Saturn Electrostatic Discharges Research Articles

Statistical study of Saturn electrostatic discharges

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. We have made a statistical study 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, we compare the results obtained for the two encounters and describe in detail the appearance, intensity, and spectrum of the Saturn electrostatic discharges (SED). We show that the SED spectrum is likely smooth in the frequency range 1–40 MHz and extends down to 20 KHz. We have determined 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). An attempt to interpret the statistical results is presented.

The Z3 zonal harmonic model of Saturn's magnetic field: Analyses and implications

The planetary magnetic field of Saturn has been studied by the spacecraft Pioneer 11 in 1979, Voyager 1 in 1980, and Voyager 2 in 1981. The field is found to be primarily dipolar and axially coincident with the rotation axis but with significant quadrupole and octupole moments. The harmonic terms are g10 = 21535 nT, g20 = 1642 nT, and g30 = 2743 nT. This model field, Z3, in conjunction with a model for an equatorial ring current represents very precisely the in situ magnetic field measurements and data on charged particle absorption by satellites and rings within 8 Rs of the planet. However, this axisymmetric model fails to explain the periodic modulation of Saturn's kilometric radiation (SKR) or Saturn's electrostatic discharges (SED). This enigma of Saturn's magnetosphere remains unsolved in spite of extensive reconsideration of all available data bearing on this issue.

Saturn's electrostatic discharges: Could lightning be the cause?

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 10 h10 m 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.

Saturn's electrostatic discharges: Properties and theoretical considerations

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 R s. 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 R s 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.

Atmospheric storm explanation of saturnian electrostatic discharges

The Voyager Planetary Radio Astronomy (PRA) experiments detected an impulsive (15–400 ms), broadband (20 kHz to 40 MHz) radio emission component that persisted throughout the two Saturn encounter periods1,2. These bursts were grouped into episodes which recurred with a period of about 10 h 10 min, distinctly faster than the saturnian rotation period of 10 h 39.4 min (refs 1–5). This periodicity, coupled with the occurrence dependence on distance from Saturn, led Warwick et al.1 to conclude that the bursts were related to the Saturn system and coined the term SED for Saturn electrostatic discharges. We have analysed these SED episodes paying particular attention to their occurrence in both time and frequency as a function of spacecraft–Saturn geometry during the Voyager 1 encounter. We conclude that the characteristics of SED are best explained by a long-lived atmospheric lightning storm or systems of storms in Saturn's equatorial zone. From our model, we infer the ionospheric electron density near Saturn's noon meridian and over the night hemisphere.

A search for Saturn electrostatic discharges in the Voyager plasma wave data

Voyager plasma wave data were searched for evidence of Saturn electrostatic discharges which have previously been reported by the planetary radio astronomy team following the Voyager 1 and 2 encounters with Saturn. On the basis of the radio astronomy observations, the discharges should have been observable by the plasma wave experiment on Voyager, however, there is no evidence in the plasma wave data for the phenomenon. We analyze the statistical significance and comment on the ramifications of this null result. We entertain various explanations of the lack of plasma wave observations of electrostatic discharges, including the possibility that many events are of a much shorter duration than previously reported or that there could possibly be a nonlinear distortion in the radio astronomy receiver which could artificially broaden the spectrum of the discharges.

Shot noise from grain and particle impacts in Saturn's ring plane

The ring plane event detected by the Voyager 1 and 2 Planetary Radio Astronomy experiment is distinct from Saturn kilometric radiation (SKR) and from Saturn electrostatic discharges (SED). It consists of radio noises recorded only during Saturnian ring plane crossings. Several models are tested. The electrostatic noise on the antennas resulting from the passage of electrons and ions near the antennas (quasi‐thermal noise) leads to order of magnitude much lower than the observed values. Shot noise due to electrons and ions collected and/or emitted by the antennas and spacecraft can explain the noise recorded during Saturn Voyager 1 ring plane crossing and partly what is observed in the case of Voyager 2. For this latter event we must introduce the shot noise due to grain impacts. A quantitative approach of this process gives an estimation of the dust size ∼ 2.3 µm just outside the G‐ring.

The source of Saturn electrostatic discharges

During both Voyager encounters with the saturnian system, the Planetary Radio Astronomy (PRA) experiment detected strong discrete episodic bursts of radio emission, termed Saturn electrostatic discharges (SED)1–3. An examination of Voyager 2 photopolarimeter (PPS) 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.