Plasma Wave Turbulence at the Magnetopause&Colon

VOL. 84, NO. A12 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER 1, 1979

Plasma Wave Turbulence at the Magnetopause: Observations From ISEE 1 and 2

D. A. GURNETT,l R. R. ANDERSON,l B. T. TSURUTANI,2 E. J. SMITH,2 G. PASCHMANN,3 G. HAERENDEL,3 $. J. BAME,4 AND C. T. RUSSELLs

In this paper we investigateplasma wave electric and magnetic fields in the vicinity of the magneto- pauseby usingrecent measurements from the ISEE 1 and 2 spacecraft.Strong electric and magneticfield turbulenceis often observedat the magnetopause.The electricfield spectrumof this turbulencetypically extendsover an extremelylarge frequencyrange, from lessthan a few hertz to above 100 kHz, and the magneticfield turbulencetypically extends from a few hertz to about 1 kHz. The maximum intensities usually occur in the magnetopausecurrent layer and plasmaboundary layer. Somewhatsimilar turbulence spectraare also sometimesobserved in associationwith flux transfer events and possible'inclusions'of boundarylayer plasma in the magnetosphere.In addition to the broad-bandelectric and magneticfield turbulence,narrow-band electrostatic emissions are occasionallyobserved near the electron plasmafrequency in the vicinity of the magnetopause.Two possibleplasma instabilities, the electrostaticion-cyclotron instability and the lower-hybrid-driftinstability, are consideredthe primary candidatesfor explaining the broad-bandelectric field turbulence.The narrow-bandelectrostatic emissions near the local electron plasma frequency are believed to be either plasma oscillationsor electrostatic wavesnear the upper-hybrid-resonancefrequency.

1. INTRODUCTION waves. Although the magnetopauseis known to be very turbulent in the low-frequency MHD portion of the spectrum, Becauseof the many important questionswhich have been relatively little is known about the plasma wave intensitiesat raised recently concerning the physical processeswhich occur higher frequenciesin the region of primary importance for at the earth's magnetopauseboundary [Heikkila, 1975; Hae- microscopicplasma processes.Neugebauer et al. [1974] and rendel et al., 1978], the study of the magnetopausehas entered Fairfield [1976] have investigatedmagnetic measurementsof a period of increasedactivity. During the past year, magneto- whistler-mode and ion-cyclotron waves near the magneto- pausestudies have been particularly aided by the launch of pause. However, no plasma wave electric field measurements the ISEE 1 and 2 spacecraft[Ogilvie et al., 1978], which for the have yet been reported in associationwith the magnetopause. first time can provide basic information on the spatial-tem- As will be shown in this paper, strong electric and magnetic poral structureof the magnetopause.In this paper we present field turbulence is frequently observedat the magnetopause. an initial investigationof the plasma wave electric and mag- The electric field spectrumof this turbulencetypically extends netic fields associatedwith the magnetopauseusing data obtained from ISEE 1 and 2. over an extremely large frequencyrange, from lessthan a few hertz to above 100 kHz, and the magnetic field turbulence The importance of plasma wave observationsnear the mag- typically extends from a few hertz to about 1 kHz. This fre- netopauseoriginates from the possiblerole which wave-particle interactions may play in the diffusion and transport of quencyrange includesnearly all of the characteristicfrequen- ciesof the plasma, from the proton gyrofrequencyto the elec- plasma acrossthe magnetopause[Axford, 1964; Bernsteinet tron plasma frequency. The maximum intensitiesof both the al., 1964; Eviatar and Wolf, 1968; Hasegawa and Mima, 1978] electric and the magnetic field turbulence are usually confined and from the possibleeffects of plasma turbulence on energy to a region which includesthe plasma boundary layer and the dissipationand reconnectionat the magnetopause[Syrovatski, magnetopause current layer. Somewhat similar turbulence 1972; Huba et al., 1977; Haerendel, 1978]. Plasma and mag- spectra are also sometimesobserved in associationwith flux netic field measurementshave now been obtained at the mag- transfer events of the type described by Russell and Elphic netopauseunder a wide variety of conditions [Sonneruœand [1979] and possible'inclusions' of boundary layer plasma into Cahill, 1967; Hones et al., 1972; Akasofu et al., 1973; Crooker the magnetosphereas describedby Paschmannet al. [ 1979]. and Siscoe, 1975; Rosenbauer et al., 1975; Paschmann et al., To facilitate the comparison of the plasma wave measure- 1976; Eastman et al., 1976; Haerendel et al., 1978]. For a recent ments with the plasma and magnetic field measurements, review of the various plasma regimesassociated with the mag- some of the magnetopausecrossings have been selectedfrom netopause, see Eastman and Hones [1978]. These measure- the crossingspreviously analyzed by Paschmannet al. [1979] ments show that the magnetopause is often very turbulent, and Russelland Elphic [1979]. For a descriptionof the plasma with considerableevidence of large-amplitudelow-frequency wave instrumentation used in this study, see Gurnett et al. •Departmentof Physicsand Astronomy,The University of Iowa, [1978]. Descriptions of the plasma and magnetic field instru- Iowa City, Iowa 52242. mentation are given by Bame et al. [1978] and Russell [1978], 2Jet PropulsionLaboratory, California Institute of Technology, Pasadena, California 91103. respectively. 3Max-Planck-Institutfiir Physikund Astrophysik,Institut fiir ex- traterrestrichePhysik, 8046 Garching, West Germany. 2. SOME REPRESENTATIVE MAGNETOPAUSE 4Universityof California, Los Alamos ScientificLaboratory, Los CROSSINGS Alamos, New Mexico 87545. 5Departmentof Planetaryand SpaceScience, University of Califor- To illustrate the intensity and primary characteristicsof nia at Los Angeles,Los Angeles,California 90024. plasma wavesobserved at the magnetopausea seriesof mag- Copyright @ 1979 by the American GeophysicalUnion. netopausecrossings have been selectedfor analysis from four Paper number 9A0742. 7043 0148-0227/79/009A-0742501.00 7044 GURNETT ET AL.' MAGNETOPAUSE ELECTRIC FIELD TURBULENCE passesthrough the magnetospherein November and Decem- coordinatesis stronglysouthward, whereas for the remaining ber 1977. These passeswere selectedprimarily on the basis of crossing,on November3, the Z componentof the magneto- the plasma wave activity observed at the magnetopause. In sheathfield rangedfrom near zero to slightlysouthward. The each casean easily identified burst of plasma wave turbulence inbound crossingson November 10 and December 2 also oc- is present at the magnetopause.Because of this selectioncrite- curred unusuallyclose to the earth, at --•7.4R• and --•6.7R•, rion the magnetopausecrossings shown probably cannot be whereasthe crossingson the other two days are closeto the regarded as typical, since they were selectedon the basis of nominal magnetosheathposition. enhanced plasma wave intensities.Further studieswill therefore be needed to fully characterizethe entire range of plasma Crossingsof Novernber 10, 1977 wave turbulence which can occur at the magnetopauseand Figure 1 givesan overall view of the plasmawave electric the factors which control the intensity of the turbulence. The and magneticfields detected by ISEE 1 on this day. The field presentstudy is intended mainly to illustrate the relationships strengthsin each channel are shown on a logarithmicscale observed for casesin which some plasma wave turbulence is with a dynamic range of 100 dB. For the electricfield the field known to be present. strengthsvary from about 0.1 pV m-' to 10.0 mV m-I from The crossingsselected occur over a range of local times ex- the baselineof onechannel to the baselineof the next higher tending from local morning, --•0600hours, to near the subsolar channel.For the magneticfield plots the backgroundnoise point, --•1200 hours. In all casesthe latitude of the crossingis level is adjusted such that it is near the bottom of the scale. relatively low, lessthan 27 ø. For three of the setsof crossings, The backgroundnoise levels of the magneticsensors are given two on November 10 and one on December 2, the Z com- by Gurnett et al. [1978]. The solid line indicates the maximum ponent of the magnetosheathmagnetic field in solar ecliptic field strength computed over intervals of 144 s, and the solid

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'['SEE I, DAY 514, NOV. I0, 1977

Fig. 1. The plasma wave electricand magneticfield data from ISEE 1 for a representativepass through the magnetosphere.The enhancedelectric and magneticfield intensitiesat the inbound and outboundmagnetopause crossings are clearly evident. GURNETT ET AL.: MAGNETOPAUSEELECTRIC FIELD TURBULENCE 7045

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I'SEE-2, DAY 314, NOV. I0, 1977 Fig. 2. A detailedcomparison of the plasmawave electricfield intensitiesdetected by ISEE 2 with the corresponding plasmaparameters obtained from the LASL/MPI fastplasma analyzer for the inboundseries of magnetopausecrossings in Figure1. Theelectron and proton densities, Ne andN•,, in cm-3, andelectron and proton temperatures, Teand Te, in degreeskelvin, are indicatedby dotsand solidcurves, respectively. The plasmapressure P, in unitsof 10-8 dynescm- 2 on the left scale,is shownby the solid curve in the bottom panel, and the magneticfield pressure,given as magneticfield strengthin gammason the right scale,is shownby the dottedcurve in the bottompanel. The dottedareas indicate the positionof the magnetopausecurrent layer crossing (see Figure 3). black area indicatesthe averagefield strengthcomputed over The ISEE 2 electric field intensity variations for the in- the same interval. On the inbound pass, the spacecraftap- bound seriesof magnetopausecrossings are shownin greater proachesthe earth from the daysideof the magnetosphere,detail in Figure 2 and comparedwith the plasma parameters passesthrough perigee at about1800 UT in the localevening, obtained from the Los Alamos Scientific Laboratory/Max- and recedesaway from the earth on the local morningside of Planck-Institut (LASL/MPI) fast plasma analyzer. A corre- the magnetosphere.The approximatepositions of the bow spondingcomparison of the ISEE 1 electric and magnetic shock, the inbound magnetopausecrossings, and the out- field intensitiesand the magnetic field variations from the boundmagnetopause crossings are indicatedat the top of Fig- UCLA flux gate magnetometerson ISEE I and 2 is shownin ure 1. On the inbound portion of the pass,a seriesof magneto- Figure 3. The orthogonalNLM coordinatesystem used for pausecrossings and near encountersoccurs from about 1440 the magneticfield measurementshas the N axisperpendicular to 1512 UT. These crossingsand near encounterscan be to the magnetopauseand the L axis along the projection of clearlyidentified in Figure1 by the greatlyenhanced electric the solar magnetosphericZ axis onto the magnetopause.The and magneticfield intensitiesduring this interval.This series M axis completesthe right-handcoordinate system. See Rus- of crossingsoccurs near the subsolarpoint at a magneticlati- sell and Elphic[1979] for the methodof determiningthe mag- tude of about 15.2 ø and a local time of about 12.2 hours. On netopausenormal. Unfortunately,during this portion of the the outboundportion of the pass,another series of magneto- passthe ISEE 1 plasmaanalyzer was turned off, so no direct pausecrossings occurs from about2220 to 2240UT, with sim- wave-plasmacomparisons could be made with the ISEE 1 ilar enhancementsevident in the electricand magneticfield data. However, as can be seen from the ISEE I and 2 mag- intensities.This seriesof crossingsoccurs at a magnetic lati- netic field comparisonsin Figure 3, the time delays between tude of about 17.8 ø and a local time of about 7.5 hours. the two spacecraftare sufficientlysmall (<1 min) that the 7046 GURNETT ET AL.: MAGNETOPAUSE ELECTRIC FIELD TURBULENCE

lOOK ii! ....II i i I iiI • Ill I--- I ! Iii I creasein the plasma density.The third crossingat 1458:30UT 56.2K i , .... • i ill -- ii ---- has a very clearly defined boundary layer which extendsfrom 31.1K Ii .... II I ..... •' '- about 1459 to 1500 UT. The abrupt increasesin the plasma 17.8K • •- -L..-- I ' I t •*' • IO. OK density from about 1502 to 1504:30UT and from about 1510 '•- 5.62K to 1513:30 UT have been interpreted by Paschmann et al. • 3.11K z [1979] as either inclusions of magnetosheathlike plasma ._• 1.00K within the magnetosphere or a sudden switch-on of the 562 .... ,,, II" ' , " - boundary layer. Other events of interest during this crossing 311 178 II Ill• I • • .• . •. include the flux transfer events (FTE) identified by Russell I00 ...... II ...... and Elphic [1979] at 1435:30, 1447, and 1455 UT. These events 56.2 ...... I - IIII I ' ' are most easily identified by the characteristicvariation of the 31.1 ...... I I I 17.8 I •' II I I I BN component of the magnetic field, usually consistingof a I0.0 II brief positive excursionfollowed by a somewhatasymmetric 5.6 i[ i i i i •11 i i [i ii i ii1.11 iii IIii ii iI i i i i i i i i i i negativeexcursion.

:3. ILK Essentiallyall of the eventsdescribed above have a clearly 1.78K identifiable signature in the plasma wave electric and mag- [.00K netic field data. The electric field spectrumvariations are most 562 i i k --'Li'& I .....

:311 , lli•.IJai ' ,.ll-- lllll i.,•i • I -•lI I &'A i i I,j•I i I•1 i i , easily seen for the ISEE 1 data in Figure 3, since the ISEE 2 178 data have relatively high levels at low frequenciesapparently IO0 causedby interference from the spacecraftsolar arrays. The 56.2 ., IiIi ' • L.._ ß • I -- --• ii --

:3l .I kl flux transfer eventsat 1435:30, 1447, and 1455 UT, the magne- 17.8 • .'.._'. , , • __•., _ i ---i I i '' - ii i-- i topause current layer and boundary layer crossingsat 1440, I0.0 i .... 1 I I .... -• - -B 1447, and 1458:30UT, and the plasma inclusionsat 1503 and I I'----I111• ' ' I' ''•1•'I I I I' I I t I I ....II I ll''•''- I .... T' ' 1512 UT all have a very similar signature, consistingof a 16o 120J I I I I ' • L broad bandwidth burst of electrostatic noise extending from below 5.6 Hz up to about 100 kHz and a correspondingbroad bandwidth burst of magnetic noise extending from below 5.6 o-• , Hz up to about 1 kHz. The broad bandwidth burst of electric -40 i i field noise associatedwith each of these magnetopausecross- iVl ings can also be easily identified in the high-resolutionsweep frequency receiver data shown in Plate 1. The width of the -40-4-80 ...... i' I_. J..,' I[ I [I ' [- noise burst at each magnetopausecrossing corresponds very well to the combined width of the magnetopausecurrent layer 40 I ir[t I ir-u . ir'c and the plasma boundary layer. The inner earthward boundary of the region of enhancedplasma wave turbulence is ex- -4ø1'1 .... I .... I''''1 .... I .... I .... I .... I''' tremely sharp and coincidesalmost exactly with the abrupt UT(HRM•N) •430 •440 •450 •500 drop in the plasma density from values characteristicof the Z SEE-•, OAY 3•4, NOV. •0, •977 magnetosheath/boundarylayer plasma to values character- Fig. 3. A detailed comparison of the plasma wave electric and istic of the outer magnetosphere.The outer boundary of the magnetic field intensitieswith the UCLA magnetic field measure- region of enhanced plasma wave turbulence is not as sharp, ments for the inbound seriesof magnetopausecrossings in Figure 1. suggestingthat the turbulenceextends outward into the mag- The times indicated by the dashed lines marked FTE correspond to the flux transfer events discussedby Russell and Elphic [1979]. Both netosheath to varying degrees, depending on the frequency. the flux transfer events and the crossingsof the magnetopausecurrent As can be seenfrom Figure 1, the magnetosheathalways has a layer are characterized by greatly enhanced electric and magnetic substantial level of electric field turbulence [see Rodriquez, field intensities. 1979],which in somefrequency ranges,from about 100 Hz to 3 kHz, for example, is nearly as intense as the noise at the plasmaparameter variations can be considerednearly identi- magnetopause. cal on the time scales shown. Detailed comparisons of the plasma wave, plasma, and The plasma and magnetic field characteristicsfor the mag- magneticfield data for the outboundseries of magnetopause netopausecrossings in Figures 2 and 3 have been previously crossingson November 10 are shown in Figures 4 and 5. analyzed and discussedby Paschrnannet al. [1979] and Russell These crossingsshow many of the same characteristicsas the and Elphic [1979].These results will be briefly reviewedfor the inbound pass near local noon except that the boundaries are purposeof comparisonwith the wave data. The vertical dot- not as sharp and the transition takes place over a larger spatial ted areas in Figure 2 indicate the positionsof the magneto- region. The magnetopausecurrent layer crossingis located at pause current layer as deducedfrom the abrupt transitionsin about 2239 UT, as determined from the UCLA magnetic field the magnetic field shown in Figure 3. The first crossingat data in Figure 5. On the magnetosphereside of the current 1440 UT appears to have a thin plasma boundary layer from layer,several regions of boundarylayer plasma are evident in about 1440 to 1441 UT. This interpretation is somewhat un- Figure 4 from about 2228 to 2232 UT, 2235 to 2237 UT, and certain becausesmall magnetic field variations are still present 2237:30 to 2239 UT. A close approach to the boundary layer in this region, which could indicate that the spacecraftis still plasmaalso occurs from about 2222 to 2224 UT. Each of these in the current layer. The second crossingat 1447 UT appar- regions is characterizedby strongly enhanced electric and ently has no boundary layer, since the onset of the magnetic magneticfield intensitiesextending over a broad range of fre- field transition is essentially coincident with the abrupt in- quencies.Several intense narrow-band bursts of electricfield GURNETT ET AL.: MAGNETOPAUSEELECTRIC FIELD TURBULENCE 7047

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lOOK wave intensity are particularly evident in the magnetic field 56.2K `31.1K channelsfrom 5.6 to about 311 Hz. Correspondingenhance- 17.8K mentsin the electricfield intensities,although they are not as IO.OK distinct,can be seenfor frequenciesextending as high as 56.2- 5.62K ;3. ILK 100 kHz. Comparisonswith the plasma data show that these 1.78K regions of enhanced wave intensities are all associatedwith 1.00K 562 large fluctuationsin the flow velocity and a generalincrease in `311 the plasma temperatureabove the nominal magnetosheath 178 values,as would be expectedin a boundary-layer-likeregion. I00 56.2 Other periods of enhancedelectric and magnetic field intensities, such as from 0302 to 0304 UT and from 0311 to 0315 17.8 UT, appear to correspondto close approachesto the bound- I0.0 5.6 ary layer region. Examinationof the UCLA magneticfield data in Figure 7 1.00K for thispass shows a seriesof multiplecrossings of the magne- 562 topausecurrent layer from about 0204 to 0209 UT, a single •'"- "" a i . &. L -. al_ ,,.iJ--_ldll.,i 178 _..Li,,.._.,L.... s...... ,.• i I --a-I. I .... distinct crossingat 0223 UT, and a final seriesof multiple I00 I Ai '•'A .....,• • Ill I I --'•B'--- crossingsfrom about 0322 to 0328 UT. Except for the final 56.2 .31 .I transitionsthe multiple current layer crossingsfrom 0204 to 17.8 0210 UT and from 0322 to 0328 UT apparentlyare associated

I0.0 ...... II i with brief excursionsinto the boundarylayer and dø not rep- 5.6 .... ' I ' I ' ' -•""' ' I ---I resentcrossings into the magnetosphere,since at no time dur- 102 ing these intervals does the plasma density drop to values

I0 I characteristicof the outer magnetosphere.As can be seen from Figure 6, the final entry into the magnetosphereon these _•_. _'1_ :'_- I ...... I [, I I ...... I _ crossingsoccurs at about 0210 and 0328 UT. In contrast,the current layer crossingat 0223 UT is very sharp and distinct 108 and apparently has only a very thin boundary layer. Com- 107 parisonswith the UCLA magneticfield data showthat the en- 106 hancedplasma wave intensitiesare closelyassociated with the regionsof irregular magnetic field fluctuations. 2O0 Crossingsof November 3, 1977 IO0

' I ' I ' I ' I ' 1 ' - Another seriesof magnetopausecrossings, observed during UT 2200 :•:. 2230 2300 the inbound passon November 3, 1977,is shownin Figures 8 R (Re) 9.37 I O. I 10.7 LT (HR) 7.3 7.5 7.6 and 9. As can be seenfrom the UCLA magneticfield data in Figure 9, five distinct crossingsof the magnetopausecurrent ISEE-I, DAY 314, NOV. I0, 1977 layer can be identified on this pass, at 0740:30, 0741:00, Fig. 4. A detailed comparisonof the plasma wave electric and 0741:20,0743:50, and 0751:30UT. The plasmadata in Figure magneticfield intensitiesand the plasmaparameters for the outbound 8 show that ISEE 1 entered the boundary layer between the magnetopausecrossing in Figure1. The protonflow velocity V•, is in units of kilometers per second. Several distinct encounterswith the first two pairs of current layer crossingsbut did not crossinto plasmaboundary layer can be seenbefore the spacecraftcrosses the the low-densityhigh-temperature region of the outer magnet- magnetopausecurrent layer at about 2239 UT (seeFigure 5). Each of osphere. The last magnetopausecurrent layer crossingat theseencounters with the boundarylayer is associatedwith enhanced 0751:30 UT is followed by a broad region of boundary layer plasma wave electric and magnetic field intensities. plasma with a density intermediate between the magnetosheathand magnetosphericdensities. At severalpoints within noise are also evident in the 31.1- and 56.2-kHz frequency this plateau region, abrupt drops in the density, and associ- channelsat about 2239-2240 UT, near the outer edge of the ated temperature increases, occur which indicate close en- magnetopausecurrent layer. counterswith the earthward edge of the boundary layer. The final crossingfrom the boundarylayer into the magnetosphere Crossingsof December 2, 1977 occurs at 0802:45 UT. Another seriesof magnetopausecrossings, observed during As can be seenfrom a detailedcomparison of Figures8 and the inboundpass on December2, 1977,is shownin Figures6 9, enhancedbroad-band electric field intensitiesextending up and 7. Three clearly definedmagnetopause crossings, at about to about 17.8 kHz are clearly observedwhen the spacecraftis 0209, 0223, and 0328 UT, can be identified on this pass.The in the boundary layer. However, in contrastto the previous plasma wave data show a considerableamount of turbulence casesanalyzed, the magneticfield intensitiesin the boundary and wave activity throughoutthe entire interval shownin Fig- layer are quite low at all frequencies.Only a very small, 3-dB ure 6, both inside and outsideof the magnetosphere.Both the increase in the magnetic field intensities above the sensor electric and the magnetic field data show enhanced broad- noiselevel can be seenin the boundary layer. band wave intensitiesnear the magnetopausecrossings, from about 0200 to 0209 UT for the first crossing,from about 0223 3. SPECTRUM, POLARIZATION, AND WAVELENGTH to 0224 UT for the secondcrossing, and from about 0318 to To identify the plasma wave modesresponsible for the elec- 0328 UT for the third crossing.These regions of enhanced tric and magneticfield turbulenceobserved at the magneto- GURNETT ET AL.: MAGNETOPAUSE ELECTRIC FIELD TURBULENCE 7049 pause it is necessaryto determine as much as possible about with E2/Af oc 1/f 2-2for the electricfield and B2/•f oc 1/f 3'3 the spectrum,polarization, and wavelength of this turbulence. for the magnetic field. The large difference, a factor of 10-100, In the following we summarize the basic characteristicsof the between the peak and the average spectral densitiesindicates waves observed near the magnetopause, using specific ex- that the field strengthsfluctuate rapidly, with many short in- amplesto illustratethe typical characteristics. tense bursts. The peak broad-band field strengthsfor the ex- amples illustrated in Figure 10, integrated over the entire fre- Spectrum quency range measured,are E = 5.2 mV m-' and B = 1.3 The most striking featuresof both the electric and the mag- gammas.These broad-band field strengthsare reasonablytyp- netic field turbulence observed at the magnetopauseare the ical of all the magnetopausecrossings investigated in this very broad bandwidth and the rapid decreasein the intensity study. with increasingfrequency. For the electric field the turbulence As can be seen from examination of some of the magneto- spectrumusually extendsfrom below a few hertz to about 100 pause crossingsdiscussed in the previous section,it is some- kHz, and for the magnetic field the spectrumextends from be- timesdifficult to distinguishthe magnetopauseboundary layer low a few hertz to about 1 kHz, above which the intensity is turbulence from the electrostatic noise commonly observed usually below the instrument noise level. Typical spectra of throughout the magnetosheath.This is in strong contrast to the magnetosheathelectric and magnetic fields are shown in the interface between the boundary layer and the magnet- Figure 10. These spectrawere selectedfrom the first magneto- osphere,which usually has a sharp drop in the electric field pausecrossing in Figure 3 and cover a 1-min interval centered noiselevels. Careful examination, however, revealssignificant on 1440:00UT. The peak spectraldensities observed over this differencesin the electricfield spectrumbetween the magneto- 1-min interval are shown by the solid curves, and the average pause and the magnetosheath.Usually the electric field in- spectral densitiesare shown by dashed curves.Except for the tensitiesat the magnetopauseare significantlylarger than the isolated peak in the electric field spectrum at about 31.1 kHz, magnetosheath, particularly at low frequencies, <100 Hz. both the electric and the magnetic field spectraldensities are Also, the magnetopause spectrum usually has a nearly con- monotone decreasingfunctions of frequency. Both spectra fit stant slopeon a log-log plot acrossthe entire frequencyrange, a power law frequency dependenceto a good approximation, whereas the magnetosheathspectrum often has a change in

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•SEE I, DAY 314, NOV. I0, 1977 Fi•. •. A detailedcomparison of the plasmawave electricand magneticfield intensitiesand the UCLA magneticfield data for the outboundmagnetopause crossin• in Figure 1. The magneticfield directions•s• • and d•s• • are in •eocentric solar ecliptic coordinates. 7050 GURNETT ET AL.' MAGNETOPAUSE ELECTRIC FIELD TURBULENCE

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ISEE I. DAY 336 DEC. 2. 1977

Fig. 6. A seriesof magnetopausecrossings in the local morningon December2, 1977,again showingenhanced plasma wave electric and magnetic field intensitiesin the region near the magnetopause. slopeat about 300 Hz, causinga distincthump in the spec- panels, from about 1 to 3 kHz, are electrostaticwaves at fre- trum at about this frequency. These differencesin spectral quenciesslightly above the local electroncyclotron frequency, characteristicsare illustrated in Figure 11, which shows se- and the emissionsin the bottom frequency panels, from about lected spectrafrom the magnetosheath,magnetopause, and 100 to 300 Hz, are electromagnetic whistler-mode chorus magnetospherefor the magnetopausecrossing at 1447UT in emissions[Tsurutani and Smith, 1974, 1977]. Figure 3. As can be seen,the magnetopauseelectric field spec- In the magnetosheathand near the magnetopausethe elec- trum is clearly the most intenseand is readily distinguished tric field spectrumis almostfeatureless, decreasing in intensity from the spectrain the magnetosheathand magnetosphere, with increasingfrequency. Many closelyspaced spikes can be primarily becauseof the very nearly linear (power law) seen in the spectrumon time scalesof a few seconds.These frequencydependence on the log-logplot. Usuallythe magne- spikelike fluctuations are probably responsiblefor the large tosheath and magnetosphericspectra show discretenarrow- ratio of peak-to-average field strengthsevident in Figure 10. band features at intermediate frequencieswhich deviate con- The enhanced electric field strengthsat the magnetopauseare siderablyfrom a power law spectrum. not as evident in Figure 12 as in Figure 4 becauseof the auto- Further details of the fine structure of the electric field spec- matic gain control usedin the wideband telemetrysystem. trum at the magnetopauseare shownby the frequency-time In addition to the broad bandwidth electrostatic noise, spectrogramsin Figure 12,which correspond to the magneto- moderately intense narrow-band electrostaticemissions are oc- pausecrossing shown in Figure 4. Prior to about 2230 UT, casionallydetected near the local electron plasma frequency. various narrow-band emissionscan be seen inside the magnet- Two spectra illustrating these types of emissions, shown osphere.The narrow-bandemissions in the top frequency in Figure 13, were selectedfrom the outbound magnetopause GURNETT ET AL.' MAGNETOPAUSE ELECTRIC FIELD TURBULENCE 7051 crossingon November 10, shown in Figures 4 and 5. Com- electricfield intensity.In mostcases studied, little or no spin parisonswith the electron densitycomputed from the plasma modulation is evident in the electric field turbulence observed measurementsshow that the frequenciesof these emissions near the magnetopause.The low frequencyof occurrenceof are very closeto the local electronplasma frequency, spinmodulation effects can be attributedin part to the large These narrow-band emissionsare almost certainly due to ei- fluctuationswhich are presentin this region,thereby making ther electronplasma oscillations at f,,- or electrostaticwaves it difficult to clearly identify any spin modulation effects near the upper hybrid resonancefrequency fuR. For the con- whichmay be present,and to the low probabilityof having ditionsusually encountered, fg- << f•,-, the electronplasma the magnetic field oriented in a favorable direction for detect- frequency and upper hybrid resonancefrequency, fuR = ing sharp spin modulation nulls. However, a few caseswere [(fg-)2+ (f•,-)2]•/2,are soclose together that it is not possible found in which spin modulationwas clearly evident. When- to distinguishbetween thesetwo types of waves. Electrostatic ever a well-defined spin modulation was detected, the electric emissionsnear the electron plasma frequencyare frequently field wasfound to be orientedperpendicular to the localmag- observedat the magnetopause,particularly in the region near netic field. These observations are consistent with the low oc- the magnetopausecurrent layer, as in Figure 5. Other ex- currenceof spinmodulation effects, since for this polarization, amples of these emissionscan be identified in the 31.1- and sharpnulls can be detectedonly when the magneticfield is 56.2-kHz channelsof Figure 1 at about 1500UT (seealso Fig- orientedvery nearly parallel to the spin plane of the electric ure 10) and in the 10.0- and 17.8-kHz channelsof Figure 8 antenna,which occursrelatively infrequently. from about 0803 to 0806 UT. When these emissions occur, A good exampleshowing sharp nulls in the electricfield in- they are usually moderatelyintense, with field strengthsrang- tensityis shownin Figure 14.This exampleis for the magne- ing from about 100/•V m-• to 1 mV m-i, and often occur in topausecrossing at about 1447 UT in Figure 3. This illustra- extremely short bursts,sometimes lasting only a few tenthsof tion shows the electric field intensities in the 100-Hz channel a second. on a time scale which clearly resolvesthe 3-s rotation of the spacecraft.As is expected,two nullsare observedper rotation. Polarization The vertical arrows in Figure 14 indicate the times at which In principle it is possibleto obtain the polarization of the the electricantenna is parallelto the projectionof the mag- plasma wave electric fields from the spin modulation of the neticfield onto the spinplane of the antenna.During this pe-

lOOK 56.2K

17.8K

IO. OK N T 5.62K 3. ILK 1.78K 1.00K 562 311 178

56.2 31.1 17.8 IO.O

5.6

I I

-90 (,

cn 300

:5 200

• 0

UT 020O 0300

TSEE I, DAY 336, DEC. 2, 1977

Fig. 7. Detailed plasmawave electricfield and UCLA magneticfield data for the magnetopausecrossings in Figure 6. 7052 GURNETT ET AL.' MAGNETOPAUSE ELECTRIC FIELD TURBULENCE

lOOK I I i i i i -m the electron plasma frequency. Such a determination would 56.2K "'•'-- • ß . I ß I I 31.1K ,• I I be very important, sinceit wouldallow us to distinguishelec-

17.8K ...... at•- '-"•.... I I • .• --• • . • - ß I I tron plasma oscillations,which should hav• the electric field IO.OK • 5.62K parallel to B, from upper hybrid resonancewaves, which should have the electric field perpendicular to B. Unfortu- z 1.78K i _ I' &"-•-ad' -•- ß nately, the very rapid temporal variations and short duration D 1.00K of these emissionshave precluded a definitive determination n- 562 ,...._..,,.L.•'• I ...... m I •"' • .... ,2,,,,,,... -•-•-f '"'-•i - "I o•' the electricfield direction.Further effortswill be made in i ' i -• the future to determine the polarization of thesewaves. •,,, 178

. 56.2 ..... •' I ...... •'--- I •'" ...... -J...... • ..... I 31.1 ----I ...... i ...... -'"' • ...... -•' - I 17.8 Wavelength I0.0 ...... - -" .... II" ...... '--1 I • I II-, I 5.6 _ _ Estimates of the approximate wavelength of the electric I, iI I,•-i , I I I I I fieldturbulence observed at the magnetopauseare important 1.00K bothto helpidentify the plasma Wave mode involved and to 562 I ...... 311 evaluatethe effects of Dopplershifts on t•e electricfield spec- z 178 trum. Although wavelengthscannot be determined directly ::::) I00 with the ISEE plasma wave instrumentation, some limits on u-'Jm56.2 the possiblewavelengths can be establishedby comparingthe c• 17.8 electric field strengths measured with different antenna rn I0.0 lengths. If waves are presentwith wavelengthsshorter than 5.6 the longestantenna used,then substantialdifferences should io2 be observedin the electricfield intensities,the longerantenna io I indicatinga smallerfield strength.If the electricfield strengths are in good agreement, then the wavelengthsmust be substantially longer than the longestantenna. To investigatethe wavelengthof the magnetopauseelectric field turbulence, comparisonshave been made between the 107 215-m tip-to-tip (v axis) electric antenna on ISEE 1 and the •o6 30-m tip-to-tip (x axis) electric antenna on ISEE 2. Since 6O these comparisonsinvolve spacecraftseparation distances of up to 500 km, crossingsmust be carefully selectedto avoid 4O large temporal variationsin the electric field structureduring 2O the time between the crossings.To minimize the effects of 0.0 UT 07 I0 20 30 40 50 O0 0810 temporal variations, the approach taken is to find crossings R(Re) 12.4 12.2 12.0 11.8 11.6 11.4 11.2 which have a very similar electric field intensity profile on LT(HR) 12.0 12.0 12.1 12.1 12.2 12.2 12.2 both ISEE 1 and 2, preferably including a relatively broad re- TSEE I, DAY 307, NOV. :.'3,1977 gion of nearly constantintensity. One suchexample which has Fig. 8. A magnetopausecrossing near local noon showinga long been chosen for analysis is the period of enhanced electric extended boundary layer region from 0751:30 to 0802:45 UT. The field intensity from about 0320 to 0325 UT on December 2, boundarylayer is characterizedby enhancedbroad-band electric field 1977, in Figure 6. During this crossinga very similar electric intensities,but only a very small increasein the low-frequency magnetic field intensities. field intensity profile can be identified on ISEE 2 at a slightly earlier time, from about 0311 to 0316 UT, in good agreement with the transit time determinedfrom the plasma and mag- riod the magneticfield direction is only about 10¸ above the netic field instruments. For comparison the electric field spin plane. The arrows therefore indicate times when the an- spectra from ISEE I and 2 for these two time intervals are tennais approximatelyparallel to the magneticfield. As can shown in Figure 15. Both the peak and the average electric be seen,the nulls in the electricfield intensityoccur almost ex- field spectrahave been computedfor each spacecraftover the actlywhen the antenna axis is aligned parallel to the magnetic selected time intervals.As can be seen,both the peak and the field, thereby confirming that the electric field is per- averageelectric field spectrafrom the two spacecraftare in ex- pendicularto the magneticfield. The depth of the nulls, some- cellent agreement,within about a factor of 2 over most of the timesas much as 20 dB, showsthat the electricfield is aligned frequency range. This close agreement is a strong indication very nearly perpendicularto B, probably to within less than that the wavelengthsare substantiallylonger than the length 5 o. Other crossingshave been found giving similar resultsfor of the longest antenna, 215 m, since if wavelengths shorter frequenciesranging from about 30 Hz to I kHz. One impor- than the longest antenna were present, then the spectralden- tant consequenceof these polarization measurementsis that sitiesWould differ by a largefactor, as much as (215/30) •- = 51 the electricfield polarization provides a cleardistinction be- for wavelengths shorter than the 30-m antenna. Only at the tween the magnetopauseelectric field turbulence and the elec- relatively high frequencies,above 10 kHz, do differencesthis trostatic noise in the magnetosheath.Rodriquez [1979] has large occur. clearly shownthat the magnetosheathelectrostatic noise is po- Although magnetopause crossingssometimes occur with larizedwith the electricfield parallel to the magneticfield. substantialdifferences in the electric field spectraldensity at Attempts have also been made to determine the electric the two spacecraft, a sufficient number of cases have been field direction of the narrow-band electrostatic emissions near found with excellentagreement between the spectraat the two GURNETT ET AL.: MAGNETOPAUSE ELECTRIC FIELD TURBULENCE 7053

BOUNDARY LAYER

lOOK 56.2K 31.1K 17.8K IO. OK

N 5.62K 3. ILK 1.78K 1.00K 562 311 178 I00 56.2 31.1 1'7.8 I i I0.0 5.6

I I

< 60- i • • 40 i • BL o 20 • 0 uJ -20 i u_ 203 I SEE 2 LAYER I i -• BM •z -2• II i o i <• i 20-r I ! • BN 0 i -20

UT 0735 0740 0745 0750 0755 0800 0805 0810 ISEE I, DAY 307, NOV. 3, 1977

Fig. 9. A detailedcomparison of the plasmawave electric field intensities and the UCLA magneticfield data for the magnetopausecrossing in Figure 8.

spacecraftto lead us to concludethat these eventsrepresent field spectrumof the magnetopauseturbulence. Without any quiescentcondition, undisturbed by temporalvariations dur- informationon the wavelengthsinvolved, it would be possible ing the time between the two crossings.To the extent that the to account for the observedfrequency spectrumentirely in spectrumin Figure 15 representsa time stationarycondition it terms of Doppler shifts,since the flow velocitiesnear the mag- can be concludedthat the wavelengthof the electric field tur- netopauseare often very large, > 100 km s-'. However, to ac- bulence must be substantiallylonger than 215 m, otherwise count for frequenciesof 10-100 kHz by Doppler shifts alone, larger differenceswould be observedin the electric field in- assuminga flow velocityof 100 km s-•, would require wave- tensities measured by the two antennas. It is possible that lengthsof 1-10 m or less.Since such short wavelengths are not wavelengthsshorter than 215 m may be presentat frequencies observedand cannot occur becauseof the Debye length re- above 10 kHz. However, even in this frequency range, limits striction, it seemsreasonably certain that the observedspec- can be placed on the wavelengthbecause of the Debye length trum cannot be accounted for by Doppler shifts alone. For XD= (eoKT/ne2)'/2. As is well known,the shortestwavelength wavelengthsgreater than 200 m and a typical flow velocity of which can occur in a plasma is approximatelyAm -- 2•rAD.For 100 km s-• the maximum Doppler shift is lessthan 500 Hz. At the case shown in Figure 15 the correspondingelectron den- frequenciesbelow about 500 Hz, Doppler shifts could, of sity and temperaturesare rte = 90 cm-3 and Te = 8 X 105øK, course, be very substantial.However, the absenceof any which give a Debye length of 6.5 m and a minimum wave- break or substantialmodification in the shapeof the spectrum length of about 40 m. in this frequencyrange suggeststhat Doppler shiftsare prob- These limitations on the wavelength have important impli- ably relatively unimportant in determining the overall shape cationswith regard to the interpretationof the overall electric of the spectrum,even at frequenciesbelow 500 Hz. 7054 GURNETT ET AL.: MAGNETOPAUSE ELECTRIC FIELD TURBULENCE

io-4 boundary layer, integrated over the frequency range from 5.6

- ISEE-I - Hz to 311 kHz, are about 5 mV m-' and 1 gamma.Because of io-6 - %./PEAK DAY314, NOV. I0,1977 - the steep spectrum, most of the contribution to these broad- band field strengthsoccurs at low frequencies,so the field T -.....x,,x• 1439:30-1440:30UT _ strengthswould be even larger if the integration were ex- - '"",.,,,, '•% MAGNETOPAUSE- tended to even lower frequencies.In all caseswhere definitive E - AV CTRA - measurementscould be made, for frequenciesless than 1 kHz, cn I _ _ the electric field is found to be oriented very nearly per- o _ _ pendicular to the local magnetic field, and the wavelengths are longer than about 215 m. The region of occurrenceof the narrow-bandelectrostatic waves near f•,- is lesscertain, since theseevents do not occur very frequently. In most casesthese emissionstend to occur near the magnetopausecurrent layer icf16 \ or in regions with large variations in the magnetic field, indicating large currents.The electricfield polarizationof these narrow-band emissions could not be determined because of their rapid fluctuationsand short duration. xx•PEAK In considering the origin of the broad-band electric field T 10_4 noise, probably the most important characteristicwhich must •,kjAVERAGE ] be considered is the very broad, nearly featureless,electric field spectrum.Normally, no distinctivenarrow-band features or cutoff can be identified in the spectrum which could provide a definite associationwith a specificplasma wave mode. %,, _ Becauseof the very large plasma flow velocitiesin the region where this noise is 'observed,it is likely that Doppler shifts •Cf¸ _

1.0 I01 102 103 104 t05 106

-4 FREQUENCY (Hz) l0

./MAGNETOPAUSE ISEE I Fig. 10. Typical electricand magneticfield spectraof the enhanced •E6 _ % DAY314, NOV I0, 1977 plasmawave turbulenceobserved near the magnetopause. __ , • 1442'00- 1443 O0 - - - __ •'•, • 144630-1447 30 -- __ -145130 ..... L¾© 4. SUMMARY AND DISCUSSION 0-12 In this study we have shown that enhancedplasma wave turbulence levels are a common feature of the magneto- ¾14 pause.Three distincttypes of plasmawave noise are observed at the magnetopause,(1) a broad-band spectrumof elec- ¾16 tric field turbulenceextending from a few hertz to about 100 kHz, (2) a broad-bandspectrum of magneticfield turbu- ¾18__ lence extendingfrom a few hertz to about 1 kHz, and (3) lessfrequently, narrow-band electrostatic emissions ne•/r the local electronplasma frequency. Both the broad-bandelectric - OPAUSE field noise and the broad-band magneticfield tend to occur in i¾2 _ -o the same region. Usually the enhancedbroad-band electric

and magneticnoise intensities occur in a region which corre- ic•4 _ SHEATH spondsclosely to the plasmaboundary layer, beingbounded by the abrupt magneticfield transitionat the magnetopause •5 6 currentlayer and the abrupt drop in the plasmadensity at the inner, earthward, edge of the plasma boundary layer. Com- parableelectric and magneticnoise intensities have also been MAGNETOSPHERE observedin regionswhich Russelland Elphic[1979] identify as ¾ro 'flux transfer events' and in regions which Paschrnannet al. [1979]identify as either isolatedinclusions of plasmainto the I01 102 I0s 104 105 I06 magnetosphereor a suddenswitch-on of the boundarylayer. FREQUENCY (Hz) The regionsof enhancedplasma wave turbulencelevels are nearly alwaysassociated with large fluctuationsin the plasma Fig. 11. A comparisonof theelectric and magnetic field spectra in flow velocity and density and irregularitiesin the magnetic the magnetosheath,magnetopause boundary layer, and magnetospherefor themagnetopause crossing at 1447UT in Figure3. The field direction and magnitude, all of which are well-known electricand magneticfield spectraat the magnetopauseare clearly characteristicsof the boundary layer plasma. Typical maxi- distinguishedfrom the magnetosheathand magnetosphereby their mum broad-bandelectric and magneticfield intensitiesin the increasedintensity, particularly at low frequencies. GURNETTET AL.: MAGNETOPAUSE ELECTRIC FIELD TURBULENCE 7055

'.C.•-6'79:•9:.:2

__/:;:,•-::-.•. . • •. ! -- . ß ......

I I I. I I '1,0 I I I I I i ! I t I

"'EL.'ECTRIC'FIELD •1 I I I I

•0 -- '• .•: ...... :•...•.. . 2.0-- ...... =•.•.... -•.... --• :-•-•½.fi...... 1.0-- ...... '...... '...... il ...... ,...... ' '...... EL.ECTRICFt-ELD I 'i' I I I

'•'.... ::::•::;•'•:.-•::::'.•: '•::•--'.....•. • '""•:':•:'•....i: -•½'•: '• :...:.. :•-•. -.:I:: :•::••.-...... ::• ...... :•::...::.•:::•:.:•:.•:•-•:..:.:•:-:;:•:•.•:•-.:½:::•:•:•:•::. •'•:•-;•:•:-'-:•:::•:•::•'•::?•?• •:•..;:;-':.•.::;.:::•::::?':•-•':'•:-,•'"'.::-.•½;.•;•-;;.•.:•-•:•.•-:•::...•::::::::::::::::::::::::::...... ::•'::.•.. ' ...... -::--:::::.•.-.'': •-i ''•:..-::.;-t:•::•:• ¾g.•.... i':-'•.... :•::;:.i"•::"(.'.•-----::.::.'•-'"'::;'...... •. ..:::::::::::::::::::::::::::::::::::::::::::•--- " ...... -...... --:•½•½-':-•::•...•: :...... ;•:--•::':• ...... •...,::•:•'-'?-•:•??-'•: .....:;•-•:-•::::•;• ;:•::•'•';.•::•:" •::'•:'•:'7%::' ...... '•.:'½'•:"•:'•' ::•'" '"%•-"...... :- ?:•-• •'•.•'•f'•'•t:•":•-•.• :•:•-:,-•"•'?•'••:•':•'-•;•:-:•' .•:•:-•-'•'••½:;•'•:•::":.... 0 '":':'?:¾'•: ....:":"•' '"':"' ...... '•':" •':'":::':½•" ....';...... •:"•...... ;•'•';::' •:':•:' '•'• ..... ;?:•'::'"• :":.....:•:•""" ::';:':::? "•:':'•":•'•' .'.. :•::...... ' ' '....

. . UT 2'20:0 2.2I'0 22.20: 2230 2240 2250 2'500- R .(Re) 9.37 9.61 9,'8-4 i0,1 !'0.3' 1'0.5 10.7 LT (HR) 7,3 7.4 7,4 7.5 '7.5 7,6 7.6:

ISEE I-, D-A"Y3:1:4., NOV'. I 0.,: 1977

Fig. 12. High-resolutionfrequency-time spectrograms of the electricfield turbulence observed for the magnetopause crossingin Figure 4. may be presentwhich couldsmear out sharpnarrow-band tio shouldvary as l/f. At low frequencies,•< 1 kHz, the energy featuresin the spectrum.At frequenciesbelow 10 kHz the densityratio for the spectrain Figure 10 has approximately maximumDoppler shift which would be consistentwith the thecorrect frequency dependence, (cB/E) • ocl/f"', to be con- wavelengthestimates is about500 Hz. Overall,the monotonic sistent with the whistler mode. However, the index of refrac- powerlaw frequencyvariation of the electricfield spectrum tion computedfrom the energydensity ratio is abouta factor givesone the strongimpression that this noiserepresents a of 10 too small when it is comparedwith the squareof the fully developedturbulence process with the electricfield en- whistler-modeindex of refraction. For example, at 10 Hz, cB/ ergy cascadingto higherand higherfrequencies (larger wave E = 94, whereasthe whistler-modeindex of refractionusing numbers)because of nonlinearinteractions. f•,- = 50 kHz andfg- '- 3.4 kHz is 270.This comparison in- Becausemagnetic noise is alwaysobserved in the samere- dicates that a substantial electrostaticcomponent is present, gionas the broad-bandelectric field noise, the questionnatu- abovewhat would be expectedfor whistler-modewaves. It is rally ariseswhether the electricfield noiseis due to elec- possiblethat thisadditional electric field could be accounted trostaticor electromagneticwaves. The magneticfield noise for by whistler-modewaves propagating with wave vectors must consist of whistler-mode waves, since no other electro- near the resonance cone, which tends to increase the cB/E magneticmode of propagationoccurs in the frequencyrange, ratio. At higher frequencies,>1 kHz, substantialelec- betweenthe proton and the electrongyrofrequencies, where tric field intensitiesstill exist at frequenciesabove the electron this noise is observed. If the electric field noise consists of gyrofrequency,fg- • 3.3 kHz, in a regionof the spectrum whistler-mode waves, then the electric-to-magnetic field en- where the whistler mode cannot propagate. Since Doppler ergydensity ratio must be equal to thesquare of thewhistler- shifts are believed to be small, it is clear that this portion of modeindex of refraction(cB/E) 2 = n•, wheren is the index of the spectrumcannot be accountedfor by whistler-mode refraction. Since the whistler-mode index of refraction waves.Because a substantialdiscrepancy is alsopresent in the squared,n• = (f,,-)2/ffg-, variesas l/f, the energydensity ra- electric-to-magneticfield ratio at low frequenciesand the elec- 7056 GURNETTET AL.: MAGNETOPAUSEELECTRIC FIELD TURBULENCE

noisehas been found to be oriented very nearly perpendicular c•4 • ' •'""1 ' ' "•'•I ' • '•'•I ' ' '• ISEE -I to the local magneticfield, similar to the resultsof this study. DAY $14, NOV. IO, 1977 The fact that the electric field, hence wave vector K, is nearly perpendicular to the local magnetic field and the low electron- 2238:52UT to-ion temperature ratio Tel T• •< I stronglylimits the possible

ELECTRON PLASMA plasma wave modes which could be involved in the genera- OSCILLATIONS - tion of this turbulence. At the present time, only two plasma wave instabilities have been identified which could possibly account for the observed characteristics of this turbulence. These instabilitiesare the electrostaticion-cyclotron instability [Ashour-Abdallaand Thorne, 1977; Swift, 1977] and the lower-hybrid-drift instability [Huba et al., 1978; Lemon and Gary, 1977]. Both of these instabilities involve wave vectors _ nearly perpendicularto the magnetic field and are driven by w currentsparallel to the magnetic field in the caseof the ion- w cyclotron instability and perpendicular to the magnetic field in the caseof the lower-hybrid-drift instability. The lower-hy- e I01 102 I0 5 104 I05 IO 6 brid-drift instability also requires a gradient in the plasma

FREQUENCY, Hz density perpendicularto the magneticfield. Both density gra- dients and currentsparallel and perpendicularto the magnetic Fig. 13. Selectedelectric field spectrafrom the magnetopausefield are thought to be presentin the magnetopauseboundary crossingin Figures4 and 5 showingthe occurrenceof intensenarrow- band emissionsnear the local electronplasma frequency. layer, so both of theseinstabilities are good candidatesfor explaining the broad-band electrostaticnoise observed in this region. The absenceof distinct lines near the ion-cyclotron tric fieldspectrum extends smoothly across the whistler-modefrequency or the lower-hybrid-resonancefrequency is prob- resonanceat fg-, it seemsalmost certain that a substantialably not a seriousconcern becauseDoppler shifts and non- contribution to the electric field spectrumis due to elec- linear effectscould act to broaden the spectrumand smear out trostaticwaves, even at frequenciesbelow fg-. theseidentifying features.Since the ion-cyclotronfrequency is Broad-band electrostaticnoise spectra comparablewith very small,fg- '" 1 Hz, in the regionwhere the broad-band thosein Figure10 have been observed before in the magneto- electrostatic noise is observed, only a small Doppler shift tail [Gurnettet al., 1976],along the auroralfield lines [Gurnett would be required to convert discreteemissions at high har- andFrank, 1977],and in the polarcusp regions of the magnet- monics of the ion-cyclotron frequency into an essentiallycon- osphere[Gurnett and Frank, 1978].All of theseobservations tinuous spectrum. One possible difficulty with the ion-cy- can be interpretedas involvingboundary-layer-like plasmas clotron mechanism is that extremely high harmonics of the involvingsubstantial currents and spatialgradients not dis- ion-cyclotron frequency would have to be excited to explain similarto the plasmaobserved in themagnetopause boundary the broad bandwidth of the electrostatic noise. If the main en- layer.In all caseswhere spin modulation measurements have ergy input to the turbulence spectrumoccurs at the lower-hy- been made, the electricfield of the broad-bandelectrostatic brid resonancefrequency, fLOR -• (fg-fg+),/2,then onewould

ELECTRICANTENNA PARALLED TO '• i

1447:25 U.T. :26 :27 :28 :29 :30 '31 :32 :33

ISEE-I, DAY 314, NOVEMBER 10, 1977

Fig. 14. Hightime resolution electric field measuremepts at 100 Hz for themagnetopause crossing at 1447UT in Fig- ure 3 showingthe occurrenceof sharpspin modulationnulls. The nulls occurwhen the electricantenna axis is oriented approximatelyparallel to the localmagnetic field, indicating that the electricfield of the broad-bandelectrostatic turbulenceis polarizedperpendicular to the magneticfield. GURNETTET AL.:MAGNETOPAUSE ELECTRIC FIELD TURBULENCE 7057

[d4 [ [ [[i[[]] [ [ [[[[[[[ [ [ ]][[[][ I [ [[]][[] [ [ [[[l[[] [ [ [[•[[[ l N • L(ISEE-I)= 215METERS - [d6 '-.•'•,. L(T_SEE-2)='30 METERS _

PEAK ',. x•, ISEE-I [d 8 "• "'• •ISEE-2 %

\',, • ', DAY336, EDC. 2, 1977 •. '• • , 10-[2

Xrn

i• [6

1.0 I 01 102 103 104 105 106

FREQUENCY, Hz

Fig.15. A comparisonof the spectrum of themagnetopause electric field turbulence using the 215-m electric dipole antennaon ISEE I andthe 30-m electric dipole antenna on ISEE 2. The closeagreement between the two spectra at frequenciesbelow about 10 kHz, indicates that the turbulence has wavelengths substantially longer than 215 m in thisfre- quency range. expectsome indication of thischaracteristic frequency in the polarizationmust be determined.If theelectric field is parallel observedelectric field spectra.In somecases, such as the spec- to the local magneticfield, then the wavesare probablyelec- trum at 2238:52UT in Figure 13,a distinctbreak in the slope tron plasmaoscillations driven unstable by field-alignedelec- of thespectrum can be seenat about50 Hz, whichis nearthe tron beams.Attempts have been made to identify such elec- lower-hybrid-resonancefrequency. However, many other tron beamsin the plasmadata withoutsuccess, possibly owing casesoccur, such as in Figure 10 and 15, for whichthe slope to the extremelyshort duration, < 1 s, of the plasmaoscillation of thespectrum isessentially constant in theregion near bursts.If the electricfield polarizationis perpendicularto the The originof thewhistler-mode magnetic noise observed in local magneticfield, then the wavesare probablyassociated the magnetopauseboundary layer involvessimilar inter- with the upper-hybrid-resonancefrequency and similarto the pretationalproblems. Since energetic, >2 keV, electronfluxes intenseupper-hybrid waves reportedby Christiansenet al. comparablewith the energyspectra typical of the outermag- [1978]and Gurnettet al. [1979].These waves are commonly netosphereare detectedin theseregions [Russell and Elphic observedin the outer regionsof the magnetosphere,and their 1979],one possibility is that the whistler-modeturbulence is originhas been investigated in somedetail by R6nnrnarket al. generatedby an anisotropy in the energetic electron flux, with [1979]and Kurth et al. [1979]. a greatlyenhanced growth rate caused by the reducedreso- Having consideredthe possibleorigins of the plasmawave nancevelocity in the relativelydense cool boundarylayer turbulenceobserved at the magnetopause,the principal and plasma,as in the mechanismof Kenneland Petschek [1966]. probablymost important question which remains is the pos- However,inspection of the electronanisotropy in the region sible effect which this turbulence may have on the overall where the whistler-modemagnetic noise is observedusually structureof the magnetopause.The existenceof greatly en- doesnot indicatea sufficientanisotropy to producewhistler- hancedplasma wave turbulencelevels at the magnetopause modegrowth over such a broadfrequency band. The steeply stronglyindicates that microscopicplasma processes play an decreasingmagnetic field spectrum, with maximum intensities importantrole in thisregion of the magnetosphere.The possi- belowa few hertz,suggests instead that the originof this tur- bilities are numerous. The broad-band electrostatic turbu- bulenceoccurs at low frequencies,the higher frequencies lencemay providethe anomalousresistivity required to ac- beingproduced by a nonlinearcascade process. Several mech- count for the magnetopauseenergy dissipation and heating anismsbased on velocityshear [D'Angelo,1973] and drift reportedby Mozer et al. [1979] and Scudderand Ogilvie wave instabilitiesexist which could accountfor the generation [1979].This turbulencemay accountfor the accelerationof of the turbulence. electronsto high energiesin the region near the magneto- Beforethe origin of the narrow-bandemissions near the pause,as discussedby Meng andAnderson [1975] and Baker electronplasma frequency can be established the electric field and Stone[1978]. Electric and magneticturbulence may play 7058 GURNETT ET AL.: MAGNETOPAUSE ELECTRIC FIELD TURBULENCE

a role in the spatial diffusion and transport of charged parti- Haerendel, G., G. Paschmann,N. Sckopke, H. Rosenbauer,and P. C. cles acrossthe magnetopause.For the moment we do not at- Hedgecock, The frontside boundary layer of the magnetosphere and the problem of reconnection,J. Geophys.Res., 83, 3195, 1978. tempt to answer these basic theoretical questions.The main Hasegawa, A., and K. Mima, Anomalous transport produced by ki- intent of this paper is to provide the essentialparameters, elec- netic Aflv•n wave turbulence,J. Geophys.Res., 83, 1117, 1978. tric and magnetic field spectra and relationshipsto plasma Heikkila, W. J., Is there an electrostaticfield tangentialto the dayside and magnetic field characteristics,needed to stimulate further magnetopauseand neutral line?, Geophys.Res: Lett., 2, 154, 1975. investigationof theseimportant problems. Hones, E. W., Jr., J. R. Asbridge, S. J. Bame, M.D. Montgomery, S. Singer, and S.-I. Akasofu, Measurementsof magnetotail plasma Acknowledgments.The authors wish to expresstheir thanks to flow made with Vela 4B, J. Geophys.Res., 77, 5503, 1972. B. U. •. Sonnerupat theMax-Planck-Institut in Garching and to S. Huba, J. D., N. T. Gladd, and K. Papadopoulos,The lower-hybrid- P. Garyand J. T. Goslingat theLos Alamos Sclentific Laboratory for drift instability as a sourceof anomalousresistivity for magnetic numeroususeful discussions. This researchwas supportedin part by field line reconnection,Geophys. Res. Lett., 4, 125, 1977. NASA through grant NGL-16-001-043 and contract NAS5-20093 Huba, J. D., N. T. Gladd, and K. Papadopoulos,Lower-hybrid-drift with the University of Iowa, through contractNAS5-20064 with the wave turbulence in the distant magnetotail, J. Geophys.Res., 83, Universityof Californiaat LosAngeles, and •through contract NAS7- 5217, 1978. 100 with the Jet Propulsion Laboratory, California Institute of Tech- Kennel, C. F., and H. E. 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