JOURNAL
OF GEOPHYSICAL
RESEARCH, VOL. 91, NO. A10, PAGES 10,989-10,994, OCTOBER
1, 1986
The Centrifugal Flute Instability and the Generation
of Saturnian
Kilometric
Radiation
S. A. CURTIS, R. P. LEPPING,AND E. C. SITTLER,JR.
Laboratoryfor ExtraterrestrialPhysics,NASA GoddardSpaceFlight Center,Greenbelt,Maryland
We showthat the sourceregionfor Saturniankilometricradiation (SKR) whichoriginatesin the high
latitude near-noondaysideionosphericcan be mappedvia the Saturn magneticfield to the outer edgeof
the daysideequatorialplasmasheet.
The plasmasheet
is known to be unstableat this boundarydue to the
outward centrifugalforcesgeneratedby the heavily mass-loadedequatorial Saturnian magnetosphere.
Also previouswork has shown that Voyager 1 observationsof MHD wavesnear the plasmasheetare
consistentwith their beingdriven by the centrifugalflute instability.The resultingfield-alignedMHD
wavesat higher latitudescan trap and accelerateelectrons.For the plasma environmentmeasuredby
Voyager,field-alignedacceleratedelectronsup to energiesof severalkeV are expectedto be deposited
into the SKR ionosphericsourceregion. A necessarycondition for the accelerationof the electronsto
severalkeV is a populationof hot electronsinsideof the plasmasheet,
near the unstableboundary.The
hot electronsare producedas a byproduct of the pick-up of exosphericatoms from Saturn'smoons.The
solar wind dependenceof the SKR is thus a consequenceof the elongationof the Saturn nightside
magnetosphere
with greatersolar wind pressureleadingto greatercentrifugalpressures
on the nightside
plasmasheet.
Although the theory presentedcorrectlygivesthe local time dependenceof the emissions,
we suggestthat the strong emissionpeak at a fixed Saturnianlongitudeis a consequence
of locally
reducedelectron plasma frequencyto electron gyrofrequencyratio. The lack of a nightsidesourceof
SKR is consistentwith the disruption of the plasmasheetdue to loss of plasma down the Saturn
magnetotail.
INTRODUCTION
The analysisof plasma measurementsby Voyager 1 in the
dayside near-equatorial plasmasheetby Sittler et al. [1983]
have shown that beyond the innermostboundary of the outer
part of the plasmasheetR • 16 Rs there exist a number of
detached plasma regions. These dense flux tubes have the
plasma density and temperaturecharacteristicsof the plasmasheetand are separatedby regionsthat are more rarefied
and considerablyhotter.
The Saturn magnetosphereis obviously internally mass-
flute instability. A consequenceof this instability processwill
be the generation of MHD waves. Near the equatorial plane
the MHD wavesare expectedto be primarily azimuthalin
direction,whereas
at higherlatitudesthe waveswouldbe primarily field-aligneddue to the finite extentOf the plasmasheet
in latitude. Thus the magnetic field-plasma interchange occurring in the plasmasheetgives rise to an impulselike dis-
placementof the field lines which can then propagateto
higher latitudes. It is precisely this morphology of magnetic
field fluctuation which is consistentwith MHD w•ves which
loaded,althoughthe sourceof this plasmaremainscontro- has been reported by R. P. Lepping et al. (unpublishedmanuversial and will probably require a comparativeanalysisof script, 1986) from an analysis of low-frequency oscillations in
outer planet magnetospheresto resolve [Chen•l and Hill, the magnetic field observedon the inbound encounter trajec1984], and sincethe systemis a rapid rotator with a period of tory of Voyager 1. The MHD waves are found to be primarily
little over 10 hours, centrifugaleffectsmay be expectedto be azimuthal in propagation direction near the equator in the
important in the outer parts of the magneotsphere.Specifi- plasmasheet. In contrast, at higher latitudes the waves are
cally, by analogywith Jupiter,the nightsideplasmasheetis
very likely to be unstableto processesthat led to plasma loss
down the tail [Hill and Dessler, 1976; Goertz, 1976]. The
sharp edgeat the stability boundarycreatedby this losswill
steepenas it is rotated toward the daysidenoon meridian.The
steepeningwill be both a consequence
of the compressionof
dayside field lines as well as additional mass-loadingdue to
photoionization throughout the magnetosphereand electron
impact ionization, beyond 5 Rs, of neutrals emitted by the
magnetosphere'sinner sourceswhich may include the Saturn
ionosphere,the rings,the icy satellites,and perhapspart of the
Titan torus. The steep dayside density gradient at the outer
boundary of the plasmasheetcoupled with the centrifugal
force experiencedat that location has been shown to be unstable by Goertz [1983]. By usingtheory developedby Mikhailovski[1976] for plasmasin a gravitational field and by noting
that the centrifugal force gives rise to an effective gravity,
Goertz has shown the outer plasmasheetboundary to be unstable to an interchangeinstability known as the centrifugal
Copyright 1986 by the AmericanGeophysicalUnion.
Paper number 6A8462.
0148-0227/86/006A-8462505.00
primarilyfield-aligned
iri propagationdirection.As wouldbe
expectedfor thesewavesbeing driven by the centrifugalflute
instability, the wave amplitudesare greatestnear the dayside
equatorial plasmasheetouter boundary.
The field-alignedMHD wavesmay be mappedinto the high
latitude ionosphere using the Saturnian magnetic field Z3
model of Connerneyet al. [1983, 1984a, b]. This model has
shownitself to possessa high .degreeof accuracyand has even
proved usefulin diagnosingsystematicinstrumentalerrors on
other spacecraft that have encountered the Saturn system
[Connerneyet al. 1984a, b]. Using the Z3 model, along field
lines to the planetary surface,we can map the noon meridian
outer edgeof the plasmasheetto approximately76øN latitude,
very near the most probable SKR sourcelocation as deduced
from Voyager observationsby Kaiser and Desch [-1984]. The
ionospheric source location deduced by Kaiser and Desch
[1982] is thus consistentwith a free energy source located on
closedfield lines which contain the outer plasmasheetboundary. The overall mapping of the high latitude radio emission
source region to the equatorial plasmasheet boundary is
shown in Figure 1. Another interestingaspectof the Voyager
radio emissionobservationsat Saturn was the strong peaks in
the emissionswhen the 115ø SLS (Saturn longitude system)
line passesthrough the noon meridian [Kaiser et al., 1984].
10,989
10,990
CURTIS ET AL.' CENTRIFUGALFLUTE INSTABILITYAND SKR
SATURNIAN KILOIvlETRIC RADIATION
SOURCE REGION
uv
AURORA
MAGNETIC FIELD
POLAR
CAP
•:•øN
NOON MERIDIAN
z
(Rs)
o
KILOMETRIC
RADIO SOURCE
DETACHED
PLASMA
EXTENDED
PLASMA SHEET
EXTENDED
PLASMA
R
D T EM
INNER
E RING
PLASMA
TORUS
ME T
INNER
PLASMA
TORUS
D
SHEET
NEUTRAL
HYDROGEN CLOUD
R
E RING
PLUMES
HOT OUTER MAGNETOSPHERE
HOT OUTER MAGNETOSPHERE
NOON
DAWN
Fig. 1. Combined
results
fromthedataof theVoyagerPlanetary
RadioAstronomy
experiment
[KaiserandDesch,
1982],fromthe Magnetometer
[Connerney
et al., 1983]andthePlasmaScience
instrument
[Sittieret al., 1983]which
showsthemappingoftheSKR sourceregionto theunstable
edgeoftheequatorialplasmasheet.
This strong emissionpeak existsin the absenceof any apparent magnetic anomaly in the near surfacefield [Connerneyet
al., 1984]. There have also been detailed studies performed
indicating that the solar wind exerts a strong control over the
generationof SKR [Desch,1982, 1983].
Another magnetospherewith even stronger internal plasma
sourcesis that of Jupiter. Jovian kilometric radiation has two
components: narrow bandwidth kilometer wavelength emission (nKOM) and broad bandwidth kilometer wavelength
emission(bKOM) [Kaiser and Desch, 1984]. Both have spectral peaks at the same wavelengths,but have different source
locations. The nKOM radiation appear to originate in the
outer edgeof the Io plasmatorus near the magneticequatorial
plane. In contrastbKOM originateseither within the Io torus
or at low altitudes in the auroral zone at L = 6. In addition,
there is a decameter wavelength emission (DAM) which is
Io-associated and appears to come from the magnetic flux
tube linking Io and the Jovian ionosphereat L = 6 [Kaiser
and Desch,1984]. Two immediate differenceswith respectto
the Saturn magnetosphereare the much stronger plasma
sourcefrom Io comparedto any in the Saturn systemand the
much stronger magnetic field at Jupiter which is about 20
times that of Saturn at the equatorial 1 bar level. With the
higher Jovian magnetic field, shorter wavelength radiation
CURTISET AL.' CENTRIFUGAL
FLUTEINSTABILITYAND SKR
such as DAM is expectedgiven the abundance of free energy
sources [Goldstein and Goertz, 1983; Hill et al., 1983]. For
comparisonwith Saturn our interest is primarily in the kilometer wavelengthrange, however. Since the centrifugalflute
instability is believed to the operative at the Io torus outer
edge [Hill, 1976], it is tempting to compare nKOM generation with SKR generation. The confinement of the emission
to the magneticequatorial region at Jupiter as opposedto the
auroral zone at Saturn may possibly be related to the much
high densitiesin the Io torus at L - 6 comparedto the Saturn
magnetosphereat L - 16. The bKOM radiation appearsto be
related to the Io-Jovian ionosphereinteraction at L- 6 and
would most likely involve a different mechanismthan the centrifugal flute instability generatingsurfaceMHD waves.Since
the locations
of the Jovian
kilometric
sources
are still
not
firmly establishedit is difficult to speculatefurther concerning
the emissiongenerationmechanisms.
Finally, there is the very different morphology between
SKR and similar kilometric radiation from earth, the so-called
auroral kilometric radiation (AKR). Whereas SKR is
characterized by a high latitude near-noon source location
[Kaiser and Desch,1982], in contrast AKR is characterizedby
a high latitude near midnight generation region [Gurnett,
1974]. In addition to this very different local time behavior the
structureof the plasmasheets
of both planets'magnetospheres
are apparently quite different: Saturn'shaving a strong internal plasma source,whereasearth's is much weaker [Sittler et
al., 1983].
In the next section, we will show how the MHD waves
observedat the outer boundary of the plasmasheetmay be
causallyrelated to the field-alignedaccelerationof electronsto
keV energies.The field-alignedelectron fluxes will constitute
the free energysourcefor the observedSKR.
THEORY
The free energy sourcesfor driving Saturn's nonthermal
radio emissionsare the centrifugal flute instability-driven
MHD waves,and also the hot electronswhich populate the
region in the vicinity of the plasmasheet's
outer boundary.In
this sectionwe will showthat the MHD waveand trappedhot
electronfree energiescan be convertedinto keV field-aligned
electronfluxesthat in turn provide the freeenergyto drive the
10,991
here e is the magnitude of the electron charge, q/is the fieldaligned potential, T• is the temperatureof the hot electrons,VA
is the Alfven speed,6B is the MHD surfacewave amplitude, B
is the ambientfieldmagnitude,V•.r is the ion thermalvelocity,
Pi is the ion gyroradius and a is the thicknessof the plasmasheet boundary. For all of these parameters the appropriate valuesare those of the plasmasheet.Typical valuesfor the
plasmasheetparameters listed here are [Sittier et al., 1983'
Lazarus and McNutt, 1983] a mean ion mass mi • 14 amu,
Pi • 500 km, Te (hot)• 1 keV, a • 10-3 LRs whereL •- 16 is
the L shell of the plasmasheetand Rs = 60,300 km is Saturn's
radius, V•450
km s-•
and V/.r• 28 km s-•
and
6BIB •-0.1 (R. P. Lepping et al., unpublished manuscript,
1986). Hence we obtain e•p/Te • 20 6B/B or e• •> 2 keV.
Hence field-alignedelectron fluxes of several keV may be expected for Voyager observedplasmasheetboundary observations. For the sake of brevity, we refer the reader to Hase•7awa
[1976] for a detailedderivation of equation (1).
The analysis presentedby Hase•7awa[1976] for the fieldaligned acceleration of elections is linear and breaks down
whenelp/Te •., 0 (fl-•), wherefl is the ratio of plasmapressure
to magnetic field pressure. Near the outer boundary of the
plasmasheetwe have from Sittier et al. [1983] fl •-, 0.1. Hence
the linear analysisis questionablefor evTe•> 2. Thus to obtain
field-alignedelectronfluxeswith E •> 2 keV, the nonlineardevelopment in the mode conversionprocessof the surface
MHD
wave into a kinetic
Alfven wave must be considered.
This has been done by Hasegawaand Mima [1976] where
they show that from a nonlinearanalysisof the mode conversion problem a solutionin the form of a solitary Alfven wave
is obtained with the same dispersioncharacteristicsas had
been obtained earlier for the kinetic Alfven wave [Hasegawa
and Chen, 1975]. Sincethe exact solitary Alfven wave solution
is good for large values of e•/Te and since the solution is
expected to have similar field-aligned acceleration effects to
that of the linear kinetic Alfven wave, acceleration of electrons
to energiessignificantlygreaterthan 1 keV is possible.
The theory presented here representsan idealization of the
physicsoperative at the plasma sheetboundary. It does illustrate however the processby which MHD wave energy may
be converted to field-aligned electron energy and underlines
the need for a hot electron population. The heated electron
population produced by the pick-up of exosphericions origiobserved SKR.
Hydromagneticsurface-waves
that are excitedeither by a nating from Saturn's moons is required since otherwise Te
MHD instability or by an externally applied perturbation would be much less than 1 keV and since e• •-, Te, the elechave been shown to resonantly mode convert to a kinetic trons will not have sufficient energy to excite the observed
Alfven wave [Hase•Tawa,1976]. Field-alignedelectron fluxes righthand polarized extraordinary mode waves under the conmay be producedas a consequence
of the componentof the ditions required by currently acceptedtheories for the generation of planetary kilometric radiation [Wu and Lee, 1979].
kinetic Alfven wave's electric field in the direction of the
The role of plasma instabilitiesin the pick-up processesof
averagelocal magneticfield. Applicationsof this theory to the
formation of auroral arcs have been made by Hase•Tawa newly-born ions in flowing plasmashas been consideredby a
[1976]. The kinetic Alfven wave is defined as an Alfven wave number of authors [Wu and Davidson, 1972; Wu and Hattie,
whosewavelengthperpendicularto the magneticfield is com- 1974' Wu et al., 1973' Hartle and Wu, 1973; Curtis, 1981'
parable to the ion gyroradiusPi. In essencethe kinetic Alfven Winske et al., 1984]. The encounter of the International Comwave representsthe solutionto an MHD surfacewave pertur- etary Explorer with the comet Giacobini-Zinner has shown
bation applied to a densityboundary whosegradientis per- that the cometary pick-up ions possessdistribution functions
pendicular to the local field direction. This well describesthe consistentwith the interaction processbetweencometary ions
conditionsobtained at the equatorial plasmasheetboundary and the solar wind being stochastic [Gloeckler et al., 1986].
at Saturn. From the applicationof Hase•Tawa's
[1976] results These observationsconfirm the earlier theoretical speculations
to auroral arcs,we obtain a relation betweenthe magnitudeof that plasma instability processesplay a large role in the interthe externallyapplied MHD surfacewave magneticfield per- actions of newly-born ions with flowing plasmas.Barbosaet
al. [1985] have consideredthe role of lower hybrid waves in
turbationsand the magnitudeof the field-alignedpotential:
the anomalous heating of plasma near Io's torus. In their
paper, Barbosa et al. show that newly picked up ions in Io's
torus can produce a ring distribution in the cororating frame
e•p V.•6B(._a_a•
1/2
10,992
CURTISETAL.' CENTRIFUGAL
FLUTEINSTABILITY
ANDSKR
of Jupiter'smagnetospheric
ions.The ion distributionfunction direct consequenceof solar wind energy input, but rather the
providesthe free energysourceto drive lower hybrid waves indirect result of reshapingthe free energy sourcedriven by
which heat the electronsand produce a hot electron compo- the massloading and associatedexoSphericion pick-up pro-
nent.The pick-upenergyof O + ionsat L--- 16 for Saturnis
cesses
in therapidlyrotatingSaturnmagnetosphere.
--•2 keV. Unfortunately, the much lower magnetic field
strengthsin Saturn'souter plasmasheetcomparedto the field
strengthsnear Io's torus do not permit a detailedstudyof the
pick-up process-related
electronheatingand associatedlower
hybrid waves. The typical electron cyclotron frequency,fe,
near Saturn's equator at R _• 16 Rsis fce "• 280Hz [Conner-
We note that the expectedlossof plasma down the Saturn
magnetotail[Goertz, 1983] due to centrifugaleffectsforms not
onlythebasisfor thefreeenergy
source
whichdrivesthe
dayside Saturn kilometric radio emissions,but also explains
the absenceof a nightsideSKR source.The disruptiveloss of
the nightsideplasmasheetresultsin the lossof the plasmasheet
ney et al., 1984] which givesa lower hybrid frequencyfor a population which in the earth's nightside magnetosphereis
heavy ion dominated plasma with a mean atomic mass of consideredto contain the earth'sAKR energysource[Wu et
_•12-16 amu of ft:n • 2 Hz. The lower hybrid frequencyis al., 1982]. Thus a windlike loss of plasmasheetplasma down
thus far below ihe frequencyrangein which the Voyager the magnetotail operates as a free energy sink at Saturn as
plasma wave receiverhas good coverage(f> 100 Hz) [Kurth opposed to the magnetotail source of plasma for the plaset al., 1983]. However, Kurth et al. [1983] have shown,using masheet at earth.
Voyager 1 inboundobservations,
that plasmanoisetentatively
Finally, in the case of the Jovian magnetospherewhich is
identified as electrostatic bursts associated with the chorus
much more heavilymass-loadedthan Saturn'smagnetosphere,
existsnear the outer edge of the equatorial plasmasheet.The the nKOM component of Jovian kilometric radiation that
authors considereda two-stream instability of electronsas the appearsto emanate from the outer edge of the Io torus may
generator of the burst. Within the framework of the theory be generated by a mechanism similar to that discussedhere.
presentedhere, the electronpopulationsforming the streams There exist other componentsof Jovian nonthermal radiation
would be composedof the field-alignedacceleratedelectrons namely DAM and bKOM, that may be closely tied to the
and the background magnetosphericelectrons.We note that, interactionof Io itselfwith the Jovianionosphere.
evenlackingthe lowerhybridwaveobservations
at Saturn,
enoughof the artifactsof the pick-up processexist,suchas hot
electrons,a corotating heavy ion population, and a hot electron temperaturegradientdirectedradially outward I-Sittleret
al., 1983], to establishthe existenceof pick-up related electron
DISCUSSION
Althoughthe theorypresented
herecangivethe solarwind
dependenceof SKR, due to the solar wind's effect on magheating.
The observedsolar wind dependenceof SKR [Desch,1982, netosphereconfiguration,and the local time peak near the
1983] results, in the theoretical framework developed here, noon meridian owing to the maximum growth of the centrifufrom a changein configurationof Saturn'smagnetospherein gal flute instability, the conditionspresentedhere so far are
responseto solarwind bulk pressurevariations.In the limit of only necessary;they are not sufficient.In particular,the depolow solar wind pressure,such as occurswhen Saturn is im- sition of field-alignedenergeticelectronswith energies1-10
mersed
in Jupiter's
magnetic
tail [Desch,1983]theconfigura- keV into the high latitude ionosphererepresentsonly one contion of Saturn's magnetosphereshould be dipole like out to dition for SKR generation.The ionospheremust still have a
distancesmuch greater than L • 16 on both the daysideand sufficientlylow electronplasmadensityso that the ratio of the
nightside. In contrast, during Voyager l's encounter with electron plasma frequency to the electron gyrofrequency
• 0.2. This limit wasobtainedthrougha detailed
Saturn, when Saturn was in the solar wind, the dayside field LOpe/LOce
lines at L = 16 were located at 16 Rs on the daysidebut were study of the ionosphericconditionsneededfor the generation
estimatedto be at •30 Rs on the nightside [Goertz, 1983]. of the earth'sauroral kilometricradiation(AKR) by Benson
The greater distancesto which the nightsideplasmasheetex- [1985] and is consistentwith theoreticalpredictionsby Wong
•'•ne•/2/Bwheretle is the ionotends is believedto give rise to a tailward wind of the plasma et al. [1982].Since(Dpe/(Dce
and lossof the nightsideplasmasheetbeyondL • 16 [Hill and spheric electron density and B is the ambient magnetic field,
Dessler, 1976; Goertz, 1976]. The nightside tailward plas- we seethat all other thingsbeingequal the strongestemissions
masheetloss,as discussedearlier, givesrise to the steepden- will occurwhen tie is minimizedfor a givenB. The importance
ratio wasdemonstrated
at Uranus,wherethe
sity gradientat the outer plasmasheetedgewhich is required of the rOve/rOce
for the centrifugalflute instability on the dayside.The centri- dominance of the nightsideemissionsover the daysideemisratios
fugal stresses
on the nightsideplasmasheet
will increasewith sionspredicted[Curtis,1985]on the basisof rov,/rOce
the increasingdistance to which the plasma travels due to expectedfor the sunlit daysideand depletednightsideionoelongationof the nightsidefield linesowing to the solar wind- spherehas apparentlybeenverified(J. Warwick et al., private
magnetosphere
interaction.We note that Sittler et al. [1983] communication,1986).For Saturn,the strongemissionpeakobservedthe plasmasheetout to L-• 25 on Voyager 2 out- ing at one longitude, 115ø SLS, when it passesthrough the
bound when Saturn may have been in Jupiter'smagnetotail. noonmeridiansuggests
that the LOpe/LOce
ratiomustbe signifiIn contrast,the plasmasheetterminatedat L = 11 for Voyager cantly different from these at other longitudes.Among the
1 outbound when Saturn was in the solar wind. Since the solar
possibilitiesthat suggestthemselves,there is the chance of a
wind pressurewill determinethe nightsidefield elongation large magneticanomaly with variation of at least a factor of 2
which, in turn, will control the gradient formation processthat or more in the near surfacemagneticfield magnitude.Howdrives the centrifugal flute instability, solar wind control of ever, such variationsare not consistentwith Voyager magneSKR is expected.We note that in the limiting caseof total tometer observations[Connerneyet al., 1984] unlesshigher
cutoff of the solar wind the azimuthal symmetry of the mag- order moments are involved. An alternative explanation
netosphereand lack of tail lossshouldyield densitygradients would involvean ionosphericplasmadepletionof perhapsa
that are below the thresholdrequired for the centrifugalflute factor of 10 relatedto somefixed atmosphericprocess.Future
instability [Goertz, 1983].
research
may revealhowonelongitude
maintains
a (.Dpe/LOce
Solar wind control in the scenariopresentedhere is not the ratio significantlydifferent from all others.
CURTIS ET AL.: CENTRIFUGALFLUTE INSTABILITYAND SKR
Although the wave observationsof R. P. Lepping et al.
(unpublished manuscript, 1986) support the mechanism that
we have proposed,the data from the Voyager encounterswith
Saturn do not allow us to uniquely determine that the proposedmechanismis the only one operative.Another potential
mechanisminvolves an extensionof a processthat was originally put forward by Cornwall et al. [1971] to explain the
observationsof stable auroral red arcs (SAR arcs) at the earth.
In the picture presentedby Cornwall et al., the outward expansion of the plasmasphereduring the recovery phase of
geomagnetic substormsinto the ring current gives a density
ratio of cold plasmasphericplasma to energetic ring current
ions favorably to the generationof ion cyclotronturbulence.A
large fraction of the turbulenceenergyis then transferredinto
electron heating via electron Landau damping. The resulting
external flux would be directed into the ionosphere via fieldaligned electron fluxes.In the caseof Saturn, the ring current
lies well within the outer edge of the plasmasheetand hence
would not be involved in a processanalogousto that at earth.
However, within the framework of the centrifugal flute instability discussedhere, we expect the mixing of hot outer magnetosphereplasma with that of the colder plasmasheetto resemblethat of the ring current-plasmaspheremixing discussed
by Cornwall et al. [1971] and the study of artificial cool
plasma injected into the ring current [Cornwall, 1972]. The
situation may be then favorable to additional field-aligned
heating of pick-up electrons and could power the observed
nonthermal
radio
emissions
at Saturn.
Further
data
is re-
10,993
Finally, solar wind control of the SKR is an indirect conse-
quence of configurational changes in the Saturnian magnetosphereinducedby solar wind pressurevariations.
Acknowledgments. We would like to thank T. W. Hill of Rice
University, C. K. Goertz of the University of Iowa, M. L. Kaiser,
M.D.
Desch, J. K. Alexander, and J. E. P. Connerney, all of
NASA/GSFC for their helpful suggestionsand comments.We would
also like to thank S. Myers for the preparationof the manuscript.
Finally, we would like to thank both reviewers for their comments
and advice.This work was supportedby the NASA Program in Planetary AtmosphericResearchRTOP 154-60-80-32.
The Editor thanks D. D. Barbosa for acting as AssociateEditor
and A. Hasegawaand B. Tsurutani for their assistancein evaluating
this paper.
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role in the formation of the free energy source or sources
which drives the SKR.
SUMMARY AND CONCLUSIONS
We have shown that as a result of centrifugal force due to
Saturn's rotating magnetosphere and satellite-derived mass
loading, MHD waves are produced as the result of the centrifugal flute instability [Goertz, 1983; R. P. Lepping et al., unpublishedmanuscript, 1986] which can acceleratefield-aligned
electronsto energiesof several keV. The means of converting
the field-aligned MHD wave free energy to field-alignedelectron energy involves the wave-mode conversion mechanism
originally due to Hasegawa [1976]. The conversion of the
MHD wavesto kinetic Alfven wavesallows in turn a coupling
of energy from the macro or MHD scale to the micro or
kinetic scale. Given that the plasmasheetboundary density
gradient is expectedto be steepestdue to photoionization of
exospheric neutrals near the noon meridian, SKR will be
strongestthere.
The field-aligned electron fluxes can then generate kilometric radiation in the right-hand extraordinary mode as predicted by Wu and Lee [1979] provided that the electron
plasma frequencyto electron gyrofrequencyratio is sufficiently
small.
Curtis, S. A., Possible nightside source dominance in nonthermal
radio emissionsfrom Uranus, Nature, 318, 47-48, 1985.
Desch, M.D.,
Evidence for solar wind control of Saturn radio emission, J. Geophys.Res.,87, 4549, 1982.
Desch, M.D., Radio emission signature of Saturn immersions in
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and A. B. Galvin, Cometary pick-up ions observednear GiacobiniZinner, Geophys.Res. Lett., 13, 251-254, 1986.
Goertz, C. K., Comment on "Longitudinal asymmetryof the Jovian
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and the periodicescapeof energeticparticles"by T.
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(Received March 6, 1986;
revisedMay 22, 1986;
acceptedJune 20, 1986.)