The source of Saturn’s periodic radio emission

David J. Southwood1,2,3 and Margaret G. Kivelson3

Received 6 October 2008; revised 28 January 2009; accepted 20 May 2009; published 1 September 2009.

[1] This paper proposes a model of the link between the pulsing radio signals fromSaturn (Saturn kilometric radiation), known to be most intense on the morningside, andthe rotating magnetic (cam) field structure identified on low-latitude orbits. On thehigh-inclination orbits of the Cassini spacecraft in late 2006, large-scale azimuthaldeviations of the magnetic field, corresponding to upward field-aligned currents, havebeen detected in the late morning sector. There is evidence that their intensity ismodulated by the rotating cam magnetic field, the periodic field structure seen deeperinside the magnetosphere. It is proposed that the local intensification of field-alignedcurrents that gives rise to the modulation of the radio signal occurs in conjunction withthe passage through the morning magnetosphere of the swept-forward sector of theperturbation field imposed beyond the shells that carry the cam currents. Themorningside magnetosphere is unique as a region in which not only does themagnetopause compress rotating plasma inside the magnetosphere but also magneticshear develops between the magnetic shells of the middle magnetosphere and an outerregion of magnetic field swept back by the solar wind. The link between radio pulsing andmagnetic rotation arises through the interaction of the cam currents with structures uniqueto this part of the magnetosphere.

Citation: Southwood, D. J., and M. G. Kivelson (2009), The source of Saturn’s periodic radio emission, J. Geophys. Res., 114,

A09201, doi:10.1029/2008JA013800.

1. Introduction

[2] A most remarkable feature of Saturn’s magnetosphereis the slow variation of the period of the pulses of radioemissions in the kilometric band. Saturn’s kilometric radiowave emission (SKR) was discovered in the Voyager era[Kaiser et al., 1980]. The power was found to varyperiodically in intensity [Desch and Kaiser, 1981] at aperiod initially supposed to be that of the rotation of thedeep interior of the planet. The source of the modulationwas puzzling. Azimuthal asymmetry of the planetary mag-netic field could account for changing intensity of radio-frequency emissions as it does for the analogous higher-frequency radiation from Jupiter [e.g., Gulkis and Carr,1966]. The internal field of Saturn appears nearly axisym-metric [Connerney, 1993; Giampieri and Dougherty, 2004],with no more than 1� of tilt relative to the spin axis, so thecause of the modulation has remained mysterious.[3] If the radio modulation is indeed related to the rotation

of the interior, observable changes of the period would notbe anticipated on the scale of a year, as is seen. Ulysses datafrom 1994 to 1996 first revealed that the period varies by oforder of 1% per year [Galopeau and Lecacheux, 2000], a

rate far too large to arise from changes in the rotationperiod of a body with massive inertia. Subsequent analysisof magnetic field data in Saturn’s magnetosphere fromVoyager spacecraft flybys demonstrated that the magneticfield also varies periodically [Espinosa and Dougherty,2000; Espinosa et al., 2003a, 2003b]. The periodic radioand magnetic signals have both been present continuouslysince 2004 in data from the Cassini Saturn Orbiter and theperiods of the two signals vary in the same manner [Gurnettet al., 2007; Andrews et al., 2008]. The base period of theSKR was on the order of 10.67 h in the Voyager epoch[Desch and Kaiser, 1981] and has been on the order of 10.8in recent years [Giampieri and Dougherty, 2004; Giampieriet al., 2006; Kurth et al., 2007, 2008]. However, while thesource of the radio signal pulses appears not to rotate withSaturn [Warwick et al., 1981], the magnetic signal is arotating structure [Southwood and Kivelson, 2007].[4] In this paper we start from the hypothesis that the

SKR pulses and the rotating cam magnetic field have acommon source. We seek to identify a mechanism that linksthe modulation of the radio pulses with the rotating mag-netic signature and accounts for observed features of theSKR signals such as the reported likely source location in apreferred local time sector (late morning) at latitudes >70�and modulation by the solar wind [Galopeau et al., 1995;Cecconi and Zarka, 2005; Zarka et al., 2007]. The source ofthe slow evolution of the periods of the linked phenomenahas yet to be identified (but see suggestion by Dessler

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1European Space Agency Headquarters, Paris, France.2Physics Department, Imperial College, London, UK.3Institute of Geophysics and Planetary Physics, University of

California, Los Angeles, California, USA.

Copyright 2009 by the American Geophysical Union.0148-0227/09/2008JA013800$09.00

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[1985]), and we make no firm proposal here, although weraise the issue at the end of the paper.

2. Heuristic Idea

[5] We start our investigation of the link from a heuristicidea. We envision schematically a phenomenon in which arotating object without axial symmetry (e.g., a cam) period-ically strikes a fixed site where a pulse is excited as shown inFigure 1. How can the magnetosphere generate such aninteraction? For the rotating element we appeal to the camcurrent system described by Southwood and Kivelson [2007]and suggest that the rotating asymmetric phenomenon thatmodulates SKR is a magnetic cam with m = 1 axialasymmetry. Let us assume that the fixed site is also a

magnetic feature. What magnetic ‘‘interaction’’ might causeradio emissions to peak? It is believed that the SKR radiosignal is generated by a cyclotron maser process [Wu andLee, 1979; Pritchett and Strangeway, 1985] in regions whereenergetic electrons precipitate along field lines [Galopeauet al., 1989]. Precipitation of electrons is common inauroral regions where field-aligned currents flow out ofthe ionosphere at relatively high latitudes, possibly on fieldlines threading bright auroral spots [Kurth et al., 2005].Thus, one would expect to find a relation between the SKRphase and the intensity of upward field-aligned current inthe morning sector. Correspondingly, if the cam currentsgovern the SKR intensity, one should find a relationshipbetween the phase of the cam rotation and the intensity offield-aligned currents in the morningside magnetosphere atdistances that map to high invariant latitudes where theytrigger SKR.[6] Let us consider why the currents driving SKR might

be localized in the morning sector. Field-aligned currentarises where there is a shear in a magnetic field transverse tothe background field. Magnetic shear is present in themorning sector of the outer magnetosphere, becominggreatest on magnetic shells separating flux tubes that extendback into the tail relatively close to the magnetopause froman inner region in which flux tubes bend little out ofmeridian planes. This magnetic shear is induced by aviscous interaction between the solar wind and the plasmainside the magnetopause. In a slowly rotating system suchas the magnetosphere of Earth, the antisolar flow driven bythe solar wind drags outer magnetospheric flux tubes anti-sunward in both the morning and evening sectors with littleasymmetry about noon. At Saturn and Jupiter, however,rapid rotation results in considerable asymmetry. On themorningside the drag of the solar wind reinforces corotationlag in the outer magnetosphere, leading to strong azimuthalshear of the field [Galopeau et al., 1995]. On the evening-side, the drag must compete with corotation lag and the shearis not large. The morning-afternoon asymmetry arising fromthis process has been confirmed at flanks of the Jovianmagnetosphere [Khurana, 2001] as shown in Figure 2. AtSaturn, as at Jupiter, the flux tubes in the rotating magneto-sphere are swept back as their equatorial crossings approachthe morningside and the sweepback manifests a strongmorning-afternoon asymmetry.[7] The morning-afternoon asymmetry of Saturn’s field is

evident in Figure 3 in which the azimuthal component of themagnetic field measured by the Cassini (FGM) magnetom-eter [Dougherty et al., 2004] is plotted for four orbits thatcross from high southern to northern latitudes on thedayside. The plots showing the spacecraft position in radialdistance and invariant latitude versus local time appear tocontain single traces because the orbits differed little. Thusthe features of the azimuthal field that are similar in alltraces characterize the background magnetic structure. (Wewill return to discuss the dips near 1100 LT that differgreatly from orbit to orbit.) If the field were swept backsymmetrically in the morning and afternoon, one wouldexpect the sign of B8 to change across noon (as well asacross the equator), and its magnitude to be symmetric. Theorbits cross the equatorial plane at �1330 LT, so if thesystem were morning-afternoon symmetric, one wouldexpect that similar patterns of B8 variation versus invariant

Figure 1. Schematic of a mechanical cam whose rotationdelivers a periodic impulse to a fixed plate. This sequenceof images should be viewed cyclically from top to bottomand again from top to bottom. The illustrated processgenerates an impulse at the same spatial location once eachrotation. It is presented as an analog to the process throughwhich Saturn’s rotating magnetized plasma can driveperiodic SKR radiation.

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latitude would be detected inbound and outbound becausethe same shells are encountered in the afternoon northernhemisphere as in the morning southern hemisphere. Instead,despite some variability, the magnitude of B8 is in the rangeof 5–15 nT in the morning hours but averages only 1 or 2 nTa few hours after noon. Within the morning sector, asCassini moves from high to low southern latitude, B8

systematically decreases; that is, the field is sheared. Wherethe field is sheared, field-aligned currents are flowing. Thecurrent system generated by the solar wind–induced sheardiffers in an important way from the cam signal. The twosystems have different latitudinal symmetries. The solarwind stress bends magnetic flux tubes out of meridianplanes –with the primary force/stress applied near theequator. The largest bends in the field are found near theequatorial crossings of the flux tubes, but the azimuthal andradial magnetic perturbations vanish there; that is, the senseof associated field-aligned currents reverses at the equator.In contrast, Southwood and Kivelson [2007] have shownthat neither the azimuthal (B8) and radial (dBr) transversefield perturbations that characterize the cam nor the field-aligned currents that generate those perturbations changesense across the equator. Any mechanism linking camrotation with the pulsing of SKR that invokes solar wind–induced stress effects must take account of the lack ofshared symmetry with respect to the equator. Evidently asthe cam currents rotate, the currents imposed by solar wind

stress will alternately add to (or oppose) the cam currents inone NS hemisphere while opposing them (or adding tothem) in the other.[8] Additional north-south asymmetry of stress-induced

shear arises from fact that the conductivity of the northernand southern ionospheres differ slightly except at equinoxas a consequence of the tilt of the rotation axis relative to theorbital plane. During the Cassini epoch thus far, the rotationaxis tilt has been that of southern summer. Accordingly, thesouthern high-latitude ionosphere on the morningside hasbeen in sunlight with a higher electrical conductivity thanthe shadowed northern hemisphere ionosphere. The con-ductivity difference is likely to be on the order of a factor of4 (S. W. H. Cowley, private communication, 2008). In sucha situation (here we focus on 2006) the sunlit southernionosphere exerts a larger drag on magnetospheric fluxtubes than does the northern ionosphere. In a dynamicsituation, the northern ionospheric feet of flux tubes wouldmove faster than those in the south. In a quasi-staticsituation where the ionosphere is rotating more or less withthe neutral atmosphere, the displacement of the northernfoot from its notional meridian in response to solar winddrag would be greater than the displacement of the southernfoot and one would expect the currents flowing into theionospheres to differ.[9] The above discussion suggests that an analysis of

magnetic shear effects, or correspondingly of sources offield-aligned currents in the magnetosphere, may hold a clueto the action of a rotating cam magnetic signal in generatingSKR. Here we use Cassini magnetometer data acquiredduring the Cassini high-inclination orbits in 2006 to detectin situ the large field-aligned currents that we believe are thelikely source of the radio signal. We develop a model thataccounts for a peak intensity radio signal coming from thesouthern hemisphere (at the present Saturn season, i.e.,southern hemisphere tilted toward the Sun) and from thelate morning sector local time (LT) sector [Galopeau et al.,1995]. The fundamental role of solar wind stresses, bothpressure and drag, in the mechanism means that the reportedmodulation of the SKR signal by the solar wind [Galopeauand Lecacheux, 2000; Zarka et al., 2007] is explainednaturally. Sadly, as noted in the conclusion, the work bringsus no further to identifying the reason for the slow variationin period or the m = 1 azimuthal structure.

3. Magnetic Periodicity at Low Latitudes

[10] We start with a brief review of the relevant observa-tions of the low-latitude magnetic perturbations modulatedat a period close to that of planetary rotation. Just prior toCassini’s arrival at Saturn, Espinosa and Dougherty [2000]and Espinosa et al. [2003a, 2003b] reanalyzed the Pioneerand Voyager magnetic data and reported periodic modula-tion of the magnetic field components. The signal did notdecay with distance from the planet as it should for aninternal source. Moreover, the signal polarization was alsoinconsistent with an internal source. Espinosa et al. [2003b]described the source as like that produced by a ‘‘magnetic’’cam shaft launching a signal into the distant magnetosphere,The arrival of Cassini in orbit around Saturn in mid-2004,showed that periodic signals are always present but the camsignal polarization (reported by Espinosa et al. [2003a]) is

Figure 2. Unit vectors showing the direction of themeasured magnetic field in Jupiter’s magnetosphere abovethe equatorial current sheet. Here the Sun is to the right, andthe view is down toward the equatorial plane. Note the clearmorning-evening asymmetry. A gray line at the tip of thedashed arrow calls attention to the area in which strongfield-aligned currents are inferred from the rapid variationwith radial distance of the field orientation. From Khurana[2001].

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seen only within a core region of the magnetosphere onmagnetic shells whose equatorial crossings are within aradial distance of about 10–15 RS of the planetary surface,ending roughly where the planetary dipole field ceases todominate the total field [Arridge et al., 2008a]. Clearperiodic magnetic signals are also detected beyond 12–15 RS (RS = Saturn radius = 60,268 km). The periodic signalscontinue out to very large distances (>30–40 RS) but with adifferent polarization. Their structure is nearly sinusoidal athigh latitude but is more irregular and much less sinusoidalat low latitudes.[11] Figure 4 is a plot from Southwood and Kivelson

[2007]. Figure 4 (top, middle) shows the two components ofthe perturbation magnetic field transverse to the offsetdipole: dBr, B8, measured during a consecutive series ofCassini equatorial passes between January and August2006. Here dBr represents the difference between themeasured Br and the SPV model field [Davis and Smith,1990] with an additional small empirical correctiondesigned to remove the effect of an equatorial ring currentfrom the dBr signal. If the oscillations evident in the plotwere imposed by rotation of a dipole (21,160 nT at theequator) tilted by 1�, near equatorial measurements would

reveal oscillations of B8 and dBr, with peaks of B8 leadingpeaks of dBr; the amplitude of oscillations in B8 wouldbecome as large as 4 nT at the equator at 4.5 RS and woulddrop to 0.37 nT by 10 RS. The oscillations of dBr would betwice as large. Both the amplitudes and the phase relationswould thus differ from the observations shown in Figure 4.[12] The data shown have 1 min time resolution but have

been filtered to smooth out short-period (<1 h) signals. Theordinate is not time but rather a phase introduced bySouthwood and Kivelson [2007] based on the time-varyingperiod defined by Kurth et al. [2007, 2008]. Like theposition of the spacecraft expressed in planetary longitude,the phase is taken to depend on both time and the spacecraftposition as if the planet/SKR source were rotating. Theordinate is thus the predicted phase at the spacecraft localtime in a system that rotates about the planet at the long-term period given by the Kurth-Lecacheux model SKRphase. We will refer to this modified phase parameter asthe ‘‘spacecraft-frame Kurth phase’’ or SFK phase. It differsfrom the longitude of a body in orbit around Saturn asdefined by Kurth et al. [2007, 2008] only by beingexpressed in radians and defined such that on the spacecraftit varies continuously. Except where the spacecraft comes

Figure 3. Plotted versus local time: the azimuthal field and position information for four separate butextremely similar high-inclination orbits in 2006. The spacecraft radial distance and invariant latitude(based on the SPV model field) are shown in the middle and bottom plots. Here ‘‘SPV’’ is theabbreviation introduced by Davis and Smith [1990] for their Saturn field model based on Pioneer andVoyager measurements. As Cassini moves from high to low southern latitude on the morningside, B8

systematically decreases; that is, the field is sheared. Although similar radial distances and invariantlatitudes are encountered in the afternoon, the amplitude of changes in B8 is much smaller. The data arecolor-coded with red, green, black, and blue representing the orbits with closest approach on 27 October,8 November, 20 November, and 2 December, respectively.

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very close to the planet this means it increases monotoni-cally, as it does on all orbits considered here. We prefer toidentify the parameter as a phase in order to avoid thesuggestion that meridian planes of the central body rotaterigidly. Figure 4 (bottom) indicates the spacecraft positionin local time (red trace) and radial distance (black trace).The data shown cover times when the spacecraft was withina radial distance of 15 RS. The orbits are almost repeatable.Periapses are all between 4 and 6 RS at local times between1100 and 1500 LT. The overall range of local time coveredis from 0400 LT through to the vicinity of midnight. Thespacecraft remains within a degree of the planetary equato-rial plane for six of the eight orbits and is within 15� for all.The peaks of the B8 signal are seen to be separated by fullcycles of SFK phase. The markers help reveal thequadrature relationship between radial and azimuthalcomponents.[13] The plot of Figure 4 illustrates not only the regularity

of the oscillations from orbit to orbit but also the orderingby SFK phase. There can be little doubt that at low latitudewithin 15 RS of the planet there is a rotating magnetic signalwith period given by the model SKR period. The amplitudeof the azimuthal component (typically �2 nT) does notappear to vary much with either local time or radialdistance. There is, however, some evidence of a decrease

in the amplitude of the radial component as one approachesthe planet.[14] Southwood and Kivelson [2007] developed a model

of the perturbation field. Because of the way that theperiodically varying transverse signals change characteracross magnetic shells crossing the equator at a distanceon the order of 10–15 RS, they speculated that the sourcecurrents for the cam flow on magnetic shells (of thebackground field). The periodically varying azimuthal mag-netic perturbation inside of the current-carrying shells issymmetric about the magnetic equator, which requires thatsheets of field-aligned current couple the northern andsouthern hemispheres. The intensity of the model currentvaries around the planet like the sine of the azimuthal angle.Figure 5 illustrates the form of the field that would beproduced by the postulated currents flowing on a dipoleshell L = 12. Near the equatorial plane and within thecurrent-carrying shell, Figure 5 (left) shows that the pertur-bation field differs little from a uniform field. As the shellrotates, an observer would detect quasi-sinusoidal oscilla-tions in dBr and B8 with dBr leading in phase, close to whatis observed near the equator and inside r = 11–15 RS.Outside the shells and near the equator, Figure 5 (right)shows that the perturbation field approximates that of anequatorial dipole. This dipole adds to the axial dipole of

Figure 4. (top, middle) Data for B8 and dBr for eight consecutive low-latitude Cassini passes inside of15 RS. (bottom) Radial distance and local time of Cassini versus SFK phase (defined in the text) for thesame passes. The origin of phase has been selected to align with the peak of B8 near closest approach oneach orbit. Vertical lines indicate full cycles of SFK phase defined in the text. From Southwood andKivelson [2007].

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Saturn’s internal field so that the total field resembles that ofa rotating dipole tilted with respect to the spin axis. Detailsof the current closure through the ionosphere are notspecified.[15] Southwood and Kivelson’s [2007] conclusion was

based on examination of data from 2004 to mid-2006,during which time the spacecraft orbit was not greatlyinclined. The range of magnetic latitudes covered wasalways within 30� of the magnetic equator and for muchof the time Cassini remained in an almost equatorial orbit.Subsequently, Andrews et al. [2008] showed that there isalso a compressional (dBq) component in the perturbationsignal thus complicating the overall picture of the source butnot changing the necessary presence of current along thebackground field.

4. Magnetic Data From High-Inclination Orbitsand Estimating Cam Phase

[16] In the last quarter of 2006, Cassini went into a seriesof highly inclined orbits. The critical data for this papercome from these orbits and from the late morning–noonsector at times when the spacecraft passed through the high-latitude southern hemisphere. Large transverse field dis-placements were detected as the spacecraft crossed magneticshells whose invariant latitude (calculated from the SPV fieldmodel) was on the order of 70�, meaning the spacecraft wason precisely the magnetic shells identified by Southwood andKivelson [2007] as likely to carry the cam source field alignedcurrent. However, the rapid motion of the spacecraft inlatitude and radial distance precludes a firm association ofthe perturbation with the low-latitude cam.[17] Complicating matters is that although the cam peri-

odicity is remarkably steady over the long-term there is jitteron a cycle by cycle basis. Kurth et al. [2008] report achange in the sense of the secular change of phase in the

radio signal starting in mid-2006 and including the period ofinterest here. Kurth et al. [2008] even find evidence for thepresence of a second, shorter radio pulse period. The newlydetected changes in period are, however, small: on the orderof one cycle in 1 year [see, e.g., Kurth et al., 2008, Figure 1].Our purpose here is not to track precisely these changes inperiod. Rather, we are interested in the potential relationshiprelation between the high-latitude magnetic signal and theequatorial signal at the time of the high-latitude encounter. Inany one cycle the uncertainty introduced by the long-termtrend identified by Kurth et al. [2008] is on the order of onedegree. This is less than the real-time jitter observed in thesignal. Accordingly, we ignore it and use a very direct way ofestimating the phase of the cam signal at the time of thespacecraft passage through the currents crossed at �70�invariant latitude (evaluated in the southern hemisphere).[18] We adopt a period in the 10.8 h range identified by

Kurth et al. [2007, 2008] and deduce the phase by extrap-olating back from the apparent phase of the cam signalidentified near the equator about 6 h after detecting thehigh-latitude current. Using data from all six passes forwhich the phase of the current system can be inferred(plotted in Figures 6–10), we show that the largest trans-verse magnetic perturbations occur when the spacecraftcrosses the high southern latitude portions of the magneticshells on which the cam currents are flowing upward fromthe southern ionosphere. The amplitudes of the large trans-verse field displacements are bigger (by a factor of 2–3) andmore abrupt than the cam signals detected at lower latitudes.[19] Figure 6 is a plot of data from a pass at high southern

latitude in November 2006. Figure 6 (top) shows the Br andBq field components versus time. A dashed trace, whichrepresents an axisymmetric offset model dipole field (SPVmodel), lies too close to the measured values to be resolved,that is to say, the dipole model describes these twocomponents of the field to within 10 nT. Figure 6 (bottom),

Figure 5. Field lines of the cam perturbation field arising from currents that vary as sin f, where tanf = y/x, flowing on L = 12 dipole shells, (left) in the z = 0 plane and (right) in the y = 0 plane. The regionwithin the current-carrying shell is gray. The curved arrow (left) and the in and out symbols (right) indicatethe sense of rotation. Note that the field approximates that of an equatorial dipole outside of the whitesurface and that the equatorial field is approximately uniform within that surface.

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Figure 6. Magnetic field near periapsis on the significantly inclined orbit of 8 November 2006. (top)Measured and model (dipole) Br and Bq components versus time. (bottom) Parameters DBr, DBq, and B8

versus time. Labels show UT, radial distance from the mass center of Saturn in RS, latitude, magneticlocal time, nominal dipole L shell, invariant latitude, and SFK phase (defined in the text) based on thework of Kurth et al. [2008]. A solid vertical marker identifies the equator crossing of the orbit. Gray linesidentify zeroes of B8.

Figure 7. Same as Figure 6, showing DBr, DBq, and B8 on 8 November 2006, but plotted versus SFKphase. Labels and the gray and solid black vertical markers are as in Figure 6. Dashed vertical markersare separated by half cycles of the SFK phase.

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which has a different ordinate scale, shows the third fieldcomponent, B8 and DBr and DBq, the differences betweenthe measured radial and southward field components andthose predicted by the SPV dipole field model for whichB8 = 0.[20] Before turning our attention to the B8 component, we

consider DBr and DBq. Separating the effects of rapidspacecraft motion in both latitude and local time requirescare but some features seem clear. The DBr signal is

dominated by a reversal in sense, almost, but not quite,symmetrical in latitude. Such a signature is consistent withthe effects of a ring current displaced slightly with respect tothe planetary equator, the offset naturally understood as thecombined effects of the antisolar tilt of the planetary axis inthe early Cassini years and the asymmetry of the interactionwith the solar wind [Hansen et al., 2005; Arridge et al.,2008b]. The ring current is also partly responsible for thedominant features of the DBq signature. There is a clear

Figure 8. Same as Figure 6 (bottom plot) but for a pass on 2 December 2006. Dashed vertical markersare separated by half cycles of the SFK phase.

Figure 9. Same as Figure 8 but for a pass on 20 November 2006.

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depression in this field component throughout the daysidepassage, but the field depression partially recovers in aregion centered on midday and extending over about anhour on each side. Fuller analysis is reserved for elsewhere,but one can note that a similar effect is seen on other passes.The changes near noon are presumably related to themagnetic compression associated with Chapman-Ferraro(CF) currents on the subsolar magnetopause. The presenceof the ring current and CF currents greatly affects how fieldlines map from the spacecraft at high latitudes to theequator, making suspect estimates of L shell based on adipole field model. On the other hand, in following a field

line from a high-latitude observing point to the ionosphere inthe same hemisphere, the contribution of the fields ofexternal sources is fractionally small at the starting point andbecomes increasingly unimportant with approach to 1 RS.The dipole field model can be used with little uncertaintybecause the deviation between the model field and the dataremains less than 11% (reached at the left-hand and right-hand edges of Figure 6).[21] Ancillary data on spacecraft position are shown

under Figure 6. Labels below correspond to the time tickmarks on the abscissa. During the interval shown (1 day),the spacecraft travels in local time from dawn to dusk. The

Figure 10. Parameters DBr, DBq, and B8 plotted versus UT as in Figure 8. Spacecraft latitude isoverplotted in gray with scale to the right. Data are for passes on (top) 27–28 October 2006 and (bottom)14–15 December 2006.

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spacecraft radial distance varies from�8 RS (dawn) to 4.7 RS

at closest approach near 1300 LT returning to �8 RS neardusk. Nominal L shell and invariant dipole latitude (calcu-lated for a centered dipole moment) are shown in the secondand third rows of data. The solid vertical marker indicates thelocation of the magnetic equator where the spacecraft passesfrom the southern to the northern hemisphere. The gray linesshow where B8 switches sign. To the left of the left-handdotted marker and to the right of the right-hand dottedmarker, the field perturbation is such that the total field isbent out of meridian planes. The signs of the radial andazimuthal fields change as the spacecraft moves from thehigh-latitude regions on the morningside (left of the firstdotted marker) to the high-latitude regions on the after-noonside (right of the second dotted marker). In bothregions the field twists out of meridian planes in the senseimposed when equatorial plasma flow lags behind corota-tion. As described above in the discussion of the effects ofsolar wind stress, the degree of bending differs on themorningside and eveningside.[22] What is most dramatic in Figure 6 is the steep

gradient in B8 seen as the spacecraft moves equatorwardfrom high southern latitudes on magnetic shells correspond-ing to an invariant southern latitude of 71–75�. (Mapping iscarried out along the SPV model dipole field.) The secondgray line is at approximately the same invariant southernlatitude in the late afternoon (1630 LT) where a B8

perturbation is again present. Here, however, the changein B8 is far smaller than in the morning sector.

5. Relationship to the Phase of the Cam Field

[23] The morningside jump in B8 takes place on nominaldipole L shells that lie between 9.5 and 15 (invariant dipolelatitudes of 71–75�; that is, roughly the range on whichSouthwood and Kivelson [2007] located currents proposedas the source of the cam magnetic signals detected at lowlatitudes in the inner magnetosphere. The relationshipbetween the signals observed at high and low latitudes isilluminated by Figure 7, which presents the data of Figure 6(bottom)differently.Figure7showsthreetraces,DBr,DBq,B8,B8, here plotted versus the SFK phase [Kurth et al., 2008]instead of universal time. Once again ancillary data onspacecraft position (radial distance, R, model dipole L shell,and local time, LT) are given below the abscissa at eachtick. Because the tick marks are evenly spaced in SFK phaseand the angular speed of the spacecraft is not uniform, thetime marks are not evenly separated. The SFK phase,indicated with dashed vertical markers, is measured inradians (arbitrarily taken as 2p at the equator crossing,implying that 0 corresponds to the spacecraft location on8 November at 1240 UT).[24] Comparison of the plots of Figures 7 and 4 illustrates

that at similar radial distances, B8 (green trace in Figure 4and solid trace in Figure 7) varies differently as thespacecraft moves rapidly in latitude from the way it varieswhen the spacecraft remains at low latitude in the innermagnetosphere. The dominant feature of the B8 signature inFigure 7 is the reversal in sense detected on 8 Novemberbetween 1932 and 2142 UT. Assuming that the changedetected is due to the spacecraft’s crossing a spatial gradientin B8, the most likely source of the abrupt perturbation is a

sheet of field-aligned current. The sense of current requiredis upward from the southern hemisphere ionosphere. Itwould thus be associated with downward moving auroralelectrons, also likely to be the source of SKR emissions.[25] As noted previously, the 2p phase marker (Figure 7,

black vertical line) has been placed where the spacecraftcrosses the planetary equator (as is evident from the radialdistance matching the L shell value). At 5 RS radial distancethis would place the spacecraft firmly within the cam fieldregion. With this in mind, note that the B8 signal has a peak,as would be found (radially) inside of the region in whichthe cam current flows into the southern ionosphere. As theamplitude of the peak (�2–3 nT) is in the range of the camperturbation (as is evident from Figure 4), it is sensible toidentify the cam as the source of the low-latitude B8 signal.The dashed vertical marker labeled p is separated by half acycle of SFK phase. The steep gradient, here encountered ata latitude near �50�, is detected at a phase at which a troughof the B8 cam signature (corresponding to current flowingupward from the southern hemisphere) would be observedinside of the current-carrying shells at the local time sectorof the spacecraft (�1100 LT). At the high latitude ofCassini, the field rotation encountered has a strong negativeB8 signature on the low-latitude side, as would be expectedif the spacecraft crossed the cam current shells. However,the peak negative amplitude (�16 nT) is perhaps 6–8 timeslarger than that of the low-latitude signal and so one cannotmake any immediate connection.[26] A second very similar example of a field-aligned

current signature from 2 December 2006, day 336 ispresented in Figure 8. Here again increments of the rota-tionally adjusted SKR model phase are marked at multiplesof p. Ticks are annotated with time, L shell, invariantlatitude, and spacecraft local time. In this case, a peak ofB8 is encountered at low latitude in the early afternoon(1300–1400 LT), just before the spacecraft crosses theequator in the vicinity of the dashed vertical marker. Amarker to the left in Figure 8 marks a half cycle earlierwhen, at the local time of the spacecraft, a trough in B8 cammagnetic signal would be observed at low latitudes. Justbeyond the marker, B8 decreases precipitously at a locationwhere the total field is 270 nT in the radial (dominant) andtheta directions. Because of the recurrence of suchsignatures in roughly the same part of the magnetosphereon multiple orbits (i.e., compare Figures 6 and 8), we ruleout any short-term time-dependent interpretation and arguethat the spacecraft crossed a sheet of field-aligned currentflowing out of the southern ionosphere. The current-producing gradient flows over a range of L shells from 11to 15 near 1000–1100 LT at latitudes near �50�. Onceagain the large negative B8 excursion matches the sense ofthe cam current expected in this sector at low latitudes butagain the local amplitude of B8 is 6–8 times larger than theamplitude of the cam at near-equatorial latitudes. A linkbetween the signatures may appear circumstantial.[27] If the high southern latitude signatures are imposed

by the rotating cam currents, the cases of Figures 7 and8 correspond to encounter with the current system near thepeak of the upward current. Evidently, a useful test of thelink to the cam current would be to examine a case in whichCassini passes through the high southern latitude morningLT sector at a different phase of the low-latitude B8 signal.

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Fortunately such examples exist. Figure 9 shows measure-ments for a pass on 20 November 2006 quite similar in localtime and latitude to the passes examined in Figures 6–8.However, in this case the spacecraft crosses the current-carrying L shells at the SFK phase of the peak in B8 at lowlatitude, and we use this peak to infer the phase. The right-hand dashed vertical marker is placed where, shortly afterpassing through the equator just sunward of 1400 LT, thespacecraft encounters a trough in B8 rather than the peakseen in a similar location in the two previous examples. Onethen infers that half a cycle earlier, a spacecraft movinginward would cross a downward cam current and thusobserve first negative and then positive B8. In Figure 9, halfa cycle before the equator crossing, as indicated by thedouble-headed arrow, the spacecraft is at high southernlatitude (again near �50�) and a gradient in B8 isencountered. Again the range of nominal L shells wherethe gradient occurs is 10–13 (�72–74� dipole invariantlatitude). The local time is between 1030 and 1130 LT. Onthis occasion, unlike the two previous examples, B8 doesnot change sign, and the amplitude its change is relativelysmall. In part, this is because the amplitude of thedisplacement in B8 present on the magnetic shells polewardof the field gradient is smaller than in other cases presented,possibly as a result of unusually low solar wind dynamicpressure. In addition, the sign of B8 does not change on theequatorward side of the gradient, consistent with theinterpretation of the observed signature as a superpositionof the sweepback current and a downward cam current.

6. Further Examples

[28] In this survey of passes from the last quarter of 2006there are only two additional passes that crossed the high-latitude regions where field-aligned currents can be iden-tified and also crossed the equator inside of the camcurrent region where B8 can be used to establish the SKRphase. The survey cuts off on 15 December 2006 before theend of the year because on the last pass in late Decemberperigee had moved so far out that the spacecraft did notpenetrate the cam current region. The top and bottom partsof Figure 10 show contrasting situations. In both cases thespacecraft crossed a localized upward current sheet near72� invariant latitude, as evidenced by a sharp rotation inwhich the B8 component reversed sign. We suggest thatduring the event plotted in Figure 10 (top), the spacecraftpassed through the cam current system at a time when it wasflowing upward but at a phase at which its intensity was lessthan its peak intensity. In the event plotted in Figure 10(bottom), we suggest that we passed through the upwardcurrent near its peak but on a day when the current intensitywas overall unusually weak, as we next discuss in detail.[29] On 27 October 2006 (Figure 10, top), the spacecraft

encounters a peak in B8 (at the dashed vertical markerlabeled 2p) just before its equatorial crossing (solid line). Alarge negative gradient of B8 at high latitude occurs about aquarter of a cycle earlier. This signature of a field-alignedcurrent is centered near the nominal 73� invariant latitudeshell. Following our interpretation of the large field rotationat this invariant latitude as a crossing of the rotating camcurrent system, the phase that we have identified implies acurrent upward from the south, and thus a localized negative

gradient of B8 versus time. Because the crossing occursroughly 1/4 of a cycle after the passage of the peak of theupward current, the magnitude of the jump is expected to besmaller than in the events of Figures 7 and 8. The relativemagnitudes are consistent with this expectation. In the casesof Figures 7 and 8 the negative excursion was 6–7 timesbigger than the equatorial cam amplitude. Here it is only 2–3 times larger that the peak equatorial amplitude, that is, thesignature is linked to a crossing of the rotating cam currentsbut not at their peak intensity.[30] The final example is from 14 December 2006

(Figure 10, bottom). The azimuthal field in the swept-backregion at the highest southern latitudes is weaker than inthe other cases we have examined and indeed, once theequator is reached, the cam signal is somewhat weaker aswell (<2 nT amplitude). On this pass, the spacecraft latitudechanges slowly, and consequently a complete cycle of thelow-latitude cam is seen in the plot, although the near-equatorial peak in B8 is obscured by the presence of ahigher-frequency disturbance with an amplitude larger thanthe cam signal. Two substantial B8 gradients are detected athigh southern latitudes on this pass. The large perturbationcentered on day 348 at 1850 UT does not readily relate tocam currents. It occurred at higher L value and invariantlatitude (�20 and �77�) and at earlier local time (�0900LT) than any of the other events in the 3 month interval thatwe analyze here. The decrease is followed immediately byan increase almost to the original value of B8. Such asignature could arise if a sheet of field-aligned currentpassed back and forth across the spacecraft. It is possiblethat the cam current system moved back and forth in latitudebut it seems to us more likely that on this pass Cassini justbrushed a higher-latitude current system of the kinddiscussed by Bunce et al. [2008] and associated by themwith the open-closed field line boundary. Closer to Saturn,Cassini encountered a second large perturbation close to thelocations of the other events presented in this paper. Thegradient at �72� invariant latitude near the beginning of day349 occurs as the spacecraft passes from nominal L = 13 to11 at local time 1100–1200 LT and shows a negative fielddisplacement at its equatorward edge. This is consistentwith the phase of the cam currents deduced on the basis ofthe peak seen after the equatorial crossing (Figure 10, thirddashed vertical marker).[31] Before discussing what we have inferred from the

data presented above, it seems appropriate to explain whywe do not discuss observations made on Cassini orbits after2006. All of the passes that we have examined here start athigh latitudes in the southern hemisphere near 70–72�dipole invariant latitude (i.e., L shells beyond 14–15) onthe morningside, pass through the equator just postnoon andcontinue into the northern hemisphere in the afternoonsector. The orbits repeatedly traverse the same portion ofthe dayside magnetosphere. We do not continue into 2007because subsequent orbits do not cover the same latitudesand local times; the features encountered differ from thosewe have presented here. On orbits that followed in Januaryand February 2007, Cassini began to encounter field linesthat map (dipole mapping) to invariant latitudes as high as80�. Large field-aligned currents were detected in regionsnear noon but are unambiguously of different origin. Theyare observed in a region where the field was greatly

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depressed, evidence that Cassini penetrated regions of directparticle access at the dayside cusp. Some of the data wereacquired coincident with an HST observation campaignwith results reported in a paper by Bunce et al. [2008].

7. Inferences From Data

[32] In interpreting the signatures presented in Figures 6–10, we emphasize the unique field and plasma configurationof the outer magnetosphere in the morning sector. It seemslikely that the draped flux tubes encountered in the morningsector are closed; numerous passes from the early Cassiniorbits showed that the magnetopause at dawn was typicallywell beyond a radial distance of 14–15 RS [Arridge et al.,2006], flaring outward away from noon. In this part of themagnetosphere and south of the equator, B8 is positive andBr is negative, corresponding to a swept-back field. The fieldconfiguration is thought to result from antisolar drag by thesolar wind that acts to increase the draping of the fieldresulting from corotation lag. By contrast, in the afternoonsector the antisolar drag of the solar wind opposes corotationlag and flux tubes bend little out of meridian planes.[33] Near L = 15 on the morningside, as the spacecraft

moves inward, there is a change of bend-back (see Figure 3)where flux tubes bent back by a combination of solar winddrag and corotation lag overlay the core region of themagnetosphere that contains the rotating cam field. Thechange of B8 (and not the dominant field components) asthe spacecraft moves inward implies that a sheet of field-aligned current is flowing upward from the southernionosphere. Analogous changes of field configuration inJupiter’s outer magnetosphere were also ascribed to field-aligned currents by Kivelson et al. [2002, Figure 7].However, in the case of Saturn, the observed change ofB8 in the high-latitude morning events we have identified issometimes more abrupt and larger than that arising from thechanged draping of the global field. The amplitude of theadditional change (and the intensity of the implied sheetcurrent) is consistent with being governed by the (local)SFK phase, which leads us to believe that the abrupt fieldrotations observed are related to the cam current system[Southwood and Kivelson, 2007]. The total currents flowingin the region of interest are largest when encountered in themorning sector at the phase corresponding to currentflowing upward from the southern ionosphere. At highlatitude (but not at low latitude), the spacecraft crosses theflux tubes carrying the cam current in a time short comparedwith a cycle of SFK phase, so we are able to identify achange of sign of B8 consistent with crossing the sheetcurrent. The largest negative displacements are those of8 November and 2 December, and these are roughly 6–7 times larger than the amplitude of the oscillating B8 fieldat the equator (2–3 nT [Southwood and Kivelson, 2007]).This increase in amplitude is consistent with the conver-gence of flux tubes with latitude. If the field-aligned camcurrents flow from ionosphere to ionosphere on dipolar fieldlines, as in the model, the amplitude of the B8 perturbationwill vary with latitude (l) as (cos l)�3. A �2–3 nTamplitude at low latitude would increase by a factor of 3.8and would thus correspond to an amplitude of �7–11 nT(or �14–22 nT peak-to-peak) at 50� latitude. The netchange in B8, which is observed to be as large as �30 nT,

can be considerably larger than the change that would arisefrom the cam current alone. More specifically, Figure 3shows that above 72� invariant latitude, B8 is on the order of15 nT, but it drops to ��5 nT below 70�. The change of20 nT adds to the signatures we have linked to cam currentsat almost the same location. It seems that jump of �30 nT inthe largest signatures that we have considered results fromthe superposition of currents from two different sources, acurrent fixed in local time that confines bend-back to theouter morning magnetosphere juxtaposed on the peak of therotating cam current. Where the amplitude of the rotatingcam currents is not at maximum, the net current is smaller.For the case of Figure 9, the high-latitude B8 that weinterpret in terms of antisolar drag by the solar wind is onlyabout 10 nT. The cam signal predicted would produce apositive change of B8 as Cassini moves across the current-carrying shell from high to low latitude. When this relativelysmall perturbation is superposed on the change of B8 relatedto bend-back, the draping of the magnetic field decreases butdoes not change sense. In other words, B8 does not reversesign but it is clear that there is still a current present.[34] The link of a portion of the strong current signature

to the swept-back configuration of the outer magnetosphereimplies that the solar wind can modify the characteristics ofthe current system. Whether the solar wind would merelyaffect the intensity of the currents or could also modify theperiodicity [Zarka et al., 2007] is a problem that we do notaddress here.

8. Summary and Proposed Scenario

[35] SKR is a pulsing signal whose dominant source islocalized and nonrotating; the magnetic cam signal is arotating signal. The signals are modulated at the sameperiod and show the same temporal drifts of that period.A simple mechanical analogy of the link between therotating cam and the pulsing radio source is shown inFigure 1, which demonstrates how a rotating object withlow-order azimuthal asymmetry can drive a localized peri-odic pulse. The feature providing low-order azimuthalasymmetry is the cam current system and the pulse repre-sents the periodic enhancement of SKR power. SKR isgenerated by energetic electrons precipitating along fieldlines linked to the high-latitude ionosphere. Those energeticelectrons are systematically present where intense upwardfield-aligned currents flow in the auroral zone [e.g., Evans,1968; Ergun et al., 2001]. The power in SKR shouldbecome most intense when the field-aligned current flowingupward from the southern morningside ionosphere is larg-est, and this happens every time the peak of the cam currentupward from the south rotates into the morning sector.[36] Our scenario for the link between SKR and the

magnetic field starts by envisaging that there is a coremagnetospheric region of magnetic shells that are pervadedby the rotating cam magnetic field signal. The regionextends to somewhere beyond 10–12 RS in the equatorialregion, at the boundary of the region in which a dipolar fielddominates [Southwood and Kivelson, 2007]. On magneticshells in the morning sector outside this region there is aregion of strongly swept back field. In the southern hemi-sphere the sweepback appears as a positive B8. It seems thatthe regions are distinct and that in the morning sector the

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swept-back region is pressed against the core region of themagnetosphere. The pressure at the interface increases withlocal time because plasma in the outer magnetosphereexperiences compression as it rotates from dawn to noon inresponse to the decreasing distance to the magnetopause (asdiscussed for Jupiter by Kivelson and Southwood [2005]).Between the outer and inner regions field-aligned currentsflow where the orientation of the azimuthal field changes.[37] The swept-back field is at best weakly modulated by

the magnetospheric periodicity and is always positive in thesouthern hemisphere. On the other hand, the cam magneticfield signal in the interior rotates fairly smoothly and thusgives rise to the regular passage of an azimuthal perturba-tion that adds significantly to the antisolar B8 of the outermagnetosphere once per cycle. Correspondingly, the field-aligned currents flowing on the boundary at L � 11–15 between the regions are modulated. In the southernhemisphere the peak field-aligned current flows upwardtoward the equator and thus would be associated withelectrons moving toward the southern ionosphere. Theelectrons are likely to be the source of the SKR signal. Theamplitude of the radio signal itself would be expected to beproportional to the current amplitude.[38] The scenario suggests a mechanism through which

the solar wind can affect the SKR signal [Zarka et al., 2007]as the degree of magnetic sweepback is likely to beassociated with solar wind stress. Moreover it also indicateswhy the morning sector is the likely source of the pulsedSKR signal. On the afternoonside the sweepback is less thanon the morningside as we have illustrated with Figure 3.This may be understood as a consequence of the fact that inthe postnoon sector rotation lag and solar wind drag forcesare opposed, thus reducing the tension produced in themagnetospheric field in the boundary layer. Moreover, asdescribed in an explanation of the morning-afternoon asym-metry of the plasma sheet in the Jovian magnetosphere[Kivelson and Southwood, 2005], flux tubes rotatingthrough the noon meridian should also be constricted bythe solar wind pressure in the subsolar region. Once pastnoon, a flux tube can expand as it continues to rotate and therotation is further aided by the net pressure of the solar windwhich is tailward and thus in the direction of rotation.[39] With the scenario given one might expect that the

SKR power would peak twice per rotation, once when thecam current flowing upward from the southern ionospherereinforces the currents that terminate field draping there andonce when the cam current flowing upward from thenorthern ionosphere reinforces the oppositely directed cur-rent that terminates field draping there. However, there aregood reasons to believe that currents flowing from thenorthern ionosphere would be weaker than those in thesouth because of the low conductivity in the northern winterseason. This would explain why the peak SKR radiationlinks most directly to the southern hemisphere source andwhy the peak emissions occur only once per rotation period.

9. Discussion and Conclusion

[40] The work here throws considerable light on thelinkage between the two dominant periodic features ofSaturn’s ionized environment, the radio pulsing and the

rotating cam signal. It casts little light on the most intriguingproblem of all, what exactly controls the periodicity of thesignals. It is generally accepted that there is a small secularchange in the period, a variation on the scale of a minute ayear. In early 1980s at the time of the Voyager flybys theperiod was determined using the radio wave instrument onthe spacecraft as 639 min + 24 ± 7 s [Desch and Kaiser,1981]. Subsequently, in the 1990s, the very sensitive radioinstrument carried by the Ulysses spacecraft revealed achange in the Saturn radio emission frequency [Galopeauand Lecacheux, 2000] and indeed over time there was aslow variation such that by the time of Cassini’s arrival atSaturn in 2004 the period was on the order of 647 min.Since then the period has both increased and, starting in2006, decreased [Kurth et al., 2007, 2008]. It may well bethat the deep planetary rotation period is substantiallydifferent from the radio/magnetic period. Anderson andSchubert [2007] have provided arguments for a 632 min,35 s period obtained from gravitational and other atmo-spheric data independent of magnetic or radio data. Subse-quently, as noted already, Kurth et al. [2008] found thatemissions at a similar period accompany the longer-periodsignal in the radio data at frequencies below the SKRfrequency.[41] It is generally accepted that the secular change in

period precludes the radio period’s being the true planetaryrotation period [Stevenson, 2006]. Attempts have been madeto link the radio/magnetic periodicity to ionized materialmotion in the magnetosphere. Gurnett et al. [2007] andGoldreich and Farmer [2007] analyze how ionized materialoriginating from geyser action and the resulting torus atEnceladus’ orbit just outside the rings of Saturn is trans-ported across the magnetic field outward and eventually lostinto the interplanetary medium. It seems clear that therotation is imposed on the transport process at least in theinner magnetosphere. However, it is less clear how the slowand fairly steady variation in rotation period would beimposed directly by such a process. Indeed the inertiaassociated with the ionized material near the magnetospher-ic equatorial plane is slight compared with that in the upperatmosphere and ionosphere and it is more likely that thejitter seen on the scale of individual cycles is what can beattributed to local magnetospheric plasma phenomena. Pe-riodic injections of energetic material that rotate about theplanet have been detected by the Cassini MIMI instrument[Paranicas et al., 2005; Mitchell et al., 2006]. These againare more likely to pick up the underlying periodicity in themagnetic field and to provide dynamics that create jitterthan to be the source of a long-term evolution of the period.[42] What else changes on the time scale of the secular

period change? The orbital period of Saturn is on the orderof 29 years and so the time scale of changes in the period(�10 years) is of the order of the time scale of seasonalchanges represented by the change in solar aspect angle.Equinox at Saturn is due in 2009. It is noteworthy that theVoyager encounters with Saturn took place shortly afterequinox one Saturn solar orbit ago.[43] What seasonal changes could impact the periodicity

observed in the two magnetospheric phenomena linked inthis paper? Hemispheric differences in the atmosphere andionosphere are sure to be modulated as the season changes.

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Wind systems in the upper atmosphere might well differbetween northern and southern hemispheres and the differ-ential insolation varies with season. From a magnetosphericpoint of view, a very important hemispheric difference isthat the different illumination of the two polar caps atsolstice causes the conductances of the two polar caps (thatare responsive to solar illumination) to differ. A furtherseasonal effect, unique to Saturn, is that the extent andlocation of the shadows cast by the large ring system in theplanetary atmosphere varies over the 29 year orbital period.This effect will also produce large hemispherical differencesin insolation and in ionization of the upper atmosphere. Inthis case it is the low-latitude atmosphere rather than thepolar caps themselves that is affected, but flows couldproduce secondary effects at high latitudes.[44] The insolation of the rings themselves will vary with

season. The rings are rocky and appear to have someassociated dust. The most dramatic evidence for electrody-namic effects in the rings is the occasional occurrence ofspokes which seem to rotate in a non-Keplerian manner[see, e.g., Mitchell et al., 2006], consistent with effectsrelated to a charged dust environment. However, the elec-trical conduction properties of the ring system as a wholeare not yet understood and so there is reluctance to speculateon any global influence of the rings on the magnetosphericperiodicities. The ionosphere and the rings or an ionizedring atmosphere are all likely to have sufficient inertia tosustain a mechanical stress sufficient to cause a departurefrom the underlying deep planetary rotation.[45] The fact that the cam magnetic field transverse to the

background field seems not to change sign at the equatorargues that the currents do not close through equatorialplasma, and thus suggests that the source field-alignedcurrents flow from hemisphere to hemisphere. The field-aligned currents exert a fortiori no force on the ionizedmedium through which they flow. They are an agent fortransmission of momentum and angular momentum. It isthus in the global closure of the currents (i.e., where theyflow across the field) that one will discover where themomentum is extracted and deposited. Closure in theplanetary ionosphere seems probable as there are not likelyto be significant conducting paths within the magnetosphereother than in equatorial regions, but is not clear whether theprimary closure is on open field lines connecting to the solarwind in the polar cap or through subauroral latitudes on fluxtubes that are closed. It is clear that the cam field-alignedcurrents are seen on those flux tubes on the interfacebetween the dipole-dominated inner magnetosphere andthe extended current sheet region of the outer magneto-sphere, that is, precisely on the magnetic shells where theplanetary dipole ceases to dominate plasma dynamics.[46] The scenario that we have presented here does not

address the question of the drift of the dominant magneto-spheric modulation, but it does provide an explanation forthe localization of peak SKR power in the morning sector,for the close link between the SKR period and the magneticperiodicity linked to the rotating cam current system, andfor a degree of solar wind control of SKR power. That thesefeatures of the model agree with observations is not fortu-itous. We looked for a way to link rotation to pulsing and tounderstand what might differ on the morningside and after-noonside of the magnetosphere in order to understand the

preferential emission sector for SKR. We believe that themodel put forward provides plausible answers. However,we recognize that the model has additional implications. Itrequires that the peak SKR power be dominated by thesource in the southern ionosphere, at least during the epochat which these observations are taken, that is, during thesouthern summer season on Saturn.[47] What do the data say about the hemisphere of

emission? The sources of SKR power were analyzed byGalopeau and Lecacheux. [2000]. They report (p. 13,095)that there are two main SKR sources in the Voyagerobservations and that ‘‘they are located at high latitude(�60�) mainly on the morningside at 0800–0900 LT[Galopeau et al., 1995]. Thus these two sources appear tobe magnetically conjugate.’’ It is not always possible toobserve both sources simultaneously, but on occasion bothsources (distinguished by the sense of polarization) areobserved and it is found that the southern hemispheresource is the stronger during the epoch for which reportsare available That observation is consistent with our model.A test of this interpretation may be possible if the Cassinimission survives significantly beyond equinox in August2009 after which the dominant source should shift to thenorthern ionosphere, still at high latitude in the morningsector.[48] Recent analysis of the relative phases of SKR power

from northern and southern sources shows that they vary inphase (D. A. Gurnett, P. Zarka, and L. Lamy, personalcommunication, 2009), an observation that does not directlyfit the model presented here. In fact, there is, in the dataused in this paper, some evidence of what could reconcilethe observation with the model. Explanation of this featureof the emissions would require consideration of more thanthe primary current system of the model. It is possible thatthe northern hemisphere emissions are generated, at least inpart, by currents that partially close the strongest upwardcurrent from the south (e.g., the field-aligned currentswhose signatures are noted in Figures 6–8). We have notcommented earlier on the reversed (closure) currents thatare present in the data. Their signatures are the sharpincreases of B8 that follows the sharp decreases marked inFigures 6–8, which imply that downward currents flow justequatorward of the dominant upward currents. Moreanalysis is needed, but the reverse local field-alignedcurrents could arise owing to the feedback associated withconductivity modification in the region where the upwardcurrents couple to Pedersen currents in the ionosphere.Conductance contrasts could result from precipitation of theaccelerated electrons that generate SKR emissions. Diver-sion of the Pedersen currents results from nonuniformconductance and can drive the reverse current that isobserved. The reversed currents themselves may requireacceleration of current-carrying electrons, this time abovethe northern ionosphere. Further work is needed toinvestigate the implications of these suggestions.

[49] Acknowledgments. Work at UCLAwas supported by NSF grantNSF ATM 02–05958 and by NASA grant NNX08AQ46G. Work atImperial College was supported by the UK Particle Physics and AstronomyResearch Council (now Science and Technology Facilities Council). Theauthors are grateful to X. Jia for creating Figure 5.[50] Zuyin Pu thanks Hua Zhao, Christopher Arridge, and the other

reviewers for their assistance in evaluating this paper.

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ReferencesAnderson, J. D., and G. Schubert (2007), Saturn’s gravity field, internalrotation, and interior structure, Science, 317(5843), 1384 – 1387,doi:10.1126/science.1144835.

Andrews, D. J., E. J. Bunce, S. W. H. Cowley, M. K. Dougherty, G. Provan,and D. J. Southwood (2008), Planetary period oscillations in Saturn’smagnetosphere: Phase relation of equatorial magnetic field oscillationsand Saturn kilometric radiation modulation, J. Geophys. Res., 113,A09205, doi:10.1029/2007JA012937.

Arridge, C. S., N. Achilleos, M. K. Dougherty, K. K. Khurana, and C. T.Russell (2006), Modeling the size and shape of Saturn’s magnetopausewith variable dynamic pressure, J. Geophys. Res., 111, A11227,doi:10.1029/2005JA011574.

Arridge, C. S., C. T. Russell, K. K. Khurana, N. Achilleos, S. W. H. Cowley,M. K. Dougherty, D. J. Southwood, and E. J. Bunce (2008a), Saturn’smagnetodisc current sheet, J. Geophys. Res., 113, A04214, doi:10.1029/2007JA012540.

Arridge, C. S., K. K. Khurana, C. T. Russell, D. J. Southwood, N. Achilleos,M. K. Dougherty, A. J. Coates, and H. K. Leinweber (2008b), Warping ofSaturn’s magnetospheric and magnetotail current sheets, J. Geophys. Res.,113, A08217, doi:10.1029/2007JA012963.

Bunce, E. J., et al. (2008), Origin of Saturn’s aurora: Simultaneous obser-vations by Cassini and the Hubble Space Telescope, J. Geophys. Res.,113, A09209, doi:10.1029/2008JA013257.

Cecconi, B., and P. Zarka (2005), Model of a variable radio period forSaturn, J. Geophys. Res., 110, A12203, doi:10.1029/2005JA011085.

Connerney, J. E. P. (1993), Magnetic fields of the outer planets, J. Geophys.Res., 98, 18,659–18,679, doi:10.1029/93JE00980.

Davis, L., and E. J. Smith (1990), A model of Saturn’s magnetic field basedon all available data, J. Geophys. Res., 95, 15,257–15,261, doi:10.1029/JA095iA09p15257.

Desch, M. D., and M. L. Kaiser (1981), Voyager measurement of therotation period of Saturn’s magnetic field, Geophys. Res. Lett., 8,253–256, doi:10.1029/GL008i003p00253.

Dessler, A. J. (1985), Differential rotation of the magnetic fields ofgaseous planets, Geophys. Res. Lett., 12, 299 –302, doi:10.1029/GL012i005p00299.

Dougherty, M. K., et al. (2004), Cassini magnetic field investigation, SpaceSci. Rev., 114, 331–383, doi:10.1007/s11214-004-1432-2.

Ergun, R. E., Y.-J. Su, L. Andersson, C. W. Carlson, J. P. McFadden, F. S.Moser, D. L. Newman, M. V. Goldman, and R. J. Strangeway (2001),Direct observation of localized parallel electric fields in a space plasma,Phys. Rev. Lett., 87(4), 045003, doi:10.1103/PhysRevLett.87.045003.

Espinosa, S. A., and M. K. Dougherty (2000), Periodic perturbations inSaturn’s magnetic field, Geophys. Res. Lett., 27, 2785 – 2788,doi:10.1029/2000GL000048.

Espinosa, S. A., D. J. Southwood, and M. K. Dougherty (2003a), How canSaturn impose its rotation period in a noncorotating magnetosphere?,J. Geophys. Res., 108(A2), 1086, doi:10.1029/2001JA005084.

Espinosa, S. A., D. J. Southwood, and M. K. Dougherty (2003b), Reana-lysis of Saturn’s magnetospheric field data view of spin-periodic pertur-bations, J. Geophys. Res., 108(A2), 1085, doi:10.1029/2001JA005083.

Evans, D. S. (1968), The observations of a near monoenergetic flux ofauroral electrons, J. Geophys. Res., 73, 2315– 2323, doi:10.1029/JA073i007p02315.

Galopeau, P. H. M., and A. Lecacheux (2000), Variations of Saturn’s radiorotation period measured at kilometer wavelengths, J. Geophys. Res.,105, 13,089–13,102, doi:10.1029/1999JA005089.

Galopeau, P., P. Zarka, and D. LeQueau (1989), Theoretical model ofSaturn’s kilometric radiation spectrum, J. Geophys. Res., 94, 8739–8755,doi:10.1029/JA094iA07p08739.

Galopeau, P. H. M., et al. (1995), Source location of Saturn’s kilometricradiation: The Kelvin-Helmholtz instability hypothesis, J. Geophys. Res.,100, 26,397–26,410, doi:10.1029/95JE02132.

Giampieri, G., and M. K. Dougherty (2004), Rotation rate of Saturn’sinterior from magnetic field observations, Geophys. Res. Lett., 31,L16701, doi:10.1029/2004GL020194.

Giampieri, G., M. K. Dougherty, E. J. Smith, and C. T. Russell (2006), Aregular period for Saturn’s magnetic field that may track its internalrotation, Nature, 441, 62–64, doi:10.1038/nature04750.

Goldreich, P., and A. J. Farmer (2007), Spontaneous axisymmetry breakingof the external magnetic field at Saturn, J. Geophys. Res., 112, A05225,doi:10.1029/2006JA012163.

Gulkis, S., and T. D. Carr (1966), Radio rotation period of Jupiter, Science,154, 257–259, doi:10.1126/science.154.3746.257.

Gurnett, D. A., A. M. Persoon, W. S. Kurth, J. B. Groene, T. F. Averkamp,M. K. Dougherty, and D. J. Southwood (2007), The variable rotationperiod of the inner region of Saturn’s plasma disk, Science, 316(5823),442–445, doi:10.1126/science.1138562.

Hansen, K. C., A. J. Ridley, G. B. Hospodarksy, N. Achilleos, M. K.Dougherty, T. I. Gombosi, and G. Toth (2005), Global MHD simulationsof Saturn’s magnetosphere at the time of Cassini approach, Geophys. Res.Lett., 32, L20S06, doi:10.1029/2005GL022835.

Kaiser, M. L., M. D. Desch, J. W. Warwick, and J. B. Pearce (1980),Voyager detection of nonthermal radio emission from Saturn, Science,209(4462), 1238, doi:10.1126/science.209.4462.1238.

Khurana, K. K. (2001), Influence of solar wind on Jupiter’s magnetospherededuced from currents in the equatorial plane, J. Geophys. Res., 106,25,999–26,016, doi:10.1029/2000JA000352.

Kivelson, M. G., and D. J. Southwood (2005), Dynamical consequencesof two modes of centrifugal instability in Jupiter’s outer magnetosphere,J. Geophys. Res., 110, A12209, doi:10.1029/2005JA011176.

Kivelson, M. G., K. K. Khurana, and R. J. Walker (2002), Sheared mag-netic field structure in Jupiter’s dusk magnetosphere: Implications forreturn currents, J. Geophys. Res., 107(A7), 1116, doi:10.1029/2001JA000251.

Kurth, W. S., G. B. Hospodarsky, D. A. Gurnett, B. Cecconi, P. Louarn,A. Lecacheux, P. Zarka, H. O. Rucker, M. Boudjada, and M. L. Kaiser(2005), High spectral and temporal resolution observations of Saturnkilometric radiation, Geophys. Res. Lett., 32, L20S07, doi:10.1029/2005GL022648.

Kurth, W. S., A. Lecacheux, T. F. Averkamp, J. B. Groene, and D. A.Gurnett (2007), A Saturnian longitude system based on a variable kilo-metric radiation period, Geophys. Res. Lett., 34, L02201, doi:10.1029/2006GL028336.

Kurth, W. S., T. F. Averkamp, D. A. Gurnett, J. B. Groene, andA. Lecacheux (2008), An update to a Saturnian longitude system basedon kilometric radio emissions, J. Geophys. Res., 113, A05222,doi:10.1029/2007JA012861.

Mitchell, C. J., M. Horanyi, O. Havnes, and C. C. Porco (2006), Saturn’sspokes: Lost and found, Science, 311(5767), 1587–1589, doi:10.1126/science.1123783.

Paranicas, C., et al. (2005), Periodic intensity variations in global ENAimages of Saturn, Geophys. Res. Lett., 32, L21101, doi:10.1029/2005GL023656.

Pritchett, P. L., and R. J. Strangeway (1985), A simulation study of kilo-metric radiation generation along an auroral field line, J. Geophys. Res.,90, 9650–9662, doi:10.1029/JA090iA10p09650.

Southwood, D. J., and M. G. Kivelson (2007), Saturnian magnetosphericdynamics: Elucidation of a camshaft model, J. Geophys. Res., 112,A12222, doi:10.1029/2007JA012254.

Stevenson, D. J. (2006), A new spin on Saturn, Nature, 441, 34–35,doi:10.1038/441034a.

Warwick, J. W., et al. (1981), Planetary radio astronomy observations fromVoyager 1 near Saturn, Science, 212(4491), 239–243, doi:10.1126/science.212.4491.239.

Wu, C. S., and L. C. Lee (1979), A theory of terrestrial kilometric radiation,Astrophys. J., 230, 621–626, doi:10.1086/157120.

Zarka, P., L. Lamy, B. Cecconi, R. Prange, and H. O. Rucker (2007),Modulation of Saturn’s radio clock by solar wind speed, Nature, 450,265–267, doi:10.1038/nature06237.

�����������������������M. G. Kivelson, Institute of Geophysics and Planetary Physics,

University of California, 6843 Slichter Hall, 405 Hilgard Avenue, LosAngeles, CA 90024, USA. (mkivelson@igpp.ucla.edu)D. J. Southwood, European Space Agency Headquarters, F-75738 Paris,

France.

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