The geometry and electron gyrofrequency were calculated for both the North and South feet of the Io-threaded flux tube at several altitudes as a function of sub-Io longitude for various multipole field models. The models predict a maximum surface gyrofrequency equal to the observed high frequency limit of the decameter-wave radio emission (DAM) and tend to favor a mechanism involving transverse propagation from a source in the Northern hemisphere. Calculations indicate that the beaming pattern of the emission may be determined by reflection from the ionosphere rather than by inherent beaming from the source region

Acuna, M. H.

Alexander, J. K.

Kaiser, M. L.

Smith, R. A.

Thompson, R. F.

English

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^i	 x..
IMPLICATIONS OF PIONEER-II
MAGNETIC FIELD MODELS FOR
JUPITER'S DEC:AMETRIC RADIO MISSION
J. K. ALEXANDER
R. A. SMITH
M. L. KAISER
M. H. ACUNA
R. F. THOMPSON
(NASA-TM-X-71002)	 IM1FLICATIONS OF PIONEER-2
MAGNETIC FIELD MODELS FOR JUPITEF'S
DECAMF.TRIC RADIO MISSION (NASA) 	 16 P HC
$3.25
	
CSCL 03P
OCTOBER 1975
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1476-10970
Unclas
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Implications of Pioneer-11 Magnetic
Field Models for Jupiter's Decametric
Radio Eml.ssion
J.K. Alexander, R.A. Smith, M.L. Kaiser,
M.H. Acuna and R.F. Thompson
Prepared for presentation at meeting
of USNC/URSI,
Boulder, Colorado (October, 1975)
`- e
IMPLICATIONS OF PIONEER-11 MAGNETIC FIELD MODELS FOR
JUPITER'S DECAMETRIC RADIO EMISSION
J.K. Alexander, R.A. Smith, M.L. Kaiser
M.H. Acuna and R.F. Thompson
Laboratory for Extraterrestrial Physics
Goddard Space Flight Center
Greenbelt, Maryland
ABSTRACT
Models of Jupiter f s magnetic field derived from measurements with
the Pioneer-11 flux gate and vector helium magnetometers have been
analyzed with a view towards understanding the origin of the decameter-
wave radio emission (DAM) from the planet. In particular, we have
calculated the geometry and electron gyrofrequency predicted for both
the North and South feet of the Io-threaded flux tube at several
altitudes as a, function of sub-Io longitude for the multipole field
models of Acuna and Ness (1975) and Smith et al. (1975). The models
predict a mri.Ximum surface gyrofrequency equal to the observed high
frequency limit of the DAM and tend. to favor a mechanism involving
transverse propagation from a source in the Northern hemisphere.
' Although a detailed understanding of the DAM cannot be derived from
} either model, our calculations indicate that the beaming pattern
of the emission may be determined by reflection from the ionosphere
rather than. by inherent beaming from the source region.
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1. Introduction
Ire first in situ measurements of fields and particles in the
Jovian magnetosphere by instruments on Pioneer 10 (closest approach =
2.85 R J ) provided a valuable new body of information regarding the
magneto-ionic environment of the planet. However, our insight into
the nature and origin of Jupiter's decameter-wave radio emission (DAM)
did not enjoy commensurate advances as a result of new findings from
the spacecraft experiments. The radio astronomical predictions
concerning the sign, gross orientation, and dipole moment of the Jovian
magnetic field were verified, but more detailed models of the magnetic
field derived from the Pioneer data did not lead to explanations
of the morphological characteristics of DAM. For example, Smith
and Wu (1974) showed that the variation of electron gyrofrequency at
the feet of the Io-threaded flux tube (IFT) with rotation of the planet
did not show any clear correspondence to the gross frequency structure
observed in dynamic spectra of the Io-controlled DAM emissions.
With the closer and retrograde encounter of Jupiter by Pioneer 11
(1.7 RJ) it became possible to improve the models of the magnetic field.
We have used those updated models as derived from the vector helium
magnetometer on Pioneer 10 and 11 (Smith, at al., 1975) and the fluxgate
magnetometer on Pioneer 11 (Acura and Nessy 1975) in order to take
a further look at the implications of our present view of the magnetic
field for understanding DAM.
1
tThe magnetic field model of Smith et al. (1975) is a 23-coeffi5ient
spherical harmonic expansion having dipole, quadrupole, and octupole
terms for internal sources and first and second order terms for external
sources. The 04 model of Acuna and Ness (1975) uses only internal
terms up tb;ough octupole order. We have adopted the working hypothesis
that the IFT is the important source region to investigate, and we
have begun our analysis with computations of the zenographic location
and vector magnetic field of the IFT as a function of System III (1965)*
sub-Io longitude.
II. The lo Fluxtube Motion
In Figure I we have plotted the surface electron gyrofrequency at
the North and South foot points of the IFT and the magnetic dip angle
at 5.91 R  (Io's orbital distance) at the zenographic equator as a
function of System III (1965) sub-Io longitude. Representative plots
of DAM occurrence probability vs, sub-Io longitude for the four Io
controlled "sources" are also shown for comparison. As was noted
by Smith and Wu (1974), the maximum value of the gyrofrequency at the
North foot of the IFT is essentially identical to the maximum observed
emission frequency of 39.5 MHz for DAM. Since the gyrofrequency at
the South foot point never exceeds 28 MHz, the North foot of the IFT
would appear to be more attractive as the source for the three high-
% The new System III (1965) longitude convention (Riddle, 1975) gives
System III longitudes ' 30 0 lower than System III (1957.0) for
observations at the time of Pioneer-11 encounter.
,
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frequency Io-controlled sources. A problem still exists in that the
sub-Io _.,ngitude of the early source (which reaches the highest
emission frequency) does not correspond to the region of maximum
I
	 gyrofrequency at the IFT foot point. Interestingly, the maximum
gyrofrequency in the Southern hemisphere (28 MHz) is the same as the
highest frequency at which Io-independent DAM is observed. This is
the maximum gyrofrequency for trapped electrons since their minimum
mirror altitudes will be determined by the weaker Southern magnetic
field. On the other hand, prE,cipitating electrons in the Northern
hemisphere could account for the Io- controlled DAM.
From the dip angle plot in Figure 1 we see that Io crosses the
dip equator at System III (1965) longitudes — 11.5 0 and 2900 . This
is close to the boundaries of the range of 'sub-Io longitudes for which
we observe Io-controlled DAM. Apparently DAM is stimulated only when
Io is north of the magnetic equator. That DAM is apparently beamed
into the ecliptic only when Io is situated over an "active hemisphere"
centered near 2000 longitude was first pointed out by Dulk (1965).
III. The IFT Footprint
Figure 2 is a plot of the IFT foot print (i.e., the locus of
foot points of the IFT for a complete rotation of the planet with respect
r
to lo) in zenographic co-ordinates. For each point along the IFT foot
print in the regions for which lo-cont 3lled DAM is observed, we have
plotted an arrow in a direction parallel to the corresponding central 3
meridian longitude for that DAM emission feature. If the Io-controlled
emission originates from near the foot of the IFT then the four
"source" regions would be at the indicated locations and the direction
of emission in the ecliptic plane would be as given by the arrows.
Note that the IFT foot point passes very close to the Northern magnetic
pole as the IFT enters the "active hemisphere" and Io-controlled
fourth and main source emission begin. if the DAM originates from
the Northern IFT foot, then the source is confined to a relatively
small zenographic area and the emission is beamed into a pattern
that intersects the ecliptic in two directions separated by — 1500.
This was first suggested by Dulk in 1965 on the basis of his analysis
of the Io control morphology.
IV. The IFT Geometry
We have also examined the magnetic field geometry for the IFT
and results from that analysis are displayed in Figure 3. Starting
with the vector orientation of the foot of the IFT in each hemisphere,
we have calculated the angle between the field line at 1.0 R J and an
observer in the ecliptic as a function of Io phase and central meridian
longitude. The IFT foot point is beyond the limb of the planet for
the crossed-hatched areas of the diagram. Since a number of theories
for the DAM emission mechanism favor radiation at right angles to the
magnetic field, the loci of 900 aspect angles are plotted for each
hemisphere. The lightly shaded areas denote the regions of the Io
s. I4
5phase-central meridian longitude plane in which the four Io-controlled
"sources" are observed.
In the North IFT foot point diagram in Figure 3, we find that the
900 aspect angle trace passes directly through the fourth and early
source regions (Io phase of 800	1100 , central meridian longitude
of 400 - 1890). The observed high frequency limit of ^ 18 MHz for the
fourth source is consistent with the finding that the IFT intersects
the surface at a point beyond the limb for much of the fourth source
longitude range. Hence, the low altitude, high gyrofrequency portion
of the Northern IFT is obscured from view for fourth source geometries.
The high longitude boundary of the early source at 180 0 is apparently
not a result of obscuration of the source region but a consequence of
the Southward passage of Io across the magnetic equator, i.e. out of
the "active hemisphere". The magnetic field geometry for the 4outhern
foot of the IFT does not show any particular correspondence v;,th
the locations of the fourth and early sources.
The situation is less clear for the main and third sources (Io
phase ^ 2400 , central meridian longitude 220 - 3600). For the Northern
foot of the IFT the aspect angle with respect to the magnetic field
is — 600 for the main source and — 80 0 for the third source. In the
Southern hemisphere these angles are — 100 0 and — 1100 for the main
and third sources, respectively. The main source region would be
beyond the limb for Io-phase > 240 0 in the South, and the third source
would 'be beyond the limb at all times if the North foot point is involved.
	
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Discussion
The observed DAM frequency range and the sense of frequency drift
of the envelopes of the source dynamic spectra tend to favor the North
foot point over the South as the more probable source location. For
example, the observed drift from low to high frequency in the fourth
source could be a result of changes in the degree to which the foot
of the Northern IFT is occulted by the planetary disk with time. We
have already noted that obscuration effects could explain the high
frequency cut-off of the fourth source. The initial low to high frequency
drift of early source dynamic spectra might also result from obscure-
tion effects resulting from the close proximity of the North foot
point of the IFT to the "horizon" for low values of Io phase and
central meridian longitude in the ,early source portion of the
diagram. The subsequent drift from high-to-low frequency in the later
stages of many early source events could result from the high-to-low
drift of the Northern IFT gyrofrequency with time as indicated in
Figure 1. For the wain and third sources the sense of variation of
IFT gyrofrequency (high-to-low) matches the sense of observed frequency
drift for main and third source dynamic spectra which is also from high-
to-low frequency.. The variation of gyrofrequency with sub-Io longitude
in the Southcrn hemisphere does not agree with the observations as well.
Limb obscuration effects in the North could contribute to the fact
that the third source does not reach as high a frequency as either
the mail. or early sources.
6
A
II
_	 I	 V
Although the discussion above has emphasized the positive aspects
of a comparison of the Northern IF`P geometry and DAM morphology, a
number of difficulties remain to be resolved. For the range of sub-lo
longitudes over which early source emission is observed, t%e IFT
electron gyrofrequency never quite reaches the maximum observed DAM
frequency (- 34 MHz vr. 39.5 MHz). The predicted limb occultation
of the North foot point for third source geometries plus the fact
that the maximum IFT gyrofrequency in that region is only — 28 MHz
do not agree with the observed high frequency limit of — 32 MHz for
the third source if the emission is at the local gyrofrequency. With
ionospheric electron densities of N 3 x 105 cm-3 (Kliore et al., 1975)
the upper hybrid resonance frequency will not be appreciably higher
than the gyrofrequency. On the other hand, a radiation mechanism
such as that discussed by Scarf (1973) where the radiation frequency
is 3/2 times the gyrofrequency would not be similarly constrained by
the magnetic field model. Furthermore, Kaiser and Alexander (1975)
have recently shown that the terrestrial kilometric emission can apparently
originate at heights well above th,: level at which the gyrofrequency equals
the emission frequency. Perhaps the same processes can occur in Jupiter's
magnetosphere.
VI. Conclusions
Taken as a whole, the comparison f Pioneer magnetic field models
with the DAM morphology provides some encouraging lines of agreement,
7
ebut it falls short of leading to any fully satisfactory picture of the
radio source. if our initial assumptions that the models portXuy
the magnetic field topology near the surface reasonably accurately
and that the Io flux tube determines the location of the source are
correct, then simple free space geometric analyses do not suffice.
We suspect that propagation effects are likely to play an important
role its the DAM problem and that such effects will have to be considered
in order to adequately account for the ri^dio observations. For example,
reflection of the radio waves vrf a surface such as the Jovian ionosphere
may be responsible for the 90 0 - 600 asymetry in the aspect angles
determined for the main and early sources. Similarly, propagation
effects may be particularly important in determining the frequency
structure of the dynamic spectra of the Io-controllled sources and
in defining the radiation pattern of the emission. This is not a
particularly new idea of course. The results of the Pioneer magnetic
field measurements have just reinforced those conclusions.
Acknowledgements
We are indebted to our colleague N.F. Ness for his comments and
suggestions during our many discussions on this work.
8
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M.
References
Acuna, M.H., and Ness, N.F., presented at Tucson Jupiter Workshop
(May 1975); to appear in Jupiter, the Giant 'Planet, T. Gehrels
ed. (Univ. of Arizona Press, 197b).
Dulk, G.A. Univ. of Colorado Ph.D, Thesis (1965).
Kaiser, M.L., and Alexander, J.K., USNC/URSI Annual Meeting, Boulder,
Colorado (Oct. 1975).
Kliore, A., Fjeldbo, G., Seidel, B.L., Sesplaukis, T.T.,
Swei,tram, D.W., and Woiceshyn, P.M., Science 188, 474 (1975).
Riddle, A.C., presented at Tucson Jupiter Workshop (May 1975); to
appear in Jupiter, The Giant Planet, T. Gehrels ad. (Univ.
of Arizona Press, 1976) .
Scarf, F.L., TRW Systems Rept. 24876-6001-RU-00 (1973)
Smith, B.J., Davis, L., Jones, D.B., Coleman, P.J., Colburn, D.S.,
Dyal, P., and Sonetc, C.P., Science 188, 45", (1975).
Smith, R.A., and Wu, C.S., Astrophys. J. (Lett.) 190, L91 (1974).
(Addendum: ibid 193, 1.101).
FIGURE CAPTIONS
Figure 1 (Lower panel) Plot of relative occurrence probability of
DAM emission versus sub-Io System III (1965) longitude
for the major Io-controlled sources. (Middle panel)
Plot of magnetic dip angle at lo as a function of sub-Io
longitude. (Top panel) Plot of ChP electron gyrofrequency
at the E?et of the lo flux tube as a function of sub-Io
7on^}.wde.
Figure 3. Plot of the footprint of the Io flux tube as a function of
System III ( 1965) longitude and zenographic latitude. The
location of the magnetic pole at the surface for the
Northern ( left panel) and Southern (right panel) hemispheres
is given by the upper case N and S respectively. The foot
points are labelled to indicate the corresponding System III
(1965) sub-Io longitude for each point. The arrows show the
direction into which DAM emission is beamed if the major
Io-controlled sources are located at the foot of the IFT.
PRECEDING PAGE BLANK NOT pffXW
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Figure 3a. Angle between the North foot of the Io flux tube and an
observer in the ecliptic (DE - 00) as a function of Io
.	
a
phase and System III (1965) central meridian longitude.
i
The hashed area denotes cases for which the foot point
at the surface would be beyond the limb-of the planet
with respect to the observer. The dashed lines are
the loci of 90 0
 aspect angle points. The shaded areas
	 l'
e
show the location 3f the major Io-controlled DAM 	 tg
emission regions.
	 t
3b. Same as 3a, but for the South foot of the Io flux tube.
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SUC - 10 LONGITUDE (SYS M,
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Figure 1
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JPL MODEL f.OR , NORTH TFT FnnT
360
300
240
120
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4	 9-1 --88 82	 79	 81	 86 91 X4.94 -
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63 63 59 53 47 44 44 47 49 53 57
	
67 67 64
	 58 54	 b)1	 51	 52	 55 59 62
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	 74
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 i1 y8$	 '	 4 84 85	 155 ! 6..x.91
05 103 102
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180	 240	 300	 360
CENTRAL MERIDIAN LONGITUDE-SYSTEM III (1965)
Figure 3a
L
,1PL MODEL I.OR j SOUTH IFT FOOT
360
300
240
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180
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95 96 96 9
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	 104 10
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114	 118	 121
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	 120	 123
	
126
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116
	
117	 114	 108	 102	 100	 100 102	 102	 104
100 100 9 .r-.
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CEN TRAL MERIDIAN LONGITUDE- SYSTEM X (1965)
Figure 3b


Implications of Pioneer-2 magnetic field models for Jupiter's decametric radio mission

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