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Produced by the NASA Center for Aerospace Information (CASI)
^ 9l^' - cis 

(RASA -LH - 1754E1) TH&FF IICthSICNIL 1, .4Y N85-19969

TBACING 01 THE JCYiAN MAGN17CSFEBBF IN THE

LOW YBLSuUENCY FANGL Final iepott (Snuthwest

BeSedcCh I05t.) 93 p hL AC`/!!F 101 CSCI 03B ULrlat

G 3/9 1 14333

THREE DIMENSIONAL RAY TRACING

OF THE

.r

JOVIAN MAGNETOSPHE9E

IN THE LOW FREQUE NCY RANGE

SwRI PROJECT NO. 15-6546

.iPL CONTRA P:T NO. 656026

FINAL REPORT

MAY 1984

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,

J.D. MENIETTI

SOUTHWEST RESEARCH INSTITUTE

P.O. DRAWER 28510

SAN ANTONIO, TX 78284

f.

5^; err
THREE DIMENSIONAL RAY TRACING

OF THE

JOVIAN MAGNETOSPHERE

IN THE LOW FREQUENCY RANGE

S wRI PROJECT NO. 15-6546

JPL CONTRACT NO. 956026

FINAL REPORT

MAY 1984

J.D. MENIETTI

SOUTHWEST RESEARCH INSTITUTE

P.O. DRAWER 28510

SAN ANTONIO, TX 78284
THREE DIMENSIONAL RAY TRACING

OF THE

JOVIAN MAGNETOSPHERE IN THE LOW FREQUENCY RANGE

SwRI Project No. 15-6456

JPL Contract No. 956026

Final Report

April 1984

J. D. Menietti

Southwest Research Institute

P. 0. Drawer 28510

San Antonio, TX 78284

This work was performed for the Jet Propulsion Laboratory,

California Institute of Technology, sponsored by the

National Aeronautics and Space Administration under

Contract NAS7-100.

0
TABLE OF CONTENTS

Paqe

f' I. SUMMARY 1 

^J

II. INTRODUCTION 3

^:. III. TECHNICAL DISCUSSION 4 

A. Previous Results 4

B. Improvements to the Computer Code 5 

C. New Results 6 

tt

IV. CONCLUSIONS 9

I .{ V. REFERENCES 11

111111

VI. FIGURE CAPTIONSS 12

VII. APPENDICES 16

t A. Appendix 1: "Three-Dimensional Ray i"racing of

it Jovian Radiation in the Low Frequency Range"

B. Appendix 2: "Identification of Decametric

r Radiation from Sources in the Southern

Hemisphere in Jupiter"

C. Appendix 3: "Jovian Decametric Arcs: An Estimate

^f of the Required Wave Normal Angles from Three

Dimensional Ray Tracing"

14 D. Appendix 4: "Identification of a Night-Side

l Component of DAM as a Jovian Counterpart to AKR"

t ^

P
TABLE OF CONTENTS

Page

I. SUMMARY 1

II. INTRODUCTION 3

III. TECHNICAL 013CUSSION 4

A. Prevtous Results 4

B. Improvements to the Computer Code 5

C. New Results 6

IV. CONCLUSIONS 9

V. REFERENCES 11

VI. FIGURE CAPTION.; 12

VII. APPENDICES 16

A. Appendix 1: " , three-Dimensional Ray '.lacing of

Jovian Radiation in the Low Frequency Range"

B. Appendix 2: "Identification of Decametric

Radiation frum Sources in the Southern

Hemisphere in Jupiter"

C. Appendix 3: "Jovian Decametric Arcs: An Estimate

of the Required wave Normal Angles from ThreeDimensional Ray Tracing"

D. Appendix 4: "Identification of a Night-Side

Component of DAM as a Jovian Counterpart to AKR"

Y

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I '

I;

4.1

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U

I. SUMMARY

The investigative efforts at Southwest Research Institute have centered

on the development and execution of a comprehensive three-dimensional ray

tracing code. This code contains a model of the Jovian magnetosphere which

includes the 0-4 magnetic field description (Acuna and Ness, 1976) and an

empirical plasma model. Decametric radiation (2-35MHz) has been the primary

subject of the analyses but some studies in the whistler mode k 10kHz) have

been performed. The whistler mode analyses yielded the approximate dimension

of Jovian lightning source regions and the approximate energy dispersion.

From the latter an estimate of the Jovian lightning emission rate was made.

These results were previously reported in the Final Report submitted to the

Jet Propulsion Laboratory in October, 1982. The decametric radiation (DAM)

studies have produced numerous publishable results.

The early work of the current contract in ray tracing of DAM did not

incorporate a model Jovian ionosphere. Sources were assumed to be at the foo

of the Io flux tube and only northern hemisphere sources were initially

considered. anission emanating in a hollow cone around the active field line

was ray traced from sources separated by 10 0 in Jovian longitude. One of the

most notable results was the observation that not only does the magnetic field

model control the frequency extent of the emission but also the curvature and

morphology of the arcs. The importance of displaying the Planetary Radio

Astronomy data in a constant-Io longitude format were clearly demonstrated in

this early work. The results were published in the Journal of Geophysical

Research (Appendix 1).

Isolated, high-curvature DAM arcs were modelled by varying the wave

normal angle at the source until the ray tracing results matched the

observations. Plots of wave normal angle versus frequency for different

a

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2

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Uoppler shifts at the soures were made which provide the first comprehensive

three-dimensional description of the relationship of these variables. The

results were compared with the theoretical work of Staelin (1981) and imply 

maximum wave normal angle of about 80 0 for the high-curvature arcs. This wo

has been submitted to the Journal of Geophysical Research ( Appendix 3).

Because the Jovian ionosphere was found to strongly influence the

higher frequency radiation, an empirical expression was used to represent th

ionosphere in the code. The ray tracing of Jovian DAM source points was

repeated including the southern hemisphere sources. The results have, for t

first time clearly delineated northern and southern hemisphere sources in th

Voyager data. The southern hemisphere arcs are lower in frequency extent an

have a narrower emission cone angle. These findings lend strong support to

the theory that arc morphology is primarily a result of wave refraction in t

magnetosphere rather than emission processes. Appendix 2 is preprint of a

paper submitted to the Journal of Geophysical Research detailing these

exciting results.

The next step in reformatting the Voyager data led to a discovery of

an lo-independent emission which is most probably the Jovian counterpart to

terrestial auroral kilometric radiation (AKR). By plotting the sub-Io

longitude versus the difference between the spacecraft and the sub-Io

longitude, an emission which has been named Jovian Auroral Decametric

Radiation (JADR) has been identified. This emission appears to emanate only

from high-latitude nightside auroral zone sources in a manner similiar to th

terrestial AKR. These results have been submitted. to the Journal of

Geophysical Research and appear in preprint form in Appendix 4.

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This work represents culmination of a three year activity involving

ray tracing studies of Jovian low frequency emisions. The program included

the development and execution of a comprehensive three-dimensional ray trac

computer code for examination of model Jovian decametric (DAM) emission. A

detailed description of this program has been submitted to the Jet Propulsi

Laboratory as "Final Report" dated October, 1982. In the present report a

brief description of the improvements to the computer code will be outline

and a presentation of the results of ray tracing of Jovian emissions will b

presented in the form of summaries with details given in the appendices. T

appendices contain copies of papers that have been accepted or submitted f

publication to the Journal of Geophysical Research.
4

III. TECHNICAL DISCUSSION

A. Previous Results

As described previously in the "Final Report" dated October, 198

(hereafter referred to as FR1982), DAM emission from northern hemisphere

source positions located at the feet of Io flux tubes separated by 10 0 in

Jovian longitude have been ray traced. The results were compared to Voyager 

and 2 observations and have been published in the Journal of Geophysical

Research. As a result of these efforts the importance of wave propagation

effects in reproducing the observed Jovian DAM arcs became apparent. The

magnetic field model and ionospheric effects were seen to be fundamentally

important. In addition, the arcs were seen to be divided into high and low

curvature classes. Initial explanations for some of the differences between

these classes were presented which included variations of initial wave normal

'angle and local magnetic field strength at the source. Appendix I is a

reprint of a paper published in the Journal of Geophysical Research entitled

"Three Dimensional Ray Tracing of Jovian Radiation in the Low Frequency Range

which details the results of this original work.

Initial studies of whistler mode emissions (1-3kHz) from lightning

sources in the Jovian cloud tops were also made as discussed in FR1982.

Whistlers were ray traced through the Jovian magnetosphere and into the Io

torus in an effort to determine the extent of the source positions

degree of energy dissipation from this emission. These results he

reported to the University of Iowa and are awaiting submission for

publication.

H

0
5

i

B. Improvements to the Computer Code

As detailed in FR1982, the ray tracing code is based on the Stix

cold plasma formulation and solves the Haselgrove ( 1954) equations for the

index of refraction. The method of solution is by Runge-Kutta integration

with a modified Hamming predicator/corrector scheme. The ambient Jovian

plasma density model is based on the Sentman -Goertz empirical results and the

Io torus plasma density is a spline fit to the published contours of Warwick

i

at al. ( 1979). The magnetic field model used is the 0-4 model of Acuna and

Ness ( 1976). Now included in the code is an ionospheric model based on the 

empirical nonlinear fitting of Hashimoto and Goldstein, ( 1983).

Atreya and Donahue, 1976 have derived a model Jovian ionosphere

based on observations. Hashimoto and Goldstein, ( 1983) have used a nonlinear

least squares fit to the model of Atreya and Donahue producing the following

analytical result:

NI=1.15x10 5 exp((1417-h/386.3)/(1+(991/(h-276))4.31),

where NI (cm-3 ) is the ionospheric electron density, h is the altitude in km

above the cloud tops and N I=0 for h < 276 km. The magnetospheric model and

the ionospheric models have been matched at the top of the ionosphere so that

1

a smooth transition occurs. With the addition of the ionosphere it was

necessary to ray trace DAM at all frequencies whose source positions reached

1 the ionosphere.

In order to display the results of the ray tracing code a plot

program was developed on an HP1000 computer which provided high quality ink

bed plots. The wave frequency versus Jovian system III longitude was

displayed on these plots with sources for each sub-Io longitude clearly

indicated by color symbols. This plotting technique allowed easy comparison

of the model arcs with Voyager 1 and 2 observations.

__.... _-.°--•--
6

C. New Results

4 As a result of the identification of the Jovian ionosphere as an

important source of wave refraction at certain DAM frequencies, it was

necessary to ray dace all the northern hemisphere DAM source positions agai

The results of Staelin, (1981) suggested a constant initial wave normal ang

of 85 0 at the source (instead of the original 90 0 used in the previous study

(cf. Appendix 1)). Figure 1 (A-D) shows the resulting model emission on a

l

frequency-versus-system III-longitude scale for arcs within the longitude

^J ranges of 10 0 -90 0 , 100 0 -180 0 , 190 0 -270 0 , and 280 0 -360 0 respectively. Figur

f f

is in the same format for the source points of the southern hemisphere. It 

Illl immediately apparent that the model southern hemisphere source emissions hav

a lower frequency cutoff than the northern hemisphere sources, and emission

cone edges are closer together in longitude. In order to directly compare t

I" model ray tracing results with actual observations it was decided to re-form

the Voyager 1 and 2 Planetary Radio Astronomy (PRA) data by sorting it with 

fixed sub-Io longitude. This allowed direct comparison with the ray tracing 

results. The reformatted data was obtained from Marshall Space Flight Cente

as "a result of the efforts of J. L. Green and N. F. Six, supported by the

r

1111 Jupiter Data Analysis Program. Comparison of the model and observations was

very good for some cases and allowed the first delineation of Jovian DAM

` emissions into southern and northern hemisphere sources. A detailed

discussion of the comparison is included in this report as Appendix 2, whic

is a preprint of a paper entitled "Identification of Decametric Radiation F

Sources in the Southern Hemisphere of Jupiter" submitted for publication to

n

the Journal of Geophysical Research.

tJ 

Q
C1

7

Careful study of the dynamic frequency-time Voyager spectrograms roveals

a multitude of arc morphologies with arcs varying in frequency extent and in

curvature. Included is a class of arc with large frequency extent and

curvature which appears often as intense, and somewhat isolated from the

surrounding arcs. By assuming a wave normal angle which varied with frequency

at the source point we were able to match the curvature of these observed

high-curvature arcs with model ray tracing results. The wave normal angles

chosen were consistent with relativistically Doppler-shifted electron emission

in RX mode (Staelin, 1981). The electron energy was found to be about 5 keV

which is in agreement with electron energies in the vicinity of the Io torus

which are hypothesized to be accelerated and produce the DAM. This work

represents the first comprehensive three-dimensional ray tracing analysis of

varying the wave normal angle. previous studies considered only a

two-dimensional analysis and consequently could not adequately reproduce

observed arcs. The results of the present analysis are detailed in Appendix 3

which is a preprint of a paper entitled "Jovian Decametric Arcs: An Estimate

of the Required Wave Normal Angles From Three-Dimensional Ray Tracing",

submitted to the Journal of Geophysical Research.

The reformatting of the Voyager PRA data has allowed rapid analysis of

both Io-dependent and Io-independent emissions. Carrying the process of

further, S. Gulkis plotted the data using the sub-Io longitude as the

ordinate and the difference between the spacecraft longitude and the sul

longitude as the coordinate. This technique immediately allowed the

identification of an Io-independent emission which has been named Joviai

Auroral Decametric Radiation (JADR). This radiation is seen only on th(

nightside and appears to have a source in the high-latitude auroral zon(

These facts indicate the emission may be the Jovian counterpart to the

d

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intensely studied terrestial AKR emissions. Appendix A is a, preprint of a

p,,Aper entitled "Identification of a Night-Side Component of DAM as a Jovia

Counterpart to AKR" submitted to the Journal of Geophysical Research which

details the exciting discovery.
9

1.

r°

IV. CONCLUSIONS

Results of three-dimensional ray tracing of Jovian low frequency

radiation have provided important information regarding 1aR propagation of 

emissions. Direct comparison of model ray tracing results with observed ar

has indicated that many arcs can be adequately explained by propagation fro

hollow emission cones with sources at the feet of Io flux tubes. Emission

occurs at frequencies slightly Doppler-shifted above the RX cutoff frequency

with wave normal angles close to 90 0 . In addition, the effect of the

magnetic field in determining the curvature and frequency extent of the arcs

is fundamental (cf. Appendix 1).

After the introduction of a Jovian ionospheric model new ray tracing

results have yielded improved model arcs for the northern hemisphere and the

first three-dimensional model arcs with source positions in the southern

hemisphere (Appendix 2). The ray tracing has allowed the first delineation 

northern and southern hemisphere emisoions of voyager data. The importance 

re-formatting the Voyager data such that the sub-Io longitude can be held

fixed has been found to be of fundamental importance in determining

lo-dependent spectral features.

Certain high-curvature arc signatures can be modelled by allowing the

wave normal angle to vary yielding a T(f) relationship that is unique to th

high-curvature arcs. one explanation for the smaller wave normal angles

necessary to model certain high-curvature arcs is relativistically

Doppler-shifted electron gyro-emission (Appendix 3). For electrons with

energies of about 10 keV (reasonable for the source particles) the wave nor

angles resulting from theoretical Doppler shifted emission are quite close 

the values obtained from ray tracing needed to match the observed arcs. Th

implication is that certain high-curvature arcs result from the gyro-emissio

of beamed electrons with energies of a few keV.

11

C`

H

0

0

__
10

Reformatting the Voyager PRA data has also allowed the identification

of an lo-independent emission which is the probable couterpart to the

terrestial AKR. Jovian Auroral Decametric Radiation may provide new insights

to the emission mechanism responsible for the production of AKR, as well as

provide valuable information about the structure of the Jovian magnetosphere.
(

1

REFERENCES

CAcuna, M. A., and N. F. Ness, "The Main Magnetic Field of Jupiter",

J. Geophys. Res., 81, 2917, 1976.

Atreya, S. K. and T. M. Donahue, "Model Ionospheres of Jupiter", in Jupiter

edited by T. Gehrels, p. 304, University of Arizona Press, Tucson, 197

Haselgrove, J., "Ray Theory and a New Method of Ray Tracking", in Report of

Conference on the Physics of the Ionosphere, pp. 355-364, London Physi

Society, London, 1955.

Hashimoto, K. and M. L. Goldstein, "A Theory of the Io Phase Asymmetry of t

Jovian Decametric Radiation", J. Geophys. Res.,88, 2010, 1983.

Sentman, D. D., and C. K. Goertz, "Whistler Mode Noise in Jupiter's Inner 

Magnetosphere", J. Geophys. Res., 83, 3151, 1978.

Staelin, D. H., "Character of the Jovian Decametric Arcs", J. Geophys. Res.

e6, 8581, 1981.

Stix, T. H., The Theory of Plasma Waves, pp. 5-34, McGraw-Hill, New York,

1962.

Warwick, J. W., J. B. Pearce, A. C. Riddle, J. K. Alexander, M. D. Desch,

M. L. Kaiser, J. R. Thieman, T. D. Carr, A. Boischot, C. C. Harvey,

and B. M. Pederson, "Voyager 1 Planetary Radio Astronomy Observations

i

Near Jupiter", Science, 204, 995, 1979.

I'

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12

FIGURE CAPTIONS

FIGURE 1: Frequency versus system III 1 65 longitude showing the model arcs

at 10 0 intervals of constant Io CML. The Io CML is labelled on

each curve. The curves represent the intersection of one "side"

of the emission cone with a radial shell at 150 Rj. The source

points for the arcs shown lie on Io flux tubes in the northern

hemisphere with longitudes in the range: A) 10 0 to 90 0f

B) 100 0 to 180 0 1 C) 190 0 to 270 0 j and D) 280 0 to 3600.

FIGURE 2: Same as Figure 1 except the source points for the arcs shown lie

on Io flux tubes in the SOUTHERN hemisphere with longitudes in

the range: A) 10 0 to 90 0 1 B) 100 0 to 180 0

t C) 190 0 to 270 0 j and

D) 280 0 to 3600.
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VII. APPENDICES

A. Appendix 1: "Three-Dimensional Ray Tracing of

i Jovian Radiation in the Low Frequency Range"

i
L i

It

Three-Dimensional Ray Tracing of the Jovian

Magnetosphere in the Low-Frequency Range

J. D, MENiETTI, I J. L. GREEN: S. GIJLKIS, 3 AND F, SIX'

Three-dimensional ray tracing of the Jovian DAM emission has been performed utilizing she 04

magnetic held model (Acuna and Ness, 1979) and a realistic plasma model. Minimal assumptions about

the emission mechanism have been made that include radiation in the rich[-hand extraordinary mode.

propagating nearly perpendicular to the field line at source points located Just above the RX cutoff

frequency along to flux tubes. We have performed ray tracing in the frequency range from 2-31 MHz

from successive to flux tubes separated by ten degrees of central meridian longitude for u full

circumference or northern hemisphere sources. Our results show unusual complexity In model arc

spectra that Is displayed In a constant to phase formal with many similarities to the Voyager PRA data.

The results suggest much of the variation in observed DAM spectral features Is a result of propagation

effects rather than emission process differences. We feel our work represents a comprehensive ray

tracing Investigation of DAM are emission and presentation of the results In a unique formal.

I[

1489

INTRODUCTION

One of the most characteristic features of the Jovian

decametric radio emissions is the presence or arc-like struc.

tures in frequency-time spectrograms. The arc-like struc.

tures were first observed by the Voyager planetary radio

astronomy experiment (PRA) [Warrick el at, 1979] and

later found in ground based observations IUnischoi er al.,

19811. The arcs consist of narrow band emissions that drift in

frequency in a repeaiable and highly organized mnnner as

shown in Plate I. The frequency range of the arc emission is

from 40 MHz down to several MHz or lower, but all arcs do

not span this full frequency range, Several different kinds 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), The

arcs often appear as a closely spaced series referred to as

"nested arcs." Detailed descriptions of the arcs can be

found elsewhere [i.e., Warrick et al., 1979; Leblanc-, 1981;

Bolschol and Aubler; 1981, Carr er al„ 1983).

Various source mechanisms have been proposed to explain the decametric arcs. A general feature of most theories

is that the emission occurs along conical 11Y.-els that are

swept past the observer as the planet rotates 44 first suggested by Dulk [ 1965), Many investigators attribute the emission

to precipitating electrons along the to magnetic flux tube that

radiate near the gyrofrequency [Galdrelch and Lynden-Bell.

1969; Smith, 1976; Goldstein and Gnerr;, 1983]. Goldstein

and Theiman [1981], Pearce (1981], Staelln [1981]. Neubaiter [1980] have shown that arc-like structures are produced by the conical sheet model although no simple choice

of parameters has been found which reproduces the data in

detail. In particular, the cone emission model does not

explain the "nested arcs." These have been attributed to

multiple reflections of Alfv rn waves [Garnett and Gnerr;,

19811, interference effects [B(isdra and Aubier, 1981; LecaSouthwest Research Institute.

r NASA Marshall Space Flight Center.

s Jet Propulsion Laboratory,

' Arecibo Observatory.

Copyright 1984 by the American Geophysical Union.

Paper number 3AI905..

0148.0227I841003A-1905S05.00

cheux or al., 1981], and local magnetic anomal

1981),

Hashimoto and Goldstein [1982] used a three

ray tracing program to investigate the observed 

probability maps of the lo-dependent Jovian 

radiation, They also Investigated conditions for w

ation in the Jovian ionosphere, Goldstein an

(1981] performed an analytical study or refra

Jovian magnetosphere and ionosphere. They de

that an extraordinary wave, initially excited nea

nal to the magnetic field, is significantly influen

local plasma density and that arc-like features th

those observed are produced by the geometry o

fit this paper, we extend the work of Has

Goldstein [ 1983] and Goldstein and Thelrna

studying the arc structures using a three-dime

tracing program.

The purpose of this paper is to perform three-

ray (racing of DAM radiation from source positio

to flux tube at frequencies greater than but nea

the right-hand extraordinary (RX) mode cutoff 

JRx - lr12 + V,c12)t + j„= )" where Jm is gyrofre

fp is plasma frequency. We have incorporate

Jovian plasma model [Sentman and Goert 19

0.4 magnetic field model (Acting and Ness, 1

DAM arcs are produced with minimal assumption

source mechanism.

RAY TRACING EQUATIONS AND

MAGNETOSPHERIC MODEL

Ray Tracing

To calculate propagation ray paths in the Jovia

sphere, we have used a three-dimensional ray 

gram, The computer code is based on the Srix 

plasma formulation of the index of refraction 

grove's [1955] set of first order differential equati

ble to numerical solution. The program was init

oped by Shau-lain [ 1966] for two-dimensional ra

the terrestial ionosphere and magnetosphere. F

pose of this three-dimensional ray tracing stud

incorporated additional equations that allow for v

the azimuthal direction. This program has been 

tested and reduces to the two dimensional results 

al. [1977j, Green and Gernert [1980], and 

H

0

P
^

ID

^ ^ ^ m m .+ N

m

R1 ^ ^ D ^'D r+

WWU lWW7

q C^

1

[

^

•

^ V ^ 1 y r ^ ^

1

ell

L

•4p

No =•• •^^

m y = X C

• •. / '^ N

w ^/^ + • (` ul a 3

s• ^^•.we y ^ Y

•• r ^WCU E 

^c

OAM

{ 'asY

v

a ^

f

-Ole

{ t_
CONTOURS OF Fp (KHz)

OICAMETtnI $OUNCE

at 01016

r^

EO 100 100

2 ?00

100 Soo

IO /IELO LINE

OECAMETER EOUNCI

REGION

SYSTEM 111 LONGITUDE : 200' S Y STEM III LONGITUDE 3 20'

Fig. I Model Jovlan magnetosphere. Contours of plasma frequency are (tom Sentmun ant/ Gaut: 119781, B field from

A( una and Yeis 119761, to torus from War% tt k er al. 119791.

L

C

L

0981,. Chosen as Inputs are a source position tin radial

distance, longitude, and latitudcl from which waves at a

specific frequency f are to he launched at a specific wave

normal angle :L. with respect to the magnetic held. Different

frequencies onginating from source points on the same field

line or on adjacent held lines would generate a set of ncsted

emission cones Depending on the get merry of these cones,

imposed by the plasma and by the 8 field, the spac"raft

would detect frequencies that decrease or increase with time

as these cones sweep over the detector.

At the source region, the program calculates an index of

refraction surface for RX mode waves, based on E field and

on background plasma parameters. Next. the program takes

an increment;,' step in the direction perpendicular to the

index of refraction surface. i.e., in the direction of the group

velocity of energy .7,w, and then determines the coordinates

of this new point on the ray path. Then. another index of

refraction surface is calculated through this new point and

the steps are repeated. The direction of each ray changes

according to Snell ' s law as it travels through the magnetized

plasma. To generate an emission cone, thirty-six rays are

launched, one every 10° around 1i at the specified wave

normal angle and frequency.

8 field and Pla.ima .Models

The ray paths calculated by the procedure dtscnbed

above are dependent on the magnetosphenc model chosen.

The description of the lovian magnetic fl-Id utilized in these

calculations (the 0 .4 model) is that published by 4cuna and

.'Ness [ 19761; the descnption of the background plasma Is that

published by Sentmun and Goers (19741, and the description the to torus is that published by Warwick et al. (19791.

Figure I is in abbreviation of this combined "lovian magnetosphere" showing contours of plasma frequency In the 200°

-'C° mendlan plane. Suspected source regions In the northern and southern hemispheres are shaded. Sphencal harmonic expansion coefficients are used to calculate the magnitude of the field B,,, the c )mponents Br, B,t , and B. and

their dcnvatives The plasma density along the ray path, and

the spatial gradients are calculated from -mpin:al fits.

As Hashimoto and Goldstein 119831 hase pointed out

large plasma densities can produce Important refractory

effects at the higher frequencies when source points near the

foot of the to flux tube extend down into the Ionosphere. We

have Incorporated the Ionospheric model introduced by

Hashimoto and Goldstein to determine the upper cutof

frequency associated with each to flux tube investigated in

this study, but we have not incorporated an ionospheric

model in our ray tracing studies. We have avoided ray

tracing at frequencies where ionospheric Influence is impor

tant We became aware of the importance of the ionosphere

in alfcc ! tng propagation at certain frequencies only after our

extensive ray tracing investigation had almost been completed. Tests at sevend individual to flux tubes have indicated

that incorporating an ionospheric model that continuously

joins with a magnetosphenc model will modify our results

only slightly at frequencies whose source points lie outside

the influence of the Ionosphere.

COMPUTATIONS

In addition to the magnetosphenc plasma model discussed

in the previous section, some assumptions regarding saurce

location, initial launch angle, and mode of propagation are

required as inputs to the ray tracing studies. Rather than

carrying out a parameter study of these vanables, we have

chosen a set of initial conditions and studied the propagation

over a full range of observer•lu-Jupiter longitude phase

space. The results of our studies are intended to show the

trends in the radiation spectra introduced by the complex

geometry of the problem Indeed, as we will discuss below
I

j

V-110' $MILS. • 100 RJ

•o• ^ -T-T—^ ^—r —T

i ^— t yNI t MHz

t MNI

^H JO MMI ::0 —`+ IO MMI

OW I 10 MNi

Go /. 10 M"

J -3O-

.J^.^1C MMI

_ 20 MNI

t MNI

•0• ^

t MNI

t MMI

_^0 ^ I 1 1 ^ l I t

s0 •

760• 110• 130• 1104 110• s0'

SYSTEM III ( 1965) LONGITUDE

Fig 2. Calculated radiation sheets for two northern hemisphere sources 12 MHz and 10 MHz) And for two southern

hemisphere sources 12 MHz and :0 MHz) The radial shell at 100 Jupiter radii ( 100 R,i is shown in mercator projection.

kit rays were launched at MY with respect to cite B meld.

our studies show that exceedingly complex spectra can he

produced by propagation erfects alone, without introducing

complexity Into the emission mechanism itself.

The initial condition s we used are is follows:

I. The source region is at the foot of the magnetic L shell

that intercepts to

2. The frequency of emission in the source region is

close to the gyrofrequency and greater than the RX cutoff

frequency.

3 The emitted radiation is in the right-hand polarized

extraordinary mode

4. The radiation is emitted in a hollow cone pattern about

the magnetic held direction. with the emission angle close to

90° iwith respect to the magnetic held)

No specific emission mechanisms are implied by our assumption; rather. they are chosen to be consistent with the

available data (i.e.. Carr et al., 19831 and to at [cast some of

the theones of radio emissions (i.e . Dulk, 1%5; .Neubauer.

1980; Goerte, 1980; Gurnett and Goertz-, 1981; Golditein and

Goerrz. 19831.

An entire senes of ray trajectones are computed for to

positioned every 10° of Jupiter longitude. Typically, lo's

position is chosen and a magnetic line of force is calculated

from to to the foot of the flux tube near Jupiter's cloud tops.

Source points are located at a set of frequencies, typically 2.

5, 10, 15, 20, 25, 30. and 35 MHz from the condition f/fr =

1.005 at the origin. This latter ratio is chosen to he consistent

with slightly Doppler-shifted emission at large wave normal

angles. Gyroemission is nut assumed. The RX cutoff frequency is calculated and compared with f. Under the

conditior f > f tx, wave propagation is possible and a senes

of rays is traced at 36 evenly spaced azimuthal direction

around the source point. We have also accounted for the

oblateness of Jupiter in our determination of f Kx -The radial

distance to the cloud tops, R(km) is give n by R = 713(0 1 -

cos-(colatitude),, 15..1). Figure 2 shows on a mercatur Droiecoun, the intersection of several families of ray trajectories

with a sphere of radius 100 R ) that surrounds Jupiter. Shown

in the figure a re a_- and It)-\fHz family that originate in the

northern hemisphere. and a 2- and =0-`1Hz family that

onginate in the southern hemisphere.

In all, more than I(M) hours of computer time ICPU) on a

CYBER 172 was required for the results reported in this

study Typically, 23 min frequency for each lo position was

required to track the ray from the source point out to 150 R,.

RESULTS

The principal results of our computations are shown in

Figures 3a and 3b. The figure shows the Irequencies that

would intersect a spacecraft were it located at R - 150 R,

and at a latitude of 3 2° The curves are parameterized by the

central meridian longitude facing to at the time of emission.

At this distance from the source location, the parallax

between the center of the planet and the source is small, and

the results apply generally to any observer on this latitude

and at a greater distance from Jupiter. The two figures

correspond to the two azimuth positions that intersect the

observer from every source point (i.e.. in general, emission

from a cone may intersect the observer in two azimuth

directions). The solid curves in Figures 3a and 3h are not in

general what a fixcd observer would see because lit is not

stationary in the Jovian - fixed longitude system. Hence a

stationary observer sees to moving from longitude to longitude.

DiscussioN

In Figures 3a and 3h are revealed the wide range of

structure in the curves resulting from varying the to flux tube

longitude of the source points Indicated by small horizontal

brackets on Figures 3a and 3b are the upper cutoff frequencies for RX mode emission at source point frequencies

slightly above the RX cutoff. These cutoffs indicate the

maximum frequency that is expected to be emitted along a

particular to flux tube if that emission is slightly above fax.

No DAM emission will escape the ionosphere if the source is

at an altitude less than the altitude of the density maximum

in the ionosphere. We have sit far not performed ray tracing

at frequencies that are determined to have sources deep in

the ionosphere but have used an Ionospheric model only to

determine what frequencies will be influenced and the cutoff

frequency. Oftentimes, huweser, one dues not observe

emission .0 higher frequencies because those frequencies are
ti

z

3

U

Z

w

W

a

♦ r

sic N - 150 NJ LAt. 3. 1 • oil to

its{;

I

ra • -

r..

77 r I ^ S° o I

7• ^ R I I '• R

^• r I I

7e 

r

h $

I^ . ti

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1/19 • 1.005 W • 00,

! v

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JI y g Yxtf:

t

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,

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h 0 •

h ~I ! ^ I I I

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ji 

^ ti^ ° p I

;

^ • I'/ll ^I III III I 111111// ^^//

10 70 00 r0 00 110 UO 100 170 100 910 • 20 700 7r0 700 210 220 1.0 IO 30

SYSTEM III LONGITUDE

Fie 3u Frequency serus system III -65 longitude showinC the model arcs at 10° miervals of constant to CML

(constant to format The to CML is labeled on each curve These cure, represent the Intersection of one "side' of the

emission cone 'AIlh a radial shell at ISO R, The horizontal bracken Indicate the maximum frequency that can be emitted

under the conditions specified in the text. Also indicated are the location of the to-B source and a modified sketch of

pert of Figure 7.15 ICurr et ul , 191411 indicating the typical signature seen on a dynamic (nonconstant lo) format,

L

refracted iwnhoui ionospheric Influence) and do not intercept the spacecraft The upper cutoff frequency indicated on

Figures 341 and 3h are thus a result of the complex interaction

of the magnetic field structure. ionosphere, magnetosphere.

and oblateness :)f Jupiter

In Figures 3a and 3h for to central meridian longitude

(CML) in the range from approximately 86` to 240 the

curves are rather straight, indicating that multiple frequen•

cies may be observed at a given Spacecraft CML. B,, ' Io

CML" we mean the sub-lo longitude For to CML in the

range from 360` through 70` the curves have greater slope.

and some show more curvature with a lower but increasing

upper cutoff frequency In the range of to CML from 250'

through 350`. the upper cutoff frequencies are lower with

complex structure shown in Figure 3h when the spacecraft

CML Is greater than the CML of lo. It should be noted that

while to may be located at a given CML- the foot of the to

flux tube is not. in general, at the same CML. We believe

much of the changing morphology seen in the curves shown

in Figures 3a and 3h is due to the azimuthal asymmetry of

the 0-4 magnetic field model. which changes the source

point field strength and latitude in a circumference of Jupiter.

Also shown superimposed on Figures 3u and 3h are

modified sketches of part of Figure 7.15 from Carr et ul.

(1983). These sketches Indicate the location and typical

signatures of the lo-dependent dynamic inon-lo-fixed) spectra. The to-B source. for instance, generally displays fairly

low curvature arcs with smoothly falling upper cutoff frequencies in general agreement with the model arcs 

3a. The to-A source typically displays arcs of mor

ture than to-B source and also displays a low

frequency with increasing spacecraft CML. The mo

of Figure 3h in the to-A source region would map 

curvature arcs on a dynamic (non-to-fixedi plot

upper cutoff frequency dots falloff as observed. For 

source the overlap between the observed to phase 

model to phase is riot as good as for the to-A and B 

(cf Table 7 4 of Carr er al 11983)). but the lo 

region includes both left-hand and nght -hand po

emissions. which indicates possibly some southe

sphere sources that were not considered in the prod

Figure 3 The to-C source region dynamic spectra sh

curvature with rapidly dropping upper cutoff freque

region of Figure 3h where the model curves s

greatest curvature and a sharp drop in the cutoff fre

While the exact arc shape as noted on Figures 3

drawn In a constant-lo format should not he direc

pared with the dynamic spectra that are drawn in a 

stant to format, it is illustrative to note the chang

dependent arc morphology as one moves to the dif

A. -B. and -C source regions on the constant-lo f

Figures 3u and 3h. It is tempting to infer that the mo

cal changes in lo-dependent arc structure as see

dynamic spectra can he largely explained by pro

effects rather than by varying emission processes.

Plotting the Voyager PRA data in a constant to fo

he a valuable tool to a better understanding of the 

L 
SIC R - c 50 R LAT r 7 2° flf0 , t 005 W-90'

34

/•

33

10

29

4

24

23

30 \

\ \ O

,. o

ao ^

n

,0 \

J

0

1 >; O \

10 ^

1I

•

2

J

[

0

U

Z

w

O

W

Cr

U.

OO

1

O

I .^

u

u

30

(((

[O- A

w 

o

10

Z

I ^YY

o °

N w , 5 S

G ^ A O .7

= ^ O I r

i

O ( D Y,^

N Y

a ^• N

N

O pp

^O }N

"^ N

I ^ Ia\\

I

J 0 0

$

I O

I I 1 

J^

JJO ,^O

'0 30 s0 70 90 1.0 130 150 +70 190 210 230 250 210 290 310 330 350 10 30

SYSTEM 111 LONGITUDE

30 30,

IO-A , 10 10-C

10

Fig. 3b. Same as Figure 3a for the other the emission cone The to-A and to-C source locations and typical

dynamic arc signatures are also indicated [Carr of al . 19931.

processes of the lo-dependent sources (cf. Green, 1983[. We

have avoided replotting Figure 3 in a dynamic-spectra format

for direct companson with the observed Voyager dynamic

spectra (although this has been done for specific examples

with reasonably good agreements, because we feel the value

Of Figure 3 is in displaying the mayor features of lo-dependent emission. The correlation found between Figures 3a

and 3b and the to - A, - 8, and -C source regions supports the

initial assumptions pertaining to the emission process, which

include RX mode emission almost perpendicular to the

magnetic field from source points just above the KX cutoff.

We are currently in the process of companng reformatted

Voyager PRA data with Figure 3 in an effort to understand

better the emission process(es) and to explain on a finer

scale any differences noted. We recognize these differences

may be due to varying wave normal angle [Goldstein and

Thieman, 19811 andlor discrepancies in the magnetic or

plasma field models. In addition, the ionosphere is row

known to be of importance at the higher frequencies at some

CML values.

CONCLUSIONS

We conclude from our results that with minimal assumptions pertaining to an emission mechanism our ray ;racing

results produce a complex spectrum Of lo-dependent DAM

arcs as shown in Figures 3a and 3b. These few assumptions

together with the incorporation of the 0.4 magnetic field

model [Acuna and ,Vess, 19791 and a plasma model due to

Senrman and Goerrz (19781 have resulted in model DAM

arcs which display different curvatures and upper cutoff

frequencies.

When the source pints are located on to flux tubes that

are at Jovian system III longitudes ranging from approximately 90°-:30 where the 0. 4 model yields high surface

magnetic field strengths, Figures ?a and 3b indicate that the

resultant DAM arcs have low curvature and upper cutoff
(Received January 31. 1983:

revised October 24, 1983:

accepted November 29. 198

+:s 

frequencies extending beyond 35 MHz. On the other hand.

when the foot of the to flux tubes lie in the range of system

111 longitudes extending from 240°-80°, where the 0.4 sur•

face field strengths reach a minimum. the resultant arcs have

lower upper cutoff frequency and a higher curvature. It is

apparent that the B field structure that produces source

points at the foot of the to flux tube ranging in latitude from

45 400 and in altitude from 0 to >2Rl around the circumfer.

ence of Jupiter produces substantial differences in the resultant DAM arcs,

We have also performed preliminary ray tracing studies,

including the ionospheric model introduced by Hashimoto

and Goldstein ( 19821 and find that the ionosphere can have

great influence on emission ray path at the higher frequencies or emission along some to flux tubes.

Our studies indicate many areas or future attention. We

recognize the importance of including the ionosphere in our

ray tracing and have begun reproducing our results including

an ionosphere for both northern and southern hemisphere

sources, In addition, we are investigating the effects of

varying the wave normal angle and Doppler shift at the

source point. We hope to explain better the higher curvature

arcs often observed in the Voyager PRA data.

Because of our apparent success at reproducing, at least

qualitatively, some of the Voyager lo-dependent DAM observations. we recognize the importance of reformatting the

Voyager PRA data in a manner similar to Figures 3a and

3b-holding the to longitude constant (cf. Green, 19831.

Arknowledgments. We would like to thank the GSFC radio

astronomy group for supplying us with Voyager I and 2 PRA data.

We would also like to thank Jerome De La Cruz for his assistance in

programming. This work was supported by NASA contract NAS7-

100 through Cal TechIJPL contract 956026.

The Editor thanks M. L. Goldstein and C. K. Goertz for their

assistance in evaluating this paper.

REFERENCFS

Acura. M. H.. and N. F. Ness. The main magnetic field of Jupiter.

J. Geopltvs. Res„ 81, 2917, 1976,

Boischot. A., and M. G, Aubier, The Jovian decametric arcs as an

interference pattern. J. Geophys, Res.. 86, 8561. 1981.

Boischot, A,. A. Lecacheux, M. L. Kaiser, M. D. Desch. J. K.

Alexander. and J. W. Warwick, Radio Jupiter after Voyager: An

overview of the planetary radio astronomy observations, J.

Geophys. Res., 86, 8213, 1981,

Carr. T. D,. M. D. Desch, and J. K. Alexander. Phenomenology of

magnciosphcric radio emissions in Pikysirs of the Jorion Mognerosphere, edited by A. J. Dessler, p. 226. Cambridge University

Press. New York, 1983.

Dulls. G. A.. lo-related radio emissions from Jupiter. Ph.D. thesis,

Univ. of Colo„ Boulder, 1965.

Goertz, C. K.. to's interaction with the plasma torus, J. Geoplga,

Res., 8.1, 2949, 1980.

Goldreich, P.. and D. Lyndon-Bell, lo, A jovian unipolar inductor.

Astropltrs. J., 116. 59. 1969.

Goldstein, M. L., and C. K. Goertz. Theories of radio emissions and

plasma waves, in Ph lies efthe Jnrion Afagnernsphere. edited by

A. J. Dessler, p. 317, Cambridge University Press, New 

1983.

Goldstein, M. L., and J. R. Thieman, The formation of arcs 

dynamic spectra of Jovian decametric bursts. 86. 8569. 19

Green, J. L„ The to decametric emission core, Radio SeL, In 

1984.

Green, J. L., and D, A. Gurnee, Ray tracing of Jovian kilo

radiation, Geophys Res. Len„ 1, 65, 1980,

Green, J, L,. D. A. Garnett. and S, D, Shawhan. The a

distribution of natural kilometric radiation. J. Geophys. R

18:5, 1977.

Garnett. D. A., and C. K, Goertz, Multiple Alfven wave refle

excited by lo: Origin or the Joviau decametric arcs. J. Ge

Res„ 86, 717. 1981.

Haselgrove, J.. Ray theory and a new method of ray track

Report of Conference on the Physics of the Ionosphere, p

364, London Physical Society, London, 1955,

Hashimoto, K,, and M. L. Goldstein. A theory of the to 

asymmetry of the Jovian decametric radiation, J. Geophys

88, 2010, 1983.

Leblanc, Y.. On the structure of the DAM Jupiter emiss

Geophys. Res., 8546, 1981.

Lecacheux, A.. Ray tracing In the to plasma torus: Applica

the PRA observations during Voyager I's closest approa

Geophys, Res., 86, 8 523, 1981.

Lecacheux, A., N, Meyer.Verner, and G. Daigne, Jupiter's 

metric radio emission: A nice problem of optics. Astro

trophvs., 94, L9, 1981.

Neubauer. F. M.. Nonlinear standing Alfven wave current sys

to: Theory, J. Geophys. Res„ 85, 1171. 1980,

Pearce. 1. B.. A hearistic model for Jovian decametric a

Geophys. Res.. 86. 8579. 1981.

Seniman. D. D., and C. K. Goertz. Whistler mode noise In Ju

inner magnetosphere. J. Genphys, Res,, 83, 3151. 1978,

Shawhan. S. D.. VLF ray tracing in a model ionosphere. Re

66-3.3, Dept. of Phys, and Astron., Univ, of Iowa, Iow

1966,

Smith, R. A., Models of Jovian decametric radiation. Jupiter

by T. Gehrels. University of Arizona Press, Tucson. 1976,

Smclin. D. H,. Character of the Jovian decametric arcs, J. Ge

Res., 86, 8581, 1981,

Stix. T. H.. The Theory of Plasma Wares, pp. 5-34. McGra

New York, 1962,

Thieman, R,. Jr.. and A. G. Smith. Detailed geometric mode

Jupiter's lo-related decametric radiation. J. Geophys, Re

2666. 1979.

Warwick. 3. W.. Models for Jupiter's decametric arcs. J. Ge

Res., 86, 8585. 1981,

Warwick, J. W., J. B. Pearce, A. C. Riddle, 1. K. Alexander

Desch, M. L. Kaiser. J, R. Thieman. T. D. Carr, A, Boisch

C. Harvey, and B. M. Pederson, Voyager I planetary 

astronomy observations near Jupiter. Srienre, 204, 995, 1

J. L, Green. NASA Marshall Space Flight Center. Huntsv

35812.

S, Gulkis, Jet Propulsion Laboratory, Pasadena. CA 91103

J. D. Menietti, Southwest Research Institute. P. 0. Drawer 2

,San Antonio. TX 78284.

F. Six. Arecibo Observatory, Arecibo. Puerto Rico 00612.
freyuenctes extending beyond 11 MHz On the other hand

when the foot of the to Aux tubes lie in the range of system

111 longitudes extending from 240"-80`, where the 0-4 surface held -strength% reach a minimum. the resultant arcs hale

lower upper cutoff Ireyuency and a higher curvature It is

Apparent that the B field structure that produces source

points at the foot of the to flux tube ranging in latitude from

V-70` and in altitude from 0 to >2R, around the circumference of Jupiter produces suhstantial differences in the resultant DAM arcs.

We hale also performed preliminary ray tracing studies.

including the ionospheric model introduced by Httshinioto

and Goldstein 119821 and find that the ionosphere can have

great influence on emission ray path at the higher freyuencles of emission along some to flux tubes.

Our studies indicate many areas of future attention. We

recognize the importance of including the ionosphere in our

ray tracing and his begun reproducing our results including

an ionosphere for both northern and southern hemisphere

sources. In addition. we are investigating the effects of

varying the wave normal angle and Dopplef shift at the

source point We hope to explain better the higher curvature

arcs often observed in the Voyager PRA data

Because of our apparent success it reproducing. at least

yualitatisely, some of the Voyager lo-dependent DAM ohser,ation%. we recognize the importance of reformatting the

Voyager PRA data in a manner similar to Figures la and

3b-holding, the to longitude constant 1ef. Green. 19831

Atkni,nledgmrnts We would like to thank the GSFC radii)

astronom y group for supplying us %ith Voyager I and : PRA data

We would also like to thank Jerome De La Cruz for his assistance in

programming This work was supported by NASA contract NAS--

IIIO through Cal Tech JPL contract 9561126

The Editor thanks M L Goldstein and C K Goertz for their

assistance in evaluating this paper

REFERE%CcS

Acuna. M. H.. and N. F Ness. The main magnetic held of Jupiter.

J Grophys Res- 81 2917, 1976

Botschot. A.. and M. G Aubier. The Jovian decametnc arc-, as an

interference pattern. J Grophys. Rrs.. 86 8561 1981

Botschot A.. A Lecacheux, M L Kaiser. M D Desch. J K

AJetander. and 1 N Kar%ick. Radio Jupiter after Voyager An

overvit% of the planetary radio astronomy observations. J

Geopb s s Res.. 86. 8213. 1981.

Carr. T D.. M. D Desch. and 1 K Alexander. Phenomenology of

mignetosphenc radio emissions in Ph% sit s of tiro j"I lun 1futlnrrosphere edited by A. 1 Dealer p 226. Cambridge Lnnersity

Press. Ne% York, 1983

Dulk. G. A . lo-related radio emissions from Jupiter. Ph D thesis.

Lniv of Colo_ Boulder. 1965

Goeriz. C K . to s interaction %ith the plasma torus. J Grophy.%

Res . 85 2949. 1980

Goldreich. P . and D Lynden Bell, lo. A Jovian unipot ar inductor.

A.ur(,phy.s J.. IS6. 59. 1%9

Gol&icm. M. L.. and C K Goeriz. Theories of radii) emissions and

plasma waves, in Ph y stcs o/the Jos run .Mugnriosphea . edited h.

A 1 Dessltr. p 317, Cambridge Lmve ►siiy Press NvA 

1991

Goldstein, M L.. and J R Thicman, The formation of arcs 

dynamic spectra of JO%ian dc^ametnc burst.. ern 9'69. 19

Green, 1. L.. The to decametnc emission core Rudro Set.. in 

1994

Green. 1 L and D A Gurnett. Ray tracing of Jovian kilom

radiation. Groph y s Res Lett l 6 4 . 1990

Green. J. L.. D A Gurnett. and S D Shawhan. The an

distribution of auroral kilometres: radulion. J Grophy s R

19:x.1977

Gurnee 1) A . and C K (iocrt[. Multiple Alfsen wave refle

ct.ued by to Origin of the Jovun decamttnc arcs J Gro

Res . 86. 717, 1.991

Hasellrove. J . Ra) theory and a new method of ray track

Report of ConJrrene a on thr Ph y si s of tire- lonosphnr. p

US4. London Physical Society. London. 1955

Hashtmoto, K . and M L Goldstein. A theory of the to p

asymmetry of the Jovian decametnc radiation. J Grophys. 

88. 2010. 1983

Leblanc. Y.. On the structure of the DAM Jluptier emiss

Groph y s. Res . 8546 1991

Lecacheux. A.. Ray tracing in the to plasma torus: Applica

the PRA observations during Voyager I s closest approa

Grophvs Rrs.. 86 8533, 1991

Lecacheux. A . N Meyer•V'ernett. and G Daigne. Jupiter ' 

metnc radio emission A nice problem of optics. Astr

trophu . 94. L9. 1981

Neubauer. F M . Nonlinear standing Alfven wave current sy s

to Theory. J Geophvs. Res.. 83. 1171. 1980

Pearce. 1 B_ A heartsUC model for Jovian decametnc a

Geophrs Res. 86 8579, 1991

Sentman. D 1) and C K Guertz. Whistler mode noise In Ju

inner magnetosphere. J Gropins Res . 8?. 3151. 1979

Shawhan. S D . VLF ray tracing in a model ionosphere. Res

bn-??. Dept, of Phy% and Astron.. Lniv of Iowa. Iowa 

I%h.

Smith. R A.. Models of Jovian decametnc radiation. Jupiter, 

b y T Gehrels Lniversin of Anzona Press Tucson. 197x,

Stachn. D H . Character of the Jovian decametnc arcs. J. Gr

He's .86. 8581. 1981

Sty x. T. H . The Throe ujPlusntu Wares. pp 5-34. McGraw 

tie% York. 1961

Th eman. R . Jr . and A G, Smith. Detailed geometric mode

Jupiter s lo-related decametnc radiation. J Grophys Re

2666.1979.

War%ick. J W.. Models for Jupiter s decametnc arcs. J. Ge

Rrs . 86. 8585. 1991

War%ick. J 1A' . J. B Pearce A C Riddle. J K Alexander. 

Desch. M L Kaiser. J R Thieman. T D Carr, A Botsch

C Harvey. and B. M Pederson, Voyager I planetary 

astronomy observations near Jupiter. Serener. 204 995. 19

1 L Green. NASA Marshall Space Flight (enter Huntsvi

35912

S Gulkrs. Jet Propulsion Laboratory, Pasadena. CA 91103

J D Meniciti. Southwest Research Institute. P O. Dra%er 2

San Antonio. TX 79294.

F Su. Arecibo Observatur). Arecibo. Puerto Rico 00612

1Recerved lanuary 31. 199?.

revised October 24. 1993.

accepted Novemher 29. 1983.1

• !tt :'- -
B. Appendix 2: "Identification of Decametric

Radiation from Sources in the Southern

Hemisphere in Jupiter"

_. + Sys rti:^t>^n.. y
IDENTIFICATION OF DECAMETRIC RADIATION FROM THE

SOUTHERN HEMISPHERE OF JUPITER

by

C

J. Douglas Menietti

Southwest Research Institute rrI

I San Antonio, Texas 78284

James L. Green

Space Science Laboratory

NASA Marshall Space Flight Center

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Huntsville, Alabama 35812

L.1

April 1984

Submitted to Journal of Geophysical Research

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ABSTRACT

A technique has been developed which 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. All

Voyager 1 or 2 PRA observations are sorted into bins (±3 0 wide) centered on

a specific Io cental meridian longitude. When the data is plotted (as a

frequency-longitude spectrogram) in this coordinate system, Io-dependent

features can be identified and used for direct comparison with model ray

tracing calculations. The ray tracing calculations are done in a model Jovia

magnetosphere where it is assumed that the decametric emissions are generated 

in the RX mode from low altitude source regions along the instantaneous Io

flux tube. In this study, we compare the observations taken for four specifi

Io longitudes (150 0 , 160 0 , 1700 , and 260 0 ) with the corresponding model ray

tracing. The Voyager observations and the model ray tracing calculations, 

when combined, clearly distinguish the northern and southern hemisphere

emission features. This method allows, for the first time, Io-dependent DAM 

emissions, generated in the southern and northern hemisphere to be delineated.
The complete spectrum of Jovian decametric (DAM) radiation was first

observed by the Voyager Planetary Radio Astronomy (PRA) experiment (Warwick 

al., 1979). one of the most striking features of the Jovian DAM emissions

seen in frequency-time spectrograms, are "arc-like" bands. The frequency

range of the arc emissions is 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 towar

increasing time (vertex early) or open toward decreasing time (vertex late).

(

In general, an arc may appear either as an isolated feature or there may be a

series of closely spaced arcs (called "nested arcs"). Detailed descriptions

of the Jovian decametric arcs can be found elsewhere (is., Warwick et al.,

r 1979; Leblanc, 19811 Boischot and Aubier, 1981; Carr, Desch, and Alexander,

1983).

C

A general feature of most propagation theories of the DAM arc producing

mechanism is that the emission occurs along conical sheets that are swept pas

C' the observer as the planet rotates as first suggested by Dulk (1967). Many

investigators attribute the emission to precipitating electrons along the Io

-I magnetic flux tube which radiate near the gyrofrequency (Goldreich and

CI

Lynden-Bell (1969); Smith (1976); Goldstein and Goertz, 1983). Goldstein and

Thieman (1981); Pearce (1981); Staelin (1981); Neubauer, (1980) have shown

C1

that arc-like structures are produced by a conical sheet model although no

simple choice of parameters has been found which reproduces the data in

detail. in particular, these simple emission cone models do not explain the

"nested arcs". However, nested arcs have been attributed to multiple

reflections of Alfven waves (Gurnett and Goertz, 1981), interference effects

a(Boischot and Aubier, 1981; Lecacheux, 1981), and local magnetic anomalies.

n
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Is"

1. l

(Warwick, 1981). As shown by Goldstein and Thieman (1981) and Menietti at 

(1984), the arc curvature which varies on a frequency-time spectrogram from

almost zero to quite dramatic loops (great arcs) may result from variations 

the wave normal angle as a function of frequency in the source region.

Hashimoto and Goldstein (1983) have suggested that the Io phaan, asymmetry o

the DAM arcs may be due to the difference in pro pagation time of Alfven wave

reflected from both the northern and southern hemisphere Jovian ionosphere.

The purpose of this paper is to compare three-dimensional ray tracing o

Jovian DAM emissions, from the southern and northern hemisphere, with the PR

measurements in order to determine the frequency-time signatures of southern

hemisphere arcs. This paper complements the northern hemisphere ray tracing

of Menietti et. al. (1984). The observations will be transformed into an

Io-Jupiter stationary coordinate system (see Green, 1984). This coordinate

system allows for the Io-dependent emission features observed by the PRA

experiment to be directly compared with the model ray tracing results. We

have chosen source regions at four sub-Io longitudes. The agreement between

the Voyager observations and the model ray tracing is good and allows the

separation of the northern and southern hemisphere emission features. With

this technique the southern and northern hemisphere decametric source

we believe, for the first time been delineated. The good agreement b

the observations and the ray tracing results provides strong evidence

basic assumptions of the model are correct. Refinements are necessar

however, to reproduce exact details of the emissions.

4 '16 u,

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THE RAY TRACING PROGRAM

The ray tracing program used for this study is discussed in some detail

in Menietti et al. (1984). Briefly, the Jovian magnetosphere used is the 0-

magnetic field model of Acuna and Ness (1976), and the Jovian background

magnetospheric plasma is given by the model proposed by Sentman, and Goertz

(1979). The Io torus plasma density is a spline fit to the published contou

of Warwick et al. (1979). In addition, the computer code contains an

empirical model of the Jovian ionosphere based on the results of Hashimoto an

Goldstein (1983). The program is based on a closed set of first order

differential equations in spherical polar coordinates specialized by Shawhan

(1966) (in two dimensions) and by Menietti at al. (1984) (in three dimension

that describe the path of rays in a magnetized plasma. The expressions for

the phase index of refraction and its derivatives are in the cold plasma

formulation of Stix (1962).

Qualitatively, the program calculates an index of refraction surface an

group velocity (magnitude and direction) for a predetermined plasma wave mod

in the model magnetosphere at some starting point. The program then takes a

incremental step in the direction of the group velocity or energy flow and

determines the coordinates of a new point on the raypath. Another index of

refraction surface is calculated at this new point and the steps are repeated

The program used here has been extensively tested and reduces to the

two-dimensional results of Green and Gurnett (1980) and Lecacheux (1!

MODEL ASSUMPTIONS

The following assumptions regarding source location, initial lai

and mode of propagation are required as inputs to the ray tracing pri
I . The source region is at the foot of the magnetic L-shell which

intercepts Io;

2. The frequency of emission in the source region is close to the

gyrofrequency and greater than the RX cutoff frequency;

3. The emitted radiation is in the right-hand polarized extraordinary

mode; and

4. The radiation is emitted in a hollow cone pattern about the magnet

field direction. The emission angle with respect to the magnetic

field was set equal to 85 0 for northern hemisphere sources and to

both 85 0 and 95 0 for southern hemisphere sources.

No specific emission mechanisms are implied by our assumptions; rather

they are chosen to be cbnsistent with the available data (i.e. Carr et al.,

1993) and at least some of the theories of radio emissions (i.e. Dulk, 1967

Neubauer, 1980, Goertz, 1960; Gurnett and Goertz, 1981; Goldstein and Goert

1983).

We have computed ray trajectories for sources located at sub-Io

longitudes located every 10 0 of Jovian longitude, for both northern and

southern hemisphere sources. Source points are located along an Io flux tu

at a set of frequencies, typically 2, 5, 10, 15, 20, 25, 30, and 35 MHz from

the condition f/fg=1.01 at the the origin. This latter ratio is chosen to 

consistent with slightly Doppler-shifted emission at a wave normal angle, T

of 85° (cf. Staelin, 1981). Gyroemission is not assumed. Under the condit

that the wave frequency is greater than the RX cutoff frequency, wave

propagation is possible and a series of rays, for a constant frequency, is

traced at 36 evenly spaced azimuthal directions around the magnetic field a

the chosen source location. In the determination of the location of the RX

t. _1

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cutoff frequency, the oblateness of Jupiter is accounted for; the radial

distance to the cloud tope is given by R - 71300 ( 1-co92(colatitude) / 15.

The intersection of the resulting generated emission cones, for all

Mfrequencies, at 150 Rj and at a latitude of 5° determines the model deeam

F

emission spectrum. It is this model decametric spectrum which is compar

s .^

R'

i

with the observations. For the purposes of this study we have chosen on

t

ffour sub-Io longitudes to compare directly with observations which we fee

?. ! clearly depict those emissions from southern hemisphere sources.

COMPARISON OF OBSERVATIONS AND RAY TRACING RESULTS

Only PRA observations made from February 21 to March 18, 1979 by Voy

f 1 and from June 26 to July 23, 1979 by Voyager 2 are used in this study.

NE

These observations are approximately centered around the encounter period 

ry

111

comprise enough data for a complete frequency -longitude study of Io-depe

f

DAM emissions to be made. Observations at Ladial distances of less than 

'

( were excluded in order to avoid the changing emission pattern due to

inner-magnetospheric propagation effects. The frequency -longitude spect

++

^y

for Voyager 1 and 2 data (sub-IO system -III longitude of 260 0 ) is shown 

Figure 1. This figure displays Voyager 1 and 2 data transformed into a

coordinate system where ; . a sub-Io system- III longitude was equal to 260

444E

(t3 0 ) The data displayed in Figure 1 is only a subset of the entire Voya

i

data. It is important to note that a band of emission extending in frequ

from about 6 to 13 MHz seen at all Jovian system -III longitudes in Figur

i (as well as in Figures 2-4) is partially due to an increase in the sensit

of the PRA instrument at these frequencies ( Schauble and Carr, 1983).

Clearly seen in Figure 1 are two rather narrow emission zones. One

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located at Jovian system-III longitudes between approximately 155 0 -185 0 

H
1

another located between about 305 0 - 330 0 . Both of these emission zones exte

to over 30 MHz. In the region between these two narrow omission zones whic

i will hereafter be referred to as northern hemisphere emissions (NHF.) is a

region of less distinct but nevertheless structured emission which extends

between about 215 0 -305 0 and in frequency from about 5-25 MHz. There is a

narrow emission feature centered at about 60 0 and extending in frequency t

^l

possibly over 30 MHL but careful examination of this feature reveals that i

is due to spacecraft noise (interference from other instruments). In

addition, the emissions seen centered around 140 0 and extending to about 26

MHz, and the emissions at about 274 0 and extending in frequency to almost 4

MHz, are also of spacecraft origin.

``

Also shown on Figures 1-4 are the results of ray tracing emiauion from

l

both northern (circles) and southern (triangles and squares) hemisphere

x

sources. The ray tracing results for the southern hemisphere sources are

shown for initial wave normal angles of 95 0 (triangles) and 85 0

(squares) i

order to investigate the dependence of the resulting arcs on this parameter

The agreement for the northern hemisphere emissions is relatively good with

respect to Jovian longitude. Seen in Figure 1 are model emissions within

Jovian longitude range 180 0 -186 0 and emissions at about 335 0 . Both of the

model northern emission cone edges are quite narrow in longitude extent.

l

(? These northern hemisphere model emission cones lie close to the range of th

CC_f

observed northern emission cones shown in the same figure, but shifted

If possibility 10 0 to higher longitudes. The frequency extent for the model

if

emission cones is less than observed, however, by about 15 MHz. The reason

for this discrepancy may to be due to inadequacies in the magnetic field

model. An increase in the magnetic field strength at the source point from

I`^J

about 7 gauss to over 10 gauss would be required for emission at 30 MHz to 

LJ
allowed. This large increase in the field strength suggests either errors in

the model at low altitudes or transient effects. since Figure 1 represents

data taken over a long period of time by two satellites it is possible that

the observed emissions at the highest frequencies are temporal effects of the

nonquiescent Jovian magnetic field.

The model southern emission cones are also shown at specific frequencies

on Figure 1 superimposed as triangles and squares for clarity. While the

agreement is not perfect, it is clear that the emissions between the northern

emission cones shown in Figure 1 are southern hemisphere emissions. The

frequency extent is less than that of the NHE and the characteristic

"arch-shape" is easily discernable. The "left" edge of the arch formed by th

observed emission from the southern hemisphere is bracketed lay the model

emissions for wave normal angles of 95 0 and 85 0 . The "right" edge of the

model emission cone is displaced from the observed right edge of the "arch" b

possibly 30 0 or so. Changing the initial wave normal angle could produce a

match with the observations, but so would the introduction of a magnetic fiel

model with a different azimuthal dependence.

Figure 2 is the same format as Figure 1 but now for a fixed sub-Io

longitude of 160 0 ,. In this figure the two observed NHE zones are betwe

about 40 0 -90 0 longitude and between about 195 0 -240 0 . The NHE zones are

distinct as they are in Figure 1. There are, for example, reasonably i

emissions in the longitude range of 40 0 -70 0 and in the frequency range

about 17-20 MHz. Another emission region appears to be located in the

longitude range of about 195 0 -230 0 and frequency range 16-23 MHz. Thec

emissions broaden the NHE zones and may represent statistical variation

yL

x
the NHE zones or they may represent emissions not modelled by the present

assumptions. The results of the ray tracing are shown on Figure 2 as circle

for the model NHE zones. The model fit is good, but it is apparent that

the observed "right" NHE zone (at about 235 0 ) is smaller in frequency exten

than the observed "left" NHE zone (at about 80 0 ), and smaller in frequency

extent than the model NHE zone. This phenomena, one side of the emission co

having a smaller frequency extent than the other, is observed frequently in

the data and is thus far not adequately modelled. One possible explanation 

the refractive effects of the Jovian ionosphere which, because of geometry,

are more extensive on one of the emission cones. This effect is currently

being investigated. The emissions between the observed NHE cones are not as

structured as In the example of Figure 1, but it is clear that intense

radiation persists close to the emission cones produced by the model ray

tracing and shown as the triangles and circles on Figure 2. While not

perfect, the qualitative agreement leaves little doubt that the emissions

between the NHE zones are from the southern hemisphere. For this example th

right edge of the emission cone is more distinct and appears to be bracketed

by the model ray tracing results for the two initial wave normal angles.

The sorted Voyager data shown in Figure 3 is for sub--Io longitude of

170 0 . For this example the southern hemisphere emission (SHE) occurs betwee

about 95 0 -210 0 at frequencies less than 25 MHz and has the characteristic

"arch-shape" described above. The ray tracing results match the SHE very

nicely, with the "right" edge of the emission cone agreeing better with th

model for T,=85 0 . The NHE zones are broad and not distinct. The "right"

zone extends from about 190 0 to about 310 0 with a peak frequency occurring

between 245 0 -265 0 . The model ray tracing results fall close to the center

this range around 260 0 . The "left" edge of the observed NHE zone is also

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very distinct, but appears to be between 35 0 -80 0 longitude, with a frequenc

peak possibly between 50 0 -80 0 . The model ray tracing results for the "left

edge of the NHE zone appear to be shifted by about 15 0 to 20 0 to the right

(higher longitudes) from the observed NHE zone. The data for this sub-Io

longitude contains a great deal of a/c noise which confuses the match with t

NHE zones. We have chosen Figure 3 with the confusion of emissions and nois

to point out the distinctive southern hemisphere emissions.

The sorted data for sub-Io longitude 150 0 is shown in Figure 4. The m

distinctive feature is the band of emission extending in frequency from abou

20 MHz to about 5 MHz and extending in longitude from about 160 0 to about

220 0 . This is identified as the "right" edge of the SHE emission and the

model ray tracing results bracket this feature reasonably well. The "left"

edge of the SHE is not as distinct and may overlap with the NHE zone at abou

75 0 . The observed NHE zones are not clearly defined. The "left" zone may

extend from about 35 0 to almost 100 0 , and in frequency to over 35 MHz. The

other NHE zone is even less well defined but may extend from about 200 0 to

over 300 0 with a strong feature at 252 0 , but emission over 25 MHz is not we

defined. The model ray tracing results for the NHE at about 75 0 and at abo

235 0 are within the observed regions of NHE, but the ray tracing results

extend to 35 MHz. We have included this figure also to demonstrate the

distinctive SHE emission even though the NHE zones are much less discrete.

DISCUSSION

To our knowledge the results presented here represent the first

identification of Jovian southern hemisphere DAM emissions. This

identification has been made posoible not only by the comprehensive

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three-dimensional ray tracing utilizing realistic magnetospheric and

ionospheric plasma and magnetic field models, but also the sorting of the

Voyager data set with respect to constant sub-Io longitudes. This has allo

the quick analysis of many months of Voyager data in a reference frame best

adapted for study of Io-dependent DAM emissions. The southern hemisphere D

emissions observed are in general lower in frequency and the emission cones

are narrower. The first of these observations can be explained by noting t

the southern hemisphere emission sources occur on weaker magnetic field line

than northern sources (cf. Acuna and Ness, 1976), and the emission is

restricted to be slightly Doppler-shifted above the RX cutoff frequency. A

narrower emission cone results because the Voyager spacecraft, being confine

approximately to the ecliptic plane, intersects the 4• mission cones from the

northern hemisphere sources closer to the central radius. In other words, 

emission cones of the southern hemisphere sources are often inside the NHE

cones. Since the magnetic field is weaker in the southern hemisphere, the

ratio, fp/fg , will be larger for the same source frequency resulting in

additional refraction and a smaller emission cone angle. The latter fact a

explains the "arch-shape" or tendency for the highest frequency emission to

occur at the same Jovian longitude which appears frequently among the southe

hemisphere sources. For consecutive frequencies the SHE cones will be

confined to a smaller range of longitudes than the corresponding NHE cone

consecutive frequencies, and the net effect is to produce a bunching of t

higher frequency ray paths.

:A:..' ^z
f

f

SUMMARY

we have presented a comparison of the results of three -dimensional r

tracing of Sovian DAM emissions from both southern and northern hemisphere

source positions. The ray tracing is based on a minimum number of assumpt

{ which include RX mode emission from source points slightly Doppler-shifted

sVV

^ above the RX cutoff. A constant initial wave normal angle of 85 0 was ass

g for the northern hemisphere sources and both 85 0 and 95 0 for the southern

hemisphere sources, consistent with the results of Staelin, 1981 for elect

with energies of several keV. The plasma model is an empirical fit which

includes the effects of the ionosphere, Io torus, and magnetosphere. The

magnetic field is the 0-4 model (Acuna and Ness, 1976). The Voyager 1 an

1 data have been sorted to hold the sub-Io longitude constant in order to

clearly display the Io-dependent features. When directly compared to the

sorted Voyager data, the ray tracing results clearly distinguish the north

and southern emission features. It is found that the southern lo-depende

DAM emissions are, in general, lower in frequency, and the emission cone e

i

are observed closer together in longitude along the spacecraft trajectory 

expected from the model.

t This study shows that the identification of southern hemisphere DAM

emission establishes strong support that dovian DAM emission features obse

by the PRA experiment are primarily due to propagation effects. A simple

i

^j model of wave propagation at large wave normal angles from sources along t

r

Io flux tube agrees quite well with many of the Voyager observations. Th

,- I

f( e are, of course, discrepancies that need to be addressed such as the observ

higher upper frequency cutoff of the NHE on Figure 1, for example. In

addition, there are other interesting Io-dependent features observed such 

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emission observed at the sub-Io longitude of observation (the current Io fl

tube). We are aware of the power of sorting the data in displaying what wo

otherwise be subtle features in the data set. Attempts to model specific a

features can be very frustrating on an individual basis, and the sorting

technique allows rapid display of qualitative features. We are currently

introducing new plasma and magnetic field models in an effort to improve th

model and we will be examining other frequency ranges by the same procedure

The authors are grateful to Dr. James Warwick and the Planetary Radio

Astronomy team (in particular M. Kaiser) for the use of the Voyager 1 and 2

L_1 time-averaged PRA calibrated data. Helpful discussions with Drs. N. F. Six

and S. Gulkis are gratefully acknowledged. This research was supported by

the National Aeronautics and Space Administration under the Jupiter Data

Analysis Program, through NASA contract NAS7-100 and through Cal Tech/JPL

contract 956026. The authors would also like to express their gratitude to

H the Data System Technology Program for allowing access to the central compu

facilities and to the Space plasma Computer Analysis Network (SCAN).

O
REFERENCES

11

H

Acuna M. H., and N. F. Ness, "The main magnetic field of Jupiter", J. Geophy

Res., 81, 2917, 1976.

Boischot, A. and M. G. Aubier, " The Jovian Decametric Arcs As An Interferenc

Pattern", J. Geophys. Res., 86, 8561, 1981.

Carr, T. D., M. D. Desch, and J. K. Alexander, "'Phenomenology of Magnetosphe

Radio Emission", in Physics of the Jovian Magnetosphere, A. J. Dessler

(Editor), Cambridge University Press, New York, 1983.

Dulk, G. A., "Apparent Changes in the Rotation Rate of Jupiter", Icarus, 7,

173, 1967.

Goertz, C. K., "Io's Interaction With the Plasma Torus", J. Geophys. Res., 8

2949, 1980.

Goldreich, P. and D. Lynden-Bell, "Io, A Jovian Unipolar Inductor", Astrophys

J., 156, 59, 1969.

Goldstein, M. L., and J. R. Thieman, "The Formation of Arcs in the Dynamic

Spectra of Jovian Decametric Bursts", J. Geophys. Res., 86, 8568, 1981.

Goldstein, M. L., and C. K. Goertz, "Theories of Radio Emissions and Plasma

Waves", in Physics of the Jovian Magnetosphere, A. J. Dessler (Editor),

Cambridge University Press, New York, 1983.

Green, J. L., and D. A. Gurnett, "Ray Tracing of Jovian Kilometric Radiation

Geophys. Res. Lett., 7, 65, 1980.

Green, J. L., "The Io Decametric Emission Cone", Radio Science, 19 1 556, 198

Gurnett, D. A., and C. K. Goertz, "Multiple Alfven Wave Relections Excited by

Io: Origin of the Jovian Decametric Arcs", J. Geophys. Res., 86, 717,

1981.

iq

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n

a
L}

t

i

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Hashimoto, Kozo and Melvyn L. Goldstein, "A Theory of the Io Phase Asymmet

of the Jovian Decametric Radiation", J. Geophys. Res., 88, 2010, 1983.

Leblanc, Y., "On the Arc Structure of the DAM Jupiter Emission", J. Geophys

Res., 86, 8564, 1981.

Lecacheux, Alain, "Ray Tracing in the lo Plasma Torus: Application to the 

Observations During Voyager 1's Closest Approach", J. Geophys. Res., 86,

8523, 1981.

Menietti, J. D., J. L. Green, S. Gulkis, F. N. Six, "Three Dimensional Ray

Tracing of the Jovian Magnetosphere in the Low Frequency Range", J.

Geophys. Res., 89, 1489, 1984.

Neubauer, F. M., "Nonlinear Standing Alfven Wave Current System at Io:

Theory", J. Geophys. Res., 85, 1171, 1980.

Pearce, J. B., "A Heuristic Model for Jovian Decametric Arcs", J. Geophys.

Res., 86, 8579, 1981.

Schauble, John J. and Thomas D. Carr, "Calibration of the Voyager Planetary

Radio Astronomy Radiometers Above 4 MHz", Paper presented at the Nationa

Radio Science Meeting, Boulder, Colorado, January 6, 1983.

Shawhan, S. D., "VLF Ray Tracing in a Model Ionosphere" Dept. of Physics an

Astronomy, U of Iowa Res. Rept. 66-33, 1966.

Sentman D. D., and C. K. Goertz, "Whistler Mode Noise in Jupiter's Inner

Magnetosphere", J. Geophys. Res., 83, 3151, 1978.

Smith, R. A., "Models of Jovian Decametric Radiation", Jupiter, edited by T

Gehrels, University of Arizona Press, Tucson, 1976.

Staelin, D. H., "Character of the Jovian Decametric Arcs", J. Geophys.

86, 8581, 1981.

Stix, T. H., The Theory of Plasma Waves, McGraw-Hill Book Co. Inc., 196

u

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Warwick, J. W., J. B. Pearce, A. C. Riddle, J. K. Alexander, M. D. Desch,

M. L. Kaiser, J. R. Thieman, T. D. Carr, S. Gulkis, A. Boischot, C. C.

Harvey, and B. M. Pedersen, "Voyager 1 Planetary Radio Astronomy

observations Near Jupiter", Science, 204, 995, 1979.

Warwick, J. W., "Models for Jupiter's Decametric Arcs", J. Geophys. Res., 8

8585, 1981.
I^7

11

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FIGURE CAPTIONS

Figure 1. Voyager 1 PRA high-band (1.2 to 40,6 MHz) data (from

February 21 to March 16, 1979) and Voyager 2 PRA high-band data from (June 

to July 23, 1979) have been placed in bins of spacecraft system III longitu

(1965) when Io is within _3° around 260 0 system III longitude. Superimpose

on the Voyager data are the model ray tracing results for source points in t

northern hemisphere (blue circles TO 85') and for source points in the

nou*_horn hemisphere (green triangles CYO=95 0 1 and red squares ( TO-85° 1). No

the narrow " arch" of emission between longitudes of about 200 0 and 300 0 whi

we believe is due to southern hemisphere sources.

Figure 2. The format is the same as Figure 1 but now Io is within t3°

around 160 0 system III longitude. In this figure the "arch" of emission is

not as distinct as in Figure 1 but there is a "ramp" of emission correspondi

to one edge of the southern hemisphere emission cone which is bracketed by 

model ray tracing results between .longitudes about 150 0 and 2000.

Figure 3. The format is the same as Figure 1 but with Io centered aro

170 0 system III longitude. While much of a/c instrument interference is

present in this figure, the southern hemisphere emission is reasonably

distinct in its characteristic "arch" between longitudes-of about 100" and

200 0 and bracketed by the model ray tracing results.

Figure 4. The format is the same as Figure 1 but with Io centered aro

150 0 system III longitude. Only one edge of the southern hemisphere emissi

cone is distinct between longitudes about 150 0 to about 200 0 and bracketed 

the model ray tracing results.

L:

H

H

a

Q

r

0
1.

1

a

Y

42

3 N

 Ly^

t [^

G f

{

3

IIr

bn^

F

C. Appendix 3: "Jovian Decametric Arcs: An Estimate

of the Required Wave Normal Angles from ThreeDimensional Ray Tracing"
I

^

1

I"

l

L.

f

C3

f!

I.f

c ^^

I

c

JOVIAN DECAMETRIC ARCS% AN ESTIMATE OF THE REQUIRED WAVE NORMAL

ANGLES FROM THREE-DIMENSIONAL RAY TRACING

J. D. Menietti l , J. L. Green 2 , S. Gulkis 3 and N. F. Six4

1 Southwest Research Institute, Post Office Drawer 28510, San Antonio, TX 7

2 Marshall Space Flight Center, Huntsville, AL 35812

3 Jet Propulsion Laboratory, Pasadena CA 91103

4Arecibo Observatory, Arecibo, Puerto Rico
a

t

4

ti

a

H

[1

Abstract. A three-dimensional ray tracing code which incorporates the 0.4

magnetic field model (Acuna and Ness, 1979) and a realistic plasma model has

been used to model high-curvature decametric arcs observed by the Voyager

Planetary Radio Astronomy (PRA) instrument. Two examples of intense,

isolated, vertex-late, high-uurvature arcs were singled out for study. The

source point wave normal angle was incremented at each of a full range of

frequencies until the model rays identically matched the observed arcs of the

PRA data. Several different Doppler shifts were assumed at the source point.

By this procedure, an accurate relationship between the wave normal angle, T

and the frequency was obtained, with the variation being 70 0 < T < 80 0 , and

`YMAx=80 0

 for the arcs considered. As the assumed Doppler shift at the sourc

point increased, the value o g TMAx was found to decrease, but the general

shape of the T(f) curve was unaffected. By comparing the ray tracing results

with the independent results of Staelin (1981) regarding emission from beamin

electrons, it was found that larger Doppler shifts (f/fg > 1.1) at the source

point produce 'YMAx > 70 0 , in agreement with the ray tracing results for the

two arcs considered, only for v Il > 0.7 and v II /c > 0.32 (where vo = electron

velocity parallel to B). Actual values of v il /v are unknown, but independent

observations indicate that vp/c m 0.1. Since the low-curvature arcs are

believed to result from larger wave normal angles, our results indicate an

upper limit to the Doppler shift, of Jovian DAM emissions of f/fg < 1.1.
f.

I

L.1

LP

Introduction

Frequency-time spectrograms of the Voyager I and II low-frequency (i MHz 

40 MHz) observations clearly reveal Decametric (DAM) "arc" signatures with

various radii of curvature (8oischot, 19811 Lecacheux, 1981). in the top

panel of Figure 1 are displayed DAM arcs observed by Voyager 1 on day 197 of

1979. The low-curvature arcs seen from about 0510 UT to 0550 UT represent o

class of event while high-curvature arcs seem to represent a different class

A typical example of a high-curvature arc observed by Voyager 1 on day 72 of

1979 is displayed in the lower panel of Figure 1, extending at both hiqh and

low frequencies from about 1150 UT to the arc vertex at about 1235 UT.

Several authors have performed ray tracing analyses utilizing model Jovian

magnetospheres in order to reproduce the DAM arc features (Goldstein and

Thieman (1981), Menietti at al. (1984), Green (1984)). These initial effor

displayed some success in reproducing the characteristic arc shape with a

minimum number of assumptions about the emission mechanism. Goldstein and

Thieman (1981) have found, usinq an elementary ray tracing analysis, that

varying the wave normal angle was essential to accurately compare modelgenerated to observed DAM arcs. Goldstein and Thieman obtained an empirica

relationship to describe the dependence of wave normal angle, T, on emission

frequency, f. Their results indicated that 7 is not a linear function of f

but rather has a variable peak at a mid-range frequency. These results were

based on a two-dimensional ray tracing code and represent an initial analysi

Hashimoto and Goldstein (1983) published the first three-dimensional Jovian

ray tracing results which offered a convincing explanation of the DAM

assymmetry of the source position relative to the central meridian long

of Io. A major contribution of their work was in pointing out the

significance of the ionosphere in refracting DAM emissions. Their anal

0

H

0

D
which assumed R-X mode emission with sources located just above the R-X cut

frequency (fRX ), indicated that the wave normal angle at the source point

decreased with increasing ratio of f/f RX at the source point. Hashimoto a

Goldstein match the occurrence probability of DAM emission in the Io-CML ph

plane with three-dimensional ray tracing results for three different initia

wave normal angles. Their results indicate that wave normal angle, T = 85 

I'

ll

the source point produced the best agreement with ground-based observatin 

Menietti at al., 1984 have performed three-dimensional ray tracing of DAM

emissions and directly compared their results to Voyager observations. Th

point out that for 'a source region located just above the R-X cutoff frequ

and for an initial wave normal angle close to 90 0 , there is good qualitati

f

agreement with Voyager data. It appears (e.g. Goldstein and Thieman (1981

a

Menietti et al., 1984) that high-curvature arcs such as those shown in Figu

^f

cannot be reproduced with the initial wave normal angle equal to the same

Li

constant value at all frequencies, even when the refractive effects of the

Jovian ionosphere are included. The ionospheric model introduced by Hashi

and Goldstein (1983) together with the Acuna and Ness 0-4 magnetic field mo

S will affect northern hemisphere sources at mid to high latitudes only for f 

ri 20 MHz. Ionospheric refraction cannot explain the low frequency refractio

necessary to produce the observed signature of the high-curvature arcs.

11

In this paper, three-dimensional ray tracing is used to examine variabl

wave normal angle effects on the resulting DAM arc structure at frequencie

just above the R-X cutoff and with several assumed ratios of f/fg at the s

point. We use whatever wave normal angle is necessary to match Voyager

observations of two intense, vertex-late, high-curvature DAM arcs. The re

indicate that one possible explanation of the double-valued frequency natu

7 the high-curvature arcs is Doppler-shifted gyroemission from a beam of

s^ electrons with E m 10 keV.

a
1 1,

U

B

An independent investigation of Doppler-shifted gyroemission utilizinq t

results of Staelin (1981) has allowed us to set an approximate upper limit 

the Doppler shift of the stimulating electrons. Wave normal angles which fa

in the range of the ray tracing results, i.e. TMAX > 70 1 are possible for

Doppler-shifted gyroemission for f/fg > 1.1 only if V II /V > .7 and V II /c > .3

This work represents the first comprehensive three-dimensional ray tracing

investigation of varyinq the wave normal angle and Doppler shift utilizing

realistic Jovian plasma and magnetic field models with direct comparison of 

results to Voyager observations of DAM arcs. It thus complements and extend

the results of Goldstein and Thieman (1981) and the work of Hashimoto and

Goldstein (1983).

The ray tracing code used in this study has been described by Menietti e

al., (1984) so only a brief description will be given here. The code is

three-dimensional :., nd based on the cold plasma formulation of Stix (1962) a

integrates the Haselgrove (1955) equations. The magnetic field model is th

0-4 model (Acuna, and Ness, 1976) and the plasma model is that introduced by

Sentman and Goertz (1977). The three-dimensional results reduce to the

two-dimensional results of Green et. al. (1977), Green and Gurnett (1980), 

Lecacheux (1981).

The assumed emission mechanism requires that: (1) the radiation is R-X m

(2) the source region is at the foot of the instantaneous Io flux tube and 

frequency of the emission in the source region is greater than the RX cutof

frequency, and (3) the emission cone is hollow.

High-Curvature Arcs

Often seen in the Voyager PRA data are high-curvature arcs extending ove

time intervals of thirty minutes or more with frequency ranges of less than 

MHz to over 30 MHz, (Figure 1 for example). These arcs occur isolated and

fl

I
nested in groups. Two examples of isolated high-curvature arcs are shown

plotted in Figure 2. These were observed by Voyager 1 on day 72 of 1979 and 

Voyager 2 on day 188 of 1979. in order to model this class of arc the wave

[! normal angle, `Y, (the angle between K and the magnetic field, B) was varied 

the source point for each chosen frequency. source points were chosen at a 

[.^ of frequencies: 2, 5, 10, 15, 20, 25, and 30 MHz from the condition f/f q = 

and f/fg - 1.2. These ratios were chosen to be consistent with Doppler-shif

emission at large wave normal angles. Gyroemission is not assumed for the r

Li tracing. The condition £ > fRX was satisfied at each source point. For a

specific frequency, ratio f/fg, and sub-Io longitude (Io flux tube) an initi

angle, T, was chosen to begin ray tracing and the proximity of the resulting 

() path to the spacecraft position (already known from observation of. a particu

ILJ high-curvature arc) was noted. if the resulting ray path did not intersect 

spacecraft position, a new initial wave normal angle, T, was chosen and the

process repeated. A new Io flux tube was chosen for each frequency in

agreement with observations because Io revolves sliqhtly relative to Jupiter

during the time of observation of an arc. The longitude of the currently

stimulated Io flux tube was chosen according to observations, since the

I

} observed arcs were treated as input constraints to the ray tracing process,

L_1

this procedure generated model arcs which exactly matched the observed arcs

rII

1{

111111

when T varied with frequency according to the curves shown in Figures 3 and 

Results

The results for the two high-curvature arcs observed by Voyager 1 on day 

I(

of 1979 and on day 188 of 1979 are shown in Figures 3 and 4 respectively. 

can be seen, the T(f) curves for each modelled arc are qualitatively similar

(peaking at f = 10 MHz and falling off at lower and higher frequencies). T

curves of Figure 3 were evaluated for f/fg = 1.02 and 1.2 at the source

19
l'

^i

C;

Ill

U

n

D

0

points. An increase in the Doppler shift from f/fg = 1.02 to f/f g = 1.2

shifts the curve downward approximately 3 0 while the characteristic shape i

I

maintained. The data for day 188 in Figure 4 for f/£g = 1.2 and f/f g - 1.0

li show the same general functional relationship between `Y and f as shown in

Figure 3. In Figure 4 the shift in the curves is approximately 8 0 at the

highest frequency. The arc vertex frequency (- 10 MHz) requires the larges

wave normal angle (— 79 0 ) at f/fg = 1.02 as determined for both day 72 and 

188.

For both high-curvature arcs plotted in Figure 2 the lo flux tube

intersects Jupiter at a longitude where the azimuthal variation in the B-fie

(0-4 model) is slight. The results may be explained by noting that since th

source positions for the highest frequencies along the Io flux tube are clo

to the planet by about 1 R Jr increased refraction of the higher frequencies

refracts the emission cones so that the spacecraft observes the higher

frequencies "inside" the emission cones of the lower frequencies (cf.

Goldstein and Thieman, 1981).

Doppler-Shift Gyroemission Discussion

Let us consider the case of Doppler-shifted qyroemission from high energy

field-aligned electrons. The ray tracing analysis is not dependent on an

emission mechanism and so this section is distinct from the ray tracing

results. Staelin (1981) has used the results of Goldreich and Lynden-Bell

(1969) to obtain an expression for the maximum wave normal angle expected f

Doppler-shifted R-X emission (assuming the emission phase velocity is

approximately equal to c) as

_1 f I Y{fp /fg }2 + Y-11 t

TMAX - cos l J

K (y2 -1)1/2
lJ

U

n

r

f

^j

1.:

where K - ViI/V and y is the relativistic term. The Doppler-shifted frequen

is given by

f

= 1 + V II cos L - VII) (2)

fgC 2 C

For a given ratio, Vu/c, the above equation 2 yields the Doppler shift rati

f/fg . Then, for a given value of K, y is determined and &AX can be found

precise values of K and V II appropriate for Jupiter are unknown. Using V II 

(as Staelin suggests is consistent with observed decametric S burst frequen

drifts) and choosing an intermediate value of K = .5 gives y = 1.02 and TMA

78 0 (if we assume fp/fg << 1 which is the case near most of the source poin

considered). This value of ` MAX is close to the value of the peak wave nor

angle shown in Figures 3 and 4 for f/fg = 1.02. Figure 5 contains a family 

TMAX versus K curves for values of VII/c which vary from 0.1 to 0.45. This

figure differs from Figure 2 of Staelin (1981) in that we have labelled cur

as a function of the assumed Doppler shift ratio as shown in Table ]:. The 

of y varies slightly for each curve as shown. These curves demonstrate tha

larger Doppler shifts (as shown in curves 4 and 5 for f/fg = 1.1 and f/f g = 

respectively) produce values of TMAX > 70 0 consistent with the ray tracing

results shown in the curves of Figures 3 and 4 only if K > .7 and VII > .32c

Accepting VII a .1c as a reasonable value, our results indicate that f/f g a 

is an upper limit to the Doppler shift at the sourcepoint for the arcs

considered in this study.

H

H

H
TABLE 1

{

PARAMETERS USED IN FIGURE 5

Curve f/fg VII/c Ymin YMAX

(I

1 1.01 0.10 1.01 1.03

2 1.02 0.14 1.01 1.04

3 1.04 0.20 1.02 1.09

4 1.10 0.32 1.05 1.12

^j 5 1.20 0.45 1.10 1.20

GConclusions

U

In the Voyager PRA data set one frequently observes either isolated or 

^^ groups of arcs that have a larger curvature and are difficult to explain i

19

L1

assumes T - constant n 90 0 . Two examples of isolated, vertex-late,

high-curvature arcs have been matched through ray tracing by allowing the 

normal angle to vary with frequency. The T(f) relationship that produced 

spectral features was peaked around 10 MHz and fell off on either side in

qualitative agreement with the earlier two-dimensional ray tracing results 

r

I

Goldstein and Thieman ( 1981).

If a gyroemission mechanism is assumed that produces Doppler-shifted

emission, the results of Staelin (1981) predict a frequency dependence of 

wave normal angle. An energetic, field-aligned electron beam (v ll = 0.1c) 

C^ produce maximum wave normal angles in agreement with those found in the pr

models if f/fg a 1.02 and K m 0.5. Such energies (10 keV) are comparable 

LLL suggested energies of several keV for accelerated electrons near the surfa

Io which may be the source of DAM emission (cf. Shawhan, 1976; Goldstein a

Goertz, 1983). We emphasize, however, that our ray tracing results are no

r^

LJ

U

.^^'v..^ ^— ... _ T-__.

V ♦ 1'.x..4 R V' L?. .. 
l_.'

dependent on a gyroemission mechanism. Each of the `M curves were obtai

from ray tracing by requiring that the model DAM high-curvature arcs matche

those observed by Voyager 1 on day 72 (Figure 3) and day 188 (Figure 4) of 

The characteristic shape of the curves (with a peak at the vertex frequency

does not significantly change as the Doppler shift increases. The wave no

angle, `Y, varies between 70 0 and 80 0 for all Doppler shifts considered with

smaller values of `Y resulting from larger assumed Doppler shifts. Independ

of the ray tracing results, the condition that gyroemission occur for f > f

(equation 1 (cf. Staelin, 1981)) suggests that an electron beam with K a 

Vp a .1c will result in TMAX which agrees favorably with the results shown 

Figures 3 and 4. Figure 5 and the values listed in Table 1 indicate that 

Doppler shifts (f/fg a 1.1 and f/fq a 1.2) will produce TMAX > 70 0 in agre

with the ray tracing results only for K > .7 and Vp/c > .35. The value of 

vI/V appropriate for Jupiter is not known. But observations of frequency 

of Jovian S bursts imply that VI a .1c. As seen from Table 1 this implies 

for the two hiqh-curvature arcs presently modelled, curves 1 and 2 for VII /

.14 and f/fg a 1.02 best match the ray tracing results. The two arcs pres

modelled were chosen because of the large frequency dispersion and intense

emission. we believe these arcs represent emission at close to the smalle

wave normal angles and the largest Doppler shifts for observed DAM arcs

(Figures 3 and 4 indicate that a larger assumed Doppler shift at the source

point yields a smaller value of TMAX ). Our results thus indicate that f/f

1.1 is the largest to be expected for Jovian DAM arcs, with typical values 

f/fg 4 1.02. While we have chosen only vertex-late arcs for this investiga

(due to the intensity of the emission and large frequency dispersion) we ex

no differences in our conclusions for the vertex-early arcs. We have excl

radiation from Alfven wave reflections located away from the Io flux tube

0

U

i4 i/1~hai.

4y ^'
simultaneous emission from multiple flux tubes was responsible for the

production of the two isolated arcs modelled, our conclusions would have to 

modified.

ACKNOWLEDGEMENTS

The authors are indebted to Dr. James Warwick and the Planetary Radio

Astronomy team (in particular M. Kaiser) for the use of the Voyager 1 and 2 

averaged PRA calibrated data. This research was supported by the National

Aeronautics and Space Administration under the Jupiter Data Analysis Progra

(NASA contract NAS7-100) through Cal Tech/JPL contract 956026. The authors

would also like to express their gratitude to the Data System Technology Pr

for allowing access to the central computer facilities through the Space Pl

Computer Analysis Network (SCAN).
1 

U

REFERENCES

Acuna, M. H., and Ness, N. F., "Results from the GSFC Fluxgate Magnetometer 

Pioneer 11 11 , Jupiter, ad. T. Gehrels, 1976, pp. 830-847.

Boischot, A., A. Lecacheux, M. L. Kaiser, M. D. Deach, J. K. Alexander, and

J. W. Warwick, "Radio Jupiter after Voyager: An Overview of the Planet

of the Planetary Radio Astronomy Observations", J. Geophys. Res., 861

8213, 1981.

"The Angular Distribution 

Res., 82, 1825y 1977.

Jovian Kilometric Radiation

Radio Scienc, 19, 556-570

Green, J. L., D. A. Gurnett, and S. D. Shawhan,

Auroral Kilometric Radiation", J. Geophys.

Green, J. L. and D. A. Gurnett, "Ray Tracing of

Geophysical Research Letters, 7, 65, 1980.

Green, J. L., "'fhe Io Decametric Emission Cone"

1984.

Goldreich, P. and D. Lynden-Bell, "Io, A Jovian Unipolar Inductor", Astroph

J., 1956, 59, 1969.

Goldstein, Melvyn L. and J. R. Thieman, "The Formation of Arcs in the Dyna

Spectra of Jovian Decametric Bursts", 86, 8569, 1981,

Goldstein, Melvyn L. and C. K. roertz, "Theories of Radio Emissions and Pla

Waves", in Physics of the Jovian Magnetosphere, ad. A. J. Dessler, 198

Haselgrove, J., "Ray Theory and a New Method of Ray Tracking", London Physi

Society, Report on Conference on the Physics of the Ionosphere, 1955,

pp. 355-364.

Hashimoto, J. and M. L. Goldstein, "A Theory of the Io Phase Assymmetry of 

Jovian Decametric Radiation", J. Geophys. Res., 88, 2010, 1983. 

Lecacheux, Alain, "Ray Tracing in the Io Plasma Torus: Application to the 

Observastions During Voyager 1's Closest Approach", J. Geophys. Res., 

8523, 1981.

Menietti, J. D., J. L. Green, S. Gulkis, and N. F. Six, "Three Dimensional 

Tracing of Jovian Decametric Arcs, J. Geophys. Res., 89, 1489-1495, 19

Sentman, D. C. and C. K. Goertz, "Whistler Mode Noise in Jupiter's Inner

Magnetosphere", J. Geophys. Res., 83, 322, 1977.

Shawhan, S. D., "Io Sheath-Accelerated Electrons and Ions", J. Geophys. Res

81, 3373, 1976.

Staelin, David H., "Character of the Jovian Decametric Arcs", J. Geophys. R

86, 8581, 1981.

Stix, T. H., The Theory of Plasma Waves, McGraw-Hill Book Co. Inc., 5-34, 1
l

Fj

^^ f

Figure Captions

Fig. 1. Frequency-time spectrograms of Voyager low-frequency radio

emissions. The emission intensity is gray-shade coded acording to the ba

graph at the left. In the top panel low-curvature DAM arcs observed by V

1 on day 197 of 1979 are seen from about 0510 UT to about 0550 UT. In th

bottom panel is seen a typical example of a high-curvature DAM arc obsrer

Voyager 1 on day 72 of 1979 and extending from about 1150 UT to arc verte

about 1235 UT.

Fig. 2. A frequency versus system-III longitude plot of Voyager dat

two observed isolated DAM hiqh-curvature arcs: Voyager 1 for day 72 of 1

(shown in spectrogram in Figure 1)1 and Voyager 2 for day 188 of 1979.

Fig. 3. Initial wave normal angle, T, versus frequency obtained fr

tracing which resulted in a DAM arc identical in curvature to that observ

Voyager 1 on day 72 of 1979. The upper curve is for an initial Doppler-

of f/fq = 1.02, while the lower curve is for f/fg - 1.2.

Fig. 9, same format as Figure 3 except for comparison to the arc ob

by Voyager 2 on day 188 of 1979.

Fig. 5. Maximum wave normal angle, TMAX , versus K (V U /V) for a fam

curves with parameters varying as shown in Table 1. The ratio, f/fg, inc

from 1.01 (curve 1) to 1.2 (curve 5). Gamma is approximately a constant 

each curve.

0

0

0 
i

i

i

i

I^i

n

r

v

[i

79 197 nl 16 4: 0: 0 TO 6:20: 0

FPE,Xo Cr' TIPS 5'PECTT rt~

PIC

w aQ.e

r - ,or 35.6

• 1

^, IJ f ^ ^ f 16.0 161?

c ^J• ^*

,

• r l ' ^ •

' til 11.0

3350 4,

1.

J

_ ^ ^.,^ wit•• - ._^;,.^_ r._^ ,

TIME 430 .350 510 530 550 610 F}{T12 p1 86 8< 67 87 87 87 RJ SC LNG 91 103 115 127 139 151 DEG 10 LNG 100 197 2% 216 225 234 DEG

VOYAGER 1 79i 72 Mme? 13 11: 0: 0 TO 13: 0: 0

FRE000 4Cr' TIP£ SPECTROGRAM

mw

IL -16

35.6

30. 7

•_ 25 .8

a : r ice[ ^- sums a -' 20,9

67 q• _' `^' 16.0

• ^1 n

5 ^ ^^ n ,.^ `. n ^ ^ ^ 1 l . 0

4 ^ n . C :

4

yX^r^

l

3 ^ 00 1 w

3358 1

6.1

TIME 1110 1120 1130 1140 1150 1200 1210 1220 1230 1240 1250 F+i?'12 RJ 130 130 130 131 131 131 131 131 131 131 131 RJ SC LNG 219 225 231 237 243 249 255 261 267 273 2^r9 DEG j0 LNG 152 157 161 1GG 170 175 180 184 189 194 198 DEG

FIGU2E .

• s aw -^•:^ _ _ 
O N

r 1^

11 I

to to

LL LL.

k LL

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it

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(S33UDEM) 3lJNV -IVWHON 3AVM IVUINI
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C?1

01

r+

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'D 

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CO N N n n h i1 n h n n O to

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(S338J3(3) 31E)NV IVWHON 3AVM -IVIIINI (zHW) AON3nc)3ad
E

7

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w 

CD

z

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74

75

72

71

70

69

68

67

66

65

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Ll
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l:a

D. Appendix 4: "Identification of a Night-side

Component of DAM as a Jovian Counterpart to AKR"
[

r

.

t;

1.

1.^

L

1a

C

L^

1.

L3

IDENTIFICATION OF A NIGHT-SIDE COMPONENT OF DAM

AS A JOVIAN COUNTERPART TO AKR

BY

James Lauer Green

Magnetospheric Physics Branch

Space Science Laboratory

NASA/Marshall Space Flight Center, AL 35812

N. Frank Six *

Western Kentucky University

Bowling Green, Kentucky

Sam Gulkis

Jet Propulsion Laboratory

Pasadenia, California

J. D. Menietti

Southwest Research Institute

San Antonio, Texas

December 1983

Submitted to the Journal of Geophysical Research

* Now at Arecibo Observatory

Arecibo, Puerto Rico
L1

ABSTRACT

The post-encounter Voyager detection of a night-side de

metric emission which differs from pre-encounter and earth-ba

observations is examined in this study. This intense rad:at

is observed in the frequency range from about 12 to 25 MHz, 

detected in bursts lasting a couple of hours, is localized 

Jovian longitude, and is clearly an Io phase independent em

sion. A spatial survey of this emission reveals that it 

observed when the Voyager spacecraft are in the System 

(1965) longitude range from 150 to nearly 190 degrees, at po

tive mag netic latitudes, and with a local time from 0 to 

hours on the night-side of the planet. The night-side nature 

this emission prevented its detection by Earth-based rad

observations which are made in the local time range from ab

11.3 to 12.7 hours.

This night-side component of decametric radiation may 

coming from the low-altitude night-side auroral zone which 

linked to the Jovian plasmasheet by magnetic field lines t

are at Niger latitudes than Io's field lines. From all in

cations, it appears that this emission is the Jovian counterp

to the Earth's auroral kilometric radiation or AKR, hence it 

referred to here are Jovian Auroral Decametric Radiation 

JADR. Since JADR is seen nearly every time the Voyager spa

craft are in the emission pattern of the radiation, the nig

side Jovian tail field auroral zone may be continuously activ
INTRODUCTION

It is clear from satellite and Earth-based observat

that the Jovian magnetosphere provides a rich environmen

the generation of many types of free escaping radio emissi

Earth-based radio measurements as early as 1955 discov

emission from Jupiter at decametric or DAM wavelengths.

Srigg discovered in 1964 that the phase of Io has a s

influence on whether or not decametric radio emissions 

Jupiter are detected at earth. This Io effect has led t

classification of the decametric emissions as Io dependen

Io independent. :`he Io independent emissions are less in

and show Jovian longitude control,.

This paper will discuss observations from the PRA ex

ment onboard the Voyager 1 and 2 spacecraft of a new type 

independent emission at decametric wavelengths which sho

strong dependence on the observing longitude and magnetic 

tude and are seen only on the night-side of Jupiter.
-r^n

3

a

.rr

OBSERVATIONS

In order to concentrate on the Io independent emis

close examination of the data must be made to clearly ide

the Io dependent emissions. Decametric measurements have 

routinely put into a coordinate system of central meri

longitude (CML) versus Io phase (see for example Alexand

al., 1981). In this coordinate system Io dependent emis

have been classified as "sources" or regions of CML and Io 

where the probability of measuring decametric radiation is h

It has been known for sometime that the most intense decam

emissions are from sources A, B, and C. Green, 1984 

Voyager 1 and 2 observations over several months organize

Voyager data into spacecraft-Io longitude difference v

frequency for fixed positions of Io in System III ( 1 65) l

tute. As discussed in detail in Green, 1984 the purpos

plotting the Voyager data in this new coordinate system 

eliminate the relative motion of Jupiter and Io. The resu

the coordinate transformation of the Voyager data (see Fi

3, 4, 5, and 6 of Green, 1984) is the same as would be obs

by a spacecraft that.moved completely around Jupiter at a 

stant radial distance keeping Io and Jupiter fixed. I

important to note that out of several months of observa

only a relatively small amount of data is used which sati

the conditions of having Io at a specified system III longi
l^ The analysis presented here carries the work of Green, 

C

a step further by selecting a band of frequencies for each 

Io sorted data plot of spacecraft-Io longitude differenc

^G phase) (in this case from 15 to 21 MHz). Figure 1 presen

occurrence of decametric emission in the frequency range 

21 MHz over the period June 26 to July 23, 1979 versus Io 

tude. The top panel is determined from all PRA observatio

r,

the inbound and outbound leg of the Voyager 2 flyby. A 

Ij factor was used to take into account the radial dependen

the emission. An occurrence block is shown when the a

ri decametric intensity (normalized to 10 R j ) over the 15 to 

frequency range was greater than 1.1 x 10 -17 Watts/ 

r N

observations closer than 30 R; to the center of Jupiter wer

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used to avoid any possible propagation effects in the 

Jovian magnetosphere. All spacecraft observations are orga

^

v,

R

in longitude bins 3 degrees wide and plotted such that 

position is along a vertical line at the center of the 

p with the difference between spacecraft longitude and Io 

tude increasing to the right of center and decreasing 

L1 

left of center. Along the ordinate the data is organized 

degree wide Io longitude increments spaced every 10 d

apart. Labeled in all three panels of Figure 1 are the exp

ri locations of the Io dependent sources A, B, and C. Ve

features on these plots would maintain constant Io-ob

L

(

phase and hence, be labeled "Io phase dependent". Note 

periodic observations of decametric radiation are seen ext
1

-.

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from the upper left corner of Figure 1 (top panel) to the l

right. Although these observations cross into the B sourc

is certain that they are not Io phase dependent.

The middle and bottom panels of Figure 1 are plotted in 

same format as the top panel, but contain only inbound 

(middle panel) and outbound data (bottom panel) data. When 

Voyager 2 data are separated in this manner, incomplete samp

occurs at many of the longitudes. The regions of no obse

tions in this coordinate system are shown shaded. The sep

tion of the Voyager 2 data into inbound and outbound passe

performed in order to examine the local time effects in 

observation of the periodic decametric radiation pointed ou

the top panel. The PRA measurements on the inbound portio

the Voyager flybys restrict the majority of the observation

the day-side Jovian magnetosphere from 9.5 to 11.0 hours l

time (LT) and the outbound from 0 to 2.8 hours LT. A compar

between the middle and bottom panels indicates that the per

_ emission seen in the top panel extending from upper lef

lower right is observed only on the outbound leg of the Vo

2 flyby. This analysis confirms the detection of a night-

component of decametric emission from Jupiter, which is cle

Io phase independent.

The spectrogram of Figure 2 shows the frequency-time c

acteristics of a typical night-side decametric burst lasti

couple of hours. The enhancement in decametric emission at 

low frequencies, from about 8 to 16 MHz, may be attributed t

increase in sensitivity of the PRA instrument in this frequ
range recently determined by Schauble and Carr, 1983

enhanced sensitivity in this frequency range sometimes give

appearance that a broad Jovian emission band exits. In ord

4 avoid the sensitivity effect and to provide a concise pic

the entire frequency range of the night-side decametric emi

was not used (only frequencies from 15 to 21 MHz were use

i

the determination of DAM occurrence of Figure 1. usually i

8 to 16 MHz frequency range, "arc" structures (see Warwi

al., 1979a and Leblanc, 1981) are found that are typic

non-Io emissions, however, in Figure 2, at higher freque

(>12 MHz) this emission is not made up of a series of arc

• 

is much broader (as observed in time) than a single arc

frequency-time structure of this emission is seen periodi

in the Voyager 2 observations from about 25 R j until the 

sion is just barely measured above the receiver's noise lev

distances greater than 120 Rj.

The spectral characteristics of the night-side decam

emission of Figure 2 is shown in Figure 3 where the power 

emission (expressed in Watts/m 2 /Hz) has been normalized 

R j . In addition, Figure 3 shows only the maximum power 

observed at ea;rh frequency over the spacecraft longitude 

from 150 to 160 degrees (about 17 minutes of data). Note 

this emission reaches peak intensity from 14 to 25 MH

decreases rapidly at higher and lower frequencies. This 

tral shape is not characteristic of the Io dependent deca
emission (Carr et al., 1983) but it does more closely rese

the spectral shape of the Earth's auroral kilometric radia

(see for example Gurnett, 1974).

The repetition of the night-side decametric bursts ma

understood by examining if there is any dependence on the sp

craft position in magnetic latitude and longitude for obser

the emission. on a "wiggle" plot (position in magnetic lati

and radial distance with time), of the Voyager 2 outbound 

jectory in Figure 3, are dark strips where complete night-

decametric bursts are observed. These bursts are observed ex

sively at high positive magnetic latitudes with the high 

quencies seen at magnetic latitudes greater than about 9 d

grees. It is important to note that the highest frequencies 

the night-side decametric emission (20 to 22 MHz) are not m

sured when the spacecraft is at the highest magnetic di

latitude (corresponding to approximately 200 degrees longitu

but at the latitude which corresponds to the longitude (abo

150 degrees) of the believed maximum surface magnetic fiel

strength (see for example, Connerney, et al., 1981). This r

tionship is very suggestive of low altitude source regions w

have a strong dependence on Jupiter's magnetic field. In a

tion, since the maximum frequency is observed at or very n

the longitude of the maximum field strength the beaming of 

emission appears to nearly parallel to the magnetic field in

source region. This situation is very different from the b

emission cones (large initial wave normal angles) that

believed to be so characteristic of the Io A and B sources 
Green, 1984). Better understanding of the emission and propag

tion characteristics of this radiation is currently under stu

using ray tracing techniques.

The spatial distribution of the night-side DAM as suggest

by Figure 1 can be further investigated by using Voyager 1 

data as well as the Voyager 2. The Voyager outbound traje

tories, looking down from the north onto an equatorial plan

are plotted in Figure 5. The shaded regions are at the posit

of the spacecraft where the night-side DAM was measured. T

cross hatched block at the expected location of the emissi

along the Voyager 2 trajectory is due to the inability to cle

ly separate the night-side DAM emission from intense Io depe

dent source B radiation measured at the same time. It is imp

tant to note however, that the Voyager 1 measurements of t

night-side DAM abruptly stopped after a radial distance of ab

58 R i and local time of 3.9 hours. The Voyager 2 observatio

of this emission, shown in Figure 5, continue until the inte

sity of the emission is near the PRA experiment noise level 

radial distances greater than 120 R j . The overall repetition 

the Voyager 2 observations (and to some extent the Voyager 

strongly indicates that the absence of Voyager 1 night-side 

observations past 58 Rj on the outbound trajectory is due to 

spatial and not temporal effect and that the this emission ha

distinct outer boundary that Voyager 1 passed through. Combi

with the results of Figure 4, the "inner boundary" labeled 

Figure 5 is probably due to the low magnetic latitude measur

ments near the encounter.
DISCUSSION

1,! The night-side DAM emission reported here has been 

mented on in the paper by Alexander et al., 1981. This e

sion, as described here, can be easily seen in the syno

analysis of Plate 1 in the Alexander et al., 1981 paper for 

the Voyager 2 after encounter plot with traces of the emis

seen in the Voyager 1 after encounter data. The low occurr

probably of the night-side DAM from the Voyager 1 observat

C.? 

in the Alexander et al., 1981 paper is undoubtably due to 

fact that Voyager 1 was in the emission region for only a s

time (see Figure 9 of this paper). Alexander et al., 1981 

:- point out that the "Io-independent emission at frequencies a

20 MHz exhibit marked local time effects". Their interpreta

` of this emission as a feature which "probably corresponds to 

weak lo-independent B source recorded in earth-based survey

.. not the interpretation presented here. It is without que

that the night-side DAM can only be measured at high mag

latitudes (greater than 9 degrees) and on the night-sid

Jupiter, both of which, the Earth-based measurements are un

to achieve. The situation with the lower frequency extensi

this emission (less than 12 MHz) however is not at all clear 

may very well be related to the Io-independent B sourc

csiscussed by Alexander et al., 1981. Additional work needs 

be done to clarify this point.
From the above observations it is important to compare 

night-side DAM to other well known magnetospheric emission

order to gain insight on its possible origin and generation. 

fisgenerally believed that the earth's auroral kilometric ra

tion (AKR) is the terrestrial counterpart to the decame

radiation from Jupiter. These emissions have many phys

similarities such as the same polarization, the same frequ

range when normalized with respect to the planets magn

field, high latitude low altitude source regions, and the 

intense free escaping radio frequency radiation generate

11 -

their respective magnetospheres. Many generation mechan

have been proposed for both AKR and DAM which are the same 

cr , for example Melrose, 1976), However, when considering 

th

control of the intense DAM radiation there is no obvious ter

trial counterpart. Amore analagous comparison would be bet

AKR and the Io independent emissions. The night-side DAM e

f

sions, discussed here, are the only Io independent emi

which have spatial and spectral characteristics similar to 

ff The angular distribution of AKR has been extensively stu

(see Green et al., 1977, for example) and it is clear tha

!( emission exhibits latitudinal propagation effects and is 

primarily on the night-side of the earth. If the night-sid

is the Jovian counterpart to AKR, then it may be coming 

(! Jupiter's tail field auroral zone (with low altitude sou

l 

which is at higher latitudes than the field lines that Io i

sects (Connerney, et al., 1981). Because of these similari

C

the night-side decametric emissions will be referred to he
Jovian Auroral Decametric Radiation or JADR. The persistence 

JADR in the Voyager 2 data may therefore indicate that Jupit

has an extremely active and continuous night-side partic

precipitation in the Jovian tail-field auroral zone. It 

important to note that recent work by Maeda and Carr, 19

suggests that the Io-independent A source, seen on the day-si

of the Earth, may have an Jovian auroral zone origin. How th

emission relates to the JADR discussed here is not known and 

is hoped that future ray tracing studies may be successful 

determining the relationship between the lo-independent A sour

and JADR.
0

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CONCLUSIONS

We have presented an identification of one of the de

metric components generated in the Jovian magnetosphere. Th

emission, referred to here as JADR, has been measured by 

Voyager! and 2 PRA experiments and has the following ma

characteristics: 

1. JADR is an Io-phase independent emission in the frequency range from 12 to 25MHz.

2. JADR (from 16 to 25MHz) cannot be categorized as

having an "arc" like structure characteristic of muc

of the Io-dependent and -independent emissions.

3. The JADR power spectrum is not typical of the

Io-dependent or lo-independent DAM emissions.

4. There is a strong latitudinal and longitudinal beaming of JADR since it is observed at positive magneti

latitudes with the highest frequencies measured when

the largest surface magnetic field strength (near 15

degrees) is near the longitude meridian of the space

craft. ti
Fill

r!

c

c

LS

S. It is observed only on the outbound potions of the

Voyager flyby trajectories.

6. A strong night-side beaming effect is implied when 

abrupt halt to intense JADR observations from Voya

1 occurs at local times greater than 3.9 hours, wh

Voya g er 2, at local times less than 2.9 hours observes the emission to radial distances greater th

120 Rj.

A number of conclusions can be made from the obse

characteristics of the JADR emissions and their similarit

other mz3netospheric emissions. These include:

1. A possible propagation effect may be responsible f

the magnetic latitude asymmetry seen.

2. Any southern hemisphere JADR emissions would only 

observed at the lowest frequency (12 MHz) since ne

Voyager 1 or Voyager 2 were at southern magnetic l

tudes greater than about 7 degrees.

3. JADR emission may be the Jovian counterpart to the

earth's auroral kilometric radiation and emanate f

continuously active Jovian tail field auroral zone

y
ACKNOWLEDGMENTS

The authors are indebted to Dr. James Warwick and 

Planetary Radio Astronomy team (in particular M. Kaiser) 

the use of the Voyager 1 and 2 time averaged PRA calibra

data. This research was supported by the National Aeronaut

and Space Administration under the Jupiter Data Analysis P

.

gram. The authors would also like to express their gratitud

the Da .z. System Technology Program for allowing access to 

central computer facilities through the Space-plasma Compu

Analysis Network (SCAN).
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REFERENCES

1. Alexander, J.K., T.D. Carr, J.R. Thieman, J.J. Sch

and A.C. Riddle, Synoptic observations of Jupiter's 

Emissions: Average statistical properties observed 

Voyager, J. Geophys. Res., 86, 8529, 1981.

2. Bigg, E.K., Influence of the satellite Io on Jupiter

decametric emission, Nature, 203, 1008, 1964.

3. Connerney, J.E.P., M.H. Acuna, and N.F. Ness, Model

Jovian Current Sheet and Inner Magnetosphere, IL. Ge, 

Res., 86, 8370, 1981. 

4. Carr, T.D., M.D. Desch, and J.K. Alexander, Phenomen

of magnetospheric radio emission, in Physics of the 

Magnetosphere, Edited by A. J. Dessler, pp. 226-284, 

bridge University Press, New York, 1983.

5. Green, J.L., D.A. Gurnett, and S.D. Shawhan, The A

Distribution of Auroral Kilometric Radiation, J G

Res., 82, 1825, 1977.

6. Green, J.L., The Io Decametric Emission Conte, to

lished in Radio Science, April 1984.
7. Gurnett, D. A., The Earth as a Radio Source: Terrestrial

Kilometric Radiation, J. Geophys. Res., 79, 4227, 1974.

8. Leblanc, Y., On the arc structure of the DAM Jupiter emis

sion., J. Geophys. Res., 86, 8564, 1981.

9. Maeda, K. and T. D. Carr, Beam Structure of Jupiter's

Decametric Radiation, to be published in Nature, 1984.

10, Melrose, D. B., An interpretation of Jupiter's decametric

radiation and the terrestrial kilometric radiation as

direct amplified gyroemission, Astrophys. J., 207, 651,

1976.

1,1. Schauble, John. J. and Thomas D. Carr, Calibration of th

Voyager Planetary Radio Astronomy Radiometers above 4 MHz

Paper presented at the National Radio Science Meeting,

Boulder, Colorado, January 6, 1983. 

12. Warwick, J.W., J.B. Pearce, A.C. Piddle, J.K. Alexander

M.D. Desch, et al., Voyager 1 Planetary Radio Astronomy

Observations Near Jupiter, Science, 204, 995, 1979a.
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Figure 1

Occurrence of DAM emission from Voyager 2 spacecr

shown by the blacken squares when the average intens

greater than 1.1 x 1H -17 Watts/ni 2 /Hz. (normalized to 

over the frequency range from 15 to 21 MHz. This three 

figure is derived from observations taken along the ou

(bottom), inbound (middle), and both inbound and outboun

Voyager 2 trajectory. The DAM occurrences are plotted in 

difference between the spacecraft and Io versus Io Syst

( 1 65) longitude. In the top and bottom panels only, a s

tic DAM non-Io emission is observed from the upper left 

lower right corner of the plot.

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[SPACECRAFT - to] SYSTEM I11 LONGITUX

Figure 1
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Figure 2

This Voyager 2 frequency-time spectrogram shows the basi

characteristics of the JADR emissions. A night-side decametri

burst usually lasts about an hour and is first seen in Voyager 

and 2 data at high frequencies (typically 21 to 22 MHz) and as 

function of time to lower and lower frequencies until a broa

emission band appears from 8 to 12 MHz.

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Figure 3

The maximum power flux spectrum of the night-side de

metric emission is shown in this figure. Note that the spect

peak occurs from 14 to 22 MhZ and rapidly decreases at hig

and lower frequencies. 
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S/C LONGITUDE 1500-1600
Figure 4

The position of the Voyager 2 spacecraft when the nightside DAM emissions are observed in magnetic latitude and radial

distance coordinates are shown here as darkened strips during

the outbound portion of its trajectory. The characteristic

shape of these emissions as pointed out in Figure 2 were used in

determining the spacecraft positions of the shaded strips. Note

that the night-side DAM emissions are all seen at positive

magnetic latitudes with the highest frequencies observed near

the beginning of each strip.

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The dark block regions along the outbound Voyager 1 and 

near equatorial trajectories show the position of the spacecraf

when the night-side DAM was observed. Note that there are on

a few examples of this emission observed from Voyager 1. T

crossed hatched area is near the expected location for th

night-side DA,M Isut its characteristic frequency-time structur

cannot be clearly distinguished from other decametric emission

which are measured at that time. The Voyager 2 spacecra

observed the night-side DAM to radial distances greater than 10

R j (not shown in this figure) just above the receiver nois

level. The wavy lines represent qua]itative boundaries of t

emission based on these observations.

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Figure 5

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