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

Fischer, G. and Pagaran, J. A. and Zarka, P. and Delcroix, M. and Dyudina, U. A. and Kurth, W. S. and Gurnett, D. A. (2019) Analysis of a long-lived, two-cell lightning storm on Saturn. Astronomy and Astrophysics, 621 . Art. No. A113. ISSN 0004-6361. http://resolver.caltech.edu/CaltechAUTHORS:20190124-081711295

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Abstract

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.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1051/0004-6361/201833014DOIArticle
ORCID:
AuthorORCID
Fischer, G.0000-0002-0431-2381
Additional Information:© 2019 ESO. Article published by EDP Sciences. Received 13 March 2018; Accepted 14 November 2018; Published online 15 January 2019. Support from the Austrian Science Fund (FWF project P24325-N16) is gratefully acknowledged. P.Z. acknowledges support from CNES (French Space Agency) and the PNP & PNST programs from CNRS/INSU. The research at the University of Iowa was supported by NASA through contract 1415150 with the Jet Propulsion Laboratory. The authors also thank many amateur astronomers for their dedicated observations of Saturn, and especially Tomio Akutsu, Jean-Jacques Poupeau, and Maximo Suarez for the usage of their images in this paper. Cassini RPWS and ISS data are available via the Planetary Data System (PDS).
Funders:
Funding AgencyGrant Number
Fonds Wetenschappelijk Onderzoek (FWO)P24325-N16
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Centre National de la Recherche Scientifique (CNRS)UNSPECIFIED
Institut National des Sciences de l'Univers (INSU)UNSPECIFIED
NASA/JPL1415150
Subject Keywords:planets and satellites: atmospheres – planets and satellites: gaseous planets – methods: data analysis
Record Number:CaltechAUTHORS:20190124-081711295
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20190124-081711295
Official Citation:Analysis of a long-lived, two-cell lightning storm on Saturn. G. Fischer, J. A. Pagaran, P. Zarka, M. Delcroix, U. A. Dyudina, W. S. Kurth and D. A. Gurnett. A&A, 621 (2019) A113. DOI: https://doi.org/10.1051/0004-6361/201833014
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:92446
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:24 Jan 2019 17:16
Last Modified:24 Jan 2019 17:16

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