RESEARCH ARTICLE SUMMARY◥

GAS GIANT PLANETS

The low-frequency source of Saturn’skilometric radiationL. Lamy*, P. Zarka, B. Cecconi, R. Prangé, W. S. Kurth, G. Hospodarsky, A. Persoon,M. Morooka, J.-E. Wahlund, G. J. Hunt

INTRODUCTION: Planetary auroral radioemissions are powerful nonthermal cyclotronradiation produced by magnetized planets.Their remote observation provides informa-tion on planetary auroral processes and mag-netospheric dynamics. Understanding howthey are generated requires in situ measure-ments from within their source region. Dur-ing its early 2008 high-inclination orbits, theCassini spacecraft unexpectedly sampled twolocal sources of Saturn’s kilometric radiation(SKR) at 10-kHz frequencies, correspondingto the low-frequency (LF) portion of its 1- to1000-kHz typical spectrum (hence only ob-servable from space). These provided insightsinto the underlying physical excitation mech-anism. The combined analysis of radio, plasma,and magnetic field in situ measurements dem-onstrated that the cyclotron maser instability(CMI), which generates auroral kilometric radi-ation (AKR) at Earth, is a universal generationmechanism able to operate in widely differentplanetary plasma environments. The CMI re-quires accelerated (out-of-equilibrium) electronsand low-density magnetized plasma, such asauroral regions where the ratio of the electronplasma frequency fpe to the electron cyclotronfrequency fce is much lower than unity.

RATIONALE: Intensifications of the SKR spec-trum in general, and of its LF part in particular,

have been widely used as a diagnostic ofSaturn’s large-scale magnetospheric dynamics,such as the SKR rotational modulation ormajor auroral storms driven by the solar wind.However, the limited set of events encounteredin 2008 did not provide a comprehensive pic-ture of the source of SKRLFemissions andof theconditions in which the CMI can trigger them.During the 20 ring-grazing high-inclination

orbits and the preceding 7 orbits,which togetherspanned late 2016 to early 2017, the Cassinispacecraft repeatedly sampled the top of theSKR emission region, at distances of a fewplanetary radii (RS), corresponding to the lowest-frequency part of the SKR spectrum. SKRemission frequency is close to fce, itself pro-portional to the magnetic field amplitude, andso decreases with increasing distance from theplanet.We conducted a survey of these orbits toextract the averageproperties of the radio sourcesencountered and assess the ambient magneto-spheric plasma parameters that control them.

RESULTS: Throughout this set of trajectories,wewere able to identify only three SKR sources.They covered the 10- to 20-kHz range (3.5 to4.5 RS distances from the planet’s center) andwere solely found on the northern dawn-sidesector. The source regionshostednarrow-bandedemission, propagating in the extraordinary wavemode, and radiated quasi-perpendicularly to the

magnetic field lines. Their emission frequency,measured in situ, was fully consistent with theCMI mechanism driven by 6- to 12-keV elec-tron beams with shell-type velocity distribu-tion functions, as for AKR at Earth.The SKR sources were embedded within

larger regions of upward currents, themselvescoincident with the ultraviolet (UV) auroraloval, whichwas observed simultaneously withthe Hubble Space Telescope. However, unlikethe terrestrial case, the spacecraft exited the

radio source region beforeexiting themainoval, itselfstrictly coincident withthe upward current layer,when the ratio fpe/fce ex-ceeded the typical CMIthreshold of 0.1. This oc-

curred at times of sudden local increases ofthe magnetospheric electron density.

CONCLUSION: The generation conditions ofSKR LF emission appear to be strongly time-variable. The SKR spectrum additionally dis-plays a significant local time dependence, withthe lowest frequencies (the highest altitudes)reached for dawn-side radio sources. The char-acteristics of CMI-unstable electrons at 3.5 to4.5 RS imply that downward electron accel-eration took place at farther distances alongthe auroralmagnetic field lines and brings newconstraints to particle acceleration models.Finally, the LF SKR component is mainly con-trolled by local plasma conditions, namelytime-variable magnetospheric electron den-sities, which can quench the CMI mechanismeven if accelerated electrons are present. Thisexplains why the magnetic field lines hostingSKR LF sources can map to a restricted por-tion of the UV auroral oval and associatedupward current region.▪

RESEARCH | DIVING WITHIN SATURN ’S RINGS

Lamy et al., Science 362, 48 (2018) 5 October 2018 1 of 1

The list of author affiliations is available in the full article online.*Corresponding author. Email: laurent.lamy@obspm.frCite this article as L. Lamy et al., Science 362, eaat2027(2018). DOI: 10.1126/science.aat2027

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Ultraviolet auroraeSaturn’s kilometric radiation

Magnetic field line

Schematic view of Saturn’s auroral emissions.Auroral radio emissions observed above theatmosphere were remotely mapped fromCassini (the most intense emissions inred) when the spacecraft encountereda low-frequency radio source.Themagnetic field lines hosting it,sampled in situ by the probe,map to the (atmospheric)UV auroral oval, asobserved simulta-neously by the HubbleSpace Telescope.

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RESEARCH ARTICLE◥

GAS GIANT PLANETS

The low-frequency source of Saturn’skilometric radiationL. Lamy1*, P. Zarka1, B. Cecconi1, R. Prangé1, W. S. Kurth2, G. Hospodarsky2,A. Persoon2, M. Morooka3, J.-E. Wahlund3, G. J. Hunt4

Understanding how auroral radio emissions are produced bymagnetized bodies requires in situmeasurements within their source region. Saturn’s kilometric radiation (SKR) has beenwidely used as a remote proxy of Saturn’s magnetosphere.We present wave and plasmameasurements from the Cassini spacecraft during its ring-grazing high-inclination orbits,which passed three times through the high-altitude SKR emission region. Northern dawn-side,narrow-banded radio sources were encountered at frequencies of 10 to 20 kilohertz, withinregions of upward currents mapping to the ultraviolet auroral oval.The kilometric waves wereproduced on the extraordinary mode by the cyclotron maser instability from 6– to 12–kilo–electron volt electron beams and radiated quasi-perpendicularly to the auroral magneticfield lines.The SKR low-frequency sources appear to be strongly controlled by time-variablemagnetospheric electron densities.

Saturn is a powerful planetary radio emitter,dominated in intensity by Saturn’s kilo-metric radiation (SKR). This nonthermalauroral emission, equivalent to auroralkilometric radiation (AKR) at Earth, is

produced at frequencies f close to the localelectron cyclotron frequency fce, itself propor-tional to the magnetic field amplitude and thusdecreasing with increasing distances from theplanet. The SKR spectrum ranges from a fewkHz to 1 MHz (thus only observable from space),which corresponds to radio sources distributedalong auroral field lines from slightly above theionosphere up to several planetary radiiRS (1RS =60,268 km is Saturn’s radius). Remote observa-tions of SKR have long been used as a proxy ofkronian auroral processes and magnetosphericdynamics (1).During its early 2008 high-inclination passes,

the Cassini spacecraft unexpectedly encounteredtwo SKR low-frequency (LF) source regions. Thecombined analysis of radio, plasma, and mag-netic field in situ measurements showed that thecyclotronmaser instability (CMI), which generatesAKR at Earth, also produces SKR at Saturn, sug-gesting a universal process able to operate inwidely different planetary magnetospheres. TheCMI is a wave-electron resonant interaction thatrequires (i) accelerated (out-of-equilibrium)weakly

relativistic electrons, whose velocity distributionfunction displays a positive gradient along thevelocity direction perpendicular to themagneticfield, and (ii) an electron plasma frequency fpemuch lower than fce (2).When these two conditionsare fulfilled, radio emission is amplified, mainlyon the right-hand extraordinary (R-X) wavepropagation mode, at the frequency fCMI ¼fce=G þ k∥v∥=2p where k is the wave vector, vthe electron velocity, the ∥ subscript refers tothe direction parallel to the magnetic field andG ¼ 1=

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� v2=c2

pis the Lorentz factor (3).

Waves are thus amplified at the local relativisticelectron cyclotron frequency, Doppler-shifted bythe parallel propagation. To first order, thisexpression approximates to fCMI ~ fce.A first SKR source was identified on day

2008-291 (17 October) at 10 kHz (correspondingto 5 RS distance from Saturn’s center), at theunexpected Local Time (LT) of 01:00 and alongmagnetic field lines with −80° invariant latitude.Source traversal occurred during a global SKRintensification extending to low frequencies withwave electric fields of a fewmVm−1. It coincidedwith strong upward-directed field-aligned cur-rents, indicating down-going electron beams,triggered by a major solar wind–driven com-pression of the magnetosphere (4–6). The R-Xmode source was identified from measurementsof its low-frequency cutoff fSKR at about 2%below fce. Radio waves displayed strong linearpolarization at the source from which they wereradiated quasi-perpendicularly to the local mag-netic field. In this case, fSKR can be expressed asfCMI ¼ fce

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� v2=c2

pand direct measurements

of fSKR yielded CMI-resonant electrons of 6 to9 keV, in agreement with the hot (accelerated)electrons simultaneously measured in situ. Theirshell-like three-dimensional (3D) distributionfunction (referred to as horseshoe in 2D) was

also fully consistent with CMI-driven perpen-dicular emission. Sampling down-going elec-trons with shell-like distribution implies thatthey had been accelerated toward the planet inregions located farther than the radio sourcealong the flux tube (4, 7–9). A second direct con-firmation of the CMImechanismwas brought bythe agreement between the wave properties ob-served within the source region and the sim-ulated CMIwave growth rate and emission angledriven by the electron distribution function, mea-sured simultaneously (9). As at Earth, the auroralplasma surrounding the emission region was hot,with a marginal cold component, and tenuouswith fpe/fce ~ 0.1 (10). Conversely, terrestrial-likeplasma cavities were not observed within theradio source at the 2-s resolution of Cassini’selectronmeasurements. This difference, togetherwith the presence of SKR sources at larger dis-tances from the planet than AKR sources atEarth, was attributed to the faster rotation ofSaturn's magnetosphere. This means that CMIcondition (ii) is generally fulfilled at high lati-tudes without needing small-scale cavities.A second SKR source was likely traversed on

day 2008-073 (13 March) at 5 kHz (i.e., 6 RS dis-tance) (11). The LF emission tangentially ap-proached the fce curve on the correspondingdynamic spectrum (time-frequency map of waveflux density) at a more usual location, near 07:30LT, close to the SKR peak intensity region (12),and along magnetic field lines with −77° invar-iant latitude. The fpe/fce ratio was as low as 0.05,again fulfilling CMI condition (ii). The low-frequency cutoff fSKR was not accurately mea-sured, but 7-keV electrons with a shell-likedistribution function yielded CMI wave growthrates compatible with the observed SKR R-Xmode flux densities (13).This limited set of events did not provide a

comprehensive view of SKR LF sources and ofthe conditions under which the CMI can triggerthem. Intensifications of SKR spectrum in gen-eral, and of its LF component in particular, havenevertheless long been used as a sensitive diag-nostic of large-scale magnetospheric dynamics,including the SKR dual rotational modulation orauroral storms induced (or not) by the solar wind(14–25). The final high-inclination phase of theCassini mission carried the spacecraft to highlatitudes at low altitudes, providing the oppor-tunity to statistically investigate SKR sources insitu. Those orbits began with the ring-grazingorbital sequence, during which the spacecraftrepeatedly sampled the top of both northern andsouthern SKR emission regions, at distances cor-responding to the SKR lowest frequencies. Wepresent a survey of these orbits, extract the aver-age properties of the radio sources encountered,and discuss the magnetospheric parameters thatcontrol them.

Survey of ring-grazing orbits

Cassini’s 20 ring-grazing orbits (numbered251 to 270), whose periapsis was outside theF-ring, were executed between days 2016-335(30 November) and 2017-112 (22 April). They

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Lamy et al., Science 362, eaat2027 (2018) 5 October 2018 1 of 8

1Laboratoire d’Etudes Spatiales et d’Instrumentation enAstrophysique, Observatoire de Paris, Université ParisSciences et Lettres, Centre National de la RechercheScientifique, Sorbonne Université, Université Paris Diderot,Sorbonne Paris Cité, 5 Place Jules Janssen, 92195 Meudon,France. 2Department of Physics and Astronomy, Universityof Iowa, Iowa City, IA 52242, USA. 3Swedish Institute ofSpace Physics, Box 537, SE-751 21 Uppsala, Sweden.4Blackett Laboratory, Imperial College London, London SW72BW, UK.*Corresponding author. Email: laurent.lamy@obspm.fr

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passed through auroral field lines expectedto host SKR sources up to 30 kHz (40 kHz) inthe dawn-side northern (dusk-side southern)hemisphere (fig. S1). They were preceded byseven orbits (numbered 244 to 250) startingon day 2016-270 (26 September) that broughtthe spacecraft to high latitudes and at dis-tances close enough to the planet to probe SKRsources at a few kHz. We conducted a survey ofkilometric sources encountered during these27 orbits using combined observations of theRadio and PlasmaWave Science (RPWS) experi-ment (26) and the magnetometer (MAG) (27).After the Cassini Plasma Spectrometer (CAPS)

(28) ceased operating in 2012, it was no longerpossible to measure in situ low-energy electrondistribution functions. This prevented the iden-tification of SKR sources from the direct analysisof simultaneous radio and electron measure-ments based on growth rate calculation. Instead,on the basis of the reference event of day 2008-291, SKR sources were identified solely fromRPWS-HFR (high-frequency receiver) and MAGdata whenever (j) the emission LF cutoff fSKRpassed strictly below fce and (jj) persisted for atleast two consecutive RPWS-HFRmeasurements,obtained at a 16-s temporal resolution. Criterion(j) rests upon the assumption of perpendicularemission (examined below). Criterion (jj) con-servatively avoids false detections induced byspiky signals (see examples in fig. S2). The methodused to determine fSKR is reminded in (2). Oursurvey revealed only three events that unambig-uously fulfilled both criteria. They all occurredin the northern hemisphere on days 2016-339(4 December), 2016-346 (11 December), and 2017-066 (7 March), hereafter referred to as events S1,S2, and S3. No source was found in the southernhemisphere.We rejected several candidate north-ern SKR sources that fulfilled criterion (j) butnot (jj). These may be either nearby sources(close to but not crossed by Cassini), crossingsof small-sized sources lasting less than 16 s (~200to 300 km for typical spacecraft velocity), and/or emission produced at fCMI > fce. The lattercase is possible for oblique emission, where thek∥v∥ term in the expression of fCMI becomesnon-negligible (8, 9).Events S1, S2, and S3 were sampled during

intervals of relatively quiet solar wind activity(29). S1 and S2 additionally occurred close to thepredicted timing of periodic northern SKR bursts,whereas event S3 was instead in anti-phase withit (1, 22). The first two events were thus con-sistent with standard auroral activity, whereas S3indicated a more unusual case, possibly drivenby internal dynamics. The detailed wave prop-erties and characteristics of the three identifiedsources are listed in Table 1. Hereafter, we focuson events S3 and S1 as illustrative examples. Apolar projection of the magnetic footprint ofthese sources is provided in fig. S3, which showsthat they are additionally colocated with theaverage northern ultraviolet (UV) aurorae, asobserved by the Hubble Space Telescope (HST)throughout 2017 (29). Event S3 displayed themost intense radio emission and benefited from

coordinated HST observations, whereas eventS1 was sampled by RPWS-WBR (wideband re-ceiver) waveform measurements.

Example SKR source crossings

Radio, plasma, andmagnetic fieldmeasurementsduring events S3 and S1 are shown in Figs. 1 and2, respectively (S2 is shown in fig. S2). SKR is thepatchy bright emission observed above 10 kHz inFigs. 1A and 2A and fig. S2A (dynamic spectrumof the wave Poynting flux S), with dominantright-handed (RH) circular polarization in Figs. 1Band 2B and fig. S2B (dynamic spectrum of thenormalized degree of circular polarization V ) in-dicative of northern R-X mode. Some weakerleft-handed (LH) polarized SKR, indicative ofleft-hand ordinary (L-O) mode, was additionallyobserved at the low-frequency edge of the R-Xmode SKR toward the end of the intervals plottedin Figs. 1 and 2 and fig. S2, preceding theperiapsis. The weakly polarized emission visiblebelow 10 kHz after 15:30 UT (Universal Time) onFig. 1, A and B, is identified as auroral hiss, awhistler-mode emission. Narrow-banded long-duration and drifting short-duration emissionsobserved below 35 kHz are not investigated inour analysis. Localized SKR R-Xmode LF emis-sion is tangent to the fce curve near the middleof each interval. Condition fSKR < fce (Figs. 1C and2C and fig. S2C) is then used to define theoverall extent of the source region. During event

S3 (Fig. 1, A to C), a single SKR source wascontinuously observed from 15:11:48 to 15:17:25UT, corresponding to a 6050-km distance alongthe spacecraft trajectory. During event S1 (Fig. 2,A to C), multiple SKR sources were observedbetween 08:42:50 and 08:59:54 UT.Figure 3 shows restricted portions of Fig. 2A at

higher time-frequency resolution obtained fromRPWS-WBR waveform measurements (2). Faint,localized, remote SKR emissions observed wellabove fce (Fig. 3A) display recurrent drifting sub-structures with scarce positive and prominentnegative rates of ~1 kHz/min, consistent withprevious SKR waveform observations (30). Anenlargement of the 08:00-09:10UT interval (Fig. 3B)reveals the time-frequency structure of theradio source. Whereas Fig. 1A suggested a singleemission patternwith a bandwidth of a few kHzlasting for nearly 2 hours, Fig. 3B shows that itsplits into several subcomponents of variabledurations (down to a few min) and bandwidths.Those with frequencies slightly, but clearly, abovefce likely correspond to sources on nearby fluxtubes, while only the component strictly tangentto and below the fce curve unambiguously in-dicates a local source. The overplotted fSKR pro-file (as determined in Fig. 2C) provides, despiteits lower temporal resolution, a one-to-one cor-respondence between the brief episodes of fSKR <fce and the observed SKR bursts (including thecentral event between 08:45 and 08:50 UT and

Lamy et al., Science 362, eaat2027 (2018) 5 October 2018 2 of 8

Table 1. Properties of the three R-X mode SKR low-frequency sources identified during theCassini ring-grazing orbits.Where provided, uncertainties are one standard deviation.

SKR source S1 S2 S3

Time range (UT) 2016-339

08:42:50–08:59:54

(orbit 251)

2016-346

13:33:26–13:34:30

(orbit 252)

2017-066

15:11:48–15:17:25

(orbit 264). ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Cassini ephemeris:. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Latitude (°) +61.9–60.9 +59.6–59.5 +54.0–53.1. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

LT (hours) 9.06–9.41 9.72–9.74 10.32–10.43. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Distance to Saturn’s center (Rs) 4.47–4.31 4.15–4.14 3.63–3.58. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Altitude along the flux tube (Rs)

from the 1-bar atmospheric level

3.65–3.49 3.33–3.32 2.84–2.79

. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

fce (kHz) 13–14 15 22–23. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Df (kHz) 0.1–0.4 <1.5 <2. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Size range (km) 240–3400

(9 subsources)

1300

(1 source)

6050

(1 source). ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Mean fSKR/fce ratio

(range)

0.976 ± 0.006

(0.965–0.998)

0.975 ± 0.008

(0.968–0.987)

0.989 ± 0.004

(0.978–0.995). ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Mean electron energy (keV)

(range)

11 ± 3

(1–17)

12 ± 4

(7–16)

6 ± 3

(3–11). ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Median Poynting flux (W/m2)

(range)

5.9 10−13

(2.6 10−14–2.7 10−11)

1.2 10−13

(9.1 10−14–1.6 10−12)

7.3 10−12

(2.0 10−13–2.3 10−8). ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Median electric field amplitude (mV/m)

(range)

0.033 ± 0.032

(0.004–0.142)

0.014 ± 0.011

(0.008–0.035)

0.292 ± 0.884

(0.012–4.211). ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Beaming angle (°)

q = (k,B)

93 ± 9 86 ± 16 91 ± 16

. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

Upward current density (nA/m2) 157 171 105. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ..

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the shorter bursts beside it). This agreementvalidates the method employed to determinefSKR. For this specific event, we therefore con-sidered nonconsecutive RPWS-HFR episodes withfSKR < fce as isolated subsources as well. The overallsource region of event S1 thus decomposes intonine SKR subsources. Their horizontal size alongthe spacecraft trajectory, and across the en-countered flux tubes, ranged from 240 to 3400km. Their ~100- to 400-Hz bandwidth Df charac-terizes very narrow-banded emissions with Df/fce0.008 to 0.03, and thus corresponds to radiatingstructures that extended vertically 700 to 2800 kmalong the magnetic flux tubes.

Perpendicular emission and energy ofsource electrons

Figures 1 and 2C and fig. S2C (right) show thekinetic energy of CMI-unstable electrons derived

from the measurements of fSKR < fce, assumingperpendicular emission at the source and thusfSKR ¼ fce

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� v2=c2

p, as for the southern event

of 2008-291. To check this assumption, weanalyzed the wave parameters derived from thegoniopolarimetric (direction-finding) analysis ofRPWS-HFR three-antenna measurements (2).These consist of the wave Stokes parametersS, Q, U, and V (31) and the k-vector direction.From the latter, we also retrieved the 3D locusof the radio source, from the intersection of thek line-of-sight with a model iso-f = fce surface(assuming straight-line propagation, which isreasonable at high latitude), and its beamingangle q ¼ ðk;BÞ, where B is the magnetic fieldvector at the source (4, 8, 12, 32).Figure 4 displays the variation of the beaming

angle q(f) of events S3 and S1 for the time inter-vals of Figs. 1 and 2 using a standard data selec-

tion (2) forRH (LH)polarizedwaves, and thusR-X(L-O)mode. In agreement with previous studiesof the SKR beaming angle (8, 23, 32, 33), q(f)of R-Xmode emission smoothly rises from 60 ±25° at 800 kHz to 85°±15° at 30 kHz, wherethe uncertainty on q is dominated by the scatterof data points. Figure S4E shows an alternativerepresentation of q( f) in the time-frequencyplane. This systematic change from perpendic-ular to oblique emission with increasing fre-quency is most likely due to refraction of R-Xmode waves on the nearby iso-fX surface, wherefX ~ fce is the R-X mode cutoff frequency (8), asproposed for auroral radio emissions of Earthand Jupiter (34, 35). In this case, the apparentR-X mode beaming angle should vary as a func-tion of the k-vector direction projected in a planeperpendicular to B, with the strongest refrac-tion expected for wave propagation toward the

Lamy et al., Science 362, eaat2027 (2018) 5 October 2018 3 of 8

Fig. 1. Cassini radio, plasma, and magneticmeasurements acquired on day 2017-066between 13:00 and 17:00 UT (event S3)in the northern dawn-side sector. (A andB) Dynamic spectra of wave Poynting flux Sand normalized degree of circular polarization V,respectively, derived from two- and three-antenna RPWS-HFR 16-s measurements over3.5 to 1500 kHz (2). V = −1 (+1) refer to RH (LH)polarization, and thus to R-X (L-O) modeemission from the northern hemisphere. Thewhite dashed lines indicate the electron cyclotronfrequency fce as derived from MAG data. R-Xmode SKR around 20 kHz comes close to fceafter 15:00 UT. (C) Ratio of the SKR low-frequency cutoff fSKR to fce (2). Values fSKR/fce <1 define one SKR source crossing between 15:11and 15:18 (orange-shaded region on (C) and(D), extended by vertical dotted-dashed lines inthe other panels). The right-handed gray scaleprovides the kinetic electron energy inferred from

fSKR ¼ fceffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� v2=c2

p, assuming perpendicular

emission. (D) Characteristic frequenciescomparing fSKR (solid black line), fce, and fce/10(dashed black lines) and the electron plasmafrequency fpe as directly derived from RPWSLangmuir probe 32-s measurements (grayprofile) together with a lower limit fpe min

estimated from the auroral hiss upper frequencycutoff measured with RPWS-MFR 64-smeasurements (2) (red profile). (E) Magneticazimuthal component Bϕ, as from 1-min averagedMAG measurements in spherical coordinates.The green-shaded area marks a region ofpositive slope, indicative of an upward-directedcurrent region. The SKR source region startsat the beginning of the upward current region andends before Cassini exits it, when fpe suddenlyrises above fce/10. B

(nT

)

134.8763.47.66

Time (h) 144.3061.38.86

163.2145.011.2

172.7628.012.2

r (Rs)λ (°)

LT (h)

153.7455.610.4

Day 2017-066

C

D

E0

-15

-10

-5

fSK

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equator. L-Omode SKRwas also observed, overan extended spectral range of 30 to 100 kHz,with q(f) remaining confined above 75° and witha lower scattering than the R-X mode. The L-Omode cutoff frequency fO is fpe, which was in thiscase much lower than fce (Figs. 1 and 2D), so nosimilar refraction on the iso-fO surface is ex-pected. This interpretation implies that q(f) mea-sures the apparent (real) beaming angle of SKRR-X (L-O)mode affected (unaffected) by refractionclose to the source. Although they differ at highfrequencies, both remote observations of R-Xand L-O mode q(f) indicate quasi-perpendicularemission at 30 kHz, toward frequencies wherethe local radio sources have been sampled.A second method provides a direct measure-

ment of the wave emission angle within thesource from its polarization state. For perpen-dicular cyclotron emission, the polarizationellipse axial ratio AR ¼ ðT � LÞ=V , where T ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiQ2 þ U 2 þ V 2

pand L ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

Q2 þ U 2p

are thetotal and linear polarization degrees (31), canbe approximated to first order as cos q (36).Figures 1 and 2B and fig. S4, A to D, show thatthe emission is strongly linearly polarized at thesource, as observed during the southern event of2008-291. Measurement of the mean AR in eachSKR source of the three events S1, S2, and S3,restricting to the two HFR frequencies encom-passing fce with standard data selection (identi-cal to that used in Fig. 5C, discussed below), givesAR = −0.02 ± 0.26, 0.06 ± 0.28, and −0.05 ± 0.15,which results in q = 91 ± 16°, 86 ± 17°, and 93 ± 9°,respectively, all consistent with perpendicularemission.The measured values of fSKR can therefore be

safely inverted to derive kinetic energies of CMI-unstable electrons (Figs. 1C and 2C, fig. S2C, andTable 1). On average, fSKR reached a level of −2%from fce with a minimum value of −3.5%. Thecorresponding electron energies spanned the 1-to 17-keV range, withmean values between 6 and12 keV depending on the event and a standarddeviation of 3 to 4 keV, larger than the 2-keVuncertainty arising from the fSKR measurements.No correlation is found between the obtainedelectron energies and the observed wave in-tensities, measured between a few 0.01 and afew mV m−1.

Auroral plasma conditions

We now investigate the auroral plasma condi-tions to assess the validity of CMI condition (ii).Figures 1D and 2D compare the characteristicfrequencies fSKR, fce, and fpe. fpe was derivedfrom Langmuir probe (LP) 32-s measurementsand compared to the upper frequency cutoff ofauroral hiss derived independently from theRPWS-MFR (medium frequency receiver) 64-smeasurements. The LP measures the total elec-trical current carried by the ambient plasma fromvoltage sweeps, from which the local electrondensityN and thus fpe are derived (2). The voltagesweep resolution leads to a typical uncertainty onfpe of less than a factor of 2. Auroral hiss isradiated upward from below the spacecraft andpropagating beneath fpe, so that its upper fre-

quency cutoff, when marking a sharp transition,is a proxy of fpe when fpe < fce. The observed hissemissions were generally damped and exhibiteda smoothed cutoff, so we interpret the fitted fre-quencies as a lower limit on fpe, labeled fpe min. InFigs. 1D and 2D, the fpe min profiles were close to,and generally below, the LP-derived fpe profiles.For event S1 (Fig. 1D), fpe/fce was about 0.03across the whole source region, which endedwhen the value of fpe suddenly rose by a factorof 4 at 15:18 UT (and fpe min by a factor of 13 at15:20 UT), exceeding the fce/10 threshold forCMI condition (ii). For event S3 (Fig. 2D), a sim-ilar density gradient was observed in both data-

sets. The hiss cutoff frequency fpe min rose by afactor of 5 at 09:04 UT, again shortly after exitingthe SKR source region at 09:00 UT. The LP-derived fpe, determined at a higher time resolu-tion, reveals more complex variability: Althoughfpe rose by a factor of 4 at 08:42, it then showedlarge-scale fluctuations of a factor of ~3 (~2) upto (after) 09:00 UT. The SKR subsources (sam-pled at a better cadence) match the local minimaof fpe, suggesting that small-sized radio sourceswere colocated with local density cavities. Sim-ilar resultswere observedduring event S2 (fig S2D),despite lower time-resolution measurementsof fpe.

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Fig. 2. As for Fig. 1, but for measurements acquired on day 2016-339 between 07:00and 11:00 UT (event S1) in the northern dawn-side sector. (A to E) Nine SKR sources near 13 kHzwere encountered successively between 08:43 and 09:00. The overall interval is delimited by a lightorange-shaded region, while the individual SKR sources are indicated by dark orange-shaded areas.

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Figures 1E and 2E and fig. S2E show the azi-muthal component of the magnetic field measuredfrom 1-min averaged MAG data. They indicate up-ward currents in intervals dominated by a positiveslope. Each SKR source region started at (butended before) the beginning (end) of the upwardcurrent region. The upward current density wasestimated to be 106, 171, and 157 nA m−2 forevents S1, S2, and S3, respectively, well above theaverage of 80 nA m−2 derived from all the ring-grazing northern upward current regions (37).

Additional information on the correspon-dence between auroral emissions and upwardcurrents is apparent in Fig. 5, which maps thespatial distribution of SKR sources during eventS3, along with the northern UV aurorae sim-ultaneously observed by HST (29) on 2017-066(17 March) over 15:27-16:21 UT (corrected forthe light travel time). Figure 5A displays thedistribution of SKR emissions observed over thesame time interval as in Fig. 1, as seen fromCassini. As expected for cyclotron emission, SKR

lower- (higher-) frequency sources are observed atlarger (closer) distances from the planet. Figure5B displays in a polar view the magnetic foot-print of radio sources visible in Fig. 5A. The R-Xmode footprintsmap a circumpolar auroral ovalvisible from 04:00 to 22:00 LT. Its latitudinalwidth of about 8° to 10° is dominated by thescattering of goniopolarimetric results. This lon-gitudinally extended range derives from the LTrange swept by the spacecraft during the in-vestigated interval. Movie 1 displays an animation

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07:40 08:00 08:20Day 2016-339 (h) Day 2016-339 (h)

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Fig. 3. High-resolution dynamic spectra of event S1 reconstructed fromCassini radio electric waveform measurements taken on 2016-339.The spectra were acquired with the RPWS-WBR receiver over the 0- to80-kHz range (2). (A) Reconstructed dynamic spectrum between 07:30

and 09:30 UT. The dashed line indicates fce. (B) Enlargement of theencountered local SKR source. The solid red line indicates fSKR, as derivedfrom RPWS-HFR measurements in Fig. 2D. It is in agreement with high-resolution WBR measurements of SKR emission close to and below fce.

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Fig. 4. SKR beaming angle at the source as a function of frequency.(A) Time interval from 13:00 to 17:00 UT on 2017-066 (event S3)encompassing the SKR source crossed around 15:15 UT. The beaming angleis defined as q = (k,B), where k is the wave vector determined remotelyfrom RPWS-HFR three-antenna measurements, and B the magnetic fieldvector at the source (2). A standard data selection was applied withT > 0.85, |zr| < 0.05, and signal-to-noise ratio SNR±X,Z > 20 dB. The colorscale refers to the wave frequency, with red (blue) symbols correspondingto RH (LH) polarized waves and thus to R-X (L-O) mode. Crosses(diamonds) indicate k directions that intercept (do not intercept) their iso-

f = fce surface, assuming straight-line propagation. Red and blue linesindicate the median value of q, derived at each frequency for which thenumber of crosses exceeds the number of diamonds (7). The dotted linemarks emission perpendicular to the local magnetic field vector. (B) Timeinterval from 07:00 to 11:00 UT on 2016-339 (event S1) encompassingthe multiple SKR sources crossed around 08:50 UT.The data selection wasidentical to that of (A) except that SNR±X,Z > 30 dB to better removefaint non-SKR narrow-banded emissions observed up to 40 kHz (2). Bothpanels show that q increases from 60 ± 25° at 800 kHz to 85 ± 15° at30 kHz, consistent with quasi-perpendicular emission at the source.

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of Fig. 5, A and B, and illustrates how thespatial distribution of detected sources varieswith the time-variable position of the spacecraft.The L-O mode footprints are well clustered andprecisely colocated with the R-Xmode footprintsbetween 08:00 and 10:00 LT. The comparisonwith Fig. 5D illustrates that SKR sources arecolocated with the UV auroral oval (see fig. S5for a superimposition of both panels). This cor-respondence is visible both on the dawn-sidesector, where the UV oval is thin and well de-fined between 06:00 and 12:00 LT from +72° to+75° latitudes, and on the dusk-side sector, wherethe UV aurorae are more spread in latitude be-tween 14:00 and 23:00 LT and include a pole-ward arc beyond +80° between 14:00 and 18:00LT. The Cassini footprint trajectory is displayedby the gray arrow in Fig. 5, B to D, over the 13:00-17:00 UT interval. Its colored portions indicatethe upward current region and the SKR sourceregion identified in Fig. 1. The upward currentlayer matches the main UV auroral oval in size,while the SKR source was traversed along asmaller, poleward, portion of it. This can bebetter seen in Fig. 5C, which is similar to Fig.5B except that it only plots local radio sources,

Lamy et al., Science 362, eaat2027 (2018) 5 October 2018 6 of 8

Fig. 5. Spatial distribution of SKR sourcesand UV aurorae. (A) 2D locus of SKR sources inthe sky plane as seen from Cassini, from RPWS-HFR three-antenna measurements of k directions(2) on 2017-066 13:00-17:00 UT (event S3). Theapplied data selection was identical to that usedin Fig. 4A, except that SNR±X,Z > 40 dB for clarity.The color scale and definition of symbols areidentical to those of Fig. 4A. The red planetarymeridian indicates noon. The typical 2° uncer-tainty on k measurements is displayed by thebottom-right ellipse. (B) Polar view of themagnetic footprint of radio sources displayed in(A) as a function of planetocentric latitude andLT. Error bars are not represented for clarity butare similar to those studied in (12, 32). Thedotted-dashed curved lines display radio hori-zons at extremal frequencies, beyond which aradio source cannot be observed. The doublegreen arrow indicates the Cassini LT range sweptduring the interval. The gray line plots theassociated Cassini magnetic footprint, whoseorange and green portions indicate the crossingof SKR source and of the upward current regionidentified in Fig. 1. (C) Same as (B) but for theSKR source observed from 09:00 to 09:20 UTbetween 20 and 27 kHz. The data selection wasslightly relaxed with T > 0.85, |zr| < 0.2 andSNR±X,Z > 20 dB to maximize statistics. Cassini’sLT is indicated by a green dotted-dashed arrow.The radio source footprints coincide with thecrossed SKR source (orange portion of the grayline). (D) Polar view of Saturn’s H2 auroraeobserved with the Hubble Space Telescope onday 2017-066 from 15:27 to 16:21 UT, aftercorrecting for light travel time. The background-subtracted image was projected onto the 1100-km-altitude surface as a function of planetocentriccoordinates and expressed in kilo-rayleighs (kR) of H2 (2). The gray line and its colored portions are identical to those of (B) and (C). The upward currentlayer (green) matches the main auroral oval, while the SKR source crossed (orange) only matches the poleward portion of it.

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radiated near fce (2). Again, the footprints of R-Xmode located sources map to the Cassini trajec-tory and only to the inward portion of the mainoval and associated upward current layer.

Implications for the generation ofSaturn’s auroral radio emissions

We now discuss the average properties of SKRLF emissions as a whole, in the CMI frame. Theidentification of only three SKR sources in thedawn-side sector and none in the dusk-side sec-tor brings a direct in situ confirmation to the LTdependence of SKR spectrum (here studied dur-ing quiet solar wind conditions), with lowestfrequencies at dawn (16, 24). The small numberof radio sources identified along orbits with verysimilar trajectories and their limited extentadditionally indicate time-variable generationconditions.In all three cases, the sampled R-X mode

emission was found to be amplified nearly per-pendicularly to the local magnetic field, withemission frequency clearly below fce suppliedby CMI-unstable electrons of 6- to 12-keVmeanenergies. Direct perpendicular emission throughCMI requires specific shell-type distributionfunctions of the accelerated electrons in thevelocity plane, such as those already measuredfor the two southern SKR sources crossed in2008 (9, 13). The situation at Saturn thus di-rectly compares to the terrestrial case, whereAKR sources are commonly found to be asso-ciated with trapped, shell or horseshoe, or ringdistributions (3, 38, 39). It differs from the sit-uation at Jupiter, which just started to be in-vestigated in situ by the Juno spacecraft, wherethe hectometer and decameter radio emissionsinstead appear to be produced at oblique anglesfrom the local magnetic field by loss cone electrondistribution functions (40, 41). The presence of 6-to 12-keV electron beams with such distributionfunctions at 3.5 to 4.5 RS distances from Saturn’scenter, or equivalently 2.8 to 3.7 RS above the at-mosphere along flux tubes (Table 1), implies that

at least part of the acceleration by upward cur-rents occurred farther along the field lines,which constrains models of current-voltagerelationships (42).The general agreement with the CMI mech-

anism relies on fulfilling both CMI conditions.The three SKR sources encountered by Cassinimatched upward currents with current densitieslarger than the average (37), consistent with anacceleration region extending to higher altitudesand thus satisfying CMI condition (i). Then, whilethe spacecraft entered the northern SKR sourceregion when it encountered an upward currentregion, it exited it when fpe suddenly rose, ex-ceeding the typical fce/10 threshold. After ex-iting the source, the spacecraft remained forsome time inside the upward current region,which matches the main UV oval across theCassini trajectory. This suggests that at 3.5 to4.5 RS distances, SKR emission is primarily con-trolled by the electron plasma densityN throughCMI condition (ii), which quenches the radiationwhen fpe/fce typically exceeds 0.1. According tothe Knight relationship (43) and assuming aconstant current density, larger N is also ex-pected to decrease electron acceleration andweaken CMI condition (i).Figure 6 shows radio, plasma, and magnetic

measurements for three successive, quasi-identical,passes of the spacecraft into the northern dawn-side auroral region. Figure 6, D to F, correspondto event S3 (orbit 264), while Fig. 6, A to C and Gto I, correspond to the previous (orbit 263) andthe following (orbit 265) auroral passes. Thespacecraft did not fly across any local SKR sourceduring orbits 263 and 265, while it encounteredregions of upward current in both cases withcurrent densities of 29 and 108 nA m−2. On orbit265, the SKR intensity was relatively high and itsspectrum reached frequencies as low as 20 kHz,while the upward current density was as strongas that of orbit 264. Instead, on orbit 263, SKRwas comparatively faint, with a spectrum con-fined to higher frequencies and a lower upward

current density. This suggests that CMI condition(i), which requires unstable electrons at 3.6 RS,was at least satisfied for orbit 265. For bothintervals, however, fpe was systematically largerthan fce/10 within the upward current region, incontrast with orbit 264 (event S3), indicating thatthe mean level of N over hours-long intervalsstrongly varies with time from pass to pass. Theseresults thus indicate that CMI condition (ii) wasnot fulfilled for orbits 263 and 265 and likelyquenched SKR LF emission during orbit 265.The origin of time-variable magnetospheric

plasma densities at the altitude of LF SKR sourceshas now to be elucidated. The asymmetricalplasmapause-like boundary, a transition fromhigh to low plasma densities with increasinglatitudes that rotates at the northern SKRperiod,is a plausible source for time-variable N (44). Ona shorter time scale, Fig. 1 does not display anyEarth-like plasma cavity, as already noticed forthe 2008-291 southern event (4). However, Fig. 2(and possibly fig. S2) shows small-sized, recur-rent, drops of fpe colocated with SKR sourcesfor event S1 (and S2). This suggests that accelera-tion features could coincide with localized radiosources, as at Earth. Definite conclusions, how-ever, rely on knowing whether the plasma regionwas dominated by hot electrons (such as on day2008-291) or could have included a cold plasmacomponent sensitive to local potential drops andable to map plasma cavities. As the CAPS ex-periment was not operating during these orbits,these issues remain unresolved.Finally, the very narrow-banded nature of SKR

sources, particularly obvious during event S1(Fig. 3B), is consistent with either a nonsaturatedlinear wave growth, as predicted by the CMItheory and measured in laboratory [e.g., (45)],or with saturation by coherent nonlinear trapping,previously assumed in models of the SKR spec-trum (46). However, the emission levels may betoo low to reach saturation, and the observedwave intensities (Table 1) are also consistent withlinear growth.

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Fig. 6. Cassini radio, plasma, and magnetic measurements acquiredduring three successive auroral passes with similar trajectories.(D to F) Identical to Fig. 1, A, D, and E, covering the interval 2017-066 from13:00 to 17:00 UT encompassing a SKR source crossing (orange-shaded

region). (A to C) and (G to I) Same as (D) to (F) but for the intervals2017-059 09:00-13:00 UT and 2017-073 17:00-21:00 UT (i.e., the orbitbefore and after), respectively. No SKR source was detected during theseintervals, for which fpe was always larger than fce/10.

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REFERENCES AND NOTES

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ACKNOWLEDGMENTS

We thank the Cassini engineering teams in charge of processingthe data, in particular T. Averkamp in Iowa City and M.-P. Issartelin Meudon. L.L. thanks L. Spilker and the Cassini-MAPS group fortheir support for the HST program coordinated with the CassiniGrand Finale. Funding: The French coauthors acknowledge supportfrom CNES and CNRS/INSU programs of Planetology (PNP) andHeliophysics (PNST). The research at the University of Iowa wassupported by NASA through Contract 1415150 with the JetPropulsion Laboratory. Author contributions: L.L. led the analysisand wrote the paper. P.Z., B.C., and W.S.K. helped in theanalysis of HFR data, and G.H. in the understanding of WBRmeasurements. A.P. analyzed the MFR measurements anddigitized hiss emission frequencies. M.M. and J.E.W. analyzed theLP data and derived electron densities. R.P. contributed to theinvestigation of HST observations. G.H. calculated the currentdensities from MAG data. All authors discussed the results andcommented on the manuscript. Competing interests: Theauthors declare no competing interests. Data and materialsavailability: The Cassini RPWS and MAG raw data are accessiblethrough the Planetary Data System at https://pds-ppi.igpp.ucla.edu/mission/Cassini-Huygens. The HFR processed data areavailable though the LESIA-Kronos database at www.lesia.obspm.fr/kronos. The Cassini data used correspond to the intervalsdisplayed in Figs. 1, 2, and 6 and fig. S2. The UV observations wereobtained from the ESA–NASA Hubble Space Telescope (GeneralObserver program 14811): the original data can be retrievedfrom the MAST archive at http://archive.stsci.edu/hst/, and theprocessed data from the APIS service http://apis.obspm.fr hostedby Paris Astronomical Data Centre.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/362/6410/eaat2027/suppl/DC1Figs. S1 to S5References (47–56)

6 February 2018; accepted 4 September 201810.1126/science.aat2027

Lamy et al., Science 362, eaat2027 (2018) 5 October 2018 8 of 8

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The low-frequency source of Saturn's kilometric radiation

HuntL. Lamy, P. Zarka, B. Cecconi, R. Prangé, W. S. Kurth, G. Hospodarsky, A. Persoon, M. Morooka, J.-E. Wahlund and G. J.

DOI: 10.1126/science.aat2027 (6410), eaat2027.362Science 

, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382Scienceand directly measured the composition of Saturn's atmosphere.

identified molecules in the infalling materialet al.how they interact with the planet's upper atmosphere. Finally, Waite investigated the smaller dust nanograins and showet al.particles falling from the rings into the planet, whereas Mitchell

determined the composition of large, solid dustet al.kilometric radiation, connected to the planet's aurorae. Hsu present plasma measurements taken as Cassini flew through regions emittinget al.decay of free neutrons. Lamy

detected an additional radiation belt trapped within the rings, sustained by the radioactiveet al.the planet. Roussos measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process insideet al.Dougherty

planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered theregions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the

The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sampleCassini's final phase of exploration

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