May 1, 1987 - ation within the lobes of Jupiter's magnetic tail down to the lowest frequency of the ... Inside Jupiter's magnetosphere the plasma wave detectors.
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 92, NO. A5, PAGES 4701-4705, MAY
1, 1987
Polarization of Low-Frequency Electromagnetic Radiation in the Lobes of Jupiter's Magnetotail S. L. MOSES
TRW Spaceand TechnologyGroup,RedondoBeach, California W. S. KURTa
Departmentof Physicsand Astronomy,Universityof Iowa, Iowa City
C. F. KENNEL, F. V. CORONITI, AND F. L. SCARF TRW Spaceand TechnologyGroup,RedondoBeach, California
The plasmawave instrumentson the Voyager spacecrafthave detectedintenseelectromagnetic radiation within the lobes of Jupiter'smagnetic tail down to the lowest frequencyof the detector (10 Hz). During a yaw maneuverperformedby Voyager 1 in the lobe of the Jovian magnetotail,a modulation appearedin the amplitudesof wavesdetectedin the 10-, 17.8-and 31.1-Hz channelsof the plasmawave analyzer,well below the local electroncyclotronfrequencyof 260 Hz. The lowest amplitudesoccurred when the antenna axis was most nearly parallel to the magneticfield. Wave amplitudesin the 56.2-Hz and higher frequencychannelsremained nearly constantduring the maneuver.From the cold-plasma theory of electromagneticwaves,one can concludethat the plasma frequencywas betweenthe 56.2- and 31.1-Hz channelswhere the parallel-polarizedcomponentof the spectrumcuts off. This implies a tail-
lobe densitybetween3.2 x 10-5 and 1.5 x 10-5 cm-3. The left-handcutoff frequencywould then be below 10 Hz, consistentwith either the Z mode (L, X) or whistlers(R mode) in the modulated channels. However, locally generated whistler mode waves at these frequencieswould require electrons with resonantenergiesgreaterthan 25 MeV, whichare unlikelyto existin significantdensities.
INTRODUCTION
Inside Jupiter's magnetospherethe plasma wave detectors of both Voyager 1 and Voyager 2 observeda continuum of electromagneticradiation occasionallydown to 10 Hz, the lowest frequency detectable by the instrument [Scarf et al., 1979a; Gurnettet al., 1979]. Although the low-frequencycutoff of the continuum spectrumis often usedto determinethe local plasma densitywithin planetary magnetospheres,what exactly the cutoff representscan be ambiguous, since high-frequency electromagnetic modes with access to free space cut off at different frequenciesdepending upon their polarization [Stix, 1962]. The electromagnetic mode that is left-hand polarized when propagating parallel to the ambient magnetic field and is an ordinary wave when propagating in the perpendicular
direction(L, O mode)has a cutoffat f•,. The modethat is right-hand polarized for parallel propagation and is an extraordinary wave for perpendicular propagation (R, X mode)
comparing the wave amplitudes with the angle between the antenna axis and B [Gurnett and Shaw, 1973; Shaw and Gurnett, 1980]. The Voyager plasma wave instruments (PWS) also use dipole antennas, but since the spacecraft are three-axis stabilized, the routine measurement of wave polarization is not possible by the rotating dipole method. The planetary radio astronomy experiment (PRA) on Voyager is able to measure wave polarization, but at much higher frequencies,by employing the same antennas as two monopoles. The density profiles that have been published based on PWS data always
assumethat the continuumcutoffisfp [Gurnettet al., 1981], but this procedure provides only a maximum estimate for the
local value of fp due to the uncertainidentificationof the cutoff frequency, and also due to the possibility that a highdensity region between the spacecraft and the continuum source may cut off the electromagneticwaves above the local
re. Exceptfor thelattersituation,identifying thelocalvalueof
cutsoff at fa=o= fce/2+ I-(fce/2) 2 _.•_ fp211/2(fc,= electronfe with the continuumcutoffshouldbe reasonablyaccurate cyclotron frequency),which is always greater than the local plasma frequency. Since only one mode can have an oscillating E field polarized parallel to the ambient B field (ordinary waves),plasma wave measurementstaken from spinning spacecraftwith dipole antennas in the earth's magnetosphere have been able to identify both cutoff frequenciesand demonstrate
that
terrestrial
continuum
radiation
consists
of
a
whenfe > fce,sincethenfe andfR=oarecloseto oneanother.
Unfortunately this may not be true in the lobes of Jupiter's magnetic tail, where the densities have been reported to be extremely low and, therefore,are of great interest. As an example, Gurnett et al. [1980] presented a continuum spectrum measured by Voyager 1 on March 8, 1979, at 1355 UT and estimated the local plasma density to be approximately 3
x 10-6 cm-3 (fp< 17.8Hz)based onthefrequency at which
roughly equal mixture of L, O and R, X mode waves by
the wave amplitudesbegan to declinerapidly. One event has been found, however, where the cutoff fre-
Copyright 1987bytheAmerican Geophysical Union. Paper number 6A8825. 0148-0227/87/006A-8825502.00
quenciesfor the different components of the continuum spectrum can be positively identified. During the Jupiter encounter, Voyager 1 performed various scheduled maneuvers 4701
4702
MOSESET AL.' BRIEFREPORT
VOYAGER
10_6 COMPARISON OFSPECTRA AND POLARIZATIONS
1 MARCH 8, 1979
562.
100 db
//---.. /
//%_..•..•, '•
311.
J
• 10-12
178.
100.
10-14
56.2
10-16
......
eBX:0.98ø
....
eBX:86.81ø
eBX :1.78 ø
f(Hz)
lO
i
I
lOO
1000
10000
ffHz)
Fig. 3. Comparison of two spectra taken while the antenna was parallel to B (solid and dotted lines)with one taken while the antenna was perpendicularto B (dashedline).
31.1
17.6
field amplitude),one can estimatethe plasmadensityin the tail lobe and identifythe wavemodeswhich compriseJovian 10.0
continuumradiation from cold-plasma,electromagneticwave theory.
2100
2130
2200
2230
2300
2330
0000
0030
0100
YAWTAIL LOBE I PLASMA SHEET Fig. 1. Plasma wave data from the Jovian tail lobe showingelectromagneticradiation extendingdown to 10 Hz at the time of the yaw maneuver. The plasma sheetwas entered at 0005 UT.
POLARIZATION
MEASUREMENTS
Figure1 showsdatafromtheVoyager1 16-channel plasma waveanalyzertakenfrom2100on March 8 to 0100on March 9, 1979.The eightlowestfrequencychannelsare shownwith both peak (line) and average(solid)wave amplitudes.Until
one of which was a rotation of 360ø about the yaw axis that 0005 UT the electromagnetic emissionsare detecteddown to fortuitouslyoccurredwhile the spacecraftwas in the Jovian 10 Hz, after which time the spacecraftentered the denser tail lobe. This paper contains an analysisof the electro- plasmasheet.The low peakto averageratio of the waveam-
magneticwavesdetected duringthismaneuverandshowsthat wavespolarizedparallel to the ambientB field cut off at a higherfrequency than waveswith predominantly perpendicular polarizations.By assumingthat the wavesat thesefrequenciesare electromagnetic (the PWS measures only the E ANGULAR SEPARATION OF ANTENNA AXIS AND B-FIELD
plitudesin thesechannels, asidefrom isolatedspikesof high intensity,is characteristic of electromagnetic radiationwhich is not as burstyas electrostatic emissions. In thiscase,prior to entry into the plasmasheet,conventionalanalysiswould
assignan upperlimit to f•, of 10 Hz, corresponding to a plasmadensityof lessthan 10-6 cm-3. The yawmaneuver commencedat 2242:30 and ended at 2315:40, during which
90 ø
time the amplitudesof wavesin the 10-, 17.8-,and 31.1-Hz channels were modulated with minima occurring near 2247
60 ø
and 2304. The wave amplitudesfor 56.2-Hz and higherfre-
GBX
quenciesremainedalmostconstant.
30 ø
0o -2
56.2Hz 1,["t'J]l II " f
i 17.8Hz •,t • Jl! 'I
I
-6 Lu
•'•J
'
' I• I
I
•
O -6 •
The yaw maneuverwasa rotationaboutthe Y axisof the spacecraft, whichis perpendicular to the antennaaxisfor the plasmawavereceiver(thedirectionof maximumsensitivity to the E field),whichin turn is parallelto the spacecraft X axis. The averageanglebetweenthe tail lobeB fieldand the axisof rotationduringthe maneuverwas88.6+ 0.3ø,thereforeB was nearlyparallel to the plane definedby the rotating antenna axis.In Figure 2 the anglebetweenthe antennaaxis and B (OBx = cos-• (Bx/IBI))is plottedat the top on the sametime scale as the data from the 10-, 17.8-, and 31.1- and 56.2-Hz
channelsfor 30 min during the yaw maneuver.The minima
that appear in the three lowestfrequencychannelsoccur simultaneously whenthe B fieldis mostnearlyparallelto the I 2240
"I 2245
II I 2250
I
I
I
I'
2255
2300
2305
2310
antennaaxis,while no suchmodulationis evidentin the 56.2Hz channel.
Fig. 2. Plasmawave data duringthe spacecraft yaw maneuver. Figure3 showsfrequency spectraaveragedover20 s during The top panelplotsthe artec betweenthe antenna(X) axisand B. The bottom three wave analyzerchannelmeasurements showan ab- the two minima(solidand dottedlines)and the peak between the minima when the angle betweenB and X was greatest senceof waveswith E parallel to B.
MOSES ETAL.' BRIEFREPORT COMPARISON
OF WAVE
4703
POLARIZATIONS
CMA DIAGRAM
10 7
90 ø
106
fl3=45 Hz
fc/
fce=260 Hz 10 5
104
1
03
90 ø L
102
(•o
31.1 Hz 56.2
Hz
\+ 100
101
Hz
178I--Ixz 100
10-6 10'-8 10'-1010'-1210•-12 10'10 1(•-8 10-6
311Hz fUH x
10-1
10-2
90 ø
1
103
(fp/f)2
Fig.5. CMA diagramfor f•e- 260 Hz assuming fp-45 Hz. Crosses mark the locationsof the PWS waveanalyzerchannels.
ool,-.., ,/, ,_.-;,:oo 10-6 10-8 10-10 10-12 10-12 10-10 10-8
10-6
90 ø
betweenthe wave E field and B (the correlationcoefficientin all thesesinusoidalfits was > 0.95).
One concludes from this data that waveswith oscillatingE fieldsalignedparallelto B exhibita low-frequency cutoffbetween the 31.1- and 56.2-Hz channels that is not evident for
waveswith other polarizations.The magnitudeof the B field
at this time was9.3 nT, corresponding to f,:e- 260 Hz, well above the implied cutoff frequency.In the next sectionthese resultsare interpretedwith the intent of identifyingthe observedwavemodesanddeterminigthelocalplasmadensity.
010-6 ø[
[
['" q'. ,
•'-//'
10-8 10-10 10-12 10-12 10-10 10-8
• 180 o
10-6
V /M2-Hz
INTERPRETATION
Figure 5 is a plot of the Clemmow-Mullaly-Allis (CMA)
Fig. 4. Forty-eight second averagesof wave spectral density diagram for the frequencyrange relevant to this event. The (dots)plottedin polar coordinatesas a functionof Osx.Dashedlines verticalaxisis in unitsof (fee/f)2 and the horizontalaxisin represent best fit curves to the data: sinusoids for 10, 17.8 and 31.1 Hz; constant for 56.2 Hz.
(dashed line). At higher frequenciesthe wave spectra are nearly identical,while at the three lowestfrequenciesthe wave spectraldensitiesin the minima have dropped by 1 to 3 orders of magnitude. Becauseof the large variation in wave amplitude for different antenna-magnetic field geometries, this graph clearly illustrates the pitfalls in defining cutoff frequencieswithout knowledgeof the wave polarization. The data in Figure 2 is presentedin a different form in Figure 4, where 48-s averagesof the wave spectraldensities (dots)are plotted in polar coordinateswith 0ø representingthe first minimum and 180ø representingthe second.The wave E field amplitudes in the lower three channels were fitted to functionsof the form E(O•sx) = E• sin O•s x + Eo (whereE• and E 0 are constants)and the resulting curves are plotted with dashed lines; as a comparison,the dashed line in the 56.2-Hz channelgivesthe averagewave amplitude during the rotation. The modulated channelsfit the sinusoidalfunctionsvery well and later thesefunctionswill be used to determine the angle
unitsof (f;,/f)2.Plottedon the diagramare the right-hand cutofffrequency(fR=0),the upperhybridfrequency(full =
(fce+f;,2)1/2),and the left-handcutoff(fL=0= -f½•/2+ r(fc•/2)2+f;,2•1/2). Alsoindicated on thediagram arerepresentativephasevelocitysurfaces for modesthat propagatein the different regionswhose boundariesare the various fundamental frequencies.The phasevelocitysurfacesshowthe variation of wave phasevelocityas a functionof angle with respectto B, whereverticaldenotespropagationparallel to the magneticfield.The lettersidentifythe polarizationof the wave modewhenpropagatingparallel(L or R) or perpendicular(O or X) to B. For convenience we will refer to a particularmode by the nonevanescent, parallelor perpendicularpolarizations which characterizeit, such as L, O mode for a wave mode that
containsleft-handpolarizedand ordinarywaves.Sinceonly a mode containingordinary wavescan have E parallel to the ambientB field,and ordinarywavesdo not propagatebelow
f;,, thenf;, mustlie betweenthe 31.1-and 56.2-Hzchannels. Crosseslocatethe plasmawave analyzerchannelfrequencies in the CMA diagram,usingthe measuredvalueof f½•and choosing f;, -- 45 Hz. Wavesof all polarizations canpropagate in the regionssampledby the channelsabove 31.1 Hz, while
4704
Mosm
ET AL.' BRIEF REPORT
the 10-, 17.8-, and 31.1-Hz channels can only detect L, X mode (the so called Z mode) and R mode (whistler) waves. The cutoff at fR=o and the resonancesat fun andfce coalesce into a narrow frequencyrange between the 178- and 311-Hz channels, so that any changes in polarization occurring at those frequenciescannot be detected by the Voyager wave analyzer. Only the R mode can propagate at frequencies
COMPARISON OF MEASURED POLARIZATION WITH THEORETICAL VALUES .....
THEORETICAL
--
ANGLES
MEASURED
-fl:)'50
8O
-fl:) =45 z
below the left-handcutoff (to the right of the fL=0 curve in
70
z \
o
Figure 5)' however,for 31.1 Hz >fv' Second, theZ modebandis not alwaysobserved, and
MOSESET AL..' BRIEF REPORT
during those times when it doesnot appear the compositionof the continuum band cannot be inferred.Third, the presenceof L, O mode would eliminate the gap between the continuum band and the Z mode band, and in such a circumstance the Z mode would
not have been identified.
at best conclude
that
at various
Thus Kennel
et al. can
times there must be an ab-
senceof L, O mode in the continuum, but that its presenceat other times cannot be ruled out. This discussionpoints out the need for detailed study of the plasma wave composition of Jovian continuum radiation near the low-frequency cutoff of the spectrum. Acknowledgments. The authors would like to acknowledge the assistanceof J. Carpenter in this work. The magnetic field data in this
4705
Poynter, Determination of Jupiter's electron density profile from plasma wave observations,J. Geophys.Res.,86, 8199-8212, 1981. Kennel, C. F., R. F. Chen, S. L. Moses, W. S. Kurth, F V. Coroniti, F. L. Scarf, and F. F. Chen, Z-mode radiation in Jupiter's magnetosphere,J. Geophys.Res.,in press,1987. Kurth, W. S., Intense electrostaticwavesnear the upper hybrid resonance frequency, Ph.D. thesis, pp. 66-102, Univ. of Iowa, Iowa City, 1979. Scarf, F. L., D. A. Gurnett, and W. S. Kurth, Jupiter plasma wave observations:An initial Voyager 1 overview, Science,204, 991-995, 1979a.
Scarf, F. L., F. V. Coroniti, D. A. Gurnett, and W. F. Kurth, Pitch-
angle diffusionby whistlermode wavesnear the Io plasma torus, Geophys.Res.Lett., 6, 653-656, 1979b. Shaw, R. R., and D. A. Gurnett, A test of two theories for the low-
frequencycutoffsof nonthermalcontinuumradiation, J. Geophys. Res., 85, 4571-4576, 1980.
analysiswereobtainedfromthe NSSDC as suppliedby the Voyager Stix, T. H., The Theory of Plasma Waves,p. 27, McGraw-Hill, New
MAG team. This work was supported by NASA contract 954012 (TRW) and 954013 (University of Iowa).
REFERENCES
Gurnett, D. A., and R. R Shaw, Electromagnetic radiation trapped in the magnetosphereabove the plasma frequency,J. Geophys.Res., 78, 8136-8149, 1973. Gurnett, D. A., W. S. Kurth and F. L. Scarf, Plasma wave observations near Jupiter: Initial results from Voyager 2, Science,206, 987-990, 1979.
York, 1962.
Vogt, R. E., W. R. Cook, A. C. Cummings,T. L. Garrard, N. Gehrels, E. C. Stone, J. H. Trainor, A. W. Schardt, T. Conlon, N. Lal, and F. B. McDonald, Voyager 1: Energetic ions and electrons in the Jovian magnetosphere,Science,204, 1003-1007, 1979. F. V. Coroniti, C. F. Kennel, S. L. Moses, and F. L. Scarf, TRW Space and Technology Group, R1/1176, One Space Park, Redondo Beach, CA 90278.
W. S. Kurth, Department of Physicsand Astronomy,University of Iowa, Iowa City, IA 52242.
Gurnett, D. A., W. S. Kurth, and F. L. Scarf, The structure of the
Jovian magnetotailfrom plasmawave observations,Geophys.Res. Lett., 7, 53-56, 1980.
Gurnett, D. A., F. L. Scarf, W. S. Kurth, R. R. Shaw, and R. L.
(ReceivedNovember 10, 1986; revised February 2, 1987; acceptedFebruary 4, 1987.)
