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 and the electronplasma oscillations show a tendencyto occurat higherfrequencies closer to the sunbut 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