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 elec- trostaticion-cyclotron instability and the lower-hybrid-driftinstability, are consideredthe primary can- didatesfor 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-par- ticle 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 there- fore 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 BOW MAGNETOPAUSE MAGNETOPAUSE SHOCK (INBOUND) (OUTBOUND) I I I II 5IlK 178K lOOK 56.2K 51.1K 17.8K I O.OK 5.62K 5. ILK 1.78K 1.00K 562 511 178 I00 56.2 51.1 17.8 I0.0 5.6 5. ILK 1.78K N 1.00 >..y 562 o 511 z ::o 178 56.2 I I • 51.117.8 I0.0 5.6 UT 0000 0400 0800 1200 1600 2000 24O0 R(Re) 20.5 18.5 15.6 I 1.6 5.45 6.03 12.0 LT (HR) I0.0 10.6 11.5 11.7 12.7 6.22 7.78 '['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 31.1K 17.8K IO. OK 5.62K >-' 1.78K z 1.00K : 562 n-- 311 178 • 100 ,,i 56.2 31.1 17.8 I0.0 5.6 _':..... .... o.o [,, ;';'l,';',,;[,,, I - I I UT 1420 30 40 50 O0 I0 1520 R(R e) 8.5 8.2 7.9 7.6 7.4 7.I 6.8 LT (HR) 12.0 12.1 12.1 12.2 12.2 12.3 12.3 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

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