Jan 1, 1987 - results of model ray tracing calculations with the Planetary Radio Astronomy .... magnetic field model of L. Davis, Jr. (private communication,.
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL.
92, NO.
A1, PAGES
27-38, JANUARY
1, 1987
Ray Tracing of Jovian Decametric Radiation From Southern and Northern Hemisphere Sources' Comparison With Voyager Observations J. DOUGLASMENIETTI,• JAMESL. GREEN, 2 N. FRANKSix,3 AND S. GULKIS4 Becauseof a lack of readily usable information pertaining to the polarization of the Voyager 1 and 2 high-frequencyband data, a technique has been developed that aids the identification of Io-dependent decametric radiation originating from the southern hemisphere of Jupiter. This technique compares the results of model ray tracing calculations with the Planetary Radio Astronomy (PRA) observations.A large portion of the Voyager 1 and 2 PRA observationsare sorted into bins (_+3ø wide) centeredon a specificIo central meridian longitude. When the data are plotted (as a frequency-longitudespectrogram) in this coordinate system,Io-dependent featurescan be identified and compared with ray tracing calculations performedin a model Jovian magnetospherewhere it is assumedthat the decametricemissionsare generatedin the RX mode from low-altitude sourceregionsalong the instantaneousIo flux tube. Two different magnetic field models are used, and the results are contrasted. In this study, we compare the observationsfor constant sub-Io longitudes of 260ø and 300ø with the correspondingmodel ray tracings. The results permit the identification of decametric spectral features from source locations in both the northern and southern hemispheres:(1) The emission traditionally designated "Io-B" originates at the Io
flux tube footprintin the northernhemispherewhenthe sub-Io systemIII longitude2m is equal to 260ø. (2) The componenttraditionally designated"Io-C" is a combination of emissionsemanating from the Io flux tube footprints in both northern and southern hemisphereswhen Io is located at longitudes 260ø and 300ø. (3) The traditional "non-Io-A" emissionis, in fact, Io related at both Io configurationsstudied. When Io is locatedat 2m = 260ø,this emissionoriginatesin the southernhemisphereflux tube footprint. When Io is at 2,,, = 300ø, this component("non-Io-A") originatesfrom the flux tube footprint in the northern hemisphere.
Staelin [1981], and Neubauer [1980] have shown that arclike structures are produced by a conical sheet model, although no simple choice of parameters has been found which reproduces the data in detail. As shown by Goldstein and Thieman [1981] and Menietti et al. [1984b], the arc curvature may result from variations of the wave normal angle as a function of frequency in the source region. In all of these models, only DAM emission from northern hemisphere sourceswas considered. The first observation of DAM emissionsidentified as having sourcesin the southern hemisphere of Jupiter was reported by Desch [1978]. The information leading to this classification included the fact that Jupiter's southern hemisphere faced the earth and the emissions displayed a left-hand polarization below 18 MHz. These emissions were identified from ground
INTRODUCTION
The complete spectrum of Jovian decametric (DAM) radiation was first observed by the Voyager Planetary Radio Astronomy (PRA) experiment [Warwick et al., 1979]. One of the most striking features of the Jovian DAM emissions in frequency-time spectrogramsis "arclike" bands [cf. Boischotet al., 1981]. The frequency range of the arc emissionsis from 1 to almost 40 MHz, but not all arcs span this complete frequency range. Several different types of arcs have been recognized in the data, depending on their curvatures which either open toward increasing time (vertex early)or open toward decreasing time (vertex late). In general, either an arc may appear as an isolated feature, or there may be a series of nested arcs. In addition, the occurrence of many, but not all DAM arcs is related to the position of the satellite Io [cf. Alexander et al., 1981]. Detailed descriptions of the Jovian decametric arcs can be found elsewhere [i.e., Warwick et al., 1979; Leblanc, 1981; Boischot and Aubier, 1981; Cart et al.,
observations
1983]. A general feature of most propagation theories of the DAM
arc-producingmechanismis that the emission'occursalong conical sheets that are swept past the observer as the planet rotates, as first suggestedby Dulk [1967]. Many investigators attribute the emission to precipitating electrons along the Io magnetic flux tube that radiate near the gyrofrequency [Goldreich and Lynden-Bell, 1969; Smith, 1976; Goldstein and Goertz, 1983]. Goldstein and Thieman [1981], Pearce [1981],
•SouthwestResearchInstitute, San Antonio, Texas.
2National Space ScienceData Center, NASA Goddard Space Flight Center, Greenbelt, Maryland.
3NASA Marshall SpaceFlight Center,Huntsville,Alabama. 4JetPropulsionLaboratory,Pasadena,California. Copyright 1987 by the American Geophysical Union. Paper number 6A8429. 0148-0227/87/006A-8429505.00
Io-C
emissions.
Hashimoto
and
Goldstein
order
to locate
the sources of the emissions.
The observations
have been transformed into an Io-Jupiter stationary coordinate system [-seeGreen, 1984]. This coordinate system allows for the Io-dependent emission features observed by the PRA experiment to be directly compared with the model ray trac- ß ing results. We have chosen source regions at constant sub-Io longitudes of 260ø and 300ø for this study. The agreement between the Voyager observations and the model ray tracings allows identification of the origin of several of the emission components. This agreement is better for the P10+Pll (3, 0) magnetic field model of L. Davis, Jr. (private communication, 1986) than for the 0-4 model of Acuna and Ness [1976], as will be shown.
27
as
[1983] performed three-dimensional ray tracing and, by directly comparing their results to ground-based data, found reasonably good agreement with the results of Desch [1978]. To date, because of the difficulty in analyzing the PRA polarization data, there has not been a comprehensive study classifying the Voyager observations in terms of the hemisphere of origin: northern or southern. The purpose of this paper is to compare three-dimensional ray tracing results with the Voyager PRA measurementsin
28
MENIETTI ETAL.'JOVIAN DAM RADIATION SOURCES THE RAY TRACING
PROGRAM
The ray tracingprogramusedfor thisstudyis discussed in somedetail by Menietti et al. [1984a-].We use two Jovian magneticfieldmodels'the 0-4 magneticfieldmodelof Acuna and Ness[1976] and the P10+Pll (3, 0) modelof L. Davis, Jr. (privatecommunication, 1986).This lattermodelis an improvedversionof the D4 magneticfieldmodel[Smithet al., 1976].It is basedon the resultsof Pioneer10 and 11 and uses
We have computedray trajectoriesfor sourceslocatedat sub-IosystemIII longitudes of 260ø and 300ø,for bothnorthern and southernhemispheresources.Sourcepoints are lo-
catedalongan Io flux tubeat a setof frequencies, typically2, 5, 10, 15,20, and 25 MHz. In the determinationof the location of the RX cutoff frequency,the oblatenessof Jupiteris accounted for, the radial distance(in kilometers)to the cloud
topsbeinggivenby R = 71,300(1- cos2 (colatitude)/15.4).
The intersectionof the resultinggeneratedemissioncones,for three interior orders (dipole, quadrapole,and octupole)and all frequencies at 150 Jovianradii (Rs)and at a Jovigraphic zero exterior orders.The Jovian backgroundmagnetospheric latitude of 5ø,determinesthe model decametricemissionspecplasmais givenby themodelproposed by Sentman andGoertz trum that would be observedby the Voyager spacecraftat
[1978]. The Io torus plasmadensityis a splinefit to the published contours of Warwicket al. [1979].Bettermodelsof
that distance and latitude. It is this model decametric spec-
trum that is comparedwith the observations.The results the Io torus exist [cf. BaStehaland Sullivan,1981], but the allow identification of the source locations of several compotorus interactsonly weakly with DAM emissions,and the nents of the decametric radiation. Warwick et al. model is adequatefor our purposes.In addiCOMPARISON OF OBSERVATIONS AND tion the computercodecontainsan empiricalmodel of the RAY TRACING RESULTS Jovianionosphere basedon theresultsof Hashimoto andGoldstein[1983]. The programis basedon a closedset of firstOnly PRA observations madefrom February21 to March order differential equationsin sphericalpolar coordinates 18, 1979,by Voyager1 and from June26 to July23, 1979,by specialized by Shawhah[1966] (in two dimensions) and by Voyager2 are usedin this study.Theseobservations are apMenietti et al. [1984a] (in threedimensions) that describethe proximatelycenteredaroundthe encounterperiodand com-
path of raysin a magnetized plasma.The expressions for the priseenoughdatafor a complete frequency-longitude studyof phaseindexof re&action anditsderivatives areobtained from Io-dependentDAM emissions as discussed by Green[1984].
the cold plasmaformulationof Stix [1962]. Observationsat radial distanceof lessthan 30 Rs were excludQualitatively,the programcalculatesan indexof refraction ed in order to avoid the effectsof large angularvelocityof the
surfaceand group velocity(magnitudeand direction)for a predeterminedplasmawave mode in the model magnetosphereat somestartingpoint. The programthen takesan incrementalstep in the directionof the group velocityor energyflowanddetermines the coordinates of a newpointon the ray path.Anotherindexof refractionsurface is calculated at the next point, and the stepsare repeated.The program
spacecraft with respectto thepresumed sourceregions.
usedhemhasbeenextensively testedandreduces to thetwo-
1984,Figure2], wherethe sub-IosystemIII longitudeis, for example,50ø.Emissionin a givenfrequencybandcan be displayedin termsof the spacecraft angulardisplacement from Io, positivein the directionof increasing systemIII longitude. We next imagineIo to be locatedabovea new centralmeridian longitude(CML), 10ø greaterthan in the previousposition (Io moves3ø in its orbit, and Jupiterrotates13ø on its
dimensional results ......... cheux [!981]. MODEL
and '" ........
[1980] and Leca-
ASSUMPTIONS
The •following assumptions regardingsourcelocation,initial launchangle,and modeof propagationare requiredas inputs
It is instructive to examine the Voyager data set in Io's frame of reference.Holding Io at one particular Jovian system
III longitude,we examinethe decametricemissions received by Voyagerfrom all longitudes as thoughthe spacecraft circumnavigated the planet,telemetering resultsback to an observer on Io. This situation is illustrated in Figure 1 [cf. Green,
to the ray tracing program'
1. The sourceregionis at the foot of the Io magneticflux tube.
2. We have chosenfrequenciesgreaterthan the RX cutoff
frequency, and we haveassumed a constantDopplershiftof
f/f• = 1.01,consistent with the resultsof Meniettiet al.
DAM
EMISSION
[198463,wheref is the wave frequencyandf• the electron gyrofre•quency. The resultsof the latter authorssuggestthat small.variations of the ratio f/f• will not changeour conclusions
• •
3. Theemittedradiation isin theright-hand-polarized extraordinar• mode.
4. The radiationis emittedin a hollowconepatternabout the magneticfielddirection.The emissionwavenormalangles with re•spect to the magneticfield, ½, were 85ø and 70ø for northern•hemispheresourcesand 95ø and 110ø (half-cone-
XO(Xll I:SO ø) (hill = 140ø)
angles•oft85ø and 70ø, respectively, for outgoingwaves)for southern hemispheresources.
No •specific emissionmechanismsare implied by our assumptions; rather,they are chosento be consistent with the availabledata [i.e., Cart et al., 1983] and at leastsomeof the theories of radio emissions[i.e., Dulk, 1967' Goldsteinand Goertz,,1983].
Fig. 1. In theIo frameof reference, therotationof Jupiterandthe orbital motion of Io are halted. Io's CML
is fixed at 50ø. The deca-
metric radiation detectedby the Voyager PRA experimentis presented as a functionof the spacecraft longitudeoffsetfrom Io, positivein the directionof increasing JoviansystemIII longitude.The longitude offsetis the differencebetweenthe Voyager'•mand the Io '•m-
MENIETTI ET AL.: JOVIAN DAM
VOYAGER
RADIATION SOURCES
I + 2 SUM
FREQUENCY
BAND
29
15.1- 21.2MHz
o's POSITION IN SYS. IE LONGITUDE
-180. -150.
-120.
-90.
-60.
-:30.
O.
:30.
60.
90.
120.
150.
180.
VOYAGER LONGITUDE OFFSETFROM I0 (VoyXm:- ZoX:m) Fig. 2. Voyager 1 and 2 PRA data merged for frequenciesin the band 15.1-21.2 MHz, presented in a coordinate systemwhich is centeredon Io's position (0 on the abscissa).Displacement right or left of center on any horizontal line in this figure is measured as the angular separation of the observer (Voyager) from Io, positive in the direction of increasing Jovian systemIII longitude. The sub-Io longitude varies along the ordinate in 10ø increments. Vertical emissionfeatures imply constant angular offset from Io. Diagonal features from upper left to lower right signify constant system III longitude, as indicated by the dot-dash lines. The boundaries of the traditional "sources"Io-A, lo-13, and Io-C are those determinedfrom earth-basedobservations.The horizontal dashedlines at sub-Io longitudesof 260ø and 300ø indicate the data which are compared with the ray tracing results in this study. Note that lo-13 and Io-C sourcesare samples in this geometry.
axis). Once again the decametric emission in a given frequency band can be plotted as a function of the angular offset of the spacecraftfrom Io. By repeating this process36 times, a strip of emission which is a function of the observer's (Voyager's) angular offset from Io can be obtained for each of 36 sub-Io system III longitudes, located 10ø apart. For our studies, the decametric emission utilized to produce such a plot is that received by the spacecraftwhen the sub-Io longitude is +3 ø from a given 10ø increment in systemIII longitude. For example, the strip of data for Io at CML - 50ø is actually what the spacecraftreceivesas Io moves acrosslongitudes in the range 47ø to 53ø. The bin size on the horizontal scale (spacecraft maximum angular displacementfrom Io) is 6ø. A plot of the decametric emission in the frequency band 15.1-21.2 MHz, received by Voyager 1 and 2, is shown in Figure 2, which is assembledby stacking the strips of data described above. Io's position in Jupiter's system III longitude is indicated on the ordinate. The advantage of the Io frame of referenceis that it immediately makes obvious Io's role in the emission process. This coordinate systemeliminates the rotation of Jupiter and the orbital motion of Io. It shows what is measured by an observer (Voyager) who carries his receiver to various angular offsetsfrom Io while the moon is anchored at one longitude (+ 3ø) in Jupiter's magnetic field. The traditional sourcesA, B, and C are designated by their CML-Io phase boundaries as determined from earth-based observations near 20 MHz [Carr et al., 1983]. Vertical featuresin Figure 2 signify Io control. As examples, note the traditional
sources, Io-B and Io-C. This emission
occurs at a fixed and well-defined angular offset from Io. Loci of constant systemIII longitude are indicated by the diagonal dot-dash lines in Figure 2. Emission features aligned along such diagonal lines signifyJovian systemIII longitude control
(magnetic field control). We believe that the band of emission between longitudes 200ø and 275ø, which contains the traditional Io-A and non-Io-A sources, is such a feature. This emis-
sion appears to be independent of Io's position in Jupiter's magnetic field. Emission in the frequency range 15.1-21.2 MHz has been included in Figure 2 if its intensity exceeded a threshold value of 4500 millibels, a value accepted as a background level. All intensities above the threshold value were equally weighted. We have chosen to examine, in this study, the data corresponding to sub-Io longitudes of 260ø __.3ø and 300ø _+ 3ø. These geometries are designated by the horizontal dashed lines in Figure 2. Note that the Voyager data set corresponding to the config-
uration Io '•III = 260øsamplesboth of the traditional sources Io-B and Io-C, whereas the Io '•III--300ø data set does not intersect the Io-B component. We have performed ray tracing calculations from sources located in both the northern and southern hemispheres along Io flux tubes separated by 10ø increments of longitude around Jupiter. Becauseof the structure of Jupiter's magnetic field (as described by current field models), emission at large wave normal angles from sourceslocated at the feet of Io flux tubes in the southern hemisphere for sub-Io longitudes in the range
of about 250ø < Io '•III '• 300øshouldbe observedby a spacecraft near the jovigraphic equator (such as Voyager 1 and 2). However, DAM emission from the southern hemisphere is lower in frequency, in general, than DAM emission from the northern hemisphere because of the weaker field strength in the south polar region. Much of the emissionfrom the southern hemisphere should occur in the frequency range 5 MHz < f < 15 MHz. The intensity calibration in this range is erroneous, principally becausethe antenna sensitivity has not been taken into account [Schauble and Carr, 1983]. For this
30
MENIETTI ET AL..' JOVIAN DAM
reason we have chosen two Io flux tubes, Io at longitude 260ø and 300ø, for this study. For these sub-Io longitudes much of the data lie at frequenciesgreater than 15 MHz. When the Voyager 1 and 2 data for all frequenciesbetween 1 and 40 MHz are displayed in a frequency versus spacecraft
(observer)•iii plot centeredon the sub-Io longitude of 260ø, Plate 1 results.The data displayed in Plate 1 are only a subset of the entire Voyager data, containing high band emissions received when Io was within the range of systemIII longitude' 260 ø +_ 3ø. It is important to note that the horizontal band of emission of vertical extent 6-13 MHz seen at all spacecraft longitudes is largely the result of an increasein the sensitivity of the PRA instrument at these frequencies [Schauble and Carr, 1983]. The most obvious
features
in Plate
1 are two rather
narrow
emission zones, one located at Jovian system III longitudes between approximately 155ø and 185ø and another located between
about
305 ø and
330 ø. Both
of these emission
zones
have a high-frequency cutoff at 35 MHz. In the region between these two well-defined emission zones is a region of structured
emission
which
extends
from
215 ø to
305 ø and
covers the frequency range from about 5 to 25 MHz. The ray tracing results indicate that this emission originates in the southern hemisphereof the planet. Polarization measurements in the PRA high band data were unreliable during the period corresponding to these Voyager observations (M. Kaiser and M. Desch, private communication, 1986) and could not be used to aid in identifying the sourcehemisphere. Also shown in Plate 1 are the ray tracing results using northern and southern hemisphere sources in the 0-4 magnetic field model [Acuna and Ness, 1976]. We have chosen source points that lie along a single Io flux tube (sub-Io longitude of 260ø) extending into the northern and southern hemispheres,with the exact location of a sourcedependent on the local RX cutoff frequency. For each source position we have chosen two initial wave normal angles in order to investigate the dependenceof the emissionpattern on this parameter. The ray tracing from the northern hemisphere sources utilized initial wave normal anglesof 70ø and 85ø with respect to the outward directed B field. The southern hemisphere results were obtained for wave normal angles of 95ø and 110ø with respectto the inward directed B field (outgoing emission 85ø and 70ø with respect to the field line). The ray tracing for northern hemisphere sourcesalong a single Io flux tube approximates the emission features for Io-B and Io-C emissions within about 10ø of Jovian longitude. Seen in Plate 1 are ray tracing, model-generated features at about Jovian longitude
RADIATION SOURCES
emissionlies at Jovian longitudes of about 215ø and 330ø. The frequency extent is greater than that of the northern hemisphere (NH) emission, and a characteristic "arch shape" is discernible.The center of the arch of the model ray tracing emissionis located at •iii '•' 270ø-This is 10ø-15ø offset from the center of the observed archlike feature centered at ap-
proximately •III •- 255ø,and correspondingto that component of the emission traditionally labeled non-Io-A. On the basis of the close correspondenceof the ray tracing, model-generated features with the Voyager-detected emission, when Io is at the Jovian longitude 260ø, we believe that the source of the Io-B component is the footprint of the Io flux tube in the northern hemisphere,and the source of the Io-C component is the footprint of the Io flux tube in both the northern and southern hemispheres. In Plate 2 we have plotted the results of ray tracing for the same source locations and the same initial parameters except that now the magnetic field model is the P10 + Pll (3, 0) model (L. Davis, Jr., private communication, 1986). When waves are launched at ½ - 85ø with respectto the B field, they extend to higher frequency, and they intersect the Voyager trajectory at Jovian longitudes which better match the longitudes of the actual observed emissions. The results for ½ = 70ø do not seemto match the observationsas well. The ray tracing results for the southern hemisphere emissionsfor ½ = 95ø and ½ = 110ø are plotted in Plate 2 as well. The ray paths for the P10 + Pll (3, 0) model are almost identical
to those for the 0-4
model
for sources in the south-
ern hemisphere at the Io flux tube footprints. The modelgenerated spectral features extend to 25 MHz and have the same approximate slope as the observed emissions in the longitude range 200ø < •lii < 300ø,but they are shiftedby ,--10ø with respect to the observed emissions. The southern hemisphere ray tracing results for both ½--95 ø and ½--110 ø appearto be symmetricabout a line centeredat •iii '•' 270ø. In Figure 3 we presentstandard frequency-versus-timespectrograms of Voyager 2 data for days 184 (top panel) and 197 (bottom) of 1979. The data displayed in the top and bottom panels have been included in the regions of Plates 1 and 2 identified as Io-B and Io-C sources,respectively.That is, the data here displayed in standard frequency-time spectrograms are samples of the sorted data contained in Plates 1 and 2. Here represented are typical arcs observed by Voyager 2 for sources at the foot
of the Io flux tube in the northern
hemi-
sphere.The arcs in the top panel of Figure 3 are "vertex-early" arcs, and those in the bottom panel are "vertex-late." As explained by Boischot et al. [1981], these are the characteristic 185 ø and 330 ø. These lie close to the observed decametric shapes to be expected for the Io-B and Io-C sources,respecemissiontraditionally identified as Io-B and Io-C, respectively. tively. In Figure 4 is displayed a frequency-versus-timespecThe frequency extent for the model-generated emission is less trogram of Voyager 2 data for day 189, 1979. These data represent emission from the right side of the arch in Plate 2 than observed by about 10 MHz. The discrepancy in frequency as well as in longitude is probably due to inadequacies and are identified as southern hemisphere emission. Here we observe that the arcs are vertex-late arcs, but from neither the in the magnetic field model. An increasein the magnetic field traditional Io-C nor Io-B sources. We have also observed strength at the source point from about 7 G to over 10 G vertex-early arcs for data included in the left half of the arch of would be required for emission at 30 MHz to be allowed. This the southern hemisphereemission,but a high-resolution speclarge increasein the field strength suggestserrors in the magtrogram was unavailable. netic field model at low altitudes, since emissions at f > 30 Selecting the Voyager 1 and 2 data when Io is fixed over MHz have been frequently observed from earth. The small •III = 300ø+ 3ø, the frequency-spacecraftlongitude specdifference in longitude of the observed emissionsfrom the ray trogram shown as Plate 3 is obtained. This figure is analogous tracing results may be due to azimuthal differences in the to Plate 1, but now Io's position in Jupiter's magnetic field has model magnetic field compared to the actual field. The model-generated southern hemisphere (SH) emission been shifted 40ø to the new location at •iii = 300ø'Again the from a single Io flux tube is also shown in Plate 1. This reader is cautioned that the intense emissionsin the frequency
MENIETTIETAL.' JOVIANDAM RADIATION SOURCES
VOYAGER
2
79
FREQUENCY
TIME
197
JUL
16
31
6' 0' 0TO8'
0'0
SPECTROGRAM I
I
i --
--
_25.8
•1•.9
16.8
11.8
m
TIFE RJ SC LMG I0 LNG
610 87 151 •
•
6'38_ 87 163 243
87 15-7 L:•J9
VOYAGER
2
FREQUENCY
6•8 87 169 248
m
658 87 175 Z53
78B 88 181 Z57
m
718 88 187 •
79 184 JUL 3 7-0-0 TIME $PECTROGRAM
• 88 193 ZS?
•
73B 88 199 271
?•8 88 •B5 2"86
758 88 211 •
I
t
TO 9:0:0
16.8
11.8
6.1 me
TIFE RJ SC Lr•; IO LH•
718 91 • •
m
•
1.2
• 91 • L:•d5
• 91 i)89 24
'7,:•3 91 • 2•
'• 91 381 249
• 91 3•}7 •
81{3 91 313 L:58
• 9•3 33• •
8•(3 9{) 337 •77
• RJ DEG DEG
Fig.3. Frequency-versus-time spectrograms ofVoyager 2 data.Thetoppanel displays typical vertex-early arcslabeled '•s•d , astraditional Io-Bemission andincluded in thedataofPlates1 and2.Thebottom paneldisplays vertex-late arcslabeled,• ::;,n' . .:
astraditional Io-Cemission andalso included inthedataofPlates 1and2. range from about 6 to 13 MHz have been miscalibrated in the
data set [Schauble and Cart, 1983].The emissionconfinedto the relativelynarrow spacecraft longituderange 150ø-170 ø and extendingto 27 MHz is identified as the traditional "non-
Io-B" component. The emission in the longituderange330ø-
'!•'i•'
15ø is identified asthetraditional "Io-C"component. r Note
thattheraytracings from thenorthern andsouthern_ :flu•tube
footprints overlap here.On thisbasis, webelieve. •t:hgt this emission emanates fromtheIo fluxtubefootprints ip both hemispheres. Theemission in thelongitude range220•,255 ø
32
MENIETTI ETAL.'JOVIAN DAM RADIATION SOURCES ...
.•:,• ....
•-.•:•-
....
•
L:u?!.: .g.....•. .'•.L •':•m• ::•--/?•-• .e •--, .,,.•...... •....,•....-:: .... • •:-8":1% .. .
......
.......
.... ""'"' ....
.....{
................ .............
":'•"7'E:.•:.•'..:•.::.::; ...... • ........ --•:..,..• .... ;.....:..' ...... •..... ?:.::•:•:•".'::... '.-?•' "?¾ :•.,.• • .... '...... •.... •:-:•."•;..-• '":: :L•. '•:':•".: "::'":":..: •.......•.?":., :. '....•;•.':. 4..---:•2-...-?.•;:• ....... ..:::.;: •. .::.;:;•.•.:-:..,• ---•..;;.'.:..: ?.:.:-.:. ............... .......... -'•....•:..:.........';;•:'.,.:;-:...:...;;:,.,,:.:.. ;;:--:......... :;.• [. '•'--..• ...... ,•:'.:" ;--•:..;... •.:...:.::. "-.;.':::. :.... ..•? ........... -.',:.., ,• :...• . ...4 ..:: 4' :•. ':-:'-' .¸
I
I
36
MENIETTI
ET AL.' JOVIAN DAM
RADIATION
SOURCES
h-
m
z
o z
•
MENIETTI
ET AL.' JOVIAN DAM
TABLE 2. Dependence of L m on Observer Latitude (Southern Hemisphere Source) Observer/Spacecraft JovigraphicLatitude,deg
Frequency of Emission,MHz
Lm, deg
10
5
307.3
5
5
315.6
0
5
322.9
10 5
15 15
277.6 301.5
0
15
313.6
RADIATION
SOURCES
37
at two frequenciesfor a sub-Io longitude of 260ø for sourcesin both the northern and southern hemispheres(similar results
occur for a sub-Io longitude of 300ø). L m is the system III longitude of the observedemissionat a distanceof r - 150 Rs. For the northern hemisphere sources a change in the spacecraft latitude of 5ø changes the location of the observed model ray by a comparable number of degreesat 5 MHz and slightly more at the higher frequency. The general effect is to increase the distance between the left and right emission cone "edges" as the latitude
of the observer
is true because
is increased
the axis of the emission
from
0 to 10 ø. This
cone is well above
the
Jovigraphic equator. The model ray tracing results lie near the traditional sources seen by observers near Jupiter's equatorial plane are produced by radiation sheets emanating from the following Io flux tube footprints' Traditionally Labeled
"Source"
Io-B
of observation.
denceof Lni on observerlatitudeis stronger.In Table 2 we lisi the values of L m for the right "edge" of the emissioncone for
Based
½ = 110ø, for two frequencies, 5 and 15 MHz. Now it is seen
North hemisphere Io flux
300ø); south hemisphere
that as the latitude of the observerincreases,the value of L m decreases. Since the left edge of the emission cone behaves similarly, the general effect is that the distance between each edge of the emission cone decreases as the latitude of the observer increases.At observer latitude of 10ø the model ray tracing results for the southern hemisphere at tp -- 110ø overlap the Io-C emissionsonly at the lower frequencies(f< 10
footprint (Io 2111 -- 260ø)
MHz).
260 ø) North and south hemisphere Io flux tube footprints
North hemisphere Io flux
tube footprint (Io 2m -
These results are in agreement with and extend the analysis of Green [1984]. Our conclusion regarding the source location for the emissionlabeled "Io-C" does not appear to agree with Desch [1978] nor with Hashirnoto and Goldstein [1983], who conclude that those emissions originate in Jupiter's southern hemisphere. While the coincidence between our ray tracing results and the observations does not necessarily guarantee the emission has the source used in our model, the possibility is noteworthy. We do not state that the results of the latter authors (dependent on ground-based observations at a frequency near 20 MHz) are in error. Our results, however, indicate that some Io-C emissionsoriginate in both the southern and northern hemispheres. Observations indicate that Io-C emissionsare both right- and left-hand polarized. An example of dominant right-hand polarization measured by Voyager 2 during an Io-C storm at 2330 on July 2, 1979, can be seen in panel A of Figure 7.16 of Cart et al. [1983, p. 260]. Similarly, while non-Io-A emissions are dominantly right-hand polarized, these emissions are also observed to be partially lefthand polarized, consistent with sources in both the northern and southern hemispheres. We have presenteda ray tracing study of emissionoriginating at the footprints, N and S, of the instantaneous Io flux tube, for two positions of Io in system III longitude. If a number
at all three latitudes
on
Location
(Io ;tllI -- 260ø and 300ø) Non-Io-A
observations
For the southern hemisphere sources the emission cone angle is narrower, and/or the arc of the emission cone lies further away from the Jovigraphic equator. Thus the depen-
Ray Tracing Calculations
Source
tube footprint (Io ;tni= Io-C
actual
of Io flux tubes can be active sites of emission
simulta-
neously, then more overlap of the emission from both hemispheres,as it intersectsthe observer'sposition near the equatorial plane, would be detected. For the analysis in this paper we have assumedthe observer (spacecraft)was located at a constant magnetic latitude of 5ø, which is a good approximation to the average latitude of both Voyager 1 and Voyager 2 on the inbound and outbound legs for r > 100 Rj. We have, however, investigated the dependence of the model results on this parameter. In Tables 1 and 2 we summarize the result of changing the spacecraftlatitude
In summary we have presented a comparison of the results of three-dimensional ray tracing of Jovian DAM emissions from both southern and northern hemispheresourcepositions with the Voyager data cast into the frame of the observer system III longitude with Io at a fixed position in the Jovian magneticfield (Io •Lii I '-- 260ø and Io •Lii I = 300ø).The ray tracing is based on a minimum number of assumptions which include RX mode emission from source points slightly Doppler-shifted above the RX cutoff. Initial wave normal angles of 85ø and 70ø for outgoing waves were assumedfor the northern hemispheresources,and both 95ø and 110ø for outgoing waves for the southern hemisphere sources.The plasma model is an empirical fit which includes the effects of the ionosphere,Io torus, and magnetosphere.We have used both the 0-4 magnetic field model lacuna and Ness, 1976] and the P10 + Pll (3, 0) model (L. Davis, Jr., private communication, 1986). The Voyager 1 and 2 data have been sorted to hold the sub-Io longitude constant in order to reveal the Io-dependent features. When directly compared to the sorted Voyager data, the ray tracing results permit identification of the source locations in the northern and southern hemispheres. We find that the traditional components of emission, Io-B, Io-C, and non-Io-A, can be explained as emanating from the Io flux tube footprints in the northern and southern hemispheres.A simple model of wave propagation at large wave normal angles from sources along the Io flux tube agrees well with the Voyager observations
obtained
with
Io
located
at 260 ø and
300 ø in
Jovian system III longitude. Discrepancies in the frequency extent and longitude of the ray tracing results can be attributed to errors in the Jovian magnetic field model at low altitudes.
Acknowledgments.The authors are grateful to the Planetary Radio Astronomy team (in particular M. Kaiser) for the use of the Voyager 1 and 2 time-averaged PRA calibrated data. This research was supported by the National Aeronautics and Space Administration under the Outer Planets Data Analysis Program, through
38
MENIETTI ET AL.: JOVIAN DAM
NASA contract NASW-4045. The authors would also like to express their gratitude to the Data System Technology Program for allowing accessto the central computer facilities and to the Space Physics Analysis Network (SPAN). We would also like to thank the referees for their suggestions. The Editor
thanks
K. Hashimoto
and A. Lecacheux
for their assist-
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(ReceivedFebruary 19, 1986; revisedSeptember 2, 1986; accepted September 22, 1986.)
