A model of Jupiter's magnetosphere in which there is a circum-Jovian outflow of plasma from an inner corotating magnetodisc is compared with the observations of the Pioneer flybys

Prakash, A.

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https://ntrs.nasa.gov/search.jsp?R=19760011940 2020-03-22T17:12:29+00:00Z
4	 ^
(NASA-CF- 146421)
 CCMFARISCN CF 4 MODEL O r
	N76-19028
JIIFITEE O S MAGNETOSPHEFE WITH PIONEER CATA
(Cornell Univ.)	 15 P HC $3.50
	 CSCL 03E
unclas
1 G3/91	 18571
COMPARISON OF' A 140DEL OF JUPITER'S
1	 MAGNETOSPHERE WITH PIONEER DATA
i
I A. Prakash
Labor3tory of Plasma Studies
Cornell University
	
i	 Itraca, N.Y. 1.4853
	
•	 August 20, 1975
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ABSTRACT
The model of Jupiter's magnetosphere presented by Prakash
at the 1974 Frascati symposium (see Prakash and Brice, 1975),
in whJc:t there is a circum-Jov'.an outflow of plasma from an
inner corotating magnetodisc, is compared with the observations
of the Pioneer flybys,
-2-
i
	
From the plasma measurements of the Pioneer 10 flyby of
Jupiter, Wolfe et al. (1974) found that the size of Jupiter's
f	 magnetosphere is much larger (. 96RJ) than the extent (w 53RJ)
i	 predicted by scaling the Earth's magnetosphere (e.g., Brice
and Ioannidi.s, 1971). In order to explain the 10 hr period-
icities in the flux of energetic particles observed in the
Jovian magnetosphere by Pioneer 10 ( Fillius and McIlwain, 1974;
McKibben et al., 1974; Trainer et al., 1974; Van Allen et al.,
1974), Van Allen et al. Invoked a rigidly corotating magneto-
disc with particles concentrated in the equatorial plane.
Frori wneir magnetic field data, Smith et al. (1974) concluded
that the magnetodisc was warped toward the equator, although
there exists some controversy with the University of Iowa
interpretation of little warping (e.g., Van Allen et al., 1975).
In the model of Jupiter's magnetosphere presented by Prakash
at the 1974 Frascati symposium (see Prakash and Brice, 1975),
the rigidly corotating magnetodisc is envisaged to be of limited
extent (LcRJ) and from its edges there is a c:ntrifugally driven
outflow of thermal plasma. Although the model is similar to
the independently proposed, model of Hill et al. (1974), the
two pictures of the magnetosphere have significant differences.
Whereas Hill et al. confine the outflow into the magneto-tail,
the model presented by Prakash has circum-Jovian outflow.
Moreover, the model explicitly takes into account the effect
of the tilt of the dipole on the outflow which gives the
outflowing plasma the appearance of a wavy disc, as shown
schematically in Fig. 1. Also, the model envisaged the possi-
bility that tho outflow may be intermittent, perhaps in the
form, of blobs of plasma, and that the radius of the corotating
region ( ^RJ) was a time dependent parameter, owing to fluctua-
tions in the (thermal) plasma density distribution inside
the magnetosphere,arising from auroral precipitation etc.
ti
Fa
i
-3-
Since the Pioneer 10 and 11 plasma detectors are in no position
to directly detect the outflow predicted by the model., we
here point out the indirect e ,ridence from the Pioneer data
which would be consistent with the model. A more up to date
version of some of the features of the model is also included.
McDonough et a1. (1975) have recently obtained a revised
version of the Toannidis and Brice (1971) diffusive equili-
brium curve of plasma density in the inner Jovian magnetosphere,
resulting from the photoelectron eseu;^e flux from the iono-
sphere as calculated by Swartz at al. (1975). Using this result,
we draw Fig. 2 which updates the Fig. 5 of Prakash and Brice
(1975). The main rationale of the model is that since Jupiter
is surrounded by an insulating atmosphere, interchange motions
of the field lines are capable of causing an .1 N density drop,
which, on the assumption of a maximum (thermal) plasma density
of — 100 cm-3 (before interchange motions become dominant),
intersects the -r L 8 Alfvenic limit curve (Toannidis and Brice,
1971; Michel and Sturrock, 1974) inside the Jovian plasmasphere,
at 
o. 
The Alfvenic limit for density is the density for whir:h
the local Alfven speed becomes equal to the corotation speed.
The model postulates that from 	 there is a eircum-Jovian
outflow of plasma, with frozen-in field lines, 
c 
is identified
as the edge of the corotating magnetodisc and the plasma flow
is taken to be tangential to the magnetodisc, as an approxi-
mation. The dashed Alfven limit of Fig. 2 has been drawn for
a dipole field with a surface (L=1) value B,= 8 gauss, rather
than the actual value of B,, — 4 gauss of Smith's D2 model, in order to
make the magnitude of the magnetic field in the outer magneto-
sphere	 close to the actual. value obsery-A there. The value
r..
of Lc so obtained - is —50. Hot-ever, because the exact density
distribution within the Jovian magnetosphere is unknown, the
model takes Lc to be a time dependent parameter to be deduced
from spacecraft data. The tilt of the dipole results in a
wavy disc of outfloding plasma. The equation of the surface
of this disc can be written as
Le ^„„ 90 c^ Cwt — '`L^y —1 — )
_	 Boz 
C
wt _ 
xLiaz_1 —^1
where	 !p =
o	 \	 t
x2fi^
and	
'	 Le
Here W is the angular speed of Jovian rotation, t is the tilt
of the dipole axis with respect to the rotation axis ( eo = 10,6
if Smith's D2 model is used),	 the radius of the corotating
Inner magnetodisc, and VA = W Lc is the Alfven speed at I,, . For
a given (x,y), the equation gives the variation of z as a function
time t. The coordinate system used is the Jovicentric system with
the z-axis along the spin vector, x-axis towards the Sun, and the
y-axis pointing west to give a right hand coordinate system. It
is essentially the Jovicentric solar ecliptic coordinate system,
It should be noted . that the equation of the outfloti•ting wavy plasma
disc given above assumes that the equatorward warping of the inner
corotating magnetodisc is negligible. The field spiral angle is given by
The angle (p is defines: as the acute angle between
 B(
	
1- 	 and radius vectoi;j _,
1T
-1111	 1.1­1
l	 ;5
^Y
i
The outflow of the plasma, carrying frozen-in field lines,
	
A
les4s to a spiral field in the region beyond Lc . I1g, 3 shows
this spiraling as a function of 
c	
In the outbound pass of 	 E
Pioneer 10, Smith et cl, (1974) obtained a field 'lag' of
^• 35° at. 96R J, This would require an Lc of 74. The model
so omitted to the data is shown in Fig, 4, According to the
model, the dynamic pressure of the outflowing plasma contributes
to the observed (e.g.., Wolfe et al., 1974) distention of the
	 z
magnetospheric boundary of the scaled magnetosphere. The 	 }
distention is expected to have a morning-evening asymmetry
(see Fig. 7 of Prakash and Brice, 1975) and the dashed curve
in Fig, 4 is a schematic representation of the boundary.
Fil.lius and McIlwaf.n (1974) plotted the positions of their
energetic particle flux minima as a function of System UI (1957)
longitude (see their Fig, 10) and pointed out that the locking-
in of the phase of the inbound minima at- 50R  could be an
indication of a boundary that separates two different regions
of the magnetosphere. In their outbound data, such a boundary
is indicated at about 75R J, which is in agreement with
Fig. 4(in the outbound region) according to which Lc = 74,
The indication of an inbound boundary at L 
—50 in T:.} *.us'
data could be interpreted as due to the time depender. r, :, of o
or due to a compression of Lc by the solar wind on the dayside.
The disorderliness of the magnetic field data in the Pioneer
10 inbound traversal of the outer magnetosphere (Smith et al.,
1974) is interpreted as due to the effects of compression by
the solar wind which enhances magnetic merging of the oppositely
directed spiral field lines (e.g., Gold, 1974; Simpson et al.
1974; Prakash and Brice, 1975), 	
j
The Pioneer 10 data of McKibben et al. (1974), (see their
Fig, 2), can also be interpreted to indicate a boundary at
i
r
	 4
L X 40 inbound and at L^ 60 outbound, the distances upto which
the particle flux minima indicate rigid corotation. Since the
magnetodise need not be
	
I
' at the edges, the data of McKibben
et a1,(1974) are not inco	 Aent with the above interpretation
of L,50 inbound and C, 75 outbound. However, this inter-
pretation encounters a difficulty in the light of a preliminary
interpretation of their Pioneer 11 date by Simpson et a', (1975)
who state that the periodicities in the energetic partl e,le flux
are synchronous throughout the sunward side of the Jo^,ian magnp+ i-
sphere. However, it appears that a clarification of the conclu-
sions of Simpson et al. in harmony with the Pioneer 11 magnetic
field data and the energetic particle data of other experimenters
(e.g., F111ius et al., 1975) is needed before the situation is
considered settled. In any case, the high latitude periodicities
of Pioneer 11 present difficulties to the model discussed here,
as they do to other models,
t
	
	 Trainer et al. (1974), in their Fig. 4 and Fig. 5, plot
the Pioneer 10 differential energy spectra of electrons at
d_fferent radial distances from the planet for the inbound
and outbound trajectories. Their data show a 'coundary within
the magnetosphere such that the spectrum goes as E 2 inside
the boundary and as E-1.5  outside the boundary. Their data
are remarkably consistent with L— 50 for the boundary in
'
	
	
the inbound case and L-74 in the outbound case, and give
added support to our interpretation.
C
From the detection of a current sheet with energetic particle
'-
	
	 flux maxima in the inbound pass of Pioneer 11, a feature that
was not observable in Pioneer 10 data, Simpson et al. (1975)
conclude that the "spatial distribution of particles changes
with time over a period of many days." This is in accord with
ii
-7-
the time dependent nature of 
a 
expected in the model discussed
here. An unresolved difficulty with the model is that if the
inner magnetodise is warped as much as deduced theoretically by
Fill et al.	 then the wavy outflow is confined to an
: amplitude less than 5RJ and this makes it unlikely that the
;periodicities seen in the outer magnetosphere could represent
periodic immersion of the spacecraft in the outflow.
According to the model, the "sponginess' of the outer magneto-
.,..sphere observed by the Ames plasma detectors on Pioneer 10
and 11 (Wolfe et al.., 1974; Mihalov et al,, 1975) is to attributed
a,
in 'part to fluctuations in C, although variations in the solar
..,wind pressure undoubtedly area factor. The fact that the inbound
'. Picnedr 11 flyby showed approximately the some magnetopause
."crossings as the inbound Mass of Pioneer 10 (Mihalov et al., 1975)p
may not merely be a fortuitous coincidence but an evidence for
the distention of the magnetopause resulting from outflow of
plasma from the ;Inner magnetosphere, Sume of the multiple
boundary crossings detected by the Ames plasma analyser show
;•W 5 or — 10 hr intervals and these could also bs interpreted
.'(Prakash, 1975) as supporting the picture given by Prakash and
.,Brice ( 1975), The outflow is also expected to give rise to
;,,a double shock at the nose (e,g., Kennel et a1, 1975; Dryer, private co-
:mmunication to Prakash, 1974). Since the Ames plasma detectors
on Pioneer 10 and 11 were mounted facing the sun, it is difficult
•`to identify the second internal shock at the nose or to directly
'detect the outflowing plasma. It appears that only future
spacecraft missions, equipped with a (low energy) plasma detector
facing Jupiter, could provide conclusive evidence for or against
the outflow model of Jupiter ' s magnetosphere considered here,
`rk;s work 
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RE^ ^ RENCES
Brice, N. M., and G. A. Ioannidis, The magnetospheres of Jupiter
and Earth, Icarus, 13, 173, 1970,
Fill.ius, R. W,, and C. E. Mcll.wain, Measurements of the Jovian
radiation belts, J. Geophys. Res., 79 3 3589, 1.974.
Fillius, R. W., C. E. Mcllwain, and A. 14ogro-Campero, Radiation
belts of Jupiter: a second look, Science, 186, 465, 1975,
Gold, T., The possible combination of radiation belts and dust
rings on Jupiter, Bull. Am. Astron. Soc., 6, 378, 1974,
Hill, T. W. 5 A. J. Dessler, and F. C. Michel, Jupiter's magneto-
sphere, Geophys, Res, Lett., 1, 3, 1974,
Ioannidis, G., and N. Brice, Plasma densities in the Jovian
rdagnetosphere: plasma slingshot or Maxwell demon?,
Icarus, 14, 360 3 1971.
Kennel, C. F,, .end. F. V. Coroniti, Is Jupiter's magnetosphere
like a pulsar's or Earth's?, In The Magnetospheres
of the Earth and Jupiter (V. Formissno, Ed.), D. Reidel
Publishing Co., Dordrecht-Holland, 1975,
McDonough, T. R„ A, Prakash, and W. E. Swartz,.On Jupiter's
inner magnetospherec thermal plasma density, Eos
Trans, Am, Geophys. Union, 56, 428 3
 1975,
McKibben, R. B,, and J. A. Simpson, Evidence from charged particle
studies for the distortion of the Jovian magnetosphere,
J. Geophys. Res,, 79, 3545.
Michel, F. C,, and P. A. Sturrock, Centrifugal instability of
the Jovian magnetosphere and its interaction with the
solar wind, Planet. Space Sci, 22, 1501, 1974.
Mihalov, J. D., H. R. Collard, D. D. McKibbin, J, H. Wolfe
and D. S. Intrili.gator, Pioneer 11 encounter: preliminary
results from the Ames Research Center plasma analyser
experiment, Science, 188, 448, 1975.
r4
iy
	
YI	
- 9-
Prakash', A., Multi.plici.ty of Pioneer-Jupiter shock and magneto-
pause crossings in a model magnetosphere, Eos Trans.
Am. Geophys. Union, 56, 427, 1975.
Prakash, A., and N. M. Brice, Magnetospheres of Earth and
Jupiter after Pioneer 10, in The Magnetospheres of
the Earth and Jupiter (V. Formisano, Ed.), D. Reidel
Publishing Company, Dordrecht-Holland, 1975.
Simpson, J. A., D. Hamilton, G. Lentz, R. B. McKibben, A. Mogro-
Campero, M. Perkins, K. R. Pyle, ant A. J. Tuzzolino,
Protons and electrons in Jupiter's magnetic field,
Science, 183, 309, 1974.
Simpson, J. A., D. C. Hamilton, G. A. Lentz, R. B. McKibben,
M. Perkins, K. R. Pyle, A. J. Tuzzolino, and J. J.
O'Gallegher, Jupiter revisited; first results from
University of Chicago charged particle experiment
on Pioneer 11, Science, 188, 455 3 1975.
Smith, E. J., L. Davis, Jr., D. E. Jones, P. J. Coleman, Jr.,
D. S. Colburn, P. Dyal, C. P. Sonett, and A. M. A. Fransden,
The planetary magnetic field and magnetosphere of
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Smith, E. J., L. Davis, Jr., D. E. Jones, P. J. Coleman, Jr.,
D. S. Colburn, P. Dyal, and C. P. Sonett, Jupiter's
magnetic field, magnetosphere, and interaction with
solar wind: Eicneer 11, Science, 188, 451 3 1975.
Swartz, W.E., T. R. McDonough, and R. W. Reed, Photoelectron
escape from the ionosphere of Jupiter, J. Geophys. Res.,
80, 1195, 1975.
Trainer, J. H., F. B. McDonald, B. J. Teegarden, W, R. Webber,
and E. C. Roelof, Energetic particles in the Jovian
f	
magnetosphere, J. Geophys. Res., 79, 3600, 3.974.
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D. D. Sentaan, and II. R. Flindt, Energetic electrons
in the magnetosphere of Jupiter, Science, 183, 1974.
rf
r
1
-10-
Van Allen, J. A,, B. A. Randall, D, N. Baker, C. K. Goertz,
D. D. Sent an, M. F. Thomsen, and H. R. Flindt, Pioneer
11 observations of enargetic particles in the Jovian
magnetosphere, Science, 188, 459, 1975,
Wolfe, J. H „ J. D. Mihalov, H. R. Collard, L^. D. McKibbin,
L. A. Frank, and D. S. Intriligator, Pioneer 10
observations of the solar wind interaction with
Jupiter, J. Geophys, Res., 79, 3489 0
 1974.
L -I
FIGIM, CAPTIONS
Fig. 1. Schematic representation of the noon-midnight
meridional section of Jupiter's model magnetosphere
according to Prokash (Prokash and Brice, 1975).
The waviness of the outflowing plasma disc hos been
exaggerated -- in actuality less than one wavelength
would exist on the sunward side.
Fig. 2. An estimated plasma density distribution in Jupiter's
equatorial. plane shows that rigid corotation is unlikely
to exist beyond a critical radial distance given by
L6 50.
Fig. 3. Predicted spiraling of field lines as a function
Of Le
 -
Fig. 4. Model magnetosphere of Jupiter (now viewed from above
the N-pole) as fitted to the magnetic field spiraling
observed in the Pioneer 10 outbound pass.
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Comparison of a model of Jupiter's magnetosphere with Pioneer data

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