Ground-based and spacecraft observations of lightning activity on Saturn

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Abstract

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 12–33 MHz, 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 1.8–16 MHz. 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.

Highlights

► Joint measurements of SEDs with the Cassini and UTR-2 were carried out. ► We got a high degree of coincidence of time characteristic for observing storm. ► It forms a good basis for studying of SEDs with high time and frequency resolution. ► Ground-based radio astronomy has a potential for investigation of SED-like signals.

Introduction

After almost 30 years of studying Saturn electrostatic discharges (SED) with the help of spacecraft (Warwick et al., 1981, Kaiser et al., 1983, Zarka and Pedersen, 1983, Dyudina et al., 2007, Fischer et al., 2007) terrestrial observations have been successfully conducted using the largest decameter radio telescope, UTR-2. So far, these observations have resulted in an unambiguous registration of 70 events (Konovalenko et al., 2007) during the storm E in January–February 2006, and only two of them were matched with the data from the Cassini spacecraft.

Spacecraft observations of thunderstorm activity on other planets give us an opportunity to study them jointly with ground-based observations in more detail. The success of the first ground-based observations was the motivation to study the next SED storm. This storm (storm F in the numbering scheme used by Cassini) see, e.g. Fischer et al. (2008) started on November 27, 2007, after 21 months of “silence”. The storm F was characterized by an unusual duration – about 8 months (until July 15, 2008) – and consisted of more than 400 episodes (where an episode is defined as one rotation of Saturn with SEDs). The first task of ground-based radio telescopes is to define SED characteristics from events that were observed simultaneously. This is necessary for their identification (against terrestrial interference background) and for a further study of lightning discharges from other planets with and without spacecraft support. The progress in receiver development and computing techniques is proceeding rapidly, and the modernization of the UTR-2 radio telescope has increased the capabilities of receivers. The corresponding improvement in time resolution will allow to study the microsecond structure of discharges and define the SED power spectra. Joint SED studies will contribute to a more precise definition of radiation parameters and result in a more complete picture of storm activity on Saturn.

Section snippets

Material and methods

To interpret the joint results of ground-based and space data correctly, one has to take into account the specifics of the receivers, namely the RPWS (Radio and Plasma Wave Science) instrument onboard Cassini (Gurnett et al., 2004) and the DSPZ (Digital Spectrum analyzer Z modification) at the UTR-2 (Kozhin et al., 2007, Ryabov et al., 2010). The RPWS instrument consists of three 10-meter long antennas and its spectrum analyzer has several operating modes. A full cycle of frequency sweeping

Results and discussion

From November 27, 2007 until December 12, 2007 we conducted several observation sessions at UTR-2 with a time resolution of 20 ms. The most interesting sessions were those when both UTR-2 and Cassini registered the highest SED activity. The data obtained during these observation sessions are summarized in Table 1. SED number, SED rate (average number of flashes per second in the session) and e-folding time (τ) (burst durations can be described by an exponential distribution with an e-folding

Conclusions

In this work it is undoubtedly shown for the first time that ground-based radio telescopes can detect such low-intensity events as Saturn's lightning with a high degree of reliability.

This first step, i.e. the identification of Saturn's lightning against the background of terrestrial radio interference—provides a reliable basis for further detailed studies of the temporal and spectral characteristics of the SEDs which reflect the physical processes occurring in the atmosphere and ionosphere of

Acknowledgments

This work was initiated and partly supported by ANR Program NT05-1 42530 “Radio-Exopla”, and pursued in the frame of the NASU-CNRS PICS program “Development of LF radioastronomy with ultrahigh sensitivity and resolution” (Grant 1.33.11). This work is also partially supported by the Ukrainian State Fund for Fundamental Researches (Project F28.2/005). Cassini activities in LESIA are supported by the CNES (Centre National d'Etudes Spatiales).

References (22)

  • Grießmeier, J.-M., Zarka, P., Konovalenko, A., Fischer, G., Zakharenko, V., Stappers, B. W., Girard, J. N., Ryabov, B.,...
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