Jovian decametric arcs: An estimate of the required wave normal ...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. A10, PAGES 9089-9094, OCTOBER 1, 1984

Jovian Decametric Arcs' An Estimate of the Required Wave Normal Angles From Three-Dimensional Ray Tracing J. D. MENIETTI, ! J. L. GREEN, 2 S. GULKIS, 3 ANDN. F. S•x• A three-dimensionalray tracing code which incorporatesthe 0-4 magneticfield model (Acuna and Ness, 1976) and a realistic plasma model has been used to model high-curvaturedecametricarcs observedby the Voyager planetary radio astronomyinstrument.Two examplesof intense,isolated,vertexlate, high-curvaturearcs were singled out for study. The source point wave normal angle was incrementedat each of a full range of frequenciesuntil the model rays identically matched the observed arcs of the planetary radio astronomydata. Severaldifferent Doppler shiftswere assumedat the source point. By this procedurean accuraterelationship between the wave normal angle •P and the frequency was obtained,with the variation being 70ø < •P < 80ø, and •I'•max '-- 80ø for the arcs considered.As the assumedDoppler shift at the sourcepoint increased,the value of •Pmax was found to decrease,but the generalshapeof the •P(f) curve was unaffected.By comparing the ray tracing resultswith the independent results of Staelin (1981) regarding emissionfrom beaming electrons,it was found that larger

Dopplershifts(f/fg> 1.1)at the sourcepoint produce•I•max • 70ø, in agreement with the ray tracing results for the two arcsconsidered, onlyfor vii> 0.7and vll/c> 0.32(wherevii is theelectronvelocity parallelto B).Actualvaluesof vll/vareunknown, butindependent observations indicatethatvll/c• 0.1. Since the low-curvature arcs are believedto result from larger wave normal angles,our resultsindicate

an upperlimitto theDopplershiftofJoviandecametric emissions of f/fg < 1.1.

work was in pointing out the significanceof the ionospherein Frequency-time spectrogramsof the Voyager 1 and 2 low- refracting DAM emissions.Their analysis, which assumed frequency (1 MHz to 40 MHz) observations clearly reveal R-X mode emissionwith sourceslocated just above the R-X decametric(DAM)"arc" signatureswith various radii of cur- cutoff frequencyfRx, indicated that the wave normal angle at vature [Boischot et al., 1981; Lecacheux, 1981]. In the top the sourcepoint decreasedwith increasingratio off/fRx at the sourcepoint. Hashimoto and Goldstein match the occurrence panel of Figure 1 are displayed DAM arcs observedby Voyager 1 on day 197 of 1979. The low-curvature arcs seenfrom probability of DAM emissionin the Io-central meridian lonabout 0510 UT to 0550 UT representone classof event, while gitude phase plane with three-dimensionalray tracing results high-curvaturearcs seemto representa different class.A typi- for three different initial wave normal angles. Their results cal example of a high-curvaturearc observedby Voyager 1 on indicate that a wave normal angle of W -• 85ø at the source day 72 of 1979 is displayedin the bottom panel of Figure 1, point produced the best agreementwith ground-basedobserextendingat both high and low frequenciesfrom about 1150 vations. Menietti et al. [1984] have performed threedimensionalray tracing of DAM emissionsand directly comUT to the arc vertex at about 1235 UT. Several authors have performedray tracing analysesutilizing model Jovian mag- pared their results to Voyager observations.They point out netospheresin order to reproduce the DAM arc features that for a source region located just above the R-X cutoff [Goldstein and Thiernan, 1981; Menietti et al., 1984; Green, frequency and for an initial wave normal angle close to 90ø, 1984]. These initial efforts displayedsome successin repro- there is good qualitative agreementwith Voyager data. It apducing the characteristicarc shape with a minimum number pears[e.g., Goldsteinand Thiernan,1981; Menietti et al., 1984] of assumptionsabout the emissionmechanism.Goldsteinand that high-curvature arcs such as those shown in Figure 1 Thieman [1981] have found, using an elementary ray tracing cannot be reproduced with the initial wave normal angle analysis,that varying the wave normal angle was essentialto equal to the sameconstantvalue at all frequencies,even when accuratelycompare model-generatedto observedDAM arcs. the refractive effects of the Jovian ionosphere are included. Goldstein and Thieman obtained an empirical relationshipto The ionospheric model introduced by Hashimoto and Golddescribethe dependenceof wave normal angle W on emission stein [1983] together with the Acuna and Ness [1976] 0-4 frequencyf. Their resultsindicatedthat W is not a linear func- magneticfield model will affect northern hemispheresources tion of f but rather has a variable peak at a midrange fre- at middle to high latitudes only for f >_20 MHz. Ionospheric quency.These resultswere based on a two-dimensionalray refraction cannot explain the low-frequencyrefraction necestracing code and representan initial analysis.Hashimotoand sary to producethe observedsignatureof the high-curvature Goldstein[1983] published the first three-dimensionalJovian arcs. In this paper,three-dimensionalray tracingis usedto examray tracing resultswhich offered a convincingexplanation of the DAM asymmetry of the source position relative to the ine variable wave normal angle effectson the resulting DAM just above the R-X cutoff and with central meridian longitude of Io. A major contribution of their arc structureat frequencies INTRODUCTION

severalassumedratios of f/fg at the sourcepoint. We use •SouthwestResearchInstitute, San Antonio, Texas

2MarshallSpaceFlight Center,Huntsville,Alabama. 3jet PropulsionLaboratory,Pasadena, California. '•AreciboObservatory,Puerto Rico. Copyright1984by the AmericanGeophysicalUnion. Papernumber4A8004. 0148-0227/84/004A-8004502.00

whatever wave normal angle is necessaryto match Voyager observationsof two intense,vertex-late, high-curvature DAM arcs.The resultsindicate that one possibleexplanation of the double-valued frequencynature of the high-curvature arcs is Doppler-shifted gyroemissionfrom a beam of electrons with E •

10 keV.

An independent investigation of Doppler-shifted gyro9089

9090

MENIETTI ET AL. '. BRIEF REPORT :,•CJY•. F-•(I••'

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Fig. 1. Frequency-time spectrograms of Voyagerlow-frequency radio emissions. The emissionintensityis gray shade codedaccordingto the bar graphat the left. In the top panel,low-curvatureDAM arcsobservedby Voyager1 on day 197 of 1979are seenfrom about0510UT to about0550UT. In the bottompanelis seena typicalexampleof a high-curvature DAM arc observedby Voyager1 on day 72 of 1979and extendingfrom about 1150UT to arc vertexat about 1235UT.

MENIETTI ET AL.: BRIEF REPORT

30

The ray tracing code used in this study has been described by Menietti et al. [1984], so only a brief descriptionwill be given here. The code is three dimensionaland basedon the cold plasma formulation of Stix [1962] and integrates the Haselgro•e [1955] equations.The magneticfield model is the 0-4 model lacuna and Ness, 1976], and the plasma model is that introduced by Sentman and Goertz [1977]. The three-

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results of

Greenet al. [1977], Greenand Gurnett [1980], and Lecacheux [1981]. The assumedemissionmechanismrequiresthat (1) the radiation is R-X mode, (2) the sourceregion is at the foot of the instantaneousIo flux tube, and the frequencyof the emission in the sourceregion is greater than the RX cutoff frequency, and (3) the emissioncone is hollow.

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Fig. 2. A frequency versussystem III longitude plot of Voyager data for two observedisolated DAM high-curvaturearcs: Voyager 1 for day 72 of 1979 (shownin spectrogramin Figure 1) and Voyager 2 for day 188 of 1979.

emission utilizing the results of Staelin [1981] has allowed us to set an approximate upper limit on the Doppler shift of the stimulating electrons.Wave normal angles which fall in the range of the ray tracing results,i.e., Wmax > 70ø, are possi-

ARCS

Often seenin the Voyager planetaryradio astronomy(PRA) data are high-curvaturearcs extendingover time intervals of 30 min or more with frequencyrangesof lessthan 1 MHz to over 30 MHz (Figure 1, for example). These arcs are either isolated or nestedin groups.Two examplesof isolated highcurvature arcs are plotted in Figure 2. These were observedby Voyager 1 on day 72 of 1979 and by Voyager 2 on day 188 of 1979. In order

to model

this class of arc the wave

normal

angle W (the angle between k and the magnetic field B) was varied at the sourcepoint for each chosenfrequency.Source points were chosenat a set of frequencies:2, 5, 10, 15, 20, 25,

ble for Doppler-shifted gyroemission for f/fg > 1.1 only if and 30 MHz from the conditionf/f• = 1.02 and f/f• = 1.2. vll/v> 0.7andvll/c> 0.32.Thisworkrepresents thefirstcom- Theseratios were chosento be consistentwith Doppler-shifted prehensivethree-dimensionalray tracinginvestigationof varying the wave normal angle and Doppler shift utilizing realistic Jovianplasmaand magneticfieldmodelswith directcomparison of the results to Voyager observationsof DAM arcs. It thus complementsand extends the results of Goldsteinand Thiernan [1981] and the work of Hashirnoto and Goldstein [1983].

emission at large wave normal angles. Gyroemission is not assumedfor the ray tracing. The conditionf > fax was satis-

fied at eachsourcepoint. For a specificfrequencyratiof/fg and sub-Io longitude (Io flux tube) an initial angle W was chosento beginray tracing,and the proximity of the resulting ray path to the spacecraftposition (already known from observationof a particular high-curvaturearc) was noted. If the VOYAGER 1 DAY 72,

80-

1979

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0.7 andvll/c> 0.35.The valueof •max• 78ø(if weassume fv/f•