I I I I I I U.T.(HRMN) OI O0 0200 0300 0400 0500 R (A.U.) .857 .857 .857 .8• .856
HELLOS I, DAY 209, JULY 28, 1975 Fig. 19. A typeIII radioburst and associated electron plasma oscillations occurring a fewhours after the event in Figure 18, In this casethe onset time of the plasma oscillationsis not in good agreement with the onset time of the radio emissionat 2fp-. However,the plasmaoscillations do occurduring the periodwhen a significantlevel of radio emissionis beingdetected at 2fp-. 646 GURNETTAND ANDERSON:SOLAR WIND PLASMAWAVES
mated to be about 35 min, which is comparableto the dura- Figure 20, another type III radio burst occurred in which tion of the electronplasma oscillations. These comparisons are correspondinglyintense electron plasma oscillationsare ob- all in excellentagreement with the signatureexpected for elec- served.This eventis shownin Figure21. The electronplasma tron plasmaoscillations associated with a typeIII radio burst. oscillations in this case are of much shorter duration than About 3.5 hours after the event in Figure 18, anothertype thosein the eventin Figure 20, consistingof only a few brief III radio burst occurredwith electronplasma oscillations. This bursts,but are again unusuallyintense. The maximum electric event is shownin Figure 19. The electronplasma oscillations field intensityof the plasmaoscillations in this caseis 2.35 mV in this case consistof numeroussporadic bursts lasting for about 1 hour, from 0252 to 0352 UT, with a maximum electric Consideredtogether, these four eventsprovide substantial field strengthof about 187 #V m-1. In comparisonwith the evidenceconfirming that electronplasma oscillations are asso- event in Figure 18 this event doesnot fit the ideal signatureas ciatedwith type III radio bursts.The eventsin Figures20 and well, sincethe onsettime of the plasmaoscillations at f•,- -• 21, with plasmaoscillation intensities much larger than those 17.8 kHz does not agree with the onset time of the radio in any of the other availabledata, provide particularlystrong emissionat 2f•,- -• 35.6 kHz. Nevertheless,the plasmaoscil- evidenceof this association.The caseis not quiteas strongfor lations do occur within the interval where substantial radio the more moderate intensity (•200 #V m-1) plasma os- emissionis being detectedat 2f•,-. cillationsobserved during the eventsin Figures18 and 19.One By far the best example found to date of electron plasma obviousinterpretational difficulty is that comparablemoder- oscillationsassociated with a type III radio burst is shownin ate intensityelectron plasma oscillations often occurfor which Figure 20. This event, which occurredon day 92 (April 1), no type IlI radio emissioncan be detected.Several such cases 1976, took place during a period of exceptionalsolar flare are evidentin Figure 14. Even thoughthe absenceof a one-to- activityfrom day 83 to day 93, 1976.During this perioda total one correspondenceraises a questionof interpretation,it does of 11 separatetype IIl radio burstswere detectedby Helios 1 not conclusivelyprove that such moderateintensity plasma and 2. The type III event in Figure 20 is outstandingin that a oscillationscannot in somecases be associatedwith the type very intense burst of plasma oscillations,with a maximum III radio emission. For example, in caseswhere no radio electric field strength of 5.26 mV m -1, occurred during the emissionis detected, it is possible that these plasma os- radio burst. These electron plasma oscillationsare the most cillationsare essentiallya localizedinstability occurring in a intense ever detected by Helios and are much more intense much smallervolume than the plasmaoscillations associated than the typical electronplasma oscillation events discussed in with a type III radio burst. Direction-findingmeasurements the previous section. Although the onset of the plasma os- showthat the type Ill sourceis very large at low frequencies, cillations occurs somewhat later than the onset of the radiation subtendinghalf anglesas large as 45o-60 ø as viewed from the at 2f•,-, the temporalcorrespondence is so good in this case sun [Baumback et al., 1977]. that there can be essentiallyno doubt that the radio emission SUMMARY AND DISCUSSION and plasma oscillationsare closely associated.Preliminary comparisonswith the data from the energeticparticle and The Helios 1 plasma wave measurementsshow that the plasma experimentson Helios (E. Keppler and H. Rosen- electric field intensitiesin the solar wind are often very low, bauer, personal communication, 1976)confirm that intense much lower than thosefor comparableplasma wave measure- fluxes of low-energy(1-20 keV) electronsof solar origin are ments near the earth. During the quiet periodswhich occur in present during this event. About 4 hours after the event in the Helios 1 plasma wave data the only plasma wave activity
1•3 TYPE1'"11' RADIO BURST 178kHz(+_8%) 1(54 t I I i i i i I l-'- -F E 1(55 • 1(76 o
T 1(73 i• I00kHz (+-8%) 104ONSET • Z I• 5 TIME
64/ x• illlilt / OSCILLATIONS / •5 • • i• 6
•4 •5 i(•6 U.T. (HRMN) 0600 0700 0800 0900 I000 R (A.U.) .446 .446 .445 .445 .444
HELLOS 2 DAY 92, APRIL I, 1976
Fig. 20. A typeIII radioevent occurring in associationwith veryintense electron plasma oscillations at fp- -• 56.2kHz. The maximumintensity of theplasma oscillations in thiscase is 5.26mV m-'. Theseare the mostintense electron plasma oscillationsobserved to date with the Helios plasmawave experiment. GURNETT AND ANDERSON: SOLAR WIND PLASMA WAVES 647
- TYPE 'FFr RADIO BURST 178 kHz(_+8%) - , o_u I (•6 _ II • I III I 10 - ONSET', / '00kHz(-+8%) - I-- Id 4 TIME -•l / z -5 gd I0 I-- -6 Cn I0 \ /ELECTRON• 56.2kHz(_+8%) _ - \\ •/PLASMA _ - I • \'\ k •.•'-,'- I OSCILLATIO.. - ,...... - o 10 _
J _ \ 31.1kHz(_+8%) _
_
I I I I i I I U.T.(HRMN) I000 I100 1200 1300 1400 R (A.U.) .444 .444 .443 .442 .442
HELIOS 2 DAY 92, APRIL I, 1976 Fig. 21. A type III radio burstand associatedelectron plasma oscillations occurring a few hoursafter the eventin Figure 20. The maximum intensity of the plosma oscillationsin this case is 2.35 mV m-• which can be detected is a slight enhancementof the noise detectedby Helios I many interpretational questionsmust be levels in the frequency range from about 500 Hz to 3.0 kHz. considered. For the electron plasma oscillations the close The broadband electric field strengthof these quiet time en- tracking of the emission frequency with the local electron hancementsis very small, only about l0 #V m -•. During more plasmafrequency and the alignmentof the electricfield paral- disturbedperiods the most commonlyobserved plasma wave lel to the solar wind magneticfield stronglysupport the identi- is a sporadicemission at frequenciesbetween the electron and fication of these waves as electron plasma oscillations.The ion plasma frequencies,typically from about 1 to l0 kHz. electric field orientation specificallyshows that these waves Thesewaves are referredto as f,+ < f < f,- noise.The fre- cannot be associatedwith the upper hybrid resonance,since quencyspectrum of the f,+ < f < fp- noiseshows a systematic waves associated with this resonance would have an electric variation with radial distance from the sun, shifting toward field orientation perpendicular to the local magnetic field higher frequenciescloser to the sun.The electricfield strength [Stix, 1962]. The basic two-streaminstability responsiblefor of this noiseis very impulsive,consisting of many brief bursts the generation of electron plasma oscillationsis well known lasting for only a few secondsor lesswith peak single-channel and has been confirmed in some detail by Gurnett and Frank electricfield strengthsof a few hundred microvoltsper meter. [1975]. The principal questionsthat remain concernthe details No pronouncedvariation in the electricfield strengthof this of the double-peakedelectron distribution function required noiseis evidentwith radial distancefrom the sun. Spin modu- to produce these waves and the origin of the streamingelec- lation measurements show that the electric field of this noise is trons causingthe instability. In somecases, for example,in the closelyaligned along the directionof the solar wind magnetic event analyzed by Gurnen and Frank [1975] and the eventsin field. At higher frequencies,from about 20 to 100 kHz, narrow Figures 18-20 of this paper, it seemsreasonably certain that band electronplasma oscillations are occasionallydetected at the streamingelectrons originate from solarflare activity at the frequenciesnear the local electronplasma frequency. The elec- sun. However, other cases occur in the Helios I data for which tron plasmaoscillations are more intensethan the f,+ < f < no specificsolar flare activity is known to occur, and in these f•- noise,sometimes with electricfield strengthsas largeas a casesthe streamingelectrons may originate from local acceler- few millivolts per meter, but occur very infrequently. The ation processesin the solar wind. Further studies and com- electricfield of the electronplasma oscillations is usuallyvery parisonswith the plgsmameasurements are neededto under- impulsiveand is aligned approximatelyparallel to the direc- stand fully the conditions and circumstanceswhich lead to the tion of the solar wind magneticfield. The frequencyof the generation of electron plasma oscillationsin the solar wind. electron plasma oscillations closely tracks the local electron Since electronplasma oscillationsoccur very infrequently,it plasma frequencyas determined by the solar wind plasma seemsunlikely that these waves play a significantrole in the experimentand showsa clear systematicvariation with radial steady state propertiesof the solar wind. These waves may, distance from the sun, increasingin frequencycloser to the however, play an important role in thermalizing the low-en- sun. In all of the available data a total of 18 type llI radio ergy electronstreams which sometimesoccur in the solar wind. bursts have been detectedby Helios 1 and 2. Of these, four It is also of interest to comment on the very impulsivetem- have simultaneouslyoccurring electron plasma oscillations as- poral variations frequently observed for the plasma os- sociatedwith the type Ill radio emission.In one of thesecases cillations detected by Helios. Although the origin of these the electron plasma oscillationsare unusually intense,with a amplitude fluctuations is not known, it is suggestive,partic- maximum electric field strength of about 5.26 #V m -•. ularly for the very intenseevents such as occur in Figure 20, In examiningthe origin of the varioustypes of plasmawaves that theseimpulsive variations may be relatedto a parametric 648 GURNETT AND ANDERSON; SOLAR WIND PLASMAWAVES instability, such as the oscillating two-stream instability pro- electricfield direction, which is parallel to B (see Figure 13). posed by Papadopouloset al. [1974] to stabilize electron Of the variouspossible explanations of the f,+ ( f ( f,- streams from solar flares. noise we believe that the ion sound wave is the most likely The proper identificationof the plasmawave mode respon- mode which could be associated with this noise. If these waves siblefor the f.+ < f < f.- noiseis more difficult.This noise are ion sound waves, then there remainsthe important ques- almost certainly consistsof electrostaticwaves, since no com- tion of how these waves are generatedand what effect they parable magneticfield noise has ever been detectedin the solar may have on the macroscopicproperties of the solar wind. wind. As was mentioned earlier, even though this noiseoccurs Since these waves apparently occur over large regionsof the at frequenciesbetween electron and proton plasma frequen- solarwind and are not particularlyassociated with discontin- cies, it is not immediatelycertain that the noisehas any direct uities or other small-scalefeatures where large currentsmay be relationship to either of these frequenciesbecause of the large present,the bestpossibility for generatingthese waves appears Doppler shifts which can occur in the solar wind. If the Dop- to be the heat flux driven instability suggestedby Forslund pler shifts are small, Af/f • l, the most obvious plasma [1970]. Further detailed comparisonswith the Helios plasma wave mode which could be associated with this noise is the measurementsare clearly needed to identify definitely the in- Bunemanmode [Buneman,1958] at f• -• (m-/m + )•/3f.-. How- stability mechanisminvolved in the generationof thesewaves. ever, for the typical solar wind parameters Te = (2-6)T0 the Although the maximum intensityof f.+ < f < f.- noise is Buneman mode is not considereda very likely possibility for relatively small, with energy density ratios of typically explainingthis noise,since this modehas a higherinstability eoE2/2nkT -• 10-5, this turbulence may neverthelessplay a threshold than the ion sound wave and requires a large elec- significantrole in controlling the electronenergy distribution tron drift velocity in relation to the ions to drive the instability. in the solar wind, sincethese waves are presenta lar.ge fraction Although such large drift velocities may occur at discontin- (30-50%) of the time. uities in the solar wind, it seemsunlikely that this instability After several years of investigation [seeGurnet! and Frank, couldoccur over the largespatial regions for whichthe f.+ < f 1975] the Helios plasma wave measurementshave now finally • f.- noise is observed. provided clear evidence that intense electron plasma os- If the Doppler shiftsare large,the mostlikely possibilityfor cillations do occur in associationwith type III radio events. explainingthe f.+ < f • f.- noiseis thoughtto be ion sound Rough estimatesby Gurnet!and Frank [1975] usingthe theory waves at f -• f.+ which are Doppler-shiftedto frequencies of Papadopoulose! al. [1974] indicate that electron plasma above f.+ (requireswave vectorsK directedaway from the oscillationswith field strengthsof the order of 10 mV m -• are sun). The possibleoccurrence of an ion soundwave instability required to explain low-frequencytype III radio bursts. Field in the solar wind has been previouslysuggested and discussed strengthsnearly this large (5.26 mV m -•) have now been found by Forslund[1970]. Becauseion sound waveshave relatively in associationwith a type III burst. Although the electron short wavelengths,the Doppler shift causedby the motion of plasmaoscillation model originallyproposed by Ginzburgand the solar wind is largein comparisonto the wave frequencyof Zheleznyakov[1958] can now be consideredconfirmed, there •f.+ in the rest frame of the plasma.The frequencydetected are still many questionsremaining concerningthe exact mech- in the spacecraftframe of referenceis approximately anism by which the plasma oscillationsare convertedto radio f' = f p+ + (V/X) cos0Kv emissions.The mostsignificant question is still concernedwith the electricfield strengthof the plasmaoscillations required to where V is the solar wind velocity, X is the wavelength,and explain the power flux of the type Ill radio emission.If field is the angle between K and V. Since 0Kv is typically not near strengthsthis large are always required to produce a signifi- •r/2, the upper cutoff frequency of the observedfrequency cant level of radio emission, then it is surprising that such spectrum is mainly determined by the minimum wavelength events are not found more often consideringthe large size of (fmax = V/Xmin). The minimum wavelengthis controlled by the type Ill radio source and the extensive measurements the onsetof strong Landau damping and is approximately•min available from the Imp spacecraftnear the earth [Gurnettand = 2•r•z• where •. is the Debye length. For typical solar wind Frank, 1975]. Considerable future work is still needed to eval- temperaturesand densitiesat 1.0 AU, T = 1.0 X l05 øK, and uate the various mechanismsfor converting the plasma os- n = 5 cm-3, the minimum wavelengthis approximately•min = cillation energy to radio emissionsand to investigate further 61 m. For a solar wind velocity of 400 km s-• the correspond- and understand the spatial structure of the type Ill radio ing fmax is 6.5 k Hz, which is in good agreementwith the emission. observedupper cutoff frequencyof the f.+ • f • f.- noiseat When the Helios electric field measurementsare compared 1.0 AU (compare with Figure 12). The increasein the upper with the previous Pioneer 8 and 9 results,it is apparent that a cutoff frequency with decreasingradial distance from the sun significantdifference exists in the electricfield spectrumsand and the apparent proportionalityto f.- can be attributedto intensities detected by these two spacecraft. Of the two the densitydependence of the Debye length,which givesfmax primarytypes of plasmawaves detected by Helios(the fo ( f o: f.- if the electrontemperature remains constant. ( f,- noise and electron plasma oscillations) neither was Another possibleexplanation of the f.+ • f • f.- noiseis detectedby Pioneer 8 and 9. As can be seenby comparingthe that it is causedby electronplasma oscillationsat f -• spectrumsin Figures 12 and 15 with the instrument noise levels which are Doppler-shifted to lower frequencies(K vectors in Figure4, thef.+ < f < f.- noise'normallyoccurs with directed toward the sun). This alternative is considered un- intensitiestoo small to be detectedby Pioneer8 and 9. U nusu- likely because for a two-stream instability, such plasma os- ally intenseelectron plasma oscillations,such as those in Fig- cillations could only be producedby electronsstreaming to- ures 20 and 21, could have been detectedby Pioneer 8 and 9'
ward the sun from a sourcebeyond 1.0 AU, which seemsvery høwever,such large , intensities occur so seldom that it is not implausible. Other possibleplasma wave modes involving K surprisingthat eventsof thistype Were not observed.Thus vectors perpendicular to B, such as the electron Bernstein insofaras comparisons are possible, there is no disagreement modes at f -• nf•-, appear to be ruled out becauseof the with the Pioneer measurementsconcerning the primary types GURNETT AND ANDERSON: SOLAR WIND PLASMA WAVES 649 of plasma waves detected by Helios 1. However, when the waves in the solar wind, )kmin -- 2•rXo,to be lessthan the length waves and intensities detected by Pioneer 8 and 9 are com- of the Helios I antenna. If wavelengthsthis short occur, then pared with Helios measurements,significant differencesare the electricfield strengthsare underestimated,possibly by a evident. In describingthe Pioneer 8 and 9 results,Scarfet al. large factor if X <
Solar actit;ity. Becausethe primary Helios missionis being Baumback, M. M., W. S. Kurth, and D. A. Gurnett, Direction-finding conductedduring solar minimum conditions,whereas the Pio- measurementsof type III radio burstsout of the ecliptic plane, Solar neer 8 and 9 measurementswere made during a more active Phys., in press, 1977. period of the solar cycle (1968-1969), it is possible that Belcher,J. W., and L. Davis, Jr., Large-amplitudeAlfvfin waves in the changesin the solar activity could account for some of the interplanetary medium, 2, J. Geophys.Res., 76, 3534, 1971. Benediktov, E. A., G. G. Getmantsev, N. A. Mityakov, V. O. Rapo- differencesin the electricfield intensitiesobserved by Pioneer port, and A. F. Tarasov, Relation betweengeomagnetic activity and and Helios. This explanationcould be important for the spike- the sporadic radio emission recorded by the electron satellites, like burstsand associateddisturbances observed by Pioneer in Kosm. Issled., 6, 946, 1968. associationwith interplanetaryshocks, since the occurrenceof Buneman, O., Instability, turbulence and conductivity in a current- carrying plasma, Phys. Ret;. Lett., 1, 8, 1958. , interplanetaryshocks has been very low duringthe periodof the Burlaga, L. F., Hydromagnetic waves and discontinuitiesin the solar Helios observations.Many of the eventsdiscussed by Scarfet al. wind, Space Sci. R et;., 12, 600, 197I. [1973] are closelyassociated with interplanetaryshocks and Coleman, P. J., Jr., Hydromagnetic waves in the interplanetary haveno counterpartin the Heliosdata currentlyavat!able. In a plasma, Phys. R et;. Lett., 17, 207, 1966. few cases,electric field spectrumsqualitatively similar to the Fainberg, J., L. G. Evans, and R. G. Stone, Radio tracking of solar energeticparticles through interplanetary space,Science, 178, 743, shock-associatedspikes and long-durationevents detected by 1972. Pioneer 8 and 9 have been detectedby Imp 6 and 8; however, Forslund, D. W., Instabilities associated with heat conduction in the the intensitiesin these casesalso appear to be significantly solar wind and their consequences,J. Geophys.Res., 75, 17, 1970. smaller than the intensitiesobserved by Pioneer 8 and 9. Fredricks, R. W., Electrostatiche0ting of solar wind ions beyond 0.1 AU, J. Geophys.Res., 74, 2919, 1969. Wat;elengthsshorter than the antennalength. When the Ginzburg, V. L., and V. V. Zheleznyakov,On the possiblemechanism plasmadensity is sufficientlylarge (n >• 70 cm-3 for Te of sporadic radio emission(radiation in an isotropic plasma), Sot;. øK), it is possiblefor the minimum wavelengthof plasma Astron. A. J., 2, 653, 1958. 650 GURNETT AND ANDERSON: SOLAR WIND PLAStaAWAVES
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