Saturn Electrostatic Discharges Research Articles

Analysis of a long-lived, two-cell lightning storm on Saturn

Lightning storms in Saturn’s atmosphere can last for a few days up to several months. In this paper we analyze a lightning storm that raged for seven and a half months at a planetocentric latitude of 35° south from the end of November 2007 until mid-July 2008. Thunderstorms observed by the Cassini spacecraft before this time were characterized by a single convective storm region of ~2000 km in size, but this storm developed two distinct convective storm cells at the same latitude separated by ~25° in longitude. The second storm cell developed in March 2008, and the entire two-cell convective system was moving with a westward drift velocity of about 0.35 deg per day, which differs from the zonal wind speed. An exhaustive data analysis shows that the storm system produced ~277000 lightning events termed Saturn electrostatic discharges (SEDs) that were detected by Cassini’s Radio and Plasma Wave Science (RPWS) instrument, and they occurred in 439 storm episodes. We analyzed the SED intensity distributions, the SED polarization, the burst rates, and the burst and episode durations. During this storm Cassini made several orbits around Saturn and observed the SEDs from all local times. A comparison with optical observations shows that SEDs can be detected when the storm is still beyond the visible horizon. We qualitatively describe this so-called over-the-horizon effect which is thought to be due to a temporary trapping of SED radio waves below Saturn’s ionosphere. We also describe the first occurrence of so-called SED pre- and post-episodes, which occur in a limited frequency range around 4 MHz separated from the main episode. Pre- and post-episodes were mostly observed by Cassini located at local noon, and should be a manifestation of an extreme over-the-horizon effect. Combined radio and imaging observations suggest that some decreases in SED activity are caused by splitting of the thunderstorm into a bright cloud and a dark oval.

The behavior of spokes in Saturn’s B ring

We present the first results from Cassini ISS observations aimed at determining both the short-term and long-term behavior of the spokes in Saturn’s B ring. We have observed multiple spokes which appear between images where there was not a spoke before. The radial termini of these new spokes expand radially at ∼0.5km/s in both the inward and outward directions and both towards and away from corotation. Defining a spoke’s activity as the area-integrated optical depth over the region of the ring it covers, we find that the majority of spokes which are found in multiple images are either increasing or decreasing in activity. In addition, in analyzing the shapes and motions of spokes, we find the azimuthal profiles to be well-fitted by Gaussians for the majority of spokes. We have found that several of the imaged spokes were undergoing an “active” phase during which the spoke’s optical depth increases and the spoke grows both azimuthally and radially. We interpret these motions to be either due to the Lorentz force acting on dust grains charged by a very high temperature plasma or the group velocity of an advancing spoke-forming front. The spokes’ light scattering behavior suggests that the particles comprising them are not spherical but instead are irregularly shaped.We search for periodicities in the longer-term temporal variability of spoke activity, and for correlations between spoke activity and other processes ongoing in Saturn’s magnetosphere and atmosphere. Using the latest results on the periods and locations of the sources of Saturn’s Kilometric Radiation (SKR) obtained by the Cassini Radio and Plasma Wave experiment (RPWS) (Gurnett, D.A., Groene, J.B., Averkamp, J.B., Kurth, W.S., Ye, S.-Y., Fischer, G. [2011]. Planet. Solar Heliosph. Radio Emis. PRE VII, 51–64), we find that spoke activity observed on both sides of Saturn’s rings occurs with a period equal to, within all uncertainties, the period of the SKR emissions arising from the northern SKR source, though a period equal to that of the southern SKR source seems also to be present. The period for of the Saturn Electrostatic Discharges (SEDs) falls within the uncertainty of the spoke activity on the northern side of the rings. However, spokes have been observed during SED-quiescent periods, which strongly suggests that there is no connection between the two phenomena (Fischer, G., Gurnett, D.A., Kurth, W.S., Zarka, P., Moore, L., Dyudina, U.A., 2011. J. Geophys. Res.).Finally, we use the latest SLS4 coordinate systems and locate the peak spoke activity on both sides of the rings at ≈200° in both the northern and southern SLS4 coordinate systems on both sides of the rings. This peak is ≈100° away from the locations expected from Voyager observations in both coordinate systems.

Diurnal variation of electron density in Saturn’s ionosphere: Model comparisons with Saturn Electrostatic Discharge (SED) observations

Using the Saturn Thermosphere Ionosphere Model (STIM), we present a study of the diurnal variation of electron density, with a focus on comparisons with peak electron densities (NMAX) inferred from the low-frequency cutoff of radio emission due to lightning in the lower atmosphere, called Saturn Electrostatic Discharges (SEDs). It is demonstrated that photochemistry in Saturn’s ionosphere cannot reproduce the SED-inferred diurnal variation in NMAX unless additional production and loss sources outside of the current best estimates are considered. Additional explanations of the SED-inferred diurnal variation of NMAX are presented and analyzed, such as the possibility that the low-frequency cutoff seen in SEDs is due to the presence of sharp low-altitude layers of plasma, as frequently seen in radio occultation measurements. Finally, we outline the observational constraints that must be fulfilled by any candidate explanations of the SED-inferred diurnal variation of NMAX.

Ground-based and spacecraft observations of lightning activity on Saturn

In late 2007, Saturn electrostatic discharges (SED) were simultaneously observed at the radio telescope UTR-2 and with the Cassini spacecraft. Observations at UTR-2 were performed with a multichannel receiver in the frequency range 12–33MHz, and those performed on Cassini—with a swept frequency receiver that is part of the RPWS (Radio and Plasma Wave Science) instrument in the frequency band 1.8–16MHz. We got a very good coincidence between data of UTR-2 and Cassini. It is shown for the first time that ground-based radio astronomy lets us detect Saturn's lightning with a high degree of reliability despite terrestrial interferences. This is the necessary basis for further detailed study of the temporal and spectral characteristics of the SEDs with ground based radio telescopes. Based on six observation sessions, several parameters of SEDs were determined, in particularly a correlation of 0.77±0.15 between the average intensity of storms and the e-folding time.

IDENTIFICATION OF SATURN LIGHTNINGS RECORDED BY THE UTR-2 RADIO TELESCOPE AND CASSINI SPACECRAFT

The Saturn electrostatic discharges (SED) simultaneously recorded in the initial period of storm F at the UTR-2 radio telescope and Cassini spacecraft are investigated. The UTR-2 used the FFT-spectral receiver operating 12÷33 MHz, while the Cassini the serial spectrum analyser RPWS (Radio Plasma Wave Science) operating 1.8÷16 MHz. The ground and space data processed have shown very good agreement. E-folding time of SED and its dependence on episode intensity in the initial period of storm F were determined.

Polarization measurements of Saturn Electrostatic Discharges with Cassini/RPWS below a frequency of 2 MHz

[1] Early in 2006 the RPWS (Radio and Plasma Wave Science) instrument and the ISS (Imaging Science Subsystem) onboard the Cassini spacecraft detected a lightning storm on Saturn that lasted for about one month. The RPWS measured the so-called SEDs (Saturn Electrostatic Discharges), which are high frequency radio signals produced by lightning discharges. The ISS imaged a remarkable cloud system associated with these SEDs at a latitude of 35° South. Below the frequency of 1825 kHz the RPWS was in a mode capable of measuring the polarization of the SEDs. A surprising result was gained; SEDs appeared to be highly polarized (80%) and were exclusively right-handed polarized with a high degree of circular polarization. We will present an explanation for this diagnosis involving magneto-ionic modes and their differential absorption in the magnetoplasma of Saturn's ionosphere.

Lightning storms on Saturn observed by Cassini ISS and RPWS during 2004–2006

We report on Cassini Imaging Science Subsystem (ISS) data correlated with Radio and Plasma Wave Science (RPWS) observations, which indicate lightning on Saturn. A rare bright cloud erupt at ∼35° South planetocentric latitude when radio emissions (Saturn Electrostatic Discharges, or SEDs) occur. The cloud consisting of few consecutive eruptions typically lasts for several weeks, and then both the cloud and the SEDs disappear. They may reappear again after several months or may stay inactive for a year. Possibly, all the clouds are produced by the same atmospheric disturbance which drifts West at 0.45 °/day. As of March 2007, four such correlated visible and radio storms have been observed since Cassini Saturn Orbit Insertion (July 2004). In all four cases the SEDs are periodic with roughly Saturn's rotation rate ( 10 h 39 m ), and show correlated phase relative to the times when the clouds are seen on the spacecraft-facing side of the planet, as had been shown for the 2004 storms in [Porco, C.C., and 34 colleagues, 2005. Science 307, 1243–1247]. The 2000-km-scale storm clouds erupt to unusually high altitudes and then slowly fade at high altitudes and spread at low altitudes. The onset time of individual eruptions is less than a day during which time the SEDs reach their maximum rates. This suggests vigorous atmospheric updrafts accompanied by strong precipitation and lightning. Unlike lightning on Earth and Jupiter, where considerable lightning activity is known to exist, only one latitude on Saturn has produced lightning strong enough to be detected during the two and a half years of Cassini observations. This may partly be a detection issue.

Analysis of a giant lightning storm on Saturn

On January 23, 2006, the Cassini/RPWS (Radio and Plasma Wave Science) instrument detected a massive outbreak of SEDs (Saturn Electrostatic Discharges). The following SED storm lasted for about one month and consisted of 71 consecutive episodes. It exceeded all other previous SED observations by Cassini as well as by the Voyagers with regard to number and rate of detected events. At the same time astronomers at the Earth as well as Cassini/ISS (Imaging Science Subsystem) detected a distinctive bright atmospheric cloud feature at a latitude of 35° South, strongly confirming the current interpretation of SEDs being the radio signatures of lightning flashes in Saturn's atmosphere. In this paper we will analyze the main physical properties of this SED storm and of a single small SED storm from 2005. The giant SED storm of 2006 had maximum burst rates of 1 SED every 2 s, its episodes lasted for 5.5 h on average, and the episode's periodicity of about 10.66 h exactly matched the period of the ISS observed cloud feature. Using the low frequency cutoff of SED episodes we determined an ionospheric electron density around 10 4 cm −3 for the dawn side of Saturn.

Are Saturn electrostatic discharges really superbolts? A temporal dilemma

Saturn electrostatic discharges (SED) are freely‐propagating radio emissions detected in the high frequency (HF) radio band (1–40 MHz) associated with electrical discharge (i.e., lightning) from storms in Saturn's atmosphere. While SEDs responsible for the RF emission are considered to be very energetic superbolts (>1013 J), this determination is intimately related to the temporal nature of the discharge itself. As we demonstrate, if we assume the discharge has similar temporal properties as terrestrial cloud‐to‐ground discharges (with a stroke time scale ∼70 μs), then indeed the discharge energy has to be ∼ 1013 J in order account for the Cassini‐observed radiated HF power of ∼50 W/Hz. However, if the discharge duration is faster than the terrestrial case (i.e., ∼1 μs), the energy of the discharge can be weaker than the terrestrial case since the central peak of the emission shifts closer to the HF band. Because of the near‐flat SED spectra measured in the HF which favors a faster discharge, we conclude that the high level of radiated HF power from SEDs may have less to do with any extreme super‐bolt strength of the discharge and has more to do with the intrinsic quick time‐scale of relatively weaker discharges.

Discrimination between Jovian radio emissions and Saturn electrostatic discharges

[1] Short vertical streaks in the dynamic spectrum of the Cassini/RPWS (Radio and Plasma Wave Science) receiver in the frequency range of a few MHz can be due to Jovian radio emissions or SEDs (Saturn electrostatic discharges). Although Jupiter is increasingly far from Cassini, the peaks of decametric Jovian arcs can still be detected a few dB above the galactic background, and in some cases they look very similar to the SEDs caused by lightning in Saturn's atmosphere. We show a method for discriminating between these two phenomena by using the ratio of the measured autocorrelations in case the receiver uses at least two antennas. We analyze the special event from July 22, 2003, which was interpreted as the first indication of SEDs at a time when the spacecraft was still at a distance of 1.08 AU from Saturn, and find that it originated from Jupiter.

Saturn lightning recorded by Cassini/RPWS in 2004

During 2004 the Cassini/RPWS (Radio and Plasma Wave Science) instrument recorded about 5400 SEDs (Saturn Electrostatic Discharges), which were organized in 4 storm systems and 95 episodes. A computer algorithm with different intensity thresholds was applied to extract the SEDs from the RPWS data, and a statistical analysis on the main characteristics of these SEDs is performed. Compared to the SEDs recorded by the Voyagers in the early 1980s, some characteristics like SED rate, intensity, signal duration, or power spectrum are similar, but there are also remarkable differences with regard to time occurrence and frequency range: The first appearance of SEDs (storm 0) was recorded by RPWS from a distance of more than 300 Saturn radii at the end of May 2004, followed by storm A in mid-July, storm B at the beginning of August, and the most prominent storm C throughout most of September. There were also significant intervals of time with no detectable SED activity, e.g., SEDs were practically absent from October 2004 until June 2005. No clear indication for SEDs below a frequency of 1.3 MHz could be found. We suggest that the SED storms A, B, C, and possibly also storm 0 originate from the same storm system residing at a latitude of 35° South, which lasted for several months, waxed and waned in strength, and rotated with the Voyager radio period of Saturn. The SED source might be located in the updrafting water clouds beneath the visible cloud features detected in the Cassini images.

Effects of ring shadowing on the detection of electrostatic discharges at Saturn

A long‐standing discrepancy exists in determinations from observations and modeling of the diurnal variation of the peak electron density of Saturn's ionosphere. Using a new Saturn‐Thermosphere‐Ionosphere‐Model (STIM), we examine the suggestion by Burns et al. (1983) that Saturn's rings shadow its ionosphere causing radio penetration “holes”, thereby allowing lightning‐induced radio signals (Saturn electrostatic discharges, SEDs) to escape. This lessens the need to invoke globally enhanced loss processes to account for Voyager era observations of nighttime peak density (Nmax) as low as 103 e/cm3 from the SEDs. We find radio frequency “windows” produced by ring shadowing that were narrow and confined to low latitudes during the Voyager era, but are now broadly distributed over northern mid‐latitudes during the Cassini era. If lightning sources occur only at near‐equatorial latitudes, then the far less frequent detection of SEDs by Cassini early in its mission would be consistent with the current ionospheric morphology.

Cassini Imaging Science: Initial Results on Saturn's Atmosphere

The Cassini Imaging Science Subsystem (ISS) began observing Saturn in early February 2004. From analysis of cloud motions through early October 2004, we report vertical wind shear in Saturn's equatorial jet and a maximum wind speed of approximately 375 meters per second, a value that differs from both Hubble Space Telescope and Voyager values. We also report a particularly active narrow southern mid-latitude region in which dark ovals are observed both to merge with each other and to arise from the eruptions of large, bright storms. Bright storm eruptions are correlated with Saturn's electrostatic discharges, which are thought to originate from lightning.

Voyager electron density measurements on Saturn: Analysis with a time dependent ionospheric model

We have used a one‐dimensional chemical diffusive model of the ionosphere, in conjunction with the measured Voyager ultraviolet spectrometer (UVS) upper atmospheric temperature and composition structure, to analyze the Voyager measurements of Saturn's upper ionospheric electron densities. Electron density measurements are available from the analysis of the radio science (RSS) experiments. In addition, if interpreted as an atmospheric phenomenon the Saturn electrostatic discharges (SED), detected by the Voyager planetary radio astronomy instrument, the measurements also yield important information on electron densities at the peak. These latter data suggest a strong day‐to‐night variation in the peak electron densities with a maximum of ∼5 × 105 cm−3 near noon and a minimum of ∼103 cm−3 near midnight. The analysis of RSS data yields a peak of ∼104 cm−3 in the electron density profiles near the local terminators. As with simpler ionospheric models, we find serious difficulties in interpreting these electron density measurements because time constants in the model for the chemical sinks in the ionosphere, of mostly H+ ions, are >106 s much longer than a Saturnian day, suggesting little diurnal variation. We have investigated the effects of H2 vibrational excitation (characterized by a single vibrational temperature) and a gaseous H2O influx from Saturn's rings, as enhancers of plasma recombination, since they result in the conversion of slowly recombining H+ ions to more rapidly recombining molecular ions. This does introduce a larger diurnal variation in the peak electron densities, but not as strong as that deduced from the analysis of SED measurements. Furthermore, it also reduces the magnitude of the electron densities at the peak well below that inferred from the SED data. Based on the model results and extrapolation of calculations by Kim and Fox [1991] from Jupiter, we suggest that SEDs may not be associated with atmospheric storm systems. In our attempt to interpret the RSS measurements of electron density profile at sunrise, we have invoked an upward field‐aligned drift, H2 vibrational excitation and an influx of H2O to explain the results, and, in particular, maintain plasma peak at the measured altitude.

Radio detection of uranian lightning by Voyager 2

Within distances of ∼600,000 km from Uranus, the Planetary Radio Astronomy (PRA) experiment aboard the Voyager 2 spacecraft detected impulsive (100–300 ms) bursts of broad-band (≤900 kHz to ∼40 MHz) radio emission. This emission is very different from the uranian magnetospheric radio component, also discovered during the recent Voyager–Uranus encounter1. During the two Saturn encounters, in 1980 and 1981, the PRA experiments detected similar radio emissions2,3, which have been interpreted as radio emissions associated with lightning activity in the atmosphere of the planet4,5. By analogy with the Saturn electrostatic discharges (SED), we have adopted the term uranian electrostatic discharges (UED) for these new radio bursts. We have determined the physical characteristics of UED, and in the context of their interpretation as lightning-associated radio bursts, we derive here ionospheric peak electron densities over the uranian day- and nightside hemispheres.

Directivity of Saturn electrostatic discharges and ionospheric implications

From a detailed analysis of the intensity distribution of Saturn electrostatic discharges (SED) as a function of time during the Voyager 1 encounter with Saturn, the total beaming pattern of the SED source, which is found to be isotropic, is fetermined. This result allows for the derivation of the diurnal variations of the peak electron density over the dayside equatorial ionosphere of Saturn, and thus explains the puzzling features observed during the Voyager 1 encounter with Saturn—the longer extent of the SED visibility after closest approach and the existence of double-humped SED episodes before the encounter—by ionospheric absorption of the SED radio emission.

Voyager UVS observations of Saturn's rings and the ring atmosphere

During the Voyager 1 and 2 Saturn encounters the Voyager ultraviolet spectrometers (UVS) made numerous observations of Saturn's rings in the extreme and far ultraviolet. HI Lyman a (1216 Å) observations of the rings from a number of different aspects are used to define the extent and density of the neutral hydrogen “atmosphere” associated with the rings. Voyager 2 observations of the 520 to 1700 Å spectrum of the rings (~20 Å resolution) are used to derive the albedo of particles in the B ring at these wavelengths. These albedo measurements are compared with the laboratory reflectance spectrum of water ice longward of 1200 Å. The significance of the lack of ring reflectance in the Voyager 1 UVS data is also discussed. Finally, UVS spectra of the rings obtained in Saturn's shadow are used to establish upper limits on the presence of any emission from the neutral and ionic states of oxygen possibly associated with the Saturn electrostatic discharges (SED) discovered by the Voyager Planetary Radio Astronomy experiment.

Statistical study of Saturnian electrostatic discharges observed by the Voyager planetary radio astronomy experiment

Abstract:During the two Voyager-Saturn missions, the Planetary Radio Astronomy experiment observed sporadic broadband (20 KHz to at least 40 MHz) emissions during a few days around the closest approaches.These emissions are interpreted to arise from short-lived electrostatic discharges in the vicinity of Saturn. Their characteristics have been studied for the experiment's high band (>1.2 MHz).A difference between the periodicity of these Saturn electrostatic discharges (SED) is found, when comparing Voyager 1 and 2 observations.

An Earth-based attempt to detect S.E.D. at 21 centimeters

AbstractWe tried to observe Saturn Electrostatic Discharges at 1420 MHz using the radiotelescope at Nancay, first during Saturn 1981 opposition, the during Voyager 2 fly-by. The obtained flux upper limit indicates a spectrum steeper than about f**(-1).

On the energy of possible Saturnian lightning

The lightning hypothesis for Saturn's electrostatic discharges is discussed from the point of view of terrestrial lightning energetics. The total energy of a Saturnian lightning source is inferred to be about 10 13 J; however, some SED characteristics are not consistent with a terrestrial-like lightning source.