Philippe Jonnard

Mohammed H Modi

Karine Le Guen

Nargish Aneshwari

Mangalika Sinha

Mourad Idir

Patrick Chapon

Anouk Galtayries

Study of the Au-Cr bilayer system using X-ray reflectivity, GDOES, and ToF-SIMS

Keywords: x--ray mirror, gold, chromium, x--ray reflectivity, ToF--SIMS, GDOES

Introduction

Gold thin films are used in grazing incidence x--ray reflectivity mirrors in the 100--1500 eV soft x-ray region on synchrotron beamlines because of its flat reflectivity performance [START_REF] Attwood | Soft X--Rays and Extreme Ultraviolet Radiation[END_REF]. In order to provide a good adhesion with silicon or fused silica substrates, a binding layer of chromium is often used. These mirrors should exhibit no interdiffusion in order to get high and stable performances and thus need to be carefully characterized. To this purpose we use a combination of non--destructive techniques: soft x--ray reflectivity (XRR), glow discharge optical emission spectroscopy (GDOES) and time of flight secondary ion mass spectrometry (ToF--SIMS). The main goal is to determine the thickness of the different layers and interlayers and get some knowledge about the interactions between the layers or at the top of the substrate or at the top of the stack. This kind of methodology has been already used successfully for the study of nanometer periodic multilayers designed for x--ray optics, see Refs. [START_REF] Galtayries | Nanometer--designed Al/SiC periodic multilayers: characterization by a multi-technique approach[END_REF][START_REF] Galindo | Towards nanometric resolution in multilayer depth profiling: a comparative study of RBS, SIMS, XPS and GDOES[END_REF][START_REF] Le Guen | Characterization of EUV periodic multilayers[END_REF][START_REF] Ber | Sputter depth profiling of Mo/B4C/Si and Mo/Si multilayer nanostructures: A round--robin characterization by different techniques[END_REF][START_REF] Yuan | Interface observation of heat--treated Co/Mo2C multilayers[END_REF] for example.

Experimental details

The Au/Cr bilayer sample was prepared by electron beam evaporation from high purity Au and Cr targets. Its designed structure was : Au / Cr / SiO 2 substrate. 25 nm Au and 10 nm Cr layer thicknesses were targeted, respectively. The substrate was a float glass. After its preparation the sample was stored in ambient conditions.

XRR measurements [START_REF]X--ray and Neutron Reflectivity: Principles and Applications[END_REF] were performed at the reflectivity beamline BL3 [START_REF] Modi | Design, development and commissioning of "Soft X--ray Reflectivity Beamline" at Indus--2[END_REF] of Indus--2 synchrotron radiation source. A monochromatic 1500 eV incident photons irradiated the sample and θ -2θ scans were recorded from 0 to 16°. The intensity reflected by the sample, as measured by a silicon photodiode detector (International Radiation Detector Inc.) having a 100 % internal quantum efficiency, was normalized to the one obtained without sample to obtain the reflectance value. A nickel mesh of 90% transmission was mounted prior to the sample to measure the time variation of the incident photon flux. This enables one to account the effect of photon flux variation due to the fall in electron beam current in the storage ring. The fit of the XRR curve enable getting structural information about the studied stacks: thickness, density and roughness of the various layers.

The GDOES experiment [START_REF] Nelis | Glow Discharge as a Tool for Surface and Interface Analysis[END_REF] was performed with an apparatus Profiler 2 from Horiba Scientific. A 30 W pulsed RF power generated a plasma in Ar gas which pressure was 500 Pa. The pulse frequency and duty cycle were 3000 Hz and 0.25 repsectively. The plasma is used to both sputter the sample and excite the elements, making them emit characteristic lines. The width of the glow discharge spot on the sample was 4 mm. It took less than 2 s to sputter the whole stack. However, the fast acquisition of the optical signal makes it possible to analyse ultra--thin layers.

For the TOF--SIMS analysis [START_REF] Fearne | An Introduction to Time--of--Flight Secondary Ion Mass Spectrometry (ToF--SIMS) and its Application to Materials Science[END_REF], performed with an TOF.SIMS V apparatus from ION--TOF GmbH, a 1 keV, 50 nA Cs + ion beam was used to sputter a crater which size was 300 x 300 µm. The analysis was made on a 100 x 100 µm area at the centre of the crater by using 25 keV, 1 pA Bi + ions. First, a mass sum-spectrum was obtained by looking at the time of arrival of the ion on the detector, in order to get all the relevant ions. Then, the intensity of each selected ion (of a given mass) was followed as a function of the sputtering time. This time scale is not simply correlated to the thickness. Let us also note that the difference between the yields of two elements is not related to the difference of concentrations of these two elements (SIMS is not a quantitative technique, mainly due to the matrix effects). The analysis is made by detecting the following positive ions : Si + , CsO + , CsSiO + , CsCr + , CsCrO + , CsAu + , CsAuCr + , Cs 2 C + , CsNa + and CsCa + . The three first ones are chosen to probe the substrate; CsCrO + can indicate the possible formation of a Cr oxide; CsAuCr + can indicate the possible formation of a AuCr alloy or solid solution; the three last ones are related to contamination and impurities. Except Si, all ions are the combinations of Cs with the element of interest. Cs 2 C + is chosen instead of CsC + owing to its higher yield.

Results and discussion

Figure 1 shows the measured and fitted soft x--ray reflectivity curves of the sample together. The best fit reveals that the bilayer is made of: C (2.0nm) / Au (23.8nm) / Cr (10.2nm) / SiO 2 substrate. The roughness at the three interfaces, C--on--Au, Au--on--Cr and Cr--on--SiO 2 are respectively 0.6, 0.8 and 0.7 nm. The optical indices of bulk Cr and SiO 2 were used, but for the Au film it was necessary to decrease the refractive index, real and imaginary parts, by 3% with respect to its bulk value. This suggests that the density of the Au film is slightly lower (≈4%) than the bulk value. We show in Figure 2 the GDOES profiles of the Au, Cr major elements as well as of Na, Ca, C, O impurities or contaminants elements. Sodium and calcium are the main impurities of the float glass substrate. Carbon can come from the contamination of the substrate prior the deposition and of the sample after its preparation. Oxygen is present in the glass substrate and in air. Three main zones with two transient zones can be identified:

• 0 -0.11 s: the contaminated surface with the presence of all the studied elements except Cr.

While the presence of carbon and oxygen is easily ascribed to the air contamination, this explanation does not hold for sodium and calcium. We suggest these atoms are surfactants and stay above the growing films during the deposition process.

• 0.11 -0.53 s: the Au layer with a rather constant yield, while the yield of the other elements quickly decreases, except the Cr one increasing quickly at the end of this zone; the C signal, whose maximum is around 0.08 s, is quickly decreasing in this layer, suggesting no interphase between the contaminated and Au layers. and Ca start to increase. A small peak of the C signal is also observed in this zone. We ascribe it to the superficial contamination of the substrate prior deposition. • After 1.22 s: the substrate is reached. We show in Figure 3(a) the Si + , CsSiO + , CsO + , CsCr + , CsAu + , CsAuCr + , CsNa + and CsCa + ToF--SIMS depth profiles and in Figure 3(b) the Cs 2 C + , CrOCs + , AuCrCs + and CsCr + profiles. As in GDOES, five zones are identified:

• 0 -15 s: contaminated surface; in this zone the sputtering regime is not yet well established; the superficial zone is probed; the profile of the carbon ion is shown after and confirms the existence of the contamination layer determined from the XRR analysis.

• 15 -120 s: Au layer; the Au + yield is constant while those of the other ions are very weak; a low intensity Cr signal is significantly present as its intensity is higher than that in the silica substrate which can be considered as the background intensity; this SIMS Cr signal seems related to the low intensity tail of the GDOES Cr signal in the 0.11 - 0.53 s range (see Figure 2) and thus could show a small diffusion of Cr atoms toward the Au layer.

• 120 - 155 s: Au--Cr transient zone; both signals of CsAu + and CsCr + are present; more interesting the signal of CsAuCr + is present only in this zone; there is also of peak in the CsO + signal as in GDOES and also in the CsCrO + signal.

• 155 - 210 s: Cr layer; the CsCr + yield is more or less constant while those of the other ions are very weak; given the affinity of chromium for oxygen, a superficial oxidation of the Cr layer can take place owing to the presence of a non--negligible CsCrO + signal, whereas the CsO + signal is minimum.

• 210 -270 s: Cr--SiO 2 transient zone; signals from both the substrate and the Cr layer are present.

• 270 - 365 s: SiO 2 substrate; the presence of Si + and CsO + ions is regular; the CsCr + signal drops steeply.

• Charging effect, after 365 s: after this sputtering time, charging effects coming from the insulating nature of the glass substrate prevent from further analysis under the same conditions. Figure 3: ToF--SIMS profiles of the sample. Major elements as well as impurities are followed. The Cs 2 C + SIMS signal is present in the contaminated layer where it decreases quickly when going deeper. This confirms the carbon presence as due to a surface contamination. The GDOES results confirm the presence of the carbon--contaminated zone. From the Cs 2 C + profile it is also clear that some contamination takes place in the Cr layer and the transient zone between Cr and the substrate. In the Au and the mixed Au--Cr layers the intensity of the Cs 2 C + signal is negligible. This is also confirmed by GDOES, where the signal of C is quite intense and shows some amount of carbon in the Cr layer, not in the mixed Au--Cr layer, and in the transient zone with the substrate. The Na and Ca impurities are mainly concentrated on top of the glass substrate and on top of the sample. The CsCrO + signal is only related to the Cr and mainly present at the two interfaces (Cr--Au and Cr--SiO 2 ).

There is a fair agreement between the structure of the Au/Cr/SiO 2 sample deduced from XRR, GDOES and ToF--SIMS measurements. The XRR analysis relies on a model of stack to determine the parameters of the different layers. In the fitting process, the roughness between the layers can be due to the geometry (height variation) or the chemistry (interdiffusion) or both of them. So, we ascribe the roughnesses at the Au--on--Cr and Cr--on--SiO 2 interfaces to the transient zones observed by GDOES and ToF--SIMS. The fitting results are not significantly improved even using a five--layer model, i.e. with Au--on--Cr and Cr--on--SiO 2 interlayers. In fact, the fitting results are rather good with a simple Au/Cr two--layer mode with a superficial contamination layer, Fig. 1.

Finally, we compare ToF--SIMS and GDOES profiles in Figure 4(a) for Au and in Figure 4(b) for Cr. To superimpose both curves, the GDOES sputtering time at 0.53 s is normalized with respect to the one of SIMS at 120 s and both yields are normalized to unity. The comparison is made on a linear scale. We can see that the SIMS profile presents a better dynamics, a more stable plateau and smaller tail. As expected, given the nano--thicknesses of the deposited layers, ToF--SIMS depth profiling seems better suited for this analysis. The dynamics of the signal is rather the same for GDOES and ToF--SIMS toward the low sputtering times whereas it is better for ToF--SIMS for long sputtering times. We could infer from this, that the ToF--SIMS sputtering process induces less mixing in the sample than the GDOES sputtering process or more probably that it is important to make the analysis without influence of the crater walls. Indeed in ToF--SIMS the detected ions come from the centre of the crater (see the experimental details section) while in GDOES the detected signals come from the whole sputtered surface. It is interesting to note that both techniques give a Cr yield increase at the Au--on--Cr interface (around 130 s), however with much different amplitudes.

Conclusion

A coherent description of the Au/Cr/SiO 2 sample has been obtained by combining XRR, GDOES and ToF--SIMS measurements. XRR, which is non--destructive, enables getting the thickness of the different layers and interlayers as well determining that the gold film is slightly less dense than gold bulk. GDOES measurements are fast and enable obtaining elemental depth profiles of the major and minor elements of the sample. ToF--SIMS depth profiles present a larger dynamic range and bring supplementary chemical information in addition to the elemental one, particularly at the interfaces, as the association of an ion with another element can be detected. Finally, the structure of the sample, from the top to the bottom is:

• superficial layer, 2 nm thick, contaminated with oxygen and carbon; Na and Ca atoms are present as impurities coming from the substrate; • gold layer, 24 nm thick, having slightly lower density that of bulk gold; • Au--Cr transient layer, about 0.8 nm thick, where it is possible to find some AuCr mixing and oxidation of chromium on one or two layers (most probably Cr 3+ ); • chromium layer, 10 nm thick, contaminated with some carbon; • Cr--SiO 2 substrate transient layer, about 0.7 nm thick; Na and Ca atoms are present as impurities on top of the substrate which is also contaminated by a small amount of carbon;

• float glass substrate. Such a study shows the interest of combining complementary techniques in order to determine the in--depth composition of nanometer--thick multilayer stacks. The penetration depth of the radiation in gold being around 10 nm when used in total reflection mode, i.e. used as an x--ray mirror, our results show that the choice of the thickness of the gold layer, 20 nm, was relevant. Indeed the presence of the interdiffusion layer at the Au--on--Cr interface would have deteriorated the optical performance of the stack if the gold layer would have been 10 nm--thick or thinner. This could be also the case if the mirror would be subject to a high thermal load, as it is generally the case for the first mirrors on synchrotron beamlines, which would broaden the interdiffusion zone.

Figure 1 :

 1 Figure 1: Measured (red dots) and fitted (blue line) soft x--ray reflectivity curves of the Au/Cr bilayer sample for 1500 eV incident photon beam.

  • 0.53 -0.64 s: the Au--Cr transient zone marked by an oscillation of the Cr yield and a O peak. This could indicate a possible oxidation of the top of the Cr layer; at the end of the zone the decrease of the Cr yield is in phase with a slight shoulder of the Au yield. • 0.64 -1.08 s: the Cr layer with a yield slightly fluctuating.

• 1 .

 1 08 - 1.22 s: the Cr--SiO 2 transient zone, where the Cr signal is decreasing while those of O, Na

Figure 2 :

 2 Figure 2: GDOES profiles of the sample. Major elements as well as impurities and contaminants are followed. The signals of Na and C are divided by a factor 3, the one of Cr by 10 and the one of O by 2.

Figure 4 :

 4 Figure 4: Comparison on a linear scale of the ToF--SIMS (dotted line) and GDOES (solid line) profiles of Au (a) and Cr (b) elements. The GDOES sputtering time is normalized to the ToF--SIMS one.


L Lamy

P Zarka

B Cecconi

R Prangé

W S Kurth

G Hospodarsky

A Persoon

M Morooka

J.-E Wahlund

G J Hunt

J.-W Wahlund

J.-E Wahlund

The low-frequency source of Saturn's kilometric radiation

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Introduction

Planetary auroral radio emissions are powerful non-thermal cyclotron radiations produced by magnetized planets. Their remote observation provides a wealth of informations on planetary auroral processes and magnetospheric dynamics. Understanding how they are generated nonetheless requires in situ measurements from within their source region. During its early 2008 high-inclination orbits, the Cassini spacecraft unexpectedly sampled two local sources of Saturn's Kilometric Radiation (SKR), at 10kHz frequencies corresponding to the low frequency (LF) portion of its 1-1000 kHz typical spectrum (hence only observable from space). These events in turn provided insights into the underlying physical excitation mechanism. The combined analysis of radio, plasma and magnetic field in situ measurements demonstrated that the Cyclotron Maser Instability (CMI), which generates Auroral Kilometric Radiation (AKR) at Earth, is a universal generation mechanism able to operate in widely different planetary plasma environments. The CMI requires accelerated (out-of-equilibrium) electrons and low density magnetized plasma (such as auroral regions where the ratio of the electron plasma frequency f pe to the electron cyclotron frequency f ce is much lower than unity).

Rationale

Intensifications of SKR spectrum in general, and of its LF part in particular, have been widely used as a sensitive diagnostic of large-scale Saturn's magnetospheric dynamics, such as the SKR rotational modulation or major auroral storms driven by the solar wind. However, the limited set of events encountered in 2008 precluded to achieve a comprehensive picture of the source of SKR LF emissions and of the conditions in which the CMI can trigger them.

During the 20 ring-grazing high-inclination orbits and the preceding 7 orbits, which together spanned late 2016 to early 2017, the Cassini spacecraft repeatedly sampled the top of Saturn's Kilometric Radiation (SKR) emission region, at distances of a few planetary radii (R S ) corresponding to the lowest part of the SKR spectrum. Indeed, SKR emission frequency is close to f ce , itself proportional to the magnetic field amplitude, and thus decreases with increasing distances from the planet. We conducted a survey of these orbits to extract the average properties of the encountered radio sources and assess the ambient magnetospheric plasma parameters which control them.

Results

Throughout this large set of trajectories, similar from orbit to orbit, we were able to identify only three SKR sources. They covered the 10-20 kHz range (3.5-4.5 R S distances from the planet) and were solely found on the northern dawn-side sector. The source regions hosted narrow-banded emission, propagating in the extraordinary wave mode and radiated quasi-perpendicularly to the magnetic field lines. Their emission frequency, measured in situ, was fully consistent with the CMI mechanism driven by 6-12 keV electron beams with ring/shell velocity distribution functions, as for AKR at Earth. The SKR sources were embedded within larger regions of upward currents, themselves coincident with the ultraviolet auroral oval, which was observed simultaneously by the Hubble Space Telescope. However, unlike the terrestrial case, the spacecraft exited the radio source region before exiting the main oval/upward current region, when the ratio f pe /f ce exceeded the typical CMI threshold of 0.1. This occurred at times of sudden large-scale increases of the magnetospheric electron density.

Conclusion

These results establish that the generation conditions of SKR LF emission are strongly timevariable and illustrate the local time dependence of SKR spectrum, with the lowest frequencies (the highest altitudes) reached for dawn-side radio sources. The characteristics of CMI-unstable electrons at 3.5-4.5 R S imply that downward electron acceleration took place at farther distances along the auroral magnetic field lines and brings new constraints to particle acceleration models. Finally, the LF SKR component is mainly controlled by plasma conditions, namely time-variable magnetospheric electron densities, which can quench the CMI mechanism even if accelerated electrons are present. This explains why the magnetic field lines hosting SKR LF sources can map to a restricted portion of the ultraviolet auroral oval and associated upward current region.

Figure caption :

Schematic view of Saturn's auroral emissions. Auroral radio emissions observed above the atmosphere were remotely mapped from Cassini (most intense emissions in red) when the spacecraft encountered a low frequency radio source. The magnetic field lines hosting it, sampled in situ by the probe, map to the (atmospheric) ultraviolet auroral oval, as observed simultaneously by the Hubble Space Telescope from Earth.

Introduction

During its early 2008 high-inclination passes, the Cassini mission unexpectedly sampled two low frequency (LF) sources of Saturn's Kilometric Radiation (SKR), generally observed over the 1-1000 kHz spectral range, and thus provided the first insights into the generation of intense nonthermal auroral radio emission at a planet other than Earth [START_REF] Lamy | The Saturnian Kilometric Radiation before the Cassini Grand Finale[END_REF]. The combined analysis of radio, plasma and magnetic field in situ measurements validated the Cyclotron Maser Instability (CMI), at the origin of Auroral Kilometric Radiation (AKR) at Earth, as a universal generation mechanism able to operate in different planetary magnetospheres. The CMI is a wave-electron resonant interaction which requires (1) non-maxwellian weakly relativistic electrons (whose distribution function displays a positive gradient along the velocity direction perpendicular to the magnetic field) and (2) a ratio of the electron plasma frequency f pe over the electron cyclotron frequency f ce much lower than unity. When these two conditions are fulfilled, radio emission is amplified mainly on the right-hand extraordinary (R-X) mode at the frequency f CMI = f ce /Γ+k // v // /2π, where k is the wave vector, v the electron velocity, // refers to the direction parallel to the magnetic field and Γ = (1-v 2 /c 2 ) -1/2 is the Lorentz factor [START_REF] Treumann | The electron-cyclotron maser for astrophysical applications[END_REF]. Waves are thus amplified at the local relativistic electron cyclotron frequency Doppler shifted by the parallel propagation. At first order, this expression thus reduces to f CMI ~ f ce .

A first SKR source was identified on day 2008-291 (Oct. 17 th ) at ~10 kHz (i.e. ~5 R S distance, where 1 R S = 60268km is Saturn's radius), at the unusual Local Time (LT) of 01:00 and along -80° invariant latitude magnetic field lines, reminiscent of spiral-shaped auroral storms arising from the midnight sector. It occurred during a global SKR intensification extending to low frequencies with wave intensities of a few mV/m, coincident with unusually strong upwarddirected field-aligned currents indicative of down-going electron-beams, and was attributed to a major solar wind-driven compression of the magnetosphere [START_REF] Lamy | Properties of Saturn kilometric radiation measured within its source region[END_REF][START_REF] Bunce | Extraordinary field-aligned current signatures in Saturn's high-latitude magnetosphere: Analysis of Cassini data during revolution 89[END_REF][START_REF] Kurth | A close encounter with a Saturn kilometric radiation source region[END_REF]. The R-X mode source was identified from measurements of its low frequency cutoff f skr about 2% below f ce . Radio waves displayed strong linear polarization at the source from which they were radiated quasiperpendicularly to the local magnetic field. In this case, f skr can be expressed as f CMI = f ce (1v 2 /c 2 ) 1/2 and direct measurements of f skr yielded CMI-resonant electrons of 6-9 keV in perfect agreement with the hot (accelerated) electrons simultaneously detected in situ. Their measured shell-like 3D distribution function (referred to as horseshoe in 2D) was fully consistent with CMI-driven perpendicular emission. Sampling down-going electrons with shell-type distribution additionally implies that they had been accelerated toward the planet in regions located farther than the radio source along the flux tube [START_REF] Lamy | Properties of Saturn kilometric radiation measured within its source region[END_REF][START_REF] Schippers | Auroral electron distributions within and close to the Saturn kilometric radiation source region[END_REF][START_REF] Lamy | Emission and propagation of Saturn kilometric radiation: Magnetoionic modes, beaming pattern, and polarization state[END_REF]. A second direct confirmation of the CMI mechanism was brought by the agreement between the wave properties observed within the source region and the simulated CMI wave growth rate and emission angle driven by the electron distribution measured simultaneously [START_REF] Mutel | CMI growth rates for Saturnian kilometric radiation[END_REF]. As at Earth, the auroral plasma surrounding the emission region was hot, with a marginal cold component, and tenuous with f pe /f ce ~ 0.1 [START_REF] Hilgers | The auroral radiating plasma cavities[END_REF]. On the other hand, terrestrial-like plasma cavities were not observed within the radio source at the 2 s resolution of Cassini's electron measurements. This difference, together with the presence of SKR sources at larger distances from the planet than AKR at Earth, was attributed to the faster rotation of Saturn's magnetosphere, which means that CMI condition (2) is generally fulfilled at high latitudes without needing small-scale cavities.

A second SKR source was likely traversed as well on day 2008-073 (March 13 th ) at ~5 kHz (i.e. ~6 R S distance) [START_REF] Menietti | Saturn kilometric radiation near a source center on day 73[END_REF][START_REF] Menietti | Analysis of Saturn kilometric radiation near a source center[END_REF] The LF emission tangentially approached the f ce curve on the corresponding dynamic spectrum (time-frequency map of wave flux density) at a more usual location, near 07:30 LT, close to the SKR peak intensity region [START_REF] Lamy | An auroral oval at the footprint of Saturn's kilometric radio sources, colocated with the UV aurorae[END_REF], and along -77° invariant latitude magnetic field lines. The f pe /f ce ratio was as low as 0.05, again fulfilling CMI condition [START_REF] Treumann | The electron-cyclotron maser for astrophysical applications[END_REF]. The authors did not measure the low frequency cutoff f skr but again used the observed shelllike electron distribution at about 7 keV to calculate CMI wave growth rates compatible with the observed SKR R-X mode flux densities. This limited set of events does not provide a comprehensive view of SKR LF sources and the conditions in which the CMI can trigger them. Their understanding is however essential since intensifications of SKR spectrum in general, and of its low frequency component in particular, have long been used as a sensitive diagnostic of large-scale magnetospheric dynamics, including the SKR dual rotational modulation or night-side plasmoid ejection and associated auroral storms induced or not by the solar wind [START_REF] Kurth | An Earth-like correspondence between Saturn's auroral features and radio emission[END_REF][START_REF] Kurth | Saturn kilometric radiation intensities during the Saturn auroral campaign of 2013[END_REF][START_REF] Lamy | Saturn kilometric radiation: Average and statistical properties[END_REF][START_REF] Badman | Relationship between solar wind corotating interaction regions and the phasing and intensity of Saturn kilometric radiation bursts[END_REF][START_REF] Gurnett | Discovery of a north-south asymmetry in Saturn's radio rotation period[END_REF][START_REF] Mitchell | Recurrent energization of plasma in the midnight-to-dawn quadrant of Saturn's magnetosphere, and its relationship to auroral UV and radio emissions[END_REF][START_REF] Jackman | On the character and distribution of lower-frequency radio emissions at Saturn and their relationship to substorm-like events[END_REF][START_REF] Jackman | In situ observations of the effect of a solar wind compression on Saturn's magnetotail[END_REF][START_REF] Lamy | Variability of southern and northern SKR periodicities[END_REF][START_REF] Lamy | Multispectral simultaneous diagnosis of Saturn's aurorae through-out a planetary rotation[END_REF][START_REF] Kimura | Long-term modulations of Saturns auroral radio emissions by the solar wind and seasonal variations controlled by the solar ultraviolet flux[END_REF][START_REF] Reed | Low Frequency Extensions of the Saturn Kilometric Radiation as a proxy for magnetospheric dynamics[END_REF]. The ultimate high-inclination phase of the Cassini mission carrying the spacecraft to high latitudes at low altitudes provided an ideal occasion to tackle these questions. It started with the ring-grazing orbital sequence, during which the spacecraft repeatedly sampled the top of both northern and southern SKR emission regions, at distances corresponding to the SKR lowest frequencies. In this study, we present a survey of these orbits, extract the average properties of the crossed radio sources and discuss the magnetospheric parameters which control them.

Survey of ring-grazing orbits

The 20 ring-grazing orbits (orbits 251 to 270), whose perikrone lies outside the F-ring, were executed between days 2016-335 (Nov. 30 th ) and 2017-112 (Apr. 22 nd ), and sampled auroral field lines expected to host SKR sources radiating up to 30 kHz (40 kHz) in the dawn-side northern (dusk-side southern) hemisphere, as illustrated in supplementary Figure S1. They were preceded by 7 orbits (orbits 244 to 250) starting on day 2016-270 (Sep. 26 th ) which brought the spacecraft to high latitudes and at distances close enough to the planet to probe SKR sources at a few kHz as well. We conducted a survey of kilometric sources crossed over these 27 orbits thanks to combined observations of the Radio and Plasma Wave experiment (RPWS; 25) and the magnetometer (MAG; 26).

The lack of in situ measurements of low energy electron distribution functions from the Cassini Plasma Spectrometer (CAPS; 27) after 2012 prevented the identification of SKR sources from the direct analysis of simultaneous radio and electron measurements based on growth rate calculation. Instead, on the basis of the reference event of day 2008-291, SKR sources were identified solely from RPWS/HFR (High Frequency Receiver) and MAG data whenever (i) the emission LF cutoff f skr passed strictly below f ce (ii) for at least 2 consecutive RPWS/HFR 16-s measurements. Criterion (i) rests upon the assumption perpendicular emission which we will later show to be valid. Criterion (ii) was aimed at being conservative and avoid false detections induced by spiky signals. The method used to determine f skr (3) is described in the supplementary material. Overall, the survey revealed only 3 events which unambiguously fulfilled these two criteria. All occurred in the northern hemisphere on days 2016-339 (Dec. 4 th ), 2016-346 (Dec. 11 th ) and 2017-066 (Mar. 7 th ), hereafter labelled as events S 1 , S 2 and S 3 . No source was found in the southern hemisphere. We rejected several candidate northern SKR sources where the LF emission region was clearly tangent to f ce but which did not fulfill criterion (ii), for instance on days 2016-332 (Nov. 27 th ) or 2017-016 (Jan. 16 th ). These may be either nearby sources (close to but not crossed by Cassini) or crossings of small-sized sources lasting less than 16 s (~200-300 km for typical spacecraft velocity) and/or emission produced at f CMI > f ce , as in the case of oblique emission where the k // v // term in the expression of f CMI becomes non negligible [START_REF] Lamy | Emission and propagation of Saturn kilometric radiation: Magnetoionic modes, beaming pattern, and polarization state[END_REF][START_REF] Mutel | CMI growth rates for Saturnian kilometric radiation[END_REF].

Events S 1 , S 2 and S 3 were sampled during intervals of relatively quiet solar wind activity (C. Tao and T. Kim, personal communication). In addition, events S 1 and S 2 occurred close to the predicted rotational peak of northern SKR bursts while event S 3 was rather in anti-phase with it (1). The first two events were thus consistent with rather standard auroral activity, while the latest one indicated a more unusual case, possibly driven by internal dynamics. The detailed wave properties and characteristics of the 3 identified sources are listed in table 1 and analyzed hereafter, on the basis of the most illustrative examples of events S 3 and S 1 . A polar projection of the sources of these events is provided in Figure S2. Event S 3 displayed the most intense radio emission and benefited from coordinated observations of the northern ultraviolet (UV) aurorae by the Hubble Space Telescope (HST), while event S 1 could be sampled by RPWS/WBR waveform measurements (Wideband Receiver), the first ever obtained within a SKR source.

Examples of SKR source crossings

Radio, plasma and magnetic field measurements during events S 3 and S 1 are compared in Figures 1 and2, respectively, with identical formats. SKR is the patchy bright emission observed above 10 kHz in panel A (dynamic spectrum of the wave Poynting flux S), with dominant right-handed (RH) circular polarization in panel B (dynamic spectrum of the normalized degree of circular polarization V) indicative of northern R-X mode. Some weaker left-handed (LH) polarized SKR, indicative of left-hand ordinary (L-O) mode, was additionally observed at the low frequency edge of the R-X mode SKR on the right-hand side of each interval, preceding the perikrone. The weakly polarized emission visible below 10 kHz after 15:30 UT on Figure 1A-B is identified as auroral hiss. Narrow-banded long-duration and drifting short-duration emissions observed below 35 kHz are not considered in this study [START_REF] Ye | Source locations of narrowband radio emissions detected at Saturn[END_REF][START_REF] Taubenschuss | Saturnian low frequency drifting radio biursts : statistical properties and polarization[END_REF]. Localized SKR R-X mode LF emission is tangent to f ce near the middle of each interval. Condition f skr < f ce (panel C) is then used to precisely define the overall extent of the source region (orange-shaded area, extended by vertical dotted-dashed lines on the other panels). During event S 3 (Figure 1A-C), a single SKR source was continuously observed from 15:11:48 to 15:17:25 UT, corresponding to a 6050 km distance along the spacecraft trajectory. During event S 1 (Figure 2A-C), multiple SKR sources were observed between 08:42:50 and 08:59:54 UT.

Figure 3 shows dynamic spectra zooming on Figure 2A at unprecedented time-frequency resolution thanks to RPWS/WBR waveform measurements. Faint, localized, remote SKR emissions observed well above f ce (panel A) display recurrent drifting sub-structures with positive and (mostly) negative rates of ~1 kHz/min, in agreement with previous SKR waveform observations [START_REF] Kurth | High spectral and temporal resolution observations of Saturn kilometric radiation[END_REF]. Their analysis is beyond the scope of this study. A zoom in on the 08:00-09:10 interval (panel B) reveals the time-frequency structure of the encountered radio source. While Figure 3A suggested a single emission pattern with a bandwidth of a few kHz lasting for nearly 2 h, Figure 3B shows that it actually splits into several sub-components of variable durations and bandwidths. Those with frequencies slightly, but clearly, above f ce likely correspond to sources on nearby flux tubes, while only the component strictly tangent to and below f ce indicates a local source. The overplotted red f skr profile (as determined in Figure 2C) provides, despite its 16 s temporal resolution, a one-to-one correspondence between the brief episodes of f skr < f ce and the observed SKR bursts (be it the central one between 08:45 and 08:50 or the shorter ones beside it) here observed at a 1-s cadence. This agreement validates the method employed to determine f skr to the point that, for this specific event, we considered non-consecutive RPWS/HFR episodes with f skr < f ce as isolated sub-sources. The overall source region of event S 1 thus decomposes into 9 SKR sub-sources. Their horizontal size along the spacecraft trajectory, and across the encountered flux tubes, ranges from 240 to 3400 km. Their ~100-400 Hz bandwidth Δf characterizes very narrow-banded emissions with Δf/f ce ~ 0.008-0.03, and thus correspond to radiating structures which extended vertically ~700-2800 km along the magnetic flux tubes.

Perpendicular emission and energy of source electrons

On the right-hand side of Figures 1-2C is displayed a scale providing the energy of CMI-unstable electrons associated with the measurements of f skr < f ce , assuming perpendicular emission at the source and thus f skr = f ce (1-v 2 /c 2 ) 1/2 , as observed during the southern event of 2008-291. To check this assumption, we analyzed the wave parameters derived from the goniopolarimetric analysis of RPWS/HFR 3-antenna measurements (see the supplementary material for details). These consist of the wave Stokes parameters S, Q, U and V (as displayed on Figures 1-2 andS3) and the k-vector direction. From the latter were can additionally retrieve the 3D locus of the radio source, from the intersection of the k line-of-sight with a model iso-f=f ce surface (assuming straight-line propagation, which is reasonable at high latitude), and its beaming angle θ = (k,B), where B is the magnetic field vector at the source [START_REF] Cecconi | Goniopolarimetric study of the revolution 29 perikrone using the Cassini Radio and Plasma Wave Science instrument high-frequency radio receiver[END_REF][START_REF] Lamy | Properties of Saturn kilometric radiation measured within its source region[END_REF][START_REF] Lamy | Emission and propagation of Saturn kilometric radiation: Magnetoionic modes, beaming pattern, and polarization state[END_REF][START_REF] Lamy | An auroral oval at the footprint of Saturn's kilometric radio sources, colocated with the UV aurorae[END_REF]. Figure 4 displays the variation of the beaming angle θ(f) of events S 3 and S 1 for the time intervals of Figures 1-2 using a standard data selection with red (blue) symbols corresponding to RH (LH) polarized waves, and thus to R-X (L-O) mode. The solid colored lines provide the median value of θ at each frequency for which the number of crosses (exact determination of the source position) exceeds the number of diamonds (approximated determination of the source position). In agreement with previous studies of the SKR beaming angle (1 and refs therein), θ(f) of R-X mode emission smoothly rises from ~60°±25° at ~800 kHz to >85°±15° at 30 kHz, the uncertainty on θ being dominated by the scattering of data points. Supplementary figure S3E shows an alternative representation of θ(f) in the time-frequency plane. This systematic change from perpendicular to oblique emission with increasing frequency may be primarily due to refraction of R-X mode waves on the nearby iso-f X surface, where f X ~ f ce is the R-X mode cutoff frequency, as proposed for auroral radio emissions of the Earth and Jupiter [START_REF] Mutel | Cluster multispacecraft determination of AKR angular beaming[END_REF][START_REF] Galopeau | An oblate beaming cone for Io-controlled Jovian decameter emission[END_REF]. In this case, the apparent R-X mode beaming angle should vary as a function of the k-vector direction projected in a plane perpendicular to B, with strongest refraction expected for wave propagation toward the equator. Interestingly, L-O mode SKR was also observed, over an extended spectral range of 20-100 kHz, with θ(f) remaining confined above 75° and with a lower scattering than the R-X mode one. Indeed, the L-O mode cutoff frequency being f pe , which was here much lower than f ce (see Figures 1-2D), no similar refraction on the iso-f O surface is expected. This interpretation, whose validation is beyond the scope of this study, thus implies that θ(f) measures in fact the apparent (real) beaming angle of SKR R-X (L-O) mode affected (unaffected) by refraction. Although they differ at high frequencies, both remote observations of R-X and L-O mode θ(f) indicate quasi-perpendicular emission at 30 kHz, toward frequencies where the local radio sources have been sampled.

A second method providing a direct measurement of the wave emission angle within the source rests upon its polarization state. Indeed, for perpendicular cyclotron emission, the polarization ellipse ratio AR = (T 1/2 -L)/V, with T = (Q 2 +U 2 +V 2 ) 1/2 and L = (Q 2 +U 2 ) 1/2 being the total and linear polarization degrees [START_REF] Kraus | Wave polarization. In Radio Astronomy[END_REF], can be approximated by cos θ at first order [START_REF] Melrose | On the elliptical polarization of Jupiter's decametric radio emission[END_REF]. Figures 1-2B and Figure S3A-D show that the emission is indeed strongly linearly polarized at the source, again as it had been observed during the southern event of day 2008-291. Measurement of the mean AR in each SKR source of the 3 events S 1 , S 2 and S 3 , considering the two HFR frequencies encompassing f ce with standard data selection (SNR > 20 dB, T > 0.85, |z r | < 0.2) gives AR = -0.05±0.15, -0.07±0.27 and -0.02±0.26, which results in θ = 93±9°, 86±16° and, 91±16° respectively, all fully consistent with perpendicular emission.

The measured values of f skr can now be safely inverted to derive energies of CMI-unstable electrons, as summarized in table 1 and illustrated in Figure 1-2C. In average, f skr reached a -2% level below f ce with an extremum at -3.5%. The corresponding electron energies spanned the 1-17keV range, with mean values between 6 and 12 keV depending on the event and a standard deviation of a 3-4 keV, larger than the 2 keV resolution arising from the accuracy of f skr . We did not notice any correlation between the obtained electron energies and the observed wave intensities, measured between a few 0.01 and a few mV/m.

Auroral plasma conditions

We now investigate the auroral plasma conditions to assess the validity of CMI condition (2). Figures 12D display the characteristics frequencies f skr (solid line), f ce and f ce /10 (dashed lines) together with f pe (colored lines). f pe was derived from Langmuir Probe (LP) 32-s measurements (gray profile) and compared to the upper frequency cutoff of auroral hiss (red profile) derived independently from the RPWS/MFR (Medium Frequency Receiver) 64-s measurements. The LP measures the total electrical current carried by the ambient plasma from voltage sweeps, from which the electron density N e and thus f pe are derived. Here, the voltage sweep resolution implies a typical uncertainty on f pe less than a factor of 2. Auroral hiss is a whistler-mode emission radiated upward from below the spacecraft and propagating beneath f pe , so that its upper frequency cutoff, when marking a sharp transition, is a proxy of f pe when f pe < f ce . Here, the observed hiss emissions were generally damped and exhibited a smoothed cutoff so that the fitted frequencies were rather interpreted as a lower limit of f pe , labelled f pe min . These two methods are described in more details in the supplementary material. On Figures 12, the f pe min profiles in red were indeed close to and generally below the LP-derived f pe profiles in gray. For event S 1 , f pe /f ce was about 0.03 across the whole source region, which exactly ended when the value of f pe (f pe min ) suddenly rose by a factor of 4 (13) at 15:18 UT (15:20), overcoming the f ce /10 threshold for CMI condition [START_REF] Treumann | The electron-cyclotron maser for astrophysical applications[END_REF]. For event S 3 , a similar density gradient was observed in both datasets. The hiss cutoff frequency f pe min rose by a factor of 5 at 09:04, again shortly after the SKR source region at 09:00. The LP-derived f pe , determined with a time resolution twice better, reveals more complex variability: while f pe rose by a factor of 4 at 08:42, it then showed large-scale fluctuations of a factor of ~3 (~2) up to (after) 09:00. Interestingly, the SKR subsources (sampled at a better cadence) strikingly match the local minima of f pe , suggesting that small-sized radio sources were possibly co-located with density cavities. During event S 2 , the LP-derived f pe was obtained with too low time resolution to provide any reliable estimate of f pe /f ce .

Figures 12show the azimuthal component of the magnetic field measured from 1-min averaged MAG data. They indicate upward currents in intervals dominated by a positive slope, the main one being marked by a green-shaded region. Each SKR source region precisely started at (but ended before) the beginning (the end) of the upward current region. The upward current density was estimated to 106, 171 and 157 nAm -2 for events S 1 , S 2 and S 3 , respectively, well above the average of 80 nAm -2 derived from all the ring-grazing northern upward current regions [START_REF] Hunt | Field-aligned currents in Saturn's magnetosphere: Observations from the F-ring orbits[END_REF].

Additional information on the correspondence between auroral emissions and upward currents is brought by Figure 5, which maps the spatial distribution of SKR sources during event S 3 together with that of northern UV aurorae simultaneously observed by HST on 2017-066 (Mar. 17 th ) over 15:27-16:21 UT (light-time corrected). Panel (A) displays the distribution of SKR emissions observed over the same time interval as Figure 1 in the plane of sky as seen from Cassini. The displayed data, definition of symbols and color scales are similar to Figure 4A, except that the data selection was here more restrictive for clarity. As expected from emission close to f ce (directly proportional to |B|), SKR lower (higher) frequency sources are observed at larger (closer) distances from the planet. Panel (B) displays in a polar view the magnetic footprint of radio sources visible in panel (A). Dotted-dashed lines indicate radio horizons at extremal frequencies, beyond which no radio source can in principle be observed. The R-X mode footprints (in red) draw a clear circumpolar auroral oval visible from 04:00 to 22:00 LT. Its large latitudinal width of about 8-10° is dominated by the scattering of goniopolarimetric results. This extended range derives from the LT range swept by the spacecraft (green double arrow) during the investigated interval. Movie 1 displays an animation of panels (A-B) and illustrates how the spatial distribution of detected sources varies with the time-variable position of the spacecraft. The L-O mode footprints (in blue) are well clustered and precisely co-located with the R-X mode ones between 08:00 and 10:00 LT. The comparison with panel (D) illustrates that SKR sources are strikingly co-located with the UV auroral oval (see Figure S4 for a superimposition of both panels), both on the dawn-side sector, where the UV oval is thin and well defined between 06:00 and 12:00 LT from +72° to +75° latitudes, and on the dusk-side sector, with UV aurorae more spread in latitude between 14:00 and 23:00 LT and including a poleward arc beyond +80° between 14:00 and 18:00 LT. The Cassini footprint trajectory is displayed on panels B-D over the 13:00-17:00 UT interval by the gray line. The green and red (overlapping) portions of it refer to the upward current region and the SKR source region identified in Figure 1. The upward current layer exactly matches the main UV auroral oval in size while the SKR source was crossed along a smaller, poleward, portion of it. This can be better seen in panel (C), which is similar to (B) except that it restricts to the crossed SKR source component with slightly relaxed data selection to improve statistics (the supplementary material discusses the reliability of direction-finding results for strongly linearly polarized emissions such as those observed here). Again, the footprints of R-X mode located sources nicely map to the red portion of Cassini's trajectory and only to the inward portion of the main oval and associated upward current layer.

Discussion

We now discuss the average properties of SKR LF emissions as a whole in the CMI frame. The identification of solely 3 SKR sources in the dawn-side sector and none in the dusk-side sector brings a direct in situ confirmation to the LT dependence of SKR spectrum (here observed during quiet solar wind conditions), with lowest frequencies at dawn [START_REF] Lamy | Saturn kilometric radiation: Average and statistical properties[END_REF][START_REF] Kimura | Long-term modulations of Saturns auroral radio emissions by the solar wind and seasonal variations controlled by the solar ultraviolet flux[END_REF]. The small number of radio sources identified along orbits with very similar trajectories and their limited extent additionally reveal time-variable generation conditions. In all three cases, the sampled R-X mode emission was found to be amplified nearly perpendicularly to the local magnetic field, with emission frequency clearly below f ce supplied by CMI-unstable electrons of 6-12 keV mean energies. Direct perpendicular emission through CMI additionally requires specific shell-type distribution functions of the accelerated electrons in the phase plane such as those already measured for the 2 southern SKR sources crossed in 2008 [START_REF] Mutel | CMI growth rates for Saturnian kilometric radiation[END_REF][START_REF] Menietti | Analysis of Saturn kilometric radiation near a source center[END_REF]. The kronian situation thus directly compares to the terrestrial case, where AKR sources are commonly found to be associated with trapped or shell/horseshoe/ring distributions [START_REF] Treumann | The electron-cyclotron maser for astrophysical applications[END_REF][START_REF] Louarn | Trapped electrons as a free energy source for the auroral kilometric radiation[END_REF][START_REF] Ergun | Electron-Cyclotron Maser Driven by Charged-Particle Acceleration from Magnetic Field aligned Electric Fields[END_REF]. It differs from the situation investigated in situ by Juno at Jupiter, where the hectometer and decameter emissions appear to be produced at oblique angles from the local magnetic field by loss cone electron distribution functions [START_REF] Louarn | Generation of the Jovian hectometric radiation: First lessons from Juno[END_REF][START_REF] Louis | Io-Jupiter decametric arcs observed by Juno/Waves compared to ExPRES simulations[END_REF]. The presence of 6-12 keV electron-beams with such distribution functions at 3.5-4.5 R S distances from Saturn's center, or equivalently 2.8-3.7 R S from the atmosphere along the encountered flux tubes (see table [START_REF] Lamy | The Saturnian Kilometric Radiation before the Cassini Grand Finale[END_REF], imply that at least part of the acceleration by upward currents occurred farther along the field lines, which provide further constraints to models of current-voltage relationships [START_REF] Ray | Current-voltage relation for the Saturnian system[END_REF].

The general agreement with the CMI mechanism relies on the fulfilling of both CMI conditions mentioned in the introduction. This is first worth noting that the 3 encountered SKR sources matched upward currents with current densities larger than the average, consistent with an acceleration region extending to higher altitudes and thus satisfying CMI condition [START_REF] Lamy | The Saturnian Kilometric Radiation before the Cassini Grand Finale[END_REF]. We then noticed that, while the spacecraft entered the northern SKR source region when it encountered an upward current region, it exited it when f pe suddenly rose, exceeding the typical f ce /10 threshold. After exiting the source, the spacecraft remained for some time inside the upward current region itself precisely mapping to the main UV oval across the Cassini trajectory. This suggests that at 3.5-4.5 R S distances, SKR emission is primarily controlled by the electron plasma density N e through CMI condition (2) which quenches the radiation when f pe /f ce typically exceeds 0.1. According to the Knight relationship [START_REF] Knight | Parallel electric fields[END_REF] and assuming a constant current density, larger N e is also expected to decrease electron acceleration and weaken CMI condition (1). Figure 6 displays radio, plasma and magnetic measurements in format identical to Figure 1 A,D-E for 3 successive, quasi-identical, passes of the spacecraft in the northern dawn-side auroral region : the middle panels correspond to event S 3 investigated in Figure 1 (orbit 264), while the right and left-handed panels correspond to the previous (orbit 263) and the following (orbit 265) auroral passes. The probe did not fly across any local SKR source during orbit 263 and 265 while it encountered regions of upward current in all cases (green-shaded regions) with current densities of 29 and 108 nA m -2 , again mapping to the main UV auroral oval (the HST observation obtained during orbit 265 is not shown). On orbit 265, the SKR intensity was relatively bright and its spectrum reached low frequencies down to 20 kHz, while the upward current density was as strong as that of orbit 264. Instead, on orbit 263, SKR was comparatively fainter with a spectrum confined to higher frequencies and a lower upward current density. This suggests that CMI condition (1) requiring unstable electrons at 3.6 R S was plausible for orbit 265, at least. For both intervals though, f pe was systematically larger than f ce /10 within the upward current region, in contrast with orbit 264 (event S 3 ), therefore revealing that the mean level of N e over hours-long interval significantly varies with time from pass to pass. This further shows that CMI condition (2) was not fulfilled for any of orbits 263 and 265 and possibly quenched any SKR LF emission during orbit 265.

The origin of time-variable plasma densities at the altitude of LF SKR sources has now to be identified. The asymmetrical plasmapause-like boundary, this transition from high to low plasma densities with increasing latitudes which rotates at the northern SKR period, is a plausible source for time-variable N e [START_REF] Gurnett | The rotation of the plasmapause-like boundary at high latitudes in Saturn's magnetosphere and its relation to the eccentric rotation of the northern and southern auroral ovals[END_REF]. On a shorter timescale, Figure 1 does not display any Earth-like plasma cavity, as already noticed for the 2008-291 southern event. However, Figure 2 interestingly shows small-sized, recurrent, drops of f pe co-located with SKR sources for event S 1 (only). This suggests that acceleration features could coincide with localized radio sources, as at Earth. Definite conclusions however rely on knowing whether the plasma region was dominated by hot electrons (such as on day 2008-291) or could have included a significant cold plasma component sensitive to local potential drops and able to map plasma cavities. As the CAPS experiment was not operating during these orbits, these questions remain open.

Finally, the very narrowbanded nature of SKR sources noticed escpecially during event S 1 is consistent with either a non-saturated linear wave growth, as predicted by the CMI theory and measured in laboratory (e.g. [START_REF] Bingham | Laboratory astrophysics: Investigation of planetary and astrophysical maser emission[END_REF], or with saturation by coherent non-linear trapping previously assumed to correctly model the SKR spectrum [START_REF] Galopeau | Theoretical model of Saturn's kilometric radiation spectrum[END_REF]. However, the emission levels may be too low to even reach saturation and the observed wave intensities (table 1) are rather consistent with linear growth.

This study of SKR LF sources will form a basis to similarly analyze the Cassini proximal orbits, in which the spacecraft sampled the peak of SKR spectrum at lower altitudes (see Figure S1).

The authors thank the engineering teams in charge of processing the data, in particular T. Averkamp in Iowa City and M.-P. Issartel in Meudon. The French coauthors acknowledge support from CNES and CNRS/INSU programs of Planetology (PNP) and Heliophysics (PNST). LL thanks L. Spilker and the Cassini/MAPS group for their support to the HST program coordinated with the Cassini Grand Finale, together with C. Tao and T. Kim for inputs on solar wind conditions at Saturn. The research at the University of Iowa was supported by NASA through Contract 1415150 with the Jet Propulsion Laboratory.

LL led the analysis and wrote the paper. PZ, BC and WSK helped in the analysis of HFR data, and GH in the understanding of WBR measurements. AP analyzed the MFR measurements and digitized hiss emission frequencies. MM and JEW analyzed the LP data and derived electron densities. RP contributed to the investigation of HST observations. GH calculated the current densities from MAG data. All authors discussed the results and commented on the manuscript. 2, but for measurements acquired on day 2016-346 between 11:00 and 15:00 UT (event S 2 ) in the northern dawn-side sector. One SKR source near 15 kHz was encountered between 13:33 and 13:34 (rejecting two side isolated measurements of f SKR < f ce ). The LP-derived f pe was obtained at a ~42 s temporal resolution, lower than for events S 1 and S 3 . where k is the wave vector and B the magnetic field vector at the source derived with the SPV-CS model. This remote determination of θ relies on the knowledge of k, whose uncertainty increases with increasing L (as k is taken as the normal for the polarization ellipse). In the SKR source where linear polarization is strong, θ is therefore largely scattered.

Fig. 1 .

 1 Fig. 1. Cassini radio, plasma and magnetic measurements acquired on day 2017-066 between 13:00 and 17:00 UT (event S 3 ) in the northern dawn-side sector. The used datasets are described in details in the supplementary material. (A and B) Dynamic spectra of wave Poynting flux S and normalized degree of circular polarization V respectively derived from 2-and 3-antenna

Fig. 2 .

 2 Fig. 2. Cassini radio, plasma and magnetic measurements acquired on day 2016-339 between 07:00 and 11:00 UT (event S 1 ) in the northern dawn-side sector, in a format identical to Figure 1. Here, 9 SKR sources near 12 kHz were successively encountered between 08:43 and 09:00. The overall interval is delimited by a light orange-shaded region, while the individual SKR sources are indicated by dark orange-shaded areas.

Fig. 3 .Fig. 4 .Fig. 5 .

 345 Fig. 3. High resolution dynamic spectra of event S 1 reconstructed from Cassini radio electric waveform measurements acquired on 2016-339. They were acquired with the RPWS/WBR receiver over the 0-80 kHz range, as described in the supplementary material. (A) Reconstructed dynamic spectrum between 07:30 and 09:30 UT. The dashed line indicates f ce . (B) Zoom on the encountered local SKR source. The red solid line indicates f skr , as derived from RPWS/HFR measurements in Figure 1D. It is in excellent agreement with high resolution WBR measurements of SKR emission close to and below f ce .

Fig. 6 .Movie 1 .

 61 Fig. 6. Cassini radio, plasma and magnetic measurements acquired during 3 successive auroral passes with very comparable trajectories. (D-F) Identical to Figure 1 A,D-E covering the interval 2017-066 from 13:00 to 17:00 UT encompassing a SKR source crossing (orange-shaded region).(A-C) and (G-I) Format identical to panels D-F for the intervals 2017-059 09:00-13:00 and 2017-073 17:00-21:00, respectively. No SKR source was crossed during these intervals, for which f pe was always larger than f ce /10.

Fig. S2 .

 S2 Fig. S2.As Figs1-2, but for measurements acquired on day 2016-346 between 11:00 and 15:00 UT (event S 2 ) in the northern dawn-side sector. One SKR source near 15 kHz was encountered between 13:33 and 13:34 (rejecting two side isolated measurements of f SKR < f ce ). The LP-derived f pe was obtained at a ~42 s temporal resolution, lower than for events S 1 and S 3 .

Fig. S3 .

 S3 Fig. S3. Polar projection of the magnetic footprint of known R-X mode SKR low frequency sources. (A) Northern SKR sources identified in this study during the Cassini ring-grazing orbits. The average UV aurorae observed by HST over 2017 are superimposed as colored iso-contours (29). (B) Southern SKR sources identified during Cassini high latitudinal passes of 2008 (4,11). The gray lines represent 4h-long portions of the spacecraft footprint trajectory centered on each source.

  Fig. S4. Dynamic spectra of SKR wave parameters derived from 3-antenna RPWS/HFR measurements. (A) Wave Poynting flux S. (B) Normalized circular polarization degree V. (C) Normalized linear polarization degree L. (D) Normalized total polarization T. (E) SKR beaming angle θ = (k,B),where k is the wave vector and B the magnetic field vector at the source derived with the SPV-CS model. This remote determination of θ relies on the knowledge of k, whose uncertainty increases with increasing L (as k is taken as the normal for the polarization ellipse). In the SKR source where linear polarization is strong, θ is therefore largely scattered.

Fig. S5 .

 S5 Fig. S5. Superimposed polar view of UV aurorae and magnetic footprint of SKR sources measured on day 2017-066 as derived in Fig. 5. (A) Superimposition of Figs 5B (remote SKR sources observed from 13:00 to 17:00 UT UT). (B) Superimposition of Figs 5C (local SKR sources crossed through between 09:00 and 09:20 UT) and 5D.

Table 1 .

 1 Properties of 3 R-X mode SKR low frequency sources identified during the Cassini ring-grazing orbits. When provided, uncertainties indicate standard deviations.

	Time range (UT)	2016-339	2016-346	2017-066
		08:42:50 -08:59:54	13:33:26 -13:34:30	15:11:48 -15:17:25
		(orbit 251)	(orbit 252)	(orbit 264)

Acknowledgments:

The used data are available from the following sources. The Cassini/RPWS and MAG original data are accessible through the PDS archive at archived at https://pds.nasa.gov/. The HFR processed data are available though the LESIA/Kronos database at http://www.lesia.obspm.fr/kronos. The UV observations were obtained from the ESA/NASA Hubble Space Telescope (GO program 14811) : the original data can be retrieved from the MAST archive, and the processed data from the APIS service http://apis.obspm.fr hosted by Paris Astronomical Data Centre.

Supplementary Materials:

Materials and Methods:

The different datasets and analysis methods employed in this study are briefly described and discussed below. The characteristic frequencies employed throughout the manuscript are the electron cyclotron frequency ! "# = %& 2() and the electron plasma frequency ! *# = +% , 4( , ). / where e is the elementary electric charge, B the magnetic field amplitude, m the electron mass, N the electron plasma density and . / the vaccum permittivity.

HST observations

HST observations of the northern UV aurorae of Saturn were acquired with the Space Telescope Imaging Spectrograph (STIS, 47) at regular intervals during the Cassini ring-grazing and proximal orbits throughout 2017. They were scheduled as close as possible to the expected pass of the Cassini spacecraft within SKR sources [START_REF] Taubenschuss | Saturnian low frequency drifting radio biursts : statistical properties and polarization[END_REF]. As illustrated in Fig. S1, these were predicted as when the Cassini trajectory intercepted magnetic field lines mapping to invariant latitudes between -70° and -80°, the typical latitudinal range of kronian aurorae, and assuming emission at f ce over the 4-1000 kHz typical SKR spectral range. For that purpose, we used the Saturn-Pioneer-Voyager internal field model (48) complemented by a simple current sheet [START_REF] Gérard | Altitude of Saturn's aurora and its implications for the characteristic energy of precipitated electrons[END_REF], hereafter referred to as SPV-CS.

All the STIS images were long exposures using the time-tag mode and the SrF 2 filter which isolates H 2 auroral emissions together with planetary and sky background signals. The UV polar view displayed in Fig. 5 was obtained following the pipeline described in [START_REF] Taubenschuss | Saturnian low frequency drifting radio biursts : statistical properties and polarization[END_REF][START_REF] Gustin | Conversion from HST ACS and STIS auroral counts into brightness, precipitated power, and radiated power for H 2 giant planets[END_REF]. The mapping was made at a 1100 km altitude [START_REF] Cecconi | Direction finding and antenna calibration through analytical inversion of radio measurements performed using a system of 2 or 3 electric dipole antennas[END_REF] after the image was corrected from an empirical background model and the count rate converted into brightnesses (photon flux) derived over the full H 2 bands (70-180 nm) and expressed in kilorayleighs (kR) [START_REF] Morooka | The electron density in Saturn's magnetosphere[END_REF].

RPWS/HFR measurements

The swept-frequency High Frequency Receiver (HFR) of the Radio and Plasma Wave Science (RPWS) experiment onboard the Cassini spacecraft [START_REF] Dougherty | The Cassini magnetic field investigation[END_REF] measures the ambient wave electric power spectral density between 3.5 kHz and 16.125 MHz using three electrical antennas, labelled +X, -X and Z (monopoles also identified as u, v and w). For the time intervals investigated in this study, the HFR was operating in 3-antenna mode, sweeping the whole HFR spectral range every 16 s. From 3.5 to 1000 kHz, the measurements were obtained through four spectral bands, logarithmically spaced channels with spectral coverage ∆f/f = 5% (where ∆f is the spectral bandwidth at each sampled frequency f) between 3.5 and 325 kHz with 500 ms integration time and linearly-spaced channels at 25 kHz resolution above 325 kHz with 80 ms integration time. A 3-antenna measurement is obtained at each frequency by acquiring a pair of successive 2-antenna measurements on the +XZ and -XZ pairs of antennas. A single 2-antenna measurement consists of 2 autocorrelations measured on each antenna signal and 2 measurements of the real and imaginary parts of the cross-correlation sensed between both antenna signals. Taking the example of the +XZ pair, they are labelled A +XX , A +ZZ , C r +XZ and C i +XZ , respectively. Each 3-antenna measurement thus provides a total of 8 auto-and cross-correlations (among which only 7 are independent, A ZZ being measured twice). The signal temporal variation between the consecutive 2-antenna measurements is estimated by the parameter 0 1 = (4 566 -4 866 ) (4 566 + 4 866 ). A signal-to-noise ratio (SNR) is derived for each autocorrelation value.

The SKR low frequency cutoff f SKR displayed in Figs 1-3, 6 and S2 was determined from the method described in (4). It consists of cross-correlating each HFR spectrum with a simulated HFR response to an ideal SKR source, whose intensity as a function of frequency is modelled by a step function of variable cutoff frequency, until finding the one which provides the best fit. f SKR could thus be determined with a spectral resolution 10 times better than the original HFR one. The minimum and maximal intensities of the step function were first automatically fixed by the peak and background spectral flux densities observed on one monopole over the spectral range of the investigated SKR source region. In a second step, the peak intensity was manually decreased whenever necessary to provide a better fit to the data. The ideal step function assumes one dominant radio source, while Fig. 3 shows that several of them can overlap close to f ce , so that the crossed source is not necessarily the most intense one. SKR sources were identified by events during which f SKR < f ce over at least two consecutive RPWS/HFR measurements. A polar view of the magnetic footprint of the three northern R-X mode sources identified in this study is displayed in Fig. S3A, at latitudes coincident with the average UV auroral oval [START_REF] Taubenschuss | Saturnian low frequency drifting radio biursts : statistical properties and polarization[END_REF]. Fig. S3B displays a similar polar view of the two southern R-X mode sources identified in 2008 for comparison [START_REF] Bunce | Extraordinary field-aligned current signatures in Saturn's high-latitude magnetosphere: Analysis of Cassini data during revolution 89[END_REF][START_REF] Menietti | Analysis of Saturn kilometric radiation near a source center[END_REF]. HFR 2-and 3-antenna measurements can be inverted by goniopolarimetric (or direction-finding) inversions to retrieve the wave parameters, namely the 4 Stokes parameters S, Q, U and V and the k-vector direction defined by 2 angular coordinates [START_REF] Persoon | Polar cap electron densities from DE 1 plasma wave observations[END_REF]. 2-antenna inversions can provide 4 parameters out of 6 by assuming additional physical assumptions (for instance by fixing the source direction or neglecting the wave linear polarization). 3-antenna inversions provide the 6 wave parameters without any assumption, out of point sources. At observing latitudes beyond 30°, SKR polarization becomes strongly elliptically polarized (54), so that 3-antenna measurements are mandatory to correctly assess the wave Poynting vector (intensity S and k-vector direction) and polarization state Q, U, V, from which the normalized linear polarization degree ; = < , + = , and total polarization degree > = < , + = , + ? , 2 are derived [START_REF] Cecconi | Goniopolarimetric study of the revolution 29 perikrone using the Cassini Radio and Plasma Wave Science instrument high-frequency radio receiver[END_REF]. Goniopolarimetric results are affected by several effects described and discussed in [START_REF] Persoon | Polar cap electron densities from DE 1 plasma wave observations[END_REF][START_REF] Mutel | Cluster multispacecraft determination of AKR angular beaming[END_REF][START_REF] Lamy | An auroral oval at the footprint of Saturn's kilometric radio sources, colocated with the UV aurorae[END_REF]. In this study, we systematically removed Radio Frequency Interferences (RFI) and applied a severe selection of 3antenna data when studying wave polarization or direction by fixing (a) a threshold on the SNR (condition simultaneously applied to the four autocorrelations of each 3-antenna measurement), (b) a minimum total polarization T to remove aberrant or unpolarized signal and (c) a maximal value of |z r | to remove rapidly varying signals. In such cases, we also checked that the angle between the antenna plane and the direction of sources remained large enough to provide reliable results [START_REF] Persoon | Polar cap electron densities from DE 1 plasma wave observations[END_REF]. Fig. S4 plots dynamic spectra of S, V, L, T and of the SKR beaming angle θ over the interval 2017-066 13:00-17:00 UT, thus complementing Figs 1 and4. SKR is generally quasi-fully polarized (Fig. S4D), with circular polarization dominant at high frequencies and low observing latitudes (Fig. S4B), while linear polarization becomes increasingly large at high latitudes and toward low frequencies (Fig. S4C) down to the SKR source region (Fig. S4A). High levels of L (and low levels of V) measured for local waves close to f ce near the SKR source region confirms previous results obtained in the southern night-side hemisphere [START_REF] Mutel | CMI growth rates for Saturnian kilometric radiation[END_REF].

The 3D locus of the radio source can be retrieved from each 3-antenna gonipolarimetric measurement at the crossing of the k-vector direction of its associated iso-f = f ce surface (or the closest position to it in the absence of crossing) derived from the SPV-CS field model, assuming emission at f = f ce and straight line propagation, as described in [START_REF] Mutel | Cluster multispacecraft determination of AKR angular beaming[END_REF][START_REF] Lamy | An auroral oval at the footprint of Saturn's kilometric radio sources, colocated with the UV aurorae[END_REF][START_REF] Bunce | Extraordinary field-aligned current signatures in Saturn's high-latitude magnetosphere: Analysis of Cassini data during revolution 89[END_REF]. This method then provides a direct remote measurement of the SKR beaming angle θ = (k,B) when B is the magnetic field vector at the source, as displayed in Figs 4 andS4E, and of the magnetic footprint of the radio source, as displayed in Fig. 5 and Movie 1 (an animated version of Figs 5A-B).

For the highly linearly polarized waves close to f ce mentioned above, the k-direction is poorly determined, as it is derived from the normal to the polarization ellipse. As a result, goniopolarimetric results within SKR sources will inevitably yield noisier results when used to locate the radio sources and/or to measure their beaming angle. This explains why the values of θ on Fig. S4E and the spatial distribution of radio sources on Fig. 5C and Movie 1 are scattered for frequencies close to f ce . In addition, the 10-30 kHz window is contaminated by intense non-auroral narrow-banded emissions (for which the derived θ is wrong as narrow-banded emissions are not produced at f ce ) so that Fig. 4 restricts to the 30 to 1000 kHz range for clarity. Movie 1 illustrates the effect of SKR anisotropic beaming angle θ by mapping a portion of a wide circumpolar oval which varies with time as a function of the position of the spacecraft.

As an extension of Fig. 5, Fig. S5 finally directly superimposes the magnetic footprint of SKR remote and local sources to a polar view of the northern UV aurorae measured on day 2017-066. The footprint of visible SKR sources observed remotely coincides with the overall main auroral oval. The distribution of local SKR sources however, while noisy, illustrates a finer correspondence with the inward portion of the auroral oval only, as separately suggested by the coincidence of the source region with the early portion of the upward current layer in Fig. 1.

RPWS/Langmuir probe and MFR measurements

RPWS includes a 5 cm wide spherical Langmuir probe (LP) located at 1.5 m from the spacecraft body. The LP measures the total electrical current across the ambient plasma as a function of an input variable voltage to provide several plasma parameters. For the purpose of this study, measurements of the electric potential between the probe and the ambient plasma were used to estimate the local electron density N, and in turn f pe . Details on the LP analysis method can be found in (55). The comparison of f pe values derived from both CAPS-ELS and LP measurements for the southern SKR source event of 2008-291 [START_REF] Bunce | Extraordinary field-aligned current signatures in Saturn's high-latitude magnetosphere: Analysis of Cassini data during revolution 89[END_REF][START_REF] Schippers | Auroral electron distributions within and close to the Saturn kilometric radiation source region[END_REF] shows an excellent agreement. For the time intervals investigated in Figs 1-2 and 6, LP current sweeps were obtained every 32 s. The corresponding f pe values are displayed in gray in Figs 1-2 and6.

f pe can be alternately obtained from the sharp upper cutoff frequency of whistler-mode auroral hiss emissions when these emissions are driven into resonance at f pe and f pe < f ce (56). Upwardpropagating electromagnetic hiss emissions cannot propagate above f pe and remained trapped in the plasma. During the intervals investigated in Figs 1 and2, auroral hiss emissions were observed between 24 Hz and 12 kHz by the RPWS Medium Frequency Receiver (MFR) connected to the Z monopole antenna with a 64-s time resolution. A maximum propagation frequency of the hiss emissions was extracted by a manual search of each MFR spectrum for this interval. The hiss emissions were generally damped and displayed a smoothed cutoff, so that we interpreted their maximum propagation frequency as a lower limit of the actual value of f pe , labelled f pe min .

The comparison between the LP-derived f pe and the f pe min measurements show that the latter is generally 2-3 times larger than the former. Interestingly, both track the density gradient observed around the middle of the time intervals of Figs 1 and2.

RPWS/WBR measurements

RPWS can acquire electric (or magnetic) waveform measurements thanks to its Wideband Receiver (WBR). On day 2016-339 from about 07:20 to 10:51 UT, the WBR was connected to the X electric dipole and operating over the 0.8-75 kHz spectral range at the rate of 222,222 samples/s. A 2048 sample snapshot was obtained every second during this period. The high resolution dynamic spectra displayed in Fig. 3 were reconstructed with simple Fast Fourier Transform of the signal (2048 data points) with an effective time-frequency resolution of 1 s x 109 Hz.