Saturn Kilometric Radiation Research Articles

The reversal of the rotational modulation rates of the north and south components of Saturn kilometric radiation near equinox

It has been known for many years that Saturn emits intense radio emissions at kilometer wavelengths and that this radiation is modulated by the rotation of the planet at a rate that varies slowly on time scales of years. Recently it has been shown that the radio emission consists of two components that have different rotational modulation rates, one emitted from the northern auroral region and the other emitted from the southern auroral region. In this paper we show using radio measurements from the Cassini spacecraft that the rotational modulation rates of the northern and southern components reversed near Saturn's recent equinox, which occurred on 11 August 2009. We show that a similar reversal was also observed by the Ulysses spacecraft near the previous equinox, which occurred on 19 November 1995. The solar control implied by these reversals has important implications on how Saturn's rotation is coupled into the magnetosphere.

CMI growth rates for Saturnian kilometric radiation

We calculate growth rates for extraordinary mode Saturnian kilometric radiation generated by the electron cyclotron maser instability using a ring‐type (DGH) electron phase distribution model fitted to measured electron energy distributions in Saturn's auroral acceleration region. The observed distributions are unstable in spatially isolated regions of transverse linear size of order 103 km. Each unstable region has a growth rate consistent with amplification of background radiation to the peak observed intensity, approximately 14 electric field e‐foldings. The observed propagation direction and frequency decrement below the electron cyclotron frequency are also consistent with the CMI growth model.

Influence of hot plasma pressure on the global structure of Saturn’s magnetodisk

Using a model of force balance in Saturn's disk‐like magnetosphere, we show that variations in hot plasma pressure can change the magnetic field configuration. This effect changes (i) the location of the magnetopause, even at fixed solar wind dynamic pressure, and (ii) the magnetic mapping between ionosphere and disk. The model uses equatorial observations as a boundary condition—we test its predictions over a wide latitude range by comparison with a Cassini high‐inclination orbit of magnetic field and hot plasma pressure data. We find reasonable agreement over time scales larger than the period of Saturn kilometric radiation (also known as the camshaft period).

Extraordinary field‐aligned current signatures in Saturn's high‐latitude magnetosphere: Analysis of Cassini data during Revolution 89

During the periapsis pass of Revolution 89, specifically on day 291 of 2008, the Cassini spacecraft observed unusual field‐aligned current signatures in Saturn's high‐latitude southern hemisphere in the midnight and dawn sector. The region of open field lines was found to be contracted close to the pole, and surrounded by an unusual region containing hot keV electrons and ‘leading’ field signatures indicative of super‐corotating flow. Usual ‘lagging’ fields indicative of sub‐corotating flow were also present at lower latitudes, though of unusual strength. Unique within the ∼40 similar nightside auroral region Cassini passes during 2008, the overall field‐aligned current system thus consisted of a central region of downward current flanked by two regions of upward current. This distinctive signature coincided with the first in situ encounter of Cassini with a source region of Saturn kilometric radiation, located within the unusual poleward region of upward current adjacent to the open‐closed field line boundary. We propose that these unusual conditions relate to a major open flux closure event in Saturn's tail, possibly triggered by solar wind compression of the magnetosphere.

Variation of Saturn's UV aurora with SKR phase

It is well known that a wide range of kronian magnetospheric phenomena, including the Saturn kilometric radiation (SKR), exhibit oscillations near the planetary rotation period. However, although the SKR is believed to be generated by unstable auroral electrons, no connection has been established to date between diurnal SKR modulations and UV auroral power. We use an empirical SKR phase determined from Cassini observations to order the ‘quiet time’ total emitted UV auroral power as observed by the Hubble Space Telescope in programs during the interval 2005–2009. Our results indicate that both the northern and southern UV powers are dependent on SKR phase, varying diurnally by factors of ∼3. We also show that the UV variation originates principally from the morning half of the oval, consistent with previous observations of the SKR sources.

Longitude dependences of energetic H+ and O+ at Saturn

The charge‐energy‐mass spectrometer instrument in the Cassini spacecraft measured differential fluxes of protons (2.8–236 keV) and oxygen ions (8.8–236 keV) from July 2004 to August 2007. The fluxes were bin‐averaged in Saturn longitude system (SLS) longitude within ±5 RS of the equator and between 8 and 12 RS in radial distance (1 RS = 60,238 km) to determine their global morphology. The 3‐year time period is the range of validity of the SLS, which is based on a variable period of Saturn kilometric radiation. Fluxes at all energies of H+ and O+ display an essentially sinusoidal variation in longitude, often with peak‐to‐trough ratios of 2:1. For E < 77 keV, the maxima are consistently at ∼70° longitude (the minima are at ∼230°). For E > 77 keV; the maxima shift to ∼250°. The ion distributions closely resemble those of energetic neutral hydrogen and neutral oxygen atoms observed by the ion neutral camera onboard Cassini.

Properties of Saturn kilometric radiation measured within its source region

On 17 October 2008, the Cassini spacecraft crossed the southern sources of Saturn kilometric radiation (SKR), while flying along high‐latitude nightside magnetic field lines. In situ measurements allowed us to characterize for the first time the source region of an extra‐terrestrial auroral radio emission. Using radio, magnetic field and particle observations, we show that SKR sources are surrounded by a hot tenuous plasma, in a region of upward field‐aligned currents. Magnetic field lines supporting radio sources map a continuous, high‐latitude and spiral‐shaped auroral oval observed on the dawnside, consistent with enhanced auroral activity. Investigating the Cyclotron Maser Instability (CMI) as a mechanism responsible for SKR generation, we find that observed cutoff frequencies are consistent with radio waves amplified perpendicular to the magnetic field by hot (6 to 9 keV) resonant electrons, measured locally.

Cassini observations of narrowband radio emissions in Saturn's magnetosphere

The Radio and Plasma Wave Science instrument (RPWS) on board the Cassini spacecraft has detected numerous narrowband radio emissions in Saturn's magnetosphere from September 2005 to May 2007. Typically, the narrowband radio emissions occur after an intensification of Saturn kilometric radiation (SKR) and last for several days. These emissions often occur near 5 kHz, with a modulation period of about 10.8 h, similar to the planet's rotation period. Other apparently associated bands also occur at higher frequencies, typically from 20 to 30 kHz. Both 5 kHz and 20 to 30 kHz emissions tend to decrease in intensity with increasing time and slowly drift upward in frequency after the onset. Our study shows that the narrowband radio emissions tend to be observed on the nightside at a radial distance range from 5 to 50 Saturn radii and tend to be observed more frequently at higher latitudes. The rotational modulation of narrowband radio emissions in Saturn's magnetosphere acts like a flashing light rather than a rotating beacon, similar to the source of SKR. These emissions are believed to be generated by mode conversion from anisotropy‐driven electrostatic instabilities near the upper hybrid frequency. Comparisons with Cassini neutral particle imaging data show that transient hot plasma clouds, corotating with the planet inside the orbit of Titan, might be the source for narrowband radio emissions. Comparisons with Cassini Magnetometer data reveal that the narrowband radio emissions tend to occur 1 or 2 days after the compression of the magnetotail.

Acceleration of electrons in Titan's ionosphere

A consideration of the acceleration mechanism which supplies the fast electrons to the source of Saturnian kilometric radiation (SKR) and an interpretation of the recently reported observational indications of the influence of Titan on the SKR are presented. The proposed mechanism operates by the effect of the different magnetization of the electrons and ions in Titan's ionosphere which in the course of Titan's motion through the Saturnian magnetic field causes the creation of a charge‐separation electric field. This field has a component parallel to the magnetic field and accelerates part of the ionospheric electrons (called “runaway electrons”). The performed estimates show that the mechanism accelerates the runaway electrons up to an energy of ∼5 keV. The power of the acceleration mechanism is sufficient for SKR generation and also for the ultraviolet luminescence of Titan's atmosphere. The weakening of the SKR when Titan passes on the dayside of Saturn is due to a decrease of the magnetic field strength near the dayside magnetopause, when the Moon escapes the Saturnian magnetosphere, as well as due to the break in the magnetic connection between the electron acceleration region on Titan and the SKR sources. The latter prevents the penetration of the accelerated electrons into the radiation generation region. When Titan is on the nightside of Saturn, it enters into shell L∼14, which is stretched owing to the ring current. In this case, the electrons that accelerated in the ionosphere of Titan can reach the nightside SKR sources and activate them and therefore being the reason for the Titan influence on the SKR.

The influence of Titan on Saturn kilometric radiation

Abstract. Previous studies have shown that the occurrence probability of Saturn Kilometric Radiation (SKR) appears to be influenced by the local time of Titan. Using a more extensive set of data than the original study, we confirm the correlation of higher occurrence probability of SKR when Titan is located near local midnight. In addition, the direction finding capability of the Cassini Radio Plasma Wave instrument (RPWS) is used to determine if this radio emission emanates from particular source regions. We find that most source regions of SKR are located in the mid-morning sector of local time even when Titan is located near midnight. However, some emission does appear to have a source in the Saturnian nightside, consistent with electron precipitation from field lines that have recently mapped to near Titan.

Phase relations between energetic neutral atom intensities and kilometric radio emissions at Saturn

The intensities of energetic neutral atoms (ENAs) from Saturn's ring current were cross‐correlated with Saturn kilometric radiation (SKR, 100–400 kHz) to determine their phase relations. The cross correlations were obtained for three intervals in early 2007 when Cassini was at high northern latitudes and the Ion and Neutral Camera (INCA) could optimally view the entire ring current. During these intervals, the hydrogen, oxygen, and SKR signals all exhibited ∼10.8 h periodicities. A consistent phase relation was not observed between the ENA and the SKR. Cross‐correlation amplitudes in these intervals varied from 0.25 to 0.79, and the phase offsets varied from ∼1 h to several hours and differed according to ENA species. Not only did the SKR and ENA signal show phase offsets, but the H and O signals also displayed offsets. That is, all periodicities at ∼10.8 h had no consistent synchronization.

A north‐south difference in the rotation rate of auroral hiss at Saturn: Comparison to Saturn's kilometric radio emission

Broadband whistler‐mode emissions, commonly observed by the Cassini spacecraft at high latitudes in Saturn's magnetosphere at frequencies below about 100 Hz, have characteristics very similar to auroral hiss observed at high latitudes in Earth's magnetosphere. In contrast to terrestrial auroral hiss, which shows no obvious rotational modulation, Saturnian auroral hiss shows a very pronounced rotational modulation. We show that the rotation period of the auroral hiss is different in the northern and southern hemispheres, with a period of about 10.6 hours in the northern hemisphere and about 10.8 hours in the southern hemisphere. To within experimental error the rotation periods in the two hemispheres match the rotation periods of Saturn Kilometric Radiation, an intense radio emission generated along the auroral field lines at frequencies from about 20 to 500 kHz. These north–south asymmetries have potentially important implications on how angular momentum is transferred from the planet to the magnetospheric plasma.

An auroral oval at the footprint of Saturn's kilometric radio sources, colocated with the UV aurorae

Similarly to other magnetized planets, Saturn displays auroral emissions generated by accelerated electrons gyrating around high‐latitude magnetic field lines. They mainly divide in ultraviolet (UV) and infrared (IR) aurorae, excited by electron collisions with the upper atmosphere, and Saturn's kilometric radiation (SKR), radiated from higher altitudes by electron‐wave resonance. Whereas spatially resolved UV and IR images of atmospheric aurorae reveal a continuous auroral oval around each pole, the SKR source locus was only indirectly constrained by the Voyager radio experiment to a limited local time (LT) range on the morningside, leading to interpretation of the SKR modulation as a fixed flashing light. Here, we present resolved SKR maps derived from the Cassini Radio and Plasma Wave Science (RPWS) experiment using goniopolarimetric techniques. We observe radio sources all around the planet, organized along a high‐latitude continuous auroral oval. Observations of the Hubble Space Telescope obtained in January 2004 and January 2007 have been compared to simultaneous and averaged Cassini‐RPWS measurements, revealing that SKR and UV auroral ovals are very similar, both significantly enhanced on the dawnside. These results imply that the SKR and atmospheric aurorae are triggered by the same populations of energetic electron beams, requiring a unified model of particle acceleration and precipitation on Saturn.

Dual periodicities in energetic electrons at Saturn

The fluxes of energetic electrons (110–365 keV) in Saturn's magnetosphere were subjected to Lomb periodogram analyses for 1677 days from late 2004 to the middle of 2009. The electrons generally exhibited strong periodicities at 10.82 hours and 10.60 hours for much of the ∼4.6 year interval. The longer period was generally present all the time, while the shorter period became prominent during the last ∼600 days of the interval. The 10.8‐hour signal appeared strongly in observations made north of the equator and weakly in the south, while the 10.6‐hour signal appeared only for northern observations. The dual periodicity in the electrons is similar to that recently discovered in the Saturn kilometric radiation. Periods in the 10.6 to 10.8 hour range are similar to the sun‐referenced periods of neutral winds measured in Saturn's polar atmospheres.

The source of Saturn's periodic radio emission

This paper proposes a model of the link between the pulsing radio signals from Saturn (Saturn kilometric radiation), known to be most intense on the morningside, and the rotating magnetic (cam) field structure identified on low‐latitude orbits. On the high‐inclination orbits of the Cassini spacecraft in late 2006, large‐scale azimuthal deviations of the magnetic field, corresponding to upward field‐aligned currents, have been detected in the late morning sector. There is evidence that their intensity is modulated by the rotating cam magnetic field, the periodic field structure seen deeper inside the magnetosphere. It is proposed that the local intensification of field‐aligned currents that gives rise to the modulation of the radio signal occurs in conjunction with the passage through the morning magnetosphere of the swept‐forward sector of the perturbation field imposed beyond the shells that carry the cam currents. The morningside magnetosphere is unique as a region in which not only does the magnetopause compress rotating plasma inside the magnetosphere but also magnetic shear develops between the magnetic shells of the middle magnetosphere and an outer region of magnetic field swept back by the solar wind. The link between radio pulsing and magnetic rotation arises through the interaction of the cam currents with structures unique to this part of the magnetosphere.

On the character and distribution of lower‐frequency radio emissions at Saturn and their relationship to substorm‐like events

With the arrival of the Cassini spacecraft at Saturn in July 2004, there have been quasi‐continuous observations of Saturn kilometric radiation (SKR) emissions. Exploration of the nightside magnetosphere has revealed evidence of plasmoid‐like magnetic structures and other phenomena indicative of the Kronian equivalent of terrestrial substorms. In general, there is a good correlation between the timing of reconnection events and enhancements in the auroral SKR emission. Eight of nine reconnection events studied occur at SKR phases where the SKR power would be expected to be rising with time. Thus, while the recurrence rate of substorm‐like events at Saturn is likely much longer than the planetary rotation timescale, the events are favored to occur at a particular phase of the rotation. We show three examples in each of which the SKR spectrum extends to lower frequencies than usual. This can be interpreted as an expansion of the auroral particle acceleration region to higher altitudes along magnetic field lines as a direct consequence of an increase in the magnetosphere‐ionosphere current density driven by substorm‐like events. We then conduct a survey of such low‐frequency extensions during the equatorial orbits of 2005–2006 and place some constraints on visibility of these radio emissions.

Elliptical polarization of Saturn Kilometric Radiation observed from high latitudes

The high‐inclination orbits of the Cassini spacecraft from autumn 2006 until spring 2007 allowed the Cassini/RPWS (Radio and Plasma Wave Science) instrument to observe Saturn Kilometric Radiation (SKR) from latitudes up to 60° for the first time. This has revealed a surprising new property of SKR: above ∼30° in observational latitude, a significant amount of SKR is strongly elliptically polarized, in marked contrast to previous observations from low latitudes, which showed only circular polarization. There are transitional latitudes where the elliptical polarization occurs in “patches” in the time‐frequency spectrograms next to regions of still completely circularly polarized SKR. From ∼45° to 60° in northern latitude, it is found that most of the SKR is elliptically polarized throughout its entire frequency range with an average degree of ∼0.7 in linear polarization. We demonstrate the ellipticity of SKR by using the concept of “apparent polarization” in case of two‐antenna measurements, but also show three‐antenna measurements from which the polarization can be unambiguously determined. Possible reasons for the variation of SKR polarization with the observer's latitude will be discussed.

Reply to comment by B. Cecconi on “Spectral features of SKR observed by Cassini/RPWS: Frequency bandwidth, flux density and polarization”

The main purpose of the paper by Galopeau et al.[2007] was to classify the spectral features of the Saturniankilometric radiation (SKR) starting from three physicalobserved parameters: the frequency bandwidth, the fluxdensity, and the pol arization. We show in the presentresponse that an unsupervised application of arbitrary auto-matic criteria during the data processing (such as a signal-to-noise ratio greater than 23 dB) can totally judge a weaknatural emission as a background noise. As a consequence,such a situation may lead to consideration of only the datapresenting a degree of circular polarization close to 100%and neglect a huge part of the data. Galopeau et al. [2007]considered a phenomenological aspect and gave an estima-tion of the Stokes parameters. This approach leads to firstrecognizing spectral components (flux density and band-width) in the frequency range from 3.5 kHz to 1200 kHz,and then deriving the Stokes parameters for each compo-nent. The Cassini/RPWS instrument provides long-lastingcoverage of radio emissions at Saturn with unprecedentedinstrumental capabilities.

Source locations of narrowband radio emissions detected at Saturn

Since Cassini's arrival at Saturn in 2004, the Radio and Plasma Wave Science instrument has detected numerous narrowband (NB) radio emission events. These emissions, mostly detected around 5 and 20 kHz, usually occur periodically for several days after intensifications of Saturn kilometric radiation. We present calculations based on an electron density profile of Saturn's plasma torus and a dipole magnetic field model showing that the NB emissions originate from the northern and southern edges of Saturn's plasma torus at L shells ∼ 8 to 10 for 5‐kHz NB and L ∼ 4 to 7 for 20‐kHz NB. In many cases, Cassini passes through the source region of the 20‐kHz NB, as indicated by intense electrostatic upper hybrid (ESUH) waves in close proximity to electromagnetic emissions on spectrograms. The positions of the spacecraft when intense ESUH waves are observed agree with the model predictions of the NB source locations. Source locations determined by goniopolarimetric (also known as direction‐finding) analysis of the NB emissions also support the above results, although sometimes the directions of arrival point toward the region interior to Saturn's plasma torus. A polarization reversal technique is applied to localize the NB emissions observed during spacecraft rotation, on the basis of the fact that the source is within the antenna plane when the apparent circular polarization degree switches sign. The NB emissions are found to be L‐O mode polarized, which is consistent with the prediction of linear/nonlinear mode conversion theory. It is also found that sometimes right‐hand polarized NB emissions are generated at second harmonic frequencies of the 20‐kHz NB; in which case, wave‐wave interactions between oppositely propagating ESUH waves may play an important role in the mode conversion process.

Energetic neutral atom (ENA) and charged particle periodicities in Saturn’s magnetosphere

The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft has observed energetic neutral atoms (ENA) and charged particles at Saturn from mid-2004 to the present. The particles often but not always reveal striking periodic behavior that seems to depend on the type of particle and spacecraft location. When subjected to a Lomb periodogram analysis, energetic electrons (>150 keV) exhibited strong frequency peaks near 10.80 h (the nominal or “base” period of Saturn kilometric radiation) during 2006–2008, but essentially no periodicity during 2005. The electron periodograms also show pronounced “double” frequency peaks in 2007 and 2008. Energetic protons (3–26 keV) show strong peaks near the same period for 2005–2007, but none for 2008. Oxygen ions at the same energies display strong peaks for 2005 and 2006, but not for 2007 and 2008. By projecting the ENA images onto Saturn’s equatorial plane or onto a plane perpendicular to the equatorial plane and then summing the data in the appropriate dimension, “strip” images can be constructed from which a time history can be derived. These time histories of ENA emissions are also subjected to a Lomb periodogram analyses. The energetic hydrogen neutrals (20–50 keV) exhibited periodic behavior only during 2007, while energetic oxygen neutrals (64–144 keV) displayed a strong SKR-like period in 2005 and 2006 but not for 2007 or 2008. Some of this behavior may be due to changing spacecraft aspect relative to the ENA emissions, and some of it may be real. This periodic behavior may be consistent with a rotating anomaly that “flashes” brightly in the midnight-to-dawn sector once per 10.8 h, with the flash parameters depending on particle species and energy.