Article

Saturn's electrostatic discharges: Properties and theoretical considerations

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Abstract

The properties of Saturn's electrostatic discharges (SED) as observed by the Voyager Planetary Radio Astronomy experiment during the two Voyager encounters with Saturn are summarized. Several models for the formation of SED are discussed in light of these observations. The most likely source regions appear to be either the equatorial zone of the planet or the dense part of the B ring near 1.80 Rs. The strenghts and weaknesses of each of these possibilities are examined. Neither possibility accounts fully for the observed SED properties in a simple way. A search for an anomaly near 1.80 Rs in the data of other experiments aboard Voyager has been carried out, and at least one and possibly more such experiments do indeed obtain anomalous data at this point in the ring system. There thus appears to be unexplained phenomena at this point, independent of the PRA data, and it is a short step to postulate that a single object may be the cause of all such phenomena.

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... 1) Dans l'anneau B, à 1.8 Rs <1 Rs = 1 rayon saturnien = 60300 km) où la période de révolution Képlerienne est égale à lOhlOmn (Warwick et al., 1981(Warwick et al., , 1932Evans et al., 1931Evans et al., , 1932Evans et al., , 1983, ou 2) dans l'atmosphère équatoriale de Saturne, où la vitesse des nuages s'additionnant à la rotation de la planète correspond également à une période d'environ lOhlOmn (1) la durée réelle d du SED <d<25 ms) est inférieure à sa durée apparente ms) . ...
... (2) la durée réelle d du SED (30<d<35 ms) est supérieure à sa durée apparente ms) , (Evans et al., 1932(Evans et al., , 1983 propagation de l'onde, on définit en outre les grandeurs utiles suivantes : (Kaiser and Alexander, 1977 ;Voot s et al., 1977;Ben son and Cal vert, 1979;Desch, 1982 The SED were first destribed by Warwick et al. [1981] and Evans et al. [1981]. The important changes in the flux density observed within 140 /.tsec in the PRA high-speed data show that the maximum size of the instantaneous radio source is less than 42 km [ Warwick et al., 1981], ...
... These results are consistent with those described by Lecacheux and Biraud [1982] for the LF PRA band. and also with the values obtained by Evans et al. [1983] from the highrate data. The différence between these results and the one (10 fu for VI) obtained by Warwick et al. [1981] probably cornes from a différence in our calibration, which is detailed in the appendix. ...
Thesis
Étude de deux types d'émissions radioélectriques planétaires distincts détectes par l'expérience de radioastronomie des sondes Voyager. 1) L'étude statistique exhaustive des caractéristiques physiques des décharges électrostatiques détectées près de Saturne a permis d'identifier ces décharges à l'émission radio associée à des orages saturniens, d'en localiser la source, de déterminer ses dimensions. Comparaison des caractéristiques avec celles des autres éclairs d'orages planétaires, détermination des variations diurnes de la densité électronique dans l'ionosphère équatoriale de Saturne. 2) Étude de l'influence (ou non) du vent solaire sur les diverses émissions basses fréquences joviennes, qui permet d'établir une classification synthétique de ces émissions, et de cerner la localisation possible de leurs sources dans la magnétosphère de Jupiter.
... The lightning on Saturn were discovered by spacecraft (SC) Voyager 1 in 1980 [1]. Their research was conducted by missions Voyager 2 and Cassini , the last has observed Saturn science 2003 till today and registered more than 10 long-term (weeks or months) storms on the planet [2]. ...
... To provide this it is needed to increase the time resolution. Before now the highest time resolution data (100 mks) were obtained by swept spectrum analyzer PRA (SC Voyager) [1,6]. Cassini RPWS has a waveform system with temporal resolution on the order of 10 ms, which could resolve the Voyager PRA 1-2 millisecond pulses [6]. ...
Conference Paper
The study of SEDs with high time resolution was made possible due to the regular modernization of the receiving equipment on the largest decameter radio telescope UTR-2. The obtained sensitivity allowed to resolve the microstructure of lightning discharges for the first time and to estimate the intensity of bursts. In addition the dispersion delay gives us a very important criterion that let reliably separate terrestrial lightning and cosmic ones.
... The Voyager 2 photopolarimeter detection of a narrow feature in Saturn's B ring combined with the appropriate Keplerian revolution period of about 10 h led Evans et al. (1982) to conclude that this ring feature might be responsible for the SED bursts. Later some authors favored lightning in atmospheric storms against the exotic ring mechanism to be the source of SEDs (Burns et al., 1983;Evans et al., 1983), as the saturnian atmosphere at the equator showed prograde wind speeds up to 450 m s −1 corresponding also to a period of 10 h 10 min. Finally, Kaiser et al. (1983) showed mainly with an argument of visibility that the characteristics of SEDs are best explained by a long-lived atmospheric equatorial lightning storm system with a longitudinal extension of about 60 • . ...
... We also note that these results are close to the e-folding times of 41 ms for SEDs detected by Voyager 1 and 38 ms for SEDs detected by Voyager 2 as calculated by Zarka and Pedersen (1983). As high time resolution measurements by the Voyagers have shown (Evans et al., 1983), it is likely that each SED is made up of many individual subevents (analogous to multiple strokes in a terrestrial flash). ...
Article
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.
... MHz that would result from an intervening ionosphere of ~10 4 cm -3 (Warwick et al. 1981(Warwick et al. , 1982. Burns et al. (1983) posited that SEDs might be the result of atmospheric lightning storms near Saturn's equator (see also Evans et al., 1983;Kaiser et al., 1983;Zarka and Pedersen, 1983). They argued that Saturn's atmosphere lags the canonical rotation rate reported for Saturn (e.g., Warwick et al., 1981); clouds within 15-20 o latitude of the equator rotated at the SED period. ...
Article
Full-text available
A number of puzzling phenomena were revealed when the Voyager spacecraft flew past Saturn in 1981 to measure the ionized portions (ionosphere) of its upper atmosphere (thermosphere). Most of these issues have remained unexplained in the intervening 25 years due to a lack of conclusive observational data. With the arrival of Cassini at Saturn in July 2004, however, a new era of observations began, providing the promise of fresh evidence and demanding the development of a contemporary theoretical framework in order to re-examine old mysteries and understand new discoveries. This dissertation presents studies of Saturn's ionosphere and inner plasmasphere based on new time-dependent photochemical and diffusive transport models that solve the ion equations of continuity in one dimension. Calculations are conducted within the overall framework of a self-consistent, three-dimensional general circulation model (GCM) of Saturn's thermosphere, and the results of these studies are combined with GCM results to provide the building blocks of a new comprehensive model, the Saturn-Thermosphere- Ionosphere-Model (STIM). The one-dimensional model calculations are used to constrain and investigate a number of unresolved issues and to make testable predictions based on those investigations. Five primary topics are addressed: (1) the additional loss processes required to bring predicted electron densities into agreement with observations, (2) the discrepancy between theory and observations regarding the diurnal variation of peak electron density, (3) the effects of shadowing by Saturn's rings on its ionosphere, (4) the yet unknown electron and ion temperatures at Saturn, and (5) the ionospheric contribution to Saturn's plasmasphere. The models show that a steady influx of water into Saturn's atmosphere--from its rings or icy satellites--is required to explain observed electron densities. Additionally, the time-variability of the water source may be the cause of frequently observed ionospheric plasma depletions. The first calculations of the time-dependent effect of attenuation of sunlight by Saturn's rings indicate that they cause large latitudinal gradients within the ionosphere, and may provide radio frequency windows through which atmospheric lightning is observed. Warm plasma temperatures are predicted in Saturn's upper atmosphere, with a strong dawn/dusk asymmetry and a large diurnal variation.
... RPWS can even use its Wideband Receiver at SED frequencies with a time resolution of a few tens of µs, and some SEDs were caught recently with this mode. Evans et al. (1983) showed that the SED amplitude envelope exhibits slowly varying fluctuations for several tens of ms as well as more rapid fluctuations lasting less than 1 ms. ...
Article
Full-text available
The Cassini mission provides a great opportunity to enlarge our knowledge of atmospheric electricity at the gas giant Saturn. Following Voyager studies, the RPWS (Radio and Plasma Wave Science) instrument has measured again the so-called SEDs (Saturn Electrostatic Discharges) which are the radio signature of lightning flashes. Observations by Cassini/ISS (Imaging Science Subsystem) have shown cloud features in Saturn’s atmosphere whose occurrence, longitudinal drift rate, and brightness were strongly related to the SEDs. In this paper we will review the main physical parameters of the SEDs. Lightning does not only give us clues about the dynamics of the atmosphere, but also serves as a natural tool to investigate properties of Saturn’s ionosphere. We will also discuss other lightning related phenomena and compare Saturn lightning with terrestrial and Jovian lightning.
... The Voyager 2 photopolarimeter detection of a narrow gap in Saturn's B ring at 1.81 R S combined with the appropriate Keplerian revolution period of about 10 h 09 min led Evans et al. (1982) and Warwick et al. (1983) to conclude that this ring feature might be responsible for the SED bursts. Not only Evans himself published a paper one year later (Evans et al., 1983) questioning his own hypothesis, but also Burns et al. (1982, 1983) were more in favor for an atmospheric lightning storm source explanation. Kaiser et al. (1983) used an argument of visibility to say that SEDs should stem from an atmospheric source on Saturn. ...
Article
On January 23, 2006, the Cassini/RPWS (Radio and Plasma Wave Science) instrument detected a massive outbreak of SEDs (Saturn Electro-static 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.
... These events detected by Voyager's Planetary Radio Astronomy (PRA) instrument were shortduration bursts observed during a fraction of the PRA radio sweep at frequencies between 20 kHz and 40 MHz. While there was some initial discussion of a ring source [Warwick et al., 1981; Evans et al., 1983], the events were found to be beamed consistent with an atmospheric source [Kaiser et al., 1983] and to have a low frequency cutoff consistent with an emission propagating from an atmospheric source through the ionosphere [Zarka, 1985]. Hence, the bursts are considered the radio emission from Saturn storm-created lightning. ...
Article
Full-text available
1] 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 (>10 13 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 ms), then indeed the discharge energy has to be $ 10 13 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 ms), 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. Citation: Farrell, (2007), Are Saturn electrostatic discharges really superbolts? A temporal dilemma, Geophys. Res. Lett., 34, L06202, doi:10.1029/ 2006GL028841.
Article
Laser-induced plasmas in various gas mixtures were used to simulate lightning in other planetary atmospheres. This method of simulation has the advantage of producing short-duration, high-temperature plasmas free from electrode contamination. The laser-induced plasma discharges in air are shown to accurately simulate terrestrial lightning and can be expected to simulate lightning spectra in other planetary atmospheres. Spectra from 240 to 880 nm are presented for simulated lightning in the atmospheres of Venus, Earth, Jupiter, and Titan. The spectra of lightning on the other giant planets are expected to be similar to that of Jupiter because the atmospheres of these planets are composed mainly of hydrogen and helium. The spectra of Venus and Titan show substantial amounts of radiation due to the presence of carbon atoms and ions and show CN Violet radiation. Although small amounts of CH4 and NH3 are present in the Jovian atmosphere, only emission from hydrogen and helium is observed. Most differences in the spectra can be understood in terms of the elemental ratios of the gas mixtures. Consequently, observations of the spectra of lightning on other planets should provide in situ estimates of the atmospheric and aerosol composition in the cloud layers in which lightning is occurring. In particular, the detection of inert gases such as helium should be possible and the relative abundance of these gases compared to major constituents might be determined.
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The results published by U.S. scientists during 1983-1986 from studies related to the magnetospheres of Jupiter, Saturn, and Uranus are discussed. Consideration is given to the magnetic fields of these planets, charged particle environments, the interactions between the planetary rings and planetary satellites, the solar wind interactions, radio emissions, and auroras. Special attention is given to observations of (1) a small flux of energetic electrons and protons in the otherwise radiation-free environment in the magnetosphere under the rings of Saturn (interpreted as interactions of Galactic cosmic rays with the rings), (2) spokes, and (3) Saturn ring erosion.
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Magnetospheric radio emissions, Saturn electrostatic discharges, inferred source locations, and emission theories are addressed.
Article
It is proposed that Saturn's electrostatic discharges (SED) might be generated in the planet's equatorial atmosphere, perhaps as lightning from a storm system. The 10h10m periodicity of the signal envelope duplicates that of Saturn's equatorial jet. The rings shield the atmosphere from solar EUV photons, and thereby substantially reduce the local ionospheric cutoff frequency to allow low-frequency SED to leak out. Many of the unusual properties of SED could be explained in terms of changes in the storm system, the relative spacecraft position in the beaming pattern of the source, local refraction of the signal by the highly disturbed ionosphere, and the influence of the ring particles on the highest frequency component of SED. A comparison of SED with planetary lightning on other planets shows that the two are similar in general character and some time behavior; the power output of SED may be higher than most planetary lightnings but that is unclear because of uncertainties in the measurements and variations in the signal's spectrum. Our simple discussion suggests that lightning could be a viable source for SED and that exotic ring mechanisms are not necessarily required.
Article
Although lightning has not yet been observed in Titan's atmosphere, the presence of condensable vapors and the deposition of a significant amount of solar energy at the surface suggest the possibility of lightning activity. Based on an understanding of the relationship of lightning activity to the amount of convective energy available on Titan, a lightning energy dissipation rate of 4 × 10−6, W/m2 can be expected. This value is much lower than that for Earth or Jupiter, and is a result of both the reduced solar flux at Titan and the absorption of sunlight by the aerosols that lie above the convective layer. For this dissipation rate, the amount of HCN and C2N2 produced by lightning should be greater than that by solar UV, but could be less than that produced by electron precipitation and galactic cosmic rays. Equilibrium calculations indicate that large mole fractions of elemental solid phase carbon will also be produced. Using a simplified model of aerosol formation, coagulation, and settling, it is estimated that a lightning-produced aerosol could have a typical optical depth of 10−2, with values as high as 0.1. The accumulation of soot over geological time might reach a meter or more in depth.
Article
Two techniques of simultaneous measurement of source direction and polarization, using three orthogonal short dipoles, are reviewed and extended to cover partially polarized waves. One technique involves forming the real part of the 3 x 3 antenna voltage coherency matrix. The other involves forming the imaginary part of the same matrix. The concept of a transfer function matrix for an arbitrary three-port antenna is then developed. This concept allows the source direction and polarization to be measured simultaneously, using an iterative method, when antennas other than short dipoles are used. The method involves transformation of the transfer function matrix among various reference frames. A novel polarization transform is then developed which greatly improves the convergence properties of the iterative method, and which permits the wave parameters to be determined even when the waves are linearly polarized. The method requires that both the real and imaginary parts of the antenna voltage coherency matrix are formed. The method is valid for partially polarized and unpolarized waves in addition to purely polarized waves. An example, which has been simulated numerically, is also presented.
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Chapter
In 1970, following up the trajectory work by G.A. Flandro, NASA’s Jet Propulsion Laboratory began to design a sophisticated ‘Mark 2’ version of its highly successful Mariner series of planetary probes. The plan was to dispatch two pairs of vehicles to investigate the outer Solar System. The first pair would employ Jovian slingshots to reach Saturn, whereupon they would be deflected on to distant Pluto. The second pair would exploit Jupiter to visit first Uranus and then Neptune. Although funding for the development of this new vehicle was not forthcoming, JPL was permitted to modify its existing design to follow up Pioneer 11 with visits to Jupiter and Saturn. Working within this restricted budget, the engineers made every effort to ensure that if the new spacecraft was still healthy at Saturn, and if additional funding could be secured at that point, it would be capable of an ‘extended’ mission. In 1977, with the launches imminent, this ‘Mariner Jupiter-Saturn’ mission was renamed ‘Voyager’. It was fortunate, of course, that this window was conducive to eventually pursuing the ‘Grand Tour’.
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During the Voyager Saturn mission the Planetary Radio Astronomy (PRA) experiment observed sporadic broadband emissions (20 KHz to at least 40 MHz) during a few days around the Voyager 1 and 2 closest approaches to the planet. These emissions are interpreted to arise from short-lived electrostatic discharges in the vicinity of Saturn. A statistical study has been made of their characteristics from the Voyager 1 and 2 high-band data of the PRA experiment (1.2 MHz to 40.2 MHz). In this paper, the results obtained for the two encounters are compared, and the appearance, intensity, and spectrum of the Saturn electrostatic discharges (SED) are described in detail. It is shown that the SED spectrum is likely smooth in the frequency range 1-40 MHz and extends down to 20 KHz. The period of the SED occurrence, which seems to be slightly different for Voyager 1 and 2 encounters (10 hours 9 min for the Voyager 1 encounter and 10 hours 00 min for the Voyager 2 encounter) is determined. An attempt to interpret the statistical results is presented.
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The necessary conditions for producing lightning discharges, the experimental possibilities for investigating extraterrestrial lightning, and the scientific objectives of planetary lightning research are briefly discussed. Present knowledge on the composition, structure, and dynamics of the atmospheres for the extraterrestrial planets and the satellites Io and Titan are reviewed in terms of their importance for the production of lightning. From the knowledge on planetary atmospheres, intensive lightning activity can be expected to exist in the Jupiter and Saturn cloud systems; this is less conclusive for Venus and unlikely for the other bodies. Electrical activity is not completely ruled out for Martian dust storms. Optical and RF wave measurements from spacecraft have yielded evidence of possible lightning activity on Venus, Jupiter, and Saturn. These observations are reviewed and discussed. The conclusion that the optical and/or RF emissions are definitely from lightning is not unambiguous and based on poor data bases. There are no coincidence measurements of optical and RF pulses, and there is no information on the characteristics of individual pulses, which would help to clarify the source.
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Chapter
Full-text available
All extraterrestrial lightning detected so far on the planets Jupiter, Saturn, Uranus, Neptune, and potentially at Venus is attributed to intracloud lightning. On Mars other types of discharges might exist within dust storms. In this commentary we will focus on the historical development of planetary lightning investigations at Jupiter, Saturn, and Venus. Special attention will be devoted to the controversies around the existence of lightning on Venus. Thereby we will highlight the importance of combined optical and radio observations for the epistemological degree of confidence in lightning detection from space as well as for its detailed investigation. Future spacecraft missions to various planets should take this into account. The main future extraterrestrial lightning investigations will be outlined. We also plead for radio observations with a time resolution of the order of microseconds and for more modelling and laboratory work in this field.
Conference Paper
Full-text available
The lightning activity in Saturn’s atmosphere has been monitored by Cassini for more than six years. The continuous observations of the radio signatures called SEDs (Saturn Electrostatic Discharges) combine favorably with imaging observations of related cloud features as well as direct observations of flash–illuminated cloud tops. The Cassini RPWS (Radio and Plasma Wave Science) instrument and ISS (Imaging Science Subsystem) in orbit around Saturn also received ground–based support: The intense SED radio waves were also detected by the giant UTR–2 radio telescope, and committed amateurs observed SED–related white spots with their backyard optical telescopes. Furthermore, the Cassini VIMS (Visual and Infrared Mapping Spectrometer) and CIRS (Composite Infrared Spectrometer) instruments have provided some information on chemical constituents possibly created by the lightning discharges and transported upward to Saturn’s upper atmosphere by vertical convection. In this paper we summarize the main results on Saturn lightning provided by this multi–instrumental approach and compare Saturn lightning to lightning on Jupiter and Earth.
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Planetary ring studies have evolved from observations of a single example, (Saturn), to related studies of a class of objects, in the last several years; in fact, this is the first IUGG review article on the subject. Therefore, it seems appropriate to cite at least review articles going back beyond 1979, if only as a guide to earlier literature (Alexander 1962, Bobrov 1970, Cook et al. 1973, Palluconi and Petengill (eds) 1973, Pollack 1975, Cook and Franklin 1977, and Cuzzi 1978; all of which deal only with Saturn's rings). More recent reviews have been written by Ip (1980 a,b); Cook (1980), Elliot (1982), and Goldreich and Tremaine (1982); also of interest are review articles in books to be published shortly; Saturn (Gehreis, ed.) and Planetary Rings (Brahic and Greenberg, eds.). Popular articles of interest have been written by Pollack (1978), Burns (1981) and Pollack and Cuzzi (1982).
Chapter
Final chapter is supplemented by a variety of dust-involved phenomena in space and in the laboratory that have not been treated in the previous chapters, including latest observations and further examples of electric reconnection and critical ionization. They are: upper atmospheric discharges to the ionosphere, nebular lightning and Chondrules formation, ball lightning, atmospheric, ionospheric, and magnetospheric effects associated with earthquakes, interplanetary dust and planetary rings, planetary lightning, and Saturn electrostatic discharges.
Article
The author presents a method for deriving the radio spectrum of electrical discharges from the properties of the time series of charges crossing the discharge gap. This result is applied to the observed spectra of both terrestrial lightning and Saturn electrical discharge(s) (SED). SED occurrence and power density are shown to have subtle, yet important, differences from these observables as they have been described in the last 5 years. It is demonstrated that throughout the episode of Voyager 1's (V1) closest approach to Saturn, SED probably occurred continuously in frequency upward at least from the upper limit of Saturn kilometric radiation at about 800 kHz. This is so despite the fact that in the dynamic spectra a strip in time and frequency in which SED do not occur extends in frequency from 1.3 MHz up to the oft-discussed lower limit of SED in the leading edge of the episode of closest approach. The greater power in SED that occurred after V1 closest approach is emphasized: it is shown to be consistent with the lower frequency of the maximum in their power spectra. The variable gap length factor is also invoked to explain the variable frequency cutoff in the range 5-15 MHz of the episodes before closest approach. The SED source moved along a single arc defining both preencounter and postencounter events. The discharge gap lengths were a continuous function of position along this arc, with the shortest gaps lying about 5{degree} west (as seen from the spacecraft) of the noon meridian of Saturn and the longest gaps lying on the nightside of the planet.
Article
We present a new technique for measuring the VHF radio centroid of nearby lightning flashes at 5-us intervals. Its ability to provide continuous positions during long ( 100 us) emissions is, we believe, new and reveals new information about the discharge process. The new technique solves many of the datahandling problems in old techniques. We have built and demonstrated this technique in one angular coordinate of the lightning flash. We present data from five flashes showing complex positional and motional patterns. The breakdown phase consists of many impulses. The average speed from impulse to impulse lies in the range of 10-100 km s -. During individual impulses, speeds measure from one to several tens of thousands of kilometers per second. At times of strong VLF bursts there is usually a similar VHF burst. Its speed is like the speeds of individual impulses. We identify VLF-associated VHF burst sources with the main electrical current flow in lightning flashes. We identify the motion from one impulse to another in the breakdown phase as being caused by avalanching electrons accelerating along paths soon to become the discharge paths within thunderclouds. The high speeds in impulses represent the gross current flow in breakdown channels not yet large enough to create large VLF emissions or flashes.
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Unusually intense lightning strokes have been observed by optical sensors on the Vela satellite. These lightning flashes are over 100 times more intense than typical lightning. The lightning superbolts are characterized by optical power in the range of 1011-1013 W, have a duration of the order of 1 ms, and have a total fadiant energy greater than 100 J. In conjunction with sferies data the Vela trigger rates indicate that about two lightning flashes in 103 exceed an optical power of 1011 W and five flashes in 107 exceed an optical power of 3×1012 W.
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Atmospherics and radio noise due to lightning are discussed. The types of source signals and propagation associated with atmospherics in three frequency ranges (below 300 kHz, between 300 kHz and 30 MHz, and above 30 MHz) are reviewed. Electrical field changes due to K pulses, leader processes, return strokes and other source effects are analyzed, and selection of a mode or ray theory for the investigation of propagation effects is considered. In addition, techniques for locating distant thunderstorms through the use of single or multistation detection networks are mentioned. Satellite measurement of atmospheric noise and the excitation by lightning of resonances in the cavity between the earth and the lower ionosphere (Schumann resonances) are also treated.
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Photographic observations of the nightside of Jupiter by the Voyager 1 spacecraft show the presence of extensive lightning activity. Detection of whistlers by the plasma wave analyzer confirms the optical observations and implies that many flashes were not recorded by the Voyager camera because the intensity of the flashes was below the threshold sensitivity of the camera. Measurements of the optical energy radiated per flash indicate that the observed flashes had energies similar to that for terrestrial superbolts. The best estimate of the lightning energy dissipation rate of 0.4 × 10−3 W/m2 was derived from a consideration of the optical and radiofrequency measurements. The ratio of the energy dissipated by lightning compared to the convective energy flux is estimated to be between 0.27 × 10−4 and 0.5 × 10−4. The terrestrial value is 1 × 10−4.
Article
It is proposed that Saturn's electrostatic discharges (SED) might be generated in the planet's equatorial atmosphere, perhaps as lightning from a storm system. The 10h10m periodicity of the signal envelope duplicates that of Saturn's equatorial jet. The rings shield the atmosphere from solar EUV photons, and thereby substantially reduce the local ionospheric cutoff frequency to allow low-frequency SED to leak out. Many of the unusual properties of SED could be explained in terms of changes in the storm system, the relative spacecraft position in the beaming pattern of the source, local refraction of the signal by the highly disturbed ionosphere, and the influence of the ring particles on the highest frequency component of SED. A comparison of SED with planetary lightning on other planets shows that the two are similar in general character and some time behavior; the power output of SED may be higher than most planetary lightnings but that is unclear because of uncertainties in the measurements and variations in the signal's spectrum. Our simple discussion suggests that lightning could be a viable source for SED and that exotic ring mechanisms are not necessarily required.
Article
The Voyager observations of electrical discharges in Saturn's rings strongly support earlier speculations on the role played by electrostatics, magnetic fields, and lightning phenomena in the primitive solar system. They also suggest conditions then by direct analogy rather than by extrapolating backwards through time from conditions now. The observed discharges show a pronounced 10h periodicity, which suggests a source in Keplerian orbit at 1.80 ± 0.01 Saturn radii (1 RS = 60,330 km). In that region, the B ring is thicker than optical depth 1.8 for about 5,000 km. At 1.805 ± 0.001 Saturn radii, however, the ring is virtually transparent for a gap of width 200 m. We conclude that a small satellite orbits Saturn at that radius and clears the gap. The gap edges must prevent diffusive filling of the gap by fine material which is especially abundant at this position in the rings and would otherwise destroy the gap in minutes. The discharges represent the satellite's interaction with the outer edge of the gap. Spoke formation may involve the interaction of ring material in the vicinity of the gap.
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Nonthermal radio emissions from the Saturn system were first detected by the Voyager planetary radio astronomy (PRA) experiment on board Voyager 1 in January 1980. Since then emission between 100 kHz and 1 MHz from the planet, termed Saturn kilometric radiation (SKR), has been received almost continuously. A description is presented of eight characteristics which have been fairly well defined by the Voyager 1 encounter. These include a very flat broadband frequency spectrum, a period of approximately 10 h 10 min, a change in the envelope shape of episodes between pre and postencounter, an intensity population structure typical of plural populations, and an episodic structure of a width of approximately 180 deg. It was found that postencounter episodes continue for about three times as long as preencounter ones, and that postencounter bursts are left-circularly polarized at high frequencies. At least one episode shows the onset of high frequency events some time before that of lower frequency ones.
Article
The boundary between the inner and outer parts of Saturn's B ring is located at the theoretical limit of stability of dust grains with large negative charge to mass ratio. A grain inside of this stability limit will move along magnetic field lines and strike Saturn if given a slight velocity component normal to the ring plane. Outside of this marginal stability radius, a perturbed grain merely oscillates back and forth through the ring plane. The theoretical location of the marginal stability radius is at 1.625 Saturn radius. Observations by Pioneer 11 and Voyager 2 in the infared see the boundary as a prominent change in ring brightness at this radius. The occultation of delta-Scorpii by the rings in the ultraviolet seen by Voyager 2 shows about a factor of two change in optical depth beginning very close to this radius.
Article
Planetary radio astronomy measurements obtained by Voyager 2 near Saturn have added further evidence that Saturnian kilometric radiation is emitted by a strong dayside source at auroral latitudes in the northern hemisphere and by a weaker source at complementary latitudes in the southern hemisphere. These emissions are variable because of Saturn's rotation and, on longer time scales, probably because of influences of the solar wind and Dione. The electrostatic discharge bursts first discovered by Voyager 1 and attributed to emissions from the B ring were again observed with the same broadband spectral properties and an episodic recurrence period of about 10 hours, but their occurrence frequency was only about 30 percent of that detected by Voyager 1. While crossing the ring plane at a distance of 2.88 Saturn radii, the spacecraft detected an intense noise event extending to above 1 megahertz and lasting about 150 seconds. The event is interpreted to be a consequence of the impact, vaporization, and ionization of charged, micrometer-size G ring particles distributed over a vertical thickness of about 1500 kilometers.
Article
Voyager 2 radio occultation measurements of Saturn's atmosphere probed to the 1.2-bar pressure level, where the temperature was 143 +/- 6 K and the lapse rate apparently equaled the dry adiabatic value of 0.85 K per kilometer. The tropopause at both mid-latitude occultation locations (36.5 degrees N and 31 degrees S) was at a pressure level of about 70 millibars and a temperature of approximately 82 K. The stratospheric structures were very similar with the temperature rising to about 140 K at the 1-millibar pressure level. The peak electron concentrations sensed were 1.7 x 10(4) and 0.64 x 10(4) per cubic centimeter in the predawn (31 degrees S) and late afternoon (36.5 degrees N) locations. The topside plasma scale heights were about 1000 kilometers for the late afternoon profile, and 260 kilometers for the lower portions and 1100 kilometers for the upper portions of the topside predawn ionosphere. Radio measurements of the masses of Tethys and Iapetus yield (7.55 +/- 0.90) x 10(20) and (18.8 +/- 1.2) x 10(20) kilograms respectively; the Tethys-Mimas resonance theory then provides a derived mass for Afimas of (0.455 +/- 0.054) x 10(20) kilograms. These values for Tethys and Mimas represent major increases from previously accepted ground-based values, and appear to reverse a suggested trend of increasing satellite density with orbital radius in the Saturnian system. Current results suggest the opposite trend, in which the intermediate-sized satellites of Saturn may represent several classes of objects that differ with respect to the relative amounts of water, ammonia, and methane ices incorporated at different temperatures during formation. The anomalously low density of lapetus might then be explained as resulting from a large hydrocarbon content, and its unusually dark surface markings as another manifestation of this same material.
Article
The Voyager 1 planetary radio astronomy experiment detected two distinct kinds of radio emissions from Saturn. The first, Saturn kilometric radiation, is strongly polarized, bursty, tightly correlated with Saturn's rotation, and exhibits complex dynamic spectral features somewhat reminiscent of those in Jupiter's radio emission. It appears in radio frequencies below about 1.2 megahertz. The second kind of radio emission, Saturn electrostatic discharge, is unpolarized, extremely impulsive, loosely correlated with Saturn's rotation, and very broadband, appearing throughout the observing range of the experiment (20.4 kilohertz to 40.2 megahertz). Its sources appear to lie in the planetary rings.
Article
Voyager 1 radio occultation measurements of Titan's equatorial atmosphere successfully probed to the surface, which is provisionally placed at a radius of 2570 kilometers. Derived scale heights plus other experimental and theoretical results indicate that molecular nitrogen is the predominant atmospheric constituent. The surface pressure and temperature appear to be about 1.6 bars and 93 K, respectively. The main clouds are probably methane ice, although some condensation of nitrogen cannot be ruled out. Solar abundance arguments suggest and the measurements allow large quantities of surface methane near its triple-point temperature, so that the three phases of methane could play roles in the atmosphere and on the surface of Titan similar to those of water on Earth. Radio occultation measurements of Saturn's atmosphere near 75 degrees south latitude reached a maximum pressure of 1.4 bars, where the temperature is about 156 K. The minimum temperature is about 91 K near the 60-millibar pressure level. The measured part of the polar ionosphere of Saturn has a peak electron concentration of 2.3 x 10(4) per cubic centimeter at an altitude of 2500 kilometers above the 1-bar level in the atmosphere, and a plasma scale height at the top of the ionosphere of 560 kilometers. Attenuation of monochromatic radiation at a wavelength of 3.6 centimeters propagating obliquely through Saturn's rings is consistent with traditional values for the normal optical depth of the rings, but the near-forward scattering of this radiation by the rings indicates effective scattering particles with larger than expected diameters of 10, 8, and 2 meters in the A ring, the outer Cassini division, and the C ring, respectively. Preliminary analysis of the radio tracking data yields new values for the masses of Rhea and Titan of 4.4 +/- 0.3 x 10(-6) and 236.64 +/- 0.08 x 10(-6) times the mass of Saturn. Corresponding values for the mean densities of these objects are 1.33 +/- 0.10 and about 1.89 grams per cubic centimeter. The density of Rhea is consistent with a solar-composition mix of anhydrous rock and volatiles, while Titan is apparently enriched in silicates relative to the solar composition.
Article
As Voyager 1 flew through the Saturn system it returned photographs revealing many new and surprising characteristics of this complicated community of bodies. Saturn's atmosphere has numerous, low-contrast, discrete cloud features and a pattern of circulation significantly different from that of Jupiter. Titan is shrouded in a haze layer that varies in thickness and appearance. Among the icy satellites there is considerable variety in density, albedo, and surface morphology and substantial evidence for endogenic surface modification. Trends in density and crater characteristics are quite unlike those of the Galilean satellites. Small inner satellites, three of which were discovered in Voyager images, interact gravitationally with one another and with the ring particles in ways not observed elsewhere in the solar system. Saturn's broad A, B, and C rings contain hundreds of "ringlets," and in the densest portion of the B ring there are numerous nonaxisymmetric features. The narrow F ring has three components which, in at least one instance, are kinked and crisscrossed. Two rings are observed beyond the F ring, and material is seen between the C ring and the planet.
Article
During both Voyager encounters with the saturnian system, the Planetary Radio Astronomy experiment detected strong discrete episodic bursts of radio emission, termed Saturn electrostatic discharges (SED). An examination of Voyager 2 photopolarimeter data now reveals a narrow feature (possibly a gap) in Saturn's B ring. A single, unique object appears to be responsible for both the SED and this feature.
Article
Voyager (Mariner Jupiter/Saturn 1977) spacecraft will carry the first experiment specifically designed to measure low-frequency nonthermal planetary radio emissions. The technical aspects of the planetary radio astronomy instrument are described here. Signals from 10-m orthogonal monopoles are processed to measure polarization and for either maximum sensitivity or observation of rapid temporal variations. The 0.3-¿V/¿kHz (i.e., -117 dBm/kHz with a 50-12 source) sensitivity and the 140-dB dynamic range achieved allow signals to be observed from near earth through planetary encounter. Stepped-or fixed-frequency operation is commandable over a range of 1.2 kHz to 40.5 MHz with internal calibration for absolute amplitude measurement.
Stability of negatively charged dust grains in Saturn's ring plane. Presented at the Tucson Saturn Conference Atmospherics and radio noise
  • T G Northrop
  • J R And
  • Hill
  • E T Pierce
  • B A Smith
  • L Soderblom
  • R Beebe
  • J Boyce
  • G Briggs
  • A Bunker
  • S A Collins
  • C J Han-Sen
  • T V Johnson
  • J L Mitchell
  • R J Ter-Rile
  • M Carr
  • A F Cook
  • J Ii
  • E Cuzzl
  • D Shoe-Maker
  • T Morrison
  • C Owen
  • J Sagan
NORTHROP, T. G., AND J. R. HILL (1982). Stability of negatively charged dust grains in Saturn's ring plane. Presented at the Tucson Saturn Conference, Tucson, Ariz. May, 1982. PIERCE, E. T. (1977). Atmospherics and radio noise. In Lightning (R. H. Golde, Ed.), pp. 351-384. Aca-demic Press, New York. SMITH, B. A., L. SODERBLOM, R. BEEBE, J. BOYCE, G. BRIGGS, A. BUNKER, S. A. COLLINS, C. J. HAN-SEN, T. V. JOHNSON, J. L. MITCHELL, R. J. TER-RILE, M. CARR, A. F. COOK, II, J. Cuzzl, J. B. POLLACK, G. E. DANIELSON, A. INGERSOLL, M. E. DAVIES, G. E. HUNT, H. MASURSKY, E. SHOE-MAKER, D. MORRISON, T. OWEN, C. SAGAN, J.
Saturn: Upper atmosphere and ionosphere Lightning activity on Jupiter. Presented at AGU Fall Meeting Saturn's electrostatic dis-charges: Could lightning be the cause
  • S K Atreya
  • J H Waite
  • T M Donahue
  • And A F Nagy
  • W J Borucki
  • A Bar-Nun
  • F L Scarf
ATREYA, S. K., J. H. WAITE, JR., T. M. DONAHUE, AND A. F. NAGY (1982). Saturn: Upper atmosphere and ionosphere. Presented at the Tucson Saturn Conference, Tucson, Ariz. May, 1982. BORUCKI, W. J., A. BAR-NUN, F. L. SCARF, AND A. F. Coot II (1981). Lightning activity on Jupiter. Presented at AGU Fall Meeting, San Francisco, De-cember, 1981. BURNS, J. A., M. R. SHOWALTER, J. N. CuzzI, AND R. H. DURISON (1983). Saturn's electrostatic dis-charges: Could lightning be the cause? Icarus 54, 280-295.