The radio emission from Jupiter in the decametric range (2 to 40 MHz) has been studied for three decades. It is known that this emission is correlated with particular longitude regions on the planet and with the position of the satellite Io. The Planetary Radio Astronomy experiment on the Voyager 1 and 2 spacecraft observed this emission, obtaining valuable new information. The objectives of this study are as follows: (1) calculate raypaths for decametric wavelength radiation in Jupiter's magnetosphere; (2) compare the model-dependent raypaths with the Voyager observations; and (3) deduce characteristics of the source regions and the influence of propagation effects

Six, N. Frank

English

NASA Technical Reports Server

RAY TRACING OF JOVIAN LOW FREQUENCY RADIATION 
NASA C o n t r a c t  NAS8-34424 
F i n a l  T e c h n i c a l  Report  
December 1 9  8 7 
t N .  F rank  S i x  
Wes te rn  Kentucky U n i v e r s i t y  
Dept .  P h y s i c s  & Astronomy 
Bowling Green ,  K Y  42101 
(bASA-CR-179262) R A Y  T B A C l b i G  Cf J O V I A N  LOU N8 8- 16  0 20 
E E E Q U E R C Y  B A C l A l l C B i  F i n a l  ErFcrt 
( G o i v e r s i t y  of k r t e r r  K e n t u c k y )  1 1  p 
CSCL 038 Unclas 
G3/91 0120505 
'Now w i t h  U n i v e r s i t i e s  Space R e s e a r c h  A s s o c i a t i o n  
139 PlcMurtr ie  Lane,  H u n t s v i l l e ,  AL 35806 
https://ntrs.nasa.gov/search.jsp?R=19880007237 2020-03-20T07:22:32+00:00Z
ACKNOWLEDGEMENTS 
This project has been a joint effort by five co- 
investigators: Dr. James Green, NASA/GSFC; Dr. Douglas Menietti, 
Southwest Research Institute; Dr. Donald Gurnett, University of 
Iowa; Dr. Samuel Gulkis, JPL/Caltech; and the author. We are 
indebted to Dr. James Warwick and the Planetary Radio Astronomy 
team (in particular Drs. M. Kaiser and M. Desch) for the use of 
the Voyager 1 and 2 data. 
INTRODUCTION 
The radio emission from Jupiter in the decametric range ( 2 -  
40 PIHz) has been studied €or three decades. It is known that 
this emission is correlated with particular longitude regions on 
the planet and with the position of the satellite Io. The 
Planetary Radio Astronomy experiment on the Voyager 1 and 2 
spacecraft observed this emission, obtaining valuable new 
information. 
The objectives of this study are the following: 
1) calculate raypaths for decametric wavelength radiation 
in Jupiter's magnetosphere; 
2) compare the model-dependent raypaths with the Voyager 
observations; and 
3 )  deduce characteristics of the source regions and the 
influence of propagation effects. 
Using a model of Jupiter's magnetized plasma environment, 
radiation raypaths have been calculated with a three-dimensional 
ray tracing program. It is assumed that energetic particles 
produce the emission in the planet's auroral zone at frequencies 
just above the electron gyrofrequencies. Families of rays have 
been launched at particular angles with respect to the magnetic 
field lines to generate conical sheets of radiation for various 
frequencies and various source locations. As the planet's 
magnetic field rotates, these warped sheets of radiation sweep 
past the observer, producing signatures in frequency versus time 
plots. These signatures match some of those found in the Voyager 
data. The greatest propagation effects occur in and around the 
Source regions in Jupiter's auroral zone. 
TECHNICAL APPROACH 
A major e f f o r t  has  been  t h e  deve lopment  of  a J o v i a n  
r a d i a t i o n  p a t h  model u s i n g  t e c h n i q u e s  w h i c h  have  p r o v e n  
s u c c e s s f u l  i n  e x p l a i n i n g  t h e  i n t e r a c t i o n s  of p a r t i c l e s  and waves 
i n  t h e  t e r r e s t r i a l  magne tosphe re .  The d e s c r i p t i o n  of  J u p i t e r ' s  
m a g n e t i c  f i e l d ,  a d o p t e d  f o r  t h i s  s t u d y ,  is t h a t  p u b l i s h e d  by 
Acuna and Ness ( 1 9 7 6 ) .  A s p h e r i c a l  harmonic  a n a l y s i s  was u s e d  t o  
c a l c u l a t e  t h e  components  of 6 and t h e i r  d e r i v a t i v e s .  
d i s t r i b u t i o n  c h o s e n  was t h a t  of  Sentman and G o e r t z  ( 1 9 7 7 )  
augmented by t h e  Warwick  t o r u s  ( 1 3 7 9 ) .  
The  plasma 
R a d i a t i o n  p a t h s  a re  g e n e r a t e d  by a t h r e e - d i m e n s i o n a l  r a y  
t r a c i n g  program based on t h e  S t i x  cold plasma f o r m u l a t i o n  of  t h e  
i n d e x  of r e f r a c t i o n  and H a s e l g r o v e ' s  s e t  of  f i r s t  o r d e r  
d i f f e r e n t i a l  e q u a t i o n s  ( 1 9 5 5 ) ,  amenable  t o  n u m e r i c a l  s o l u t i o n .  A 
source p o s i t i o n  ( r a d i a l  d i s t a n c e  from t h e  center of J u p i t e r ,  
Sys tem I11 l o n g i t u d e ,  and Sys tem I11 l a t i t u d e )  f rom which waves 
of a s p e c i f i e d  f r e q u e n c y  a re  t o  be l a u n c h e d  is i n p u t ,  a l o n g  w i t h  
t h e  specified l a u n c h  a n g l e  (wave normal  a n g l e )  w i t h  respect t o  
t h e  m a g n e t i c  f i e l d .  A t  t h e  o r i g i n  of t h e  r a y ,  t h e  program 
c a l c u l a t e s  a n  i n d e x  of r e f r a c t i o n  s u r f a c e  based on t h e  plasma and 
m a g n e t i c  f i e l d  parameters. Next t h e  program t a k e s  an  i n c r e m e n t a l  
s t e p  i n  t h e  d i r e c t i o n  p e r p e n d i c u l a r  t o  t h e  i n d e x  of r e f r a c t i o n  
s u r f a c e ,  i .e . ,  i n  t h e  d i r e c t i o n  of p r o p a g a t i o n ,  and d e t e r m i n e s  
t h e  c o o r d i n a t e s  of  t h i s  new p o s i t i o n  on  t h e  r a y p a t h .  S n e l l ' s  law 
is used  t o  c o n s e r v e  t h e  component of t h e  r e f r a c t i v e  i n d e x  
p a r a l l e l  to  p l a n e s  of  c o n s t a n t  e l e c t r o n  number d e n s i t y .  A n o t h e r  
i n d e x  of  r e f r a c t i o n  s u r f a c e  is c a l c u l a t e d  a t  t h i s  new p o i n t  and  
t h e  procedure is repeated. I n  t h i s  manner,  a r a y p a t h  is 
generated. The path of a ray through the magnetized plasma is 
determined by the following initial conditions: the frequency of 
the wave, the wave normal angle, the coordinates of the source 
(launch points), the propagation mode, and the magnetospheric 
mode 1. 
The Galilean satellite Io is known to have a pronounced 
modulating effect on the decametric radio emissions from 
Jupiter. 
made from a coordinate system in which both the central meridian 
longitude of Jupiter and the central meridian longitude on which 
Io is located are changing rapidly. Observations in this 
coordinate system tend to mask features which are fixed or nearly 
fixed relative to Io itself. We have examined the data from the 
Planetary Radio Astronomy (PRA) experiment onboard the Voyager 1 
and 2 spacecraft in a stationary coordinate system in which Io is 
fixed at particular central meridian longitudes of Jupiter. The 
reformatted data are equivalent to what an observer would detect 
if s/he could sample all longitudes around Jupiter's equatorial 
plane with the planet's rotation halted and Io stopped in its 
orbit. 
Ground based and spacecraft observations of Jupiter are 
Using the 04 magnetic field model, we have investigated the 
variation of source altitude with longitude around Jupiter at 
selected emission frequencies, assuming an oblate spheroid for 
the cloudtop surface. We have concluded that, in certain ranges 
of longitude, the higher frequency radiation (f 2 20 PlHz) 
originating from lower altitudes will be affected by the Jovian 
ionosphere. A s  a result of this study, a model ionosphere was 
added to the ray tracing program. 
REVIEW OF RESULTS 
1. C a l c u l a t e d  Raypa ths .  An e x a m i n a t i o n  of  t h e  c a t a l o g  of  
d e c a m e t r i c  r a d i a t i o n  s h e e t s  g e n e r a t e d  by t h e  r a y  t r a c i n g  
p r o c e d u r e s  d e s c r i b e d  i n  P u b l i c a t i o n  4 r e v e a l s  t h a t  s i g n a t u r e s  
s u c h  as  t h o s e  o b s e r v e d  by t h e  Voyager PRA e x p e r i m e n t  c a n  be 
r e p r o d u c e d .  The r a d i a t i o n  s h e e t s  are  warped by t h e  p lasma near 
t h e  p l a n e t  and by t h e  t w i s t  ( a z i m u t h a l  asymmetry)  i n  t h e  m a g n e t i c  
f i e l d  s u c h  t h a t  o v e r l a p p i n g  o c c u r s .  An o b s e r v e r  r i d i n g  w i t h  
Voyager  i n t e r m i t t a n t l y  crosses t h i n  s h e e t s  of r a d i a t i o n  as t h e  
source co-rotates  w i t h  t h e  p l a n e t .  I t  is a s  though t h e  o b s e r v e r  
were v iewing  t h e  r u f f l e s  i n  a t w i r l i n g  b a l l e r i n a ' s  t u t u .  The Io  
p lasma t o r u s  h a s  minimal  e f f e c t  a t  d e c a m e t r i c  w a v e l e n g t h s .  The 
major d i s t o r t i o n  by p lasma occurs a t  h i g h e r  l a t i t u d e s  n e a r e r  t h e  
s u r f a c e  of t h e  p l a n e t .  T h i s  is a l so  t h e  r e g i o n  of g rea tes t  
a z i m u t h a l  asymmetry i n  t h e  m a g n e t i c  f i e l d .  
I n  t h e  one c a s e  t e s t e d ,  t h e  h i g h  f r e q u e n c y  l i m i t  of  t h e  
d e c a m e t r i c  e m i s s i o n s  h a s  been  p r e d i c t e d .  The Voyager 2 d a t a  of 
July 1 6 ,  1 9 7 9 ,  reveals  a w e l l  d e f i n e d  boundary  t o  t h e  h i g h e s t  
f r e q u e n c i e s  o b s e r v e d .  The r a y s  f o r  f r e q u e n c i e s  j u s t  below t h i s  
boundary  p r o p a g a t e d  t h r o u g h  t h e  magne t i zed  p l a sma ,  as  e x p e c t e d .  
The r a y s  t h a t  were g e n e r a t e d  f o r  f r e q u e n c i e s  above  t h e  c u t o f f  d i d  
n o t  p r o p a g a t e .  The a l t i t u d e  of  t h e  25 MHz s o u r c e  p o i n t  a t  t h i s  
l a t i t u d e  is a p p r o x i m a t e l y  1 2 , 4 0 0  km, p r e s u m a b l e  above  t h e  J o v i a n  
i o n o s p h e r e .  We c o n c l u d e  t h a t  t h e  h i g h  f r e q u e n c y  l i m i t  t o  t h e  
e m i s s i o n ,  i n  t h i s  case,  is imposed by t h e  R-X c u t o f f .  
T h e  g r o w t h - r a t e  of  t h e  e m i s s i o n  process causes t h e  r a d i a t i o n  
t o  be c o n f i n e d  t o  nar row s h e e t s  which a re  c o n t o u r e d  by t h e  plasma 
n e a r  J u p i t e r  and by t h e  a z i m u t h a l  t w i s t  i n  t h e  m a g n e t i c  f i e l d ,  
r e s u l t i n g  i n  bends  and r u f f l e s  l i k e  t h e  f a m i l i a r  shapes of 
"aurora l  c u r t a i n s " ,  b u t  on  a g r a n d e r  scale.  The i n t e r s e c t i o n  of 
these c a l c u l a t e d  s h e e t s  of  r a d i a t i o n  w i t h  t h e  s p a c e c r a f t  p r o d u c e  
a r c - l ike  s i g n a t u r e s  l i k e  t h o s e  d e t e c t e d  by Voyagers  1 and 2.  
B o t h  t h e  s h a p e  and t h e  l o c a t i o n  ( i n  l o n g i t u d e )  of t h e  l o w  
c u r v a t u r e  a rcs  i n  t h e  da t a  are p r e d i c t e d  by D o p p l e r - s h i f t e d  g y r o -  
e m i s s i o n  w i t h  a l l  f r e q u e n c i e s  l aunched  a t  a wave normal  a n g l e  of 
a p p r o x i m a t e l y  9 0 ° .  Decametric arcs of h i g h e r  c u r v a t u r e  a p p e a r  t o  
r e p r e s e n t  a d i f f e r e n t  c lass  of e v e n t .  I n i t i a l  s t u d i e s  imply t h a t  
these h i g h e r  c u r v a t u r e  f e a t u r e s  a re  r e p r o d u c e d  by a l l o w i n g  t h e  
wave normal  a n g l e  t o  v a r y  a s  a f u n c t i o n  of  t h e  wave f r e q u e n c y .  
I t  is s u g g e s t e d  t h a t  t h e s e  e v e n t s  c o u l d  be p roduced  by h i g h e r  
e n e r g y  e l e c t r o n s  whose g y r o - e m i s s i o n  is r e l a t i v i s t i c a l l y  beamed 
closer to  t h e  B - f i e l d  d i r e c t i o n ,  i . e . ,  l a u n c h e d  w i t h  smaller wave 
normal  a n g l e s .  
2 .  I o - C e n t e r e d  C o o r d i n a t e  System. Voyager spectrograms have  
been generated as a f u n c t i o n  of f r e q u e n c y  and s p a c e c r a f t  
l o n g i t u d e  for a c o n s t a n t  Io  phase .  I n  t h i s  c o o r d i n a t e  s y s t e m ,  w e  
f i n d  m u l t i p l e  l a n e s  of radio e m i s s i o n  which a re  h i g h l y  c o r r e l a t e d  
w i t h  t h e  o r b i t a l  p o s i t i o n  of  Io. T h i s  a n a l y s i s  s u p p o r t s  a n  
ea r l i e r  c o n c l u s i o n  t h a t  t h e  d e c a m e t r i c  r a d i a t i o n  e m a n a t e s  i n  t h i n  
sheets from r e g i o n s  a s s o c i a t e d  w i t h  t h e  f o o t  o f  t h e  Io f l u x  
t u b e .  The p r i n c i p a l  r a d i o  "sources" A, B, and C a re  e a s i l y  
i d e n t i f i e d  i n  t h i s  c o o r d i n a t e  s y s t e m .  
On close i n s p e c t i o n  of t h e  Voyager d a t a ,  t r a n s f o r m e d  i n t o  
t h i s  Jup i t e r - Io  s t a t i o n a r y  c o o r d i n a t e  s y s t e m ,  one  f i n d s  t h a t  t h e  
Io -dependen t  d e c a m e t r i c  r a d i a t i o n  o c c u r s  i n  t w o  or more w i d e  
frequency bands of emission on each side of the instantaneous Io 
flux tube. As the sub-Io point on Jupiter moves to different 
longitudes, a result of the planet's rotation and Io's orbital 
motion, the wide band emissions also move to different 
longitudes. The longitudinal width of the emission regions 
detected by the spacecraft varies (sometimes as broad as 30 
degrees), and the maximum frequency measured in each of the 
regions is not always the same for a fixed sub-Io longitude. 
3 .  Emission Cone. Io-dependent decametric radiation, emanating 
as hollow emission cones from source regions at the feet of Io- 
threading field lines reproduces the signatures of the wide band 
emissions in the Voyager frequency-longitude spectrograms for 
sub-Io longitudes in the range 200° -260° .  Two distinct arc-like 
structures in the Voyager spectrograms make up the cone "edges" 
(radiation sheet intersections with the plane of Voyager's 
trajectory). The edge of the cone at higher longitude than Io is 
always composed of vertex-late arcs; the other edge at lower 
longitudes is composed of v e r t e x - e a r l y  s t r u c t u r e s .  
4 .  Maqnetic Field Models. The ray tracing results show that the 
principal controlling agent of the radiation sheet morphology is 
Jupiter's B-field structure in the source region. The pointing 
of the emission cone axis is determined by the B-field direction 
in the source region. 
The null centered at System 111 longitude = 30° in the 
probability histograms (earth-based observations) appears to be a 
manifestation of the zone of avoidance created by the asymmetric 
location of Jupiter's magnetic poles. Since the poles are not 
separated by 180° of longitude, the stop zone produced by the 
Jovian ionosphere shadows those longitudes in the range 15'-45'. 
The magnetic field models used for the ray tracing 
calculations are only approximations to the actual planetary 
magnetic field at low altitudes. The displacement of the 
measured UV auroral zone from the model-generated Io footprint is 
an indication of the uncertainty in our knowledge of the B-field. 
5. Decametric Source Locations. In a comparison of model ray 
tracings with Voyager observations for constant sub-Io longitudes 
of 260° and 300°, we have identified spectral features in the 
emission with source locations in both the northern and southern 
hemispheres of Jupiter. We find that the emission traditionally 
designated "IO-B" originates at the Io flux tube footprint in the 
northern hemisphere when the sub-Io longitude = 260'. The 
emission which has been traditionally designated " Io -C"  emanates 
from source regions at the feet of the Io flux tube in both 
northern and southern hemispheres when the sub-Io longitudes are 
260° and 300'. The traditional "non-Io-A" emission is, in fact, 
Io related at both sub-Io longitudes investigated. When the sub- 
Io longitude = 260°, this emission emanates from the southern 
hemisphere flux tube footprint. When the sub-Io longitude = 
300°, the emission originates from the flux tube footprint in the 
northern hemisphere. 
The two pairs of traditional "sources" B-C and C-D are 
believed to correspond to the four intersections, in the 
observers plane, of two hollow emission cones which emanate from 
the north and south footprints of the Io flux tube. 
In addition to the Io flux tube sourcesl there are sources 
which are independent of Io's orbital position. The highest 
frequencies (f > 36 NHz) emerge near longitude 255O, consistent 
with, a polar cap source location near longitude 215O in the 
northern hemisphere. The maximum concentration of plasma in the 
Io torus occurs in this longitude range. It is plausible to 
suspect that a weak region in the planet's magnetic field allows 
deeper penetration of energetic electrons which generate the 
higher frequency gyro emissions at this location. 
PUBLICATIONS 
The  f o l l o w i n g  repor t s  a n d  p u b l i c a t i o n s  i n  t h e  o p e n  
l i t e r a t u r e  a r e  t h e  major p r o d u c t s  o f  t h e  r e s e a r c h  p e r f o r m e d  u n d e r  
c o n t r a c t  NAS8-34424. 
1. 
2. 
3.  
4 .  
5. 
6. 
7 .  
"Ray T r a c i n g  S t u d i e s  of J u p i t e r ' s  M a g n e t o s p h e r e , ' '  NASA/ASEE 
Summer F a c u l t y  R e s e a r c h  F e l l o w s h i p  Program, M a r s h a l l  Space 
F l i g h t  C e n t e r ,  1 9 8 1 ,  F. S i x .  
" M a g n e t o s p h e r i c  Ray T r a c i n g  S t u d i e s , "  NASA/ASEE Summer 
F a c u l t y  R e s e a r c h  F e l l o w s h i p  P r o g r a m ,  M a r s h a l l  Space F l i g h t  
C e n t e r ,  1 9 8 2 ,  F. S i x .  
" A n a l y s i s  of t h e  Voyager O b s e r v a t i o n s  of J u p i t e r ' s  Decametric 
R a d i o  E m i s s i o n , "  NASA/ASEE Summer F a c u l t y  F e l l o w s h i p  P r o g r a m ,  
J e t  P r o p u l s i o n  Laboratory,  C a l i f o r n i a  I n s t i t u t e  of 
T e c h n o l o g y ,  1 9 8 3 ,  F . S i x .  
" T h r e e  D i m e n s i o n a l  Ray T r a c i n g  of t h e  J o v i a n  M a g n e t o s p h e r e  i n  
t h e  Low-Frequency  R a n g e , "  J. Geophys .  R e s . ,  89, 1 4 8 9 ,  1 9 8 4 ,  
J .  D. M e n i e t t i ,  J. L. G r e e n ,  S o  G u l k i s ,  a n d  F. S i x .  
"The Io Decametric E m i s s i o n  C o n e , "  R a d i o  Sc i . ,  1 9 ,  556, 1 9 8 4 ,  
J .  L. G r e e n .  
-
" J o v i a n  Decametric Arcs: An Est imate  of t h e  R e q u i r e d  Wave 
Normal A n g l e s  from T h r e e - D i m e n s i o n a l  Ray T r a c i n g , "  J.  
G e o p h y s .  R e s . ,  89, 9 0 8 9 ,  1 9 8 4 ,  J. D. M e n i e t t i ,  J. L. G r e e n ,  
S.  G u l k i s ,  a n d  F. S i x .  
"Ray T r a c i n g  o f  J o v i a n  Decametric R a d i a t i o n  from S o u t h e r n  a n d  
N o r t h e r n  H e m i s p h e r e  Sources: C o m p a r i s o n  w i t h  V o y a g e r  
O b s e r v a t i o n s , "  J. Geophys .  R e s . ,  92, 2 7 ,  1 9 8 7 ,  J. D. 
M e n i e t t i ,  J. L. G r e e n ,  N.  F r a n k  S i x ,  a n d  S. G u l k i s .  


Ray tracing of Jovian low frequency radiation

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