Saturn Kilometric Radiation Research Articles

Image‐Based Classification of Intense Radio Bursts From Spectrograms: An Application to Saturn Kilometric Radiation

AbstractSaturn Kilometric Radiation (SKR) is a non‐thermal auroral emission with peak emission occurring at 100–400 kHz. Its properties have been extensively studied since Cassini's arrival at Saturn until mission end with its Radio and Plasma Wave Science (RPWS) experiment. Low Frequency Extensions (LFEs) of SKR which consist of global intensifications of SKR accompanied by extensions of the main SKR band down to lower frequencies have been studied in particular. Low Frequency Extensions result from internally driven tail reconnection and from solar wind compressions of the magnetosphere, which also trigger tail reconnection. They have been cataloged through visual inspection with two approaches, using an intensity threshold for LFEs in 2006 (Reed et al., 2018, https://doi.org/10.1002/2017ja024499) and more recently O’Dwyer et al. (2023a, https://doi.org/10.25546/103103) produced a sample of LFEs detected by Cassini/RPWS by fitting their exact frequency‐time coordinates with polygons. In this study we use the latter catalog of LFEs as a training set for an image based machine learning algorithm to classify all LFEs detected by Cassini/RPWS. The inputs to the model are multi‐channel images consisting of spectrogram images in flux density and degree of circular polarization. The outputs of the model are binary masks showing the exact location of the LFE in frequency‐time space. The median Intersection Over Union across the testing and training set were calculated to be 0.97 and 0.98, respectively. The output of this study is a list of all 4,874 LFEs detected using this method. The list of LFE frequency‐time coordinates is available for use amongst the scientific community.

Rotational Modulation of the High Frequency Limit of Saturn Kilometric Radiation

AbstractThe high frequency limit (HFL) of the Saturnian Kilometric Radiation (SKR) can probe the deepest SKR sources, closest to Saturn's ionosphere. In this study, we determined and analyzed the SKR HFL throughout the entire Cassini Saturn orbital tour. The maximum frequency of the northern SKR, whose distribution peaks at ∼625 kHz, is shifted by +100 to +200 kHz from the distribution of southern SKR HFL, consistent with the magnetic field offset toward the northern hemisphere at Saturn. The uniformly observed SKR HFL in the vicinity of Saturn suggests a broad extent and beaming of the SKR source. When the observer is confined to certain locations, the rotational modulation of the SKR HFL is clearly observed. This modulation feature of the SKR HFL is statistically established and analyzed in this study. The modulation of HFL is best observed at mid‐latitudes between 10° and 40° and at almost all local times. We perform a simulation that suggests that the modulation of HFL requires the superposition of a “clock” like and a rotating source behavior. By comparing the derived HFL modulation using different longitudes with variable and fixed rotation periods, we can exclude the existence of a magnetic anomaly that was proposed in a previous study based on the Voyager data. The calculation of the least‐square periodogram confirms that the modulation observed in HFL is similar to the ones previously detected at Saturn.

Hubble Detects the Start of a New Saturn Ring Spoke Season

AbstractSaturn's ring spokes typically appear over an 8‐year duration centered on equinox. Hubble Space Telescope observations in 2021 indicate the beginning of a new spoke season as Saturn approaches equinox in 2025. The spokes show increased contrast and longevity in 2022, persisting for up to 11 hours. The spokes are visible from UV to near‐IR and are slightly bluer (i.e., less red) than the rings, but we find no significant wavelength dependence in the spectral contrast, which approximates their optical depth). Spoke rotation rates are between 606 and 626 min, consistent with either Keplerian rotation or possibly the variable rotation rate of Saturn's kilometric radiation. Spoke activity is expected to increase over the next several years.

Observations of the First Harmonic of Saturn Kilometric Radiation During Cassini's Grand Finale

AbstractClear first harmonic emissions of Saturn Kilometric Radiation are discovered during the Cassini Grand Finale orbits. Both ordinary (O) and extraordinary (X) mode fundamental emissions accompanied by X mode harmonics are observed. Analysis shows that the frequency ratio between the fundamental and harmonic emissions is 2.01 ± 0.08, and the harmonic emissions display weaker intensities than the fundamental, by 30–40 dB for the X‐X (fundamental‐harmonic) type harmonic and 10–30 dB for the O‐X type harmonic. The intensity relations between the two types of harmonics, that is, O‐X and X‐X show different patterns that we attribute to different conditions of emission at the source. Direction‐finding results shows that the fundamental and harmonic emissions are plausibly generated in the same source region. In agreement with previous studies at Earth, the generation of the two types of harmonics can be attributed to the cyclotron maser instability operating with different plasma density and electron energy distributions in the source region.

Saturn Anomalous Myriametric Radiation, a New Type of Saturn Radio Emission Revealed by Cassini.

A new radio component namely Saturn Anomalous Myriametric Radiation (SAM) is reported. A total of 193 SAM events have been identified by using all the Cassini Saturn orbital data. SAM emissions are L‐O mode radio emission and occasionally accompanied by a first harmonic in R‐X mode. SAM's intensities decrease with increasing distance from Saturn, suggesting a source near Saturn. SAM has a typical central frequency near 13 kHz, a bandwidth greater than 8 kHz and usually drifts in frequency over time. SAM's duration can extend to near 11 hr and even longer. These features distinguish SAM from the regular narrowband emissions observed in the nearby frequency range, hence the name anomalous. The high occurrence rate of SAM after low frequency extensions of Saturn Kilometric Radiation and the SAM cases observed during compressions of Saturn's magnetosphere suggest a special connection to solar wind dynamics and magnetospheric conditions at Saturn.

Classification of spectral fine structures of Saturn kilometric radiation

Abstract. The spectral fine structures of Saturn kilometric radiation (SKR) are best investigated with the wideband receiver (WBR) of Cassini's Radio and Plasma Wave Science (RPWS) instrument, with which measured radio fluxes can be displayed in time–frequency spectra with resolutions of 125 ms and ∼0.1 kHz. We introduce seven different classes of SKR fine structures ranging from dots (one class for 0-dimensional objects) over lines (four classes of 1-dimensional objects being horizontal, vertical, or with negative or positive slope) to areal features (one class for 2-dimensional objects). Additionally, we define a seventh class containing special structures named according to their appearance in time–frequency spectra. These special features are named rain, striations, worms, and caterpillar and the latter two have never been described in the literature so far. Using this newly defined classification scheme, we classify features in spectra at low frequencies in the baseband of the 80 kHz WBR and at medium frequencies around 325 kHz. A statistic of the occurrence of various classes and sub-classes shows some notable characteristics: lines with a positive slope are much more common at medium frequencies than at low frequencies and vertical lines are almost absent at low frequencies. The particular fine structure of striations (group of narrowbanded lines with predominantly negative slopes) is quite common below 80 kHz but less common near 325 kHz. At these medium frequencies, the lines rather look like interrupted striations which we term with the name “rain”. We also find rare instances of striations with a positive slope and rare instances of absorption signatures within areal features. The newly introduced sub-classes of worms (lines oscillating in frequency) and caterpillars occur almost exclusively below 80 kHz. Caterpillars have a typical bandwidth of ∼10 kHz, a constant frequency below ∼40 kHz for several hours and they are mostly observed beyond distances of 10 Saturn radii. We discuss the implications of our findings in view of the many theories about spectral fine structures of auroral radio emissions.

Effect of an Interplanetary Coronal Mass Ejection on Saturn’s Radio Emission

The Saturn Kilometric Radiation (SKR) was observed for the first time during the flyby of Saturn by the Voyager spacecraft in 1980. These radio emissions, in the range of a few kHz to 1 MHz, are emitted by electrons travelling around auroral magnetic field lines. Their study is useful to understand the variability of a magnetosphere and its coupling with the solar wind. Previous studies have shown a strong correlation between the solar wind dynamic pressure and the SKR intensity. However, up to now, the effect of an Interplanetary Coronal Mass Ejection (ICME) has never been examined in detail, due to the lack of SKR observations at the time when an ICME can be tracked and its different parts be clearly identified. In this study, we take advantage of a large ICME that reached Saturn mid-November 2014 (Witasse et al., J. Geophys. Res. Space Physics, 2017, 122, 7865–7890). At that time, the Cassini spacecraft was fortunately travelling within the solar wind for a few days, and provided a very accurate timing of the ICME structure. A survey of the Cassini data for the same period indicated a significant increase in the SKR emissions, showing a good correlation after the passage of the ICME shock with a delay of ∼13 h and after the magnetic cloud passage with a delay of 25–42 h.

The Faraday rotation effect in Saturn Kilometric Radiation observed by the CASSINI spacecraft

Non-thermal radio emissions from Saturn, known as Saturn Kilometric Radiation (SKR), are analyzed for the Faraday rotation effect detected in Cassini RPWS High Frequency Receiver (HFR) observations. This phenomenon, which mainly affects the lower-frequency part of SKR below 200 kHz, is characterized by a rotation of the semi-major axis of the SKR polarization ellipse as a function of frequency during wave propagation through a birefringent plasma medium. Faraday rotation is found in 4.1% of all HFR data recorded by Cassini above 20 degrees northern and southern magnetic latitude, from mid-2004 to late 2017. A statistical visibility analysis shows that elliptically polarized SKR from the dawn source regions, when beamed toward high latitudes into the noon and afternoon local time sectors, is most likely to experience Faraday rotation along the ray path. The necessary conditions for Faraday rotation are discussed in terms of birefringent media and sharp plasma density gradients, where SKR (mostly R-X mode) gets split into the two circularly polarized modes R-X and L-O. By means of a case study we also demonstrate how Faraday rotation provides an estimate for the average plasma density along the ray path.

Observations of Solar Type III radio bursts by Cassini/RPWS experiment

Abstract. In this study, we report on the occurrence of solar Type III radio bursts recorded by radio and plasma wave experiment (RPWS) experiment onboard Cassini spacecraft. This instrument is designed to investigate the Saturn's plasma environment and sub-auroral radio missions. RPWS/Cassini experiment allows to measure electric field over a frequency range from 1 Hz to 16 MHz (Gurnett et al., 2004). The essential observed emission is associated to the Saturnian Kilometric Radiation (SKR) which is generated in the sub-auroral regions of the magnetosphere. The capability of this experiment leads to detect Solar Type III radio bursts recorded during the increase phase of the solar activity, i.e. January 2008 to December 2014. Hence we have found more than 300 Type III solar bursts when the distance of Cassini to the Sun was about 10 AU. Observational parameters like the time occurrence, the emission frequency and the relative intensity are considered in this analysis. Those features lead us to characterize the detection conditions and to estimate the occurrence variabilities of Type III bursts.

Saturn's Nightside Dynamics During Cassini's F Ring and Proximal Orbits: Response to Solar Wind and Planetary Period Oscillation Modulations

AbstractWe examine the final 44 orbits of the Cassini spacecraft traversing the midnight sector of Saturn's magnetosphere to distances of ~21 Saturn radii, to investigate responses to heliospheric conditions inferred from model solar wind and Cassini galactic cosmic ray flux data. Clear storm responses to anticipated magnetospheric compressions are observed in magnetic field and energetic particle data, together with Saturn kilometric radiation (SKR), auroral hiss, and ultraviolet auroral emissions. Most compression events are associated with corotating interaction regions, producing ~2–3.5 day intervals of magnetospheric activity that are recurrent with the ~26 day solar rotation period (one or two such events per rotation), though one on the final pass is related to a nonrecurrent interplanetary shock possibly associated with an earlier X‐class solar flare. The response to compressions is modulated by the concurrent relative phasing of the northern and southern planetary period oscillation (PPO) systems, with long (>1 planetary rotation) SKR low‐frequency extension (LFE) intervals associated with strong field‐aligned coupling currents being favored when the two PPO systems act together to thin and thicken the tail plasma sheet during each PPO cycle. LFE onsets/intensifications are then favored at thin plasma sheet phases most unstable to reconnection, producing energetic nightside particle injections and poleward contractions of dawn‐brightened auroras. Correspondingly, solar rotation recurrent intervals of magnetospheric quiet conditions also occur with weak energetic particle fluxes and auroral emissions, associated with extended solar wind rarefactions. Overall, the results emphasize how strongly activity in Saturn's magnetosphere is modulated by concurrent heliospheric conditions.

Quasiperiodic Saturn Auroral Hiss Observed During a Cassini Proximal Orbit

AbstractSaturn auroral hiss is intense whistler mode emission similar in morphology to terrestrial auroral hiss, and is observed at high latitude very often in quasiperiodic episodes with a period of approximately 1 hr. Bader et al. (2019) report auroral pulsations that may be due to duskside magnetodisk reconnection. The source of the 1‐hr period is not definitively known but has been purported to be due to second harmonic Alfven waves standing along near planet magnetic field lines (Yates et al., 2016). Observations of auroral hiss at high latitude along Cassini proximal orbits are often excellent, and we have focused on an event for which we have concurrent ultraviolet auroral images as well as electron flux data. A series of repeating auroral hiss episodes is observed to initiate near the magnetic field line that traverses a Saturn kilometric radiation source region in each hemisphere, with periodic episodes of hiss recurring at higher L‐shells. Magnetic field lines centered on individual hiss episodes have auroral footprints that lie near and within a region of intense auroral ultraviolet emissions. These observations have a parallel in terrestrial return current electron beam‐generated auroral hiss seen near magnetic field lines supporting auroral kilometric radiation source regions. Recent findings link periodic plasma injections with Saturn reconnection sites observed preferentially on the duskside. These injections may spawn Saturn kilometric radiation source regions and periodic auroral hiss emission in nearby return current regions.

Solar Wind Dynamic Pressure Upstream From Saturn: Estimation From Magnetosheath Properties and Comparison With SKR

AbstractAn analytical method is developed by which measurements made by the Cassini spacecraft in Saturn's magnetosheath can be used to infer the upstream solar wind parameters, specifically the solar wind speed (Vsw) and the dynamic pressure (Pd). The method is validated by comparing the results with other estimates of these parameters, including the mSWiM MHD model and magnetopause and bow shock models applied to observed boundary crossings. The comparisons suggest that the new inferred Vsw are on average ~40 km/s lower than the mSWiM values, and the dynamic pressure values are slightly lower as well. We find few of the lower Pd values predicted by mSWiM, probably because Cassini would have been inside the expanded magnetosphere under such conditions. Systematic temporal variations such as interplanetary shocks do seem to be captured well, with arrival times within several days of the MHD prediction. Compared to dynamic pressures estimated from boundary crossings with well‐known magnetopause and bow shock models, the magnetosheath‐inferred dynamic pressure tends to be somewhat lower, but within the uncertainties of the analytical derivation. Comparison of the inferred dynamic pressure with observed Saturn's kilometric radiation (SKR) activity reveals several episodes of very good temporal tracking between dynamic pressure and SKR intensity, with relatively short time delays (4–5 hr), suggesting rather direct driving. Such good tracking intervals occur almost exclusively on the dawnside of the magnetosphere, where the dominant SKR source is visible. When the tracking is good, the SKR fluxes vary roughly as the square of the dynamic pressure.

Seasonal variation of north–south asymmetry in the intensity of Saturn Kilometric Radiation from 2004 to 2017

This study investigates the long-term variation of Saturn Kilometric Radiation (SKR) intensity observed by the Radio and Plasma Wave Science (RPWS) instrument on board the Cassini spacecraft from 2004 (southern summer) to 2017 (northern summer). The results show that the SKR intensity was brighter in the summer hemisphere than in the winter hemisphere, which was clearly seen in the south-to-north SKR intensity ratio. Over the long-term, the southern SKR intensity became 100 times smaller during northern summer, while the northern SKR intensity remained fairly constant. It means that the reversal in the intensity ratio was mainly caused by the long-term reduction of southern SKR intensity as Saturn’s southern hemisphere moved from summer to winter, not the enhancement of northern SKR from winter to summer. We also investigated the possible contributions from the long-term solar EUV flux and solar wind dynamic pressure during the solar cycles 23 and 24, but we found that their impact on the SKR long-term variations was less than Saturn’s seasonal changes associated with the variation of the tilt of its rotational axis with respect to the Sun. We further compared the long-term variation of the SKR intensity and the SKR period over half a Kronian year. The former showed more systematic variations which do not compare to those seen in the SKR periods.

Are Saturn's Interchange Injections Organized by Rotational Longitude?

AbstractSaturn's magnetosphere has been extensively studied over the past 13 years with the now retired Cassini mission. Periodic modulations in a variety of magnetospheric phenomena have been observed at periods close to those associated with the emission intensity of Saturn kilometric radiation (SKR). Resulting from Rayleigh‐Taylor like plasma instabilities, interchange is believed to be the main plasma transport process in Saturn's inner to middle magnetosphere. Here we examine the organization of equatorially observed interchange events identified based on high‐energy (3–22 keV) H+ intensifications by several longitude systems that have been derived from different types of measurements. The main question of interest here is as follows: Do interchange injections undergo periodicities similar to the Saturn kilometric radiation or other magnetospheric phenomena? We find that interchange shows enhanced occurrence rates in the northern longitude systems between 30° and 120°, particularly between 7 and 9 Saturn Radii. However, this modulation is small compared to the organization by local time. Additionally, this organization is weak and inconsistent with previous findings based on data with a limited time span.

Planetary Period Oscillations in Saturn's Magnetosphere: Comparison of Magnetic and SKR Modulation Periods and Phases During Northern Summer to the End of the Cassini Mission

AbstractWe compare periods and phases of Saturn planetary period oscillations determined from Cassini magnetic field and Saturn kilometric radiation (SKR) data from the beginning of 2016 to the end of mission in mid‐September 2017, encompassing northern summer solstice in May 2017. Both data sets show that the periods are almost unchanging, varying by only ~ ±0.01 hr about 10.79 hr for the northern system and 10.68 hr for the southern system, close to values attained by mid‐2015 after period coalescence between mid‐2013 and mid‐2014. The mean absolute differences between the magnetic and SKR periods are ~0.0036 hr (~13 s), consistent with estimated magnetic measurement uncertainties, while the overall mean difference is less than 0.001 hr (~2–3 s), at the limit of resolution. The relative phasing between magnetic and SKR modulations is correspondingly near constant and such that the equatorial planetary period oscillation fields of the northern/southern systems point radially outward near‐oppositely at ~14.3/2.5 hr local time at corresponding SKR maxima, with upward planetary period oscillation currents located ~2 hr postdawn for both systems, consistent with previous intervals having dawnside spacecraft apoapsides. Southern SKR emissions are found to be significantly dual modulated at both southern and northern periods in data limited to lie well within the southern shadow zone of the northern sources. These northern period modulations are shown to be approximately in phase with those in the northern emissions, consistent with a recent suggestion that bidirectional auroral electron acceleration may generate in phase SKR emissions in both hemispheres.

Saturn's Northern Aurorae at Solstice From HST Observations Coordinated With Cassini's Grand Finale

AbstractThroughout 2017, the Hubble Space Telescope (HST) observed the northern far‐ultraviolet aurorae of Saturn at northern solstice, during the Cassini Grand Finale. These conditions provided a complete viewing of the northern auroral region from Earth and a maximal solar illumination, expected to maximize the ionosphere‐magnetosphere coupling. We analyze 24 HST images concurrently with Cassini measurements of Saturn's kilometric radiation and solar wind parameters predicted by two magnetohydrodynamic models. The aurorae reveal highly variable components, down to timescales of minutes, radiating 7 to 124 GW. They include a nightside‐shifted main oval, unexpectedly frequent and bright cusp emissions, and a dayside low‐latitude component. On average, these emissions display a strong local time dependence with two maxima at dawn and premidnight, the latter being newly observed and attributed to nightside injections possibly associated with solstice conditions. These results provide a reference frame to analyze Cassini in situ measurements, whether simultaneous or not.

An SLS5 Longitude System Based on the Rotational Modulation of Saturn Radio Emissions

AbstractDespite the axisymmetry of Saturn's internal field, Saturn radio emissions like Saturn kilometric radiation (SKR) are modulated due to planetary rotation. With the completion of Cassini mission in September 2017, we now have around 14 years of observation of Saturn radio emissions, roughly from southern solstice to northern solstice. In this study, we extend the SLS4 longitude system to the end of the Cassini mission using a phase tracing method. The new Saturn longitude system (SLS5) organizes the observed SKR maxima around 0° subsolar longitude in both northern and southern hemispheres and can be used to organize other phenomena observed in Saturn's magnetosphere, for example, hot plasma injection events. SKR is modulated like a clock when the main source on the morning side is visible. To convert the observed phase to the clock phase, the phase of the morning side source, we also define a second longitude system SLS5*, which takes the spacecraft position into account based on a simple visibility model.

Low‐Frequency Extensions of the Saturn Kilometric Radiation as a Proxy for Magnetospheric Dynamics

AbstractSaturn Kilometric Radiation (SKR) is an auroral radio emission which can be detected quasi‐continuously by the Cassini spacecraft. It has been shown to respond to magnetotail reconnection and to changes in solar wind conditions and thus offers the potential to be used as a remote proxy for magnetospheric dynamics. This work has developed criteria for the selection of low‐frequency extensions (LFEs), powerful intensifications of the main SKR emission, accompanied by an expansion of the SKR to lower frequencies. Upon examination of data from the Cassini Radio and Plasma Wave Science instrument, we detect 282 LFE events which are further grouped into two categories. Shorter events (<20 h) associated with tail reconnection have a median waiting time of ∼10 h, a median duration of 3.1 h and a strong correlation with the northern and southern SKR phase systems. The 60% of the short LFEs have a reconnection event within the preceding 6 h. Longer events (>20 h), associated with increases in solar wind dynamic pressure, can last multiple planetary rotations, have a median waiting time of ∼20 days, and show no relationship with SKR phase. An analysis of the power emitted during LFEs suggests that tail reconnection is not always observed or detected in situ which may partially explain the low correlation between LFEs and tail reconnection. We conclude that short LFEs are a good proxy for reconnection in the tail.

Midnight flash model of energetic neutral atom periodicities at Saturn

AbstractThe Ion Neutral Camera on the Cassini spacecraft made images of energetic H atoms (25–55 keV) over a 3 day span in 2017. The images were projected onto the equatorial plane of Saturn, and a keogram was made by interpolating the projections in local time at 9 RS (1 RS = 60268 km). The keogram intensities show strong periodicities near the 10.79 h period of Saturn's energetic particles and exhibit a slope commensurate with corotation at that period. These periodic fluxes intensify near midnight but are weaker near noon. A “midnight flash” model can explain this behavior in terms of a searchlight rotating at 10.79 h that intensifies in the midnight sector. The model can also describe similar activity in Saturn's kilometric radiation and magnetic fields, although the “flash” must be shifted to the dawn‐to‐noon sector.

Spinning, breathing, and flapping: Periodicities in Saturn's middle magnetosphere

AbstractIn Saturn's magnetosphere, ubiquitous fluctuations with a period of ~10.7 h have been observed in Saturn kilometric radiation (SKR), auroral emissions, the magnetic field, the electron density, and energetic particle fluxes. Here we characterize previously unstudied periodicities in plasma properties inside of 15 RS near the equatorial plane. Although periodically varying magnetic perturbations rotate relatively smoothly (spinning), plasma properties do not. The phase of the peak value of plasma density or pressure perturbations can change substantially across a few hours of local time or RS. As a means of interpreting observations, we use a magnetohydrodynamic simulation that generates field‐aligned currents centered at 70° invariant latitude in Saturn's southern ionosphere and rotating at the SKR period. The simulation reproduces many periodic features of the data including not only spinning perturbations but also global‐scale compression and expansion (breathing). Simulated plasma properties are also modulated by periodic large‐scale north‐south motion (flapping) in regions beyond ~15 Saturn radii (RS), which we do not analyze here. Inside of 15 RS, plasma responds to a superposition of spinning and breathing at the spin period, developing perturbations that peak at different phases depending on what is measured and where. Strong compressional effects act impulsively over a limited range of rotation phase. Superposition of local and global‐scale variations produces phase jumps across short distances and can introduce multiple peaks in the variation of plasma properties within one rotation period, accounting for anomalies in the phase dependence of periodic fluctuations identified in the sparse data available.