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The Radio Wave Polarization of Saturn Lightning Observed by Cassini.

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Title: The Radio Wave Polarization of Saturn Lightning Observed by Cassini.
Authors: Fischer, G.1 (AUTHOR) georg.fischer@uni-graz.at, Imai, M.2 (AUTHOR), Taubenschuss, U.2 (AUTHOR), Píša, D.2 (AUTHOR), Kurth, W. S.3 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. Jun2025, Vol. 130 Issue 6, p1-17. 17p.
Subject Terms: Polarization of radio waves, Electrostatic discharges, Radio waves, Plasma waves, Circular polarization
Abstract: We investigate the polarization of the radio waves emitted by lightning in Saturn's atmosphere, which are referred to as SEDs for Saturn Electrostatic Discharges. Using the complete SED data set retrieved by Cassini's RPWS (Radio and Plasma Wave Science) instrument we found that the circular polarization of SEDs below a frequency of 2 MHz depends on the latitudinal hemisphere of the lightning storm. SEDs from several storms located at 35° $35{}^{\circ}$ South latitude display a strong right–hand sense of circular polarization, whereas SEDs from storms located at 35° $35{}^{\circ}$ and 50° $50{}^{\circ}$ North are overwhelmingly left–hand polarized. A single storm located at Saturn's equator in June 2005 shows 4 right–handed SEDs when Cassini was in the northern hemisphere and 4 left–handed SEDs when Cassini was in the southern hemisphere. The polarization characteristic is consistent with an emission in the L‐O mode, consistent with the theory of Fischer, Gurnett, et al. (2007, JGR 112, A12308, https://doi.org/10.1029/2007JA012592) that the R‐X mode is largely absorbed in Saturn's ionosphere below a frequency of 2 MHz. The sense of circular SED polarization below 2 MHz is opposite to the polarization of Saturn kilometric radiation (SKR), and thus this can be used to distinguish SEDs from SKR and to determine the hemispherical origin of the causative lightning storm. Plain Language Summary: Lightning from storms in Saturn's atmosphere emit radio waves, which are abbreviated as SEDs for Saturn Electrostatic Discharges. In this paper we measured the polarization of SEDs detected by the radio wave instrument of the Cassini spacecraft. We found that SEDs below a frequency of 2 MHz show a high degree of circular polarization, that is, the tip of the electric field vector of the wave rotates on a circle. SEDs from storms in the southern hemisphere show right–hand polarization, whereas SEDs from the northern hemisphere are overwhelmingly left–hand polarized. The sense of SED polarization can thus be used to determine the hemispherical origin of the causative lightning storm. The polarization of the SEDs is fixed in the plasma of Saturn's ionosphere, in which the SEDs are split into a left–hand ordinary (L‐O) mode wave and a right–hand extraordinary (R‐X) mode wave. We confirm the theory of Fischer, Gurnett, et al. (2007, JGR 112, A12308, https://doi.org/10.1029/2007JA012592) that the SEDs display the polarization characteristic of an L‐O mode wave since the R‐X mode is absorbed in Saturn's ionosphere below 2 MHz. Key Points: The polarization of Saturn lightning radio emissions from all thunderstorms in the Cassini era are investigatedThe radio waves are highly circular and are left–handed for storms in the northern and right–handed for storms in the southern hemisphereBelow 2 MHz Saturn lightning emissions are in L–O mode since the R–X mode is absorbed in Saturn's ionosphere [ABSTRACT FROM AUTHOR]
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Abstract:We investigate the polarization of the radio waves emitted by lightning in Saturn's atmosphere, which are referred to as SEDs for Saturn Electrostatic Discharges. Using the complete SED data set retrieved by Cassini's RPWS (Radio and Plasma Wave Science) instrument we found that the circular polarization of SEDs below a frequency of 2 MHz depends on the latitudinal hemisphere of the lightning storm. SEDs from several storms located at 35° $35{}^{\circ}$ South latitude display a strong right–hand sense of circular polarization, whereas SEDs from storms located at 35° $35{}^{\circ}$ and 50° $50{}^{\circ}$ North are overwhelmingly left–hand polarized. A single storm located at Saturn's equator in June 2005 shows 4 right–handed SEDs when Cassini was in the northern hemisphere and 4 left–handed SEDs when Cassini was in the southern hemisphere. The polarization characteristic is consistent with an emission in the L‐O mode, consistent with the theory of Fischer, Gurnett, et al. (2007, JGR 112, A12308, https://doi.org/10.1029/2007JA012592) that the R‐X mode is largely absorbed in Saturn's ionosphere below a frequency of 2 MHz. The sense of circular SED polarization below 2 MHz is opposite to the polarization of Saturn kilometric radiation (SKR), and thus this can be used to distinguish SEDs from SKR and to determine the hemispherical origin of the causative lightning storm. Plain Language Summary: Lightning from storms in Saturn's atmosphere emit radio waves, which are abbreviated as SEDs for Saturn Electrostatic Discharges. In this paper we measured the polarization of SEDs detected by the radio wave instrument of the Cassini spacecraft. We found that SEDs below a frequency of 2 MHz show a high degree of circular polarization, that is, the tip of the electric field vector of the wave rotates on a circle. SEDs from storms in the southern hemisphere show right–hand polarization, whereas SEDs from the northern hemisphere are overwhelmingly left–hand polarized. The sense of SED polarization can thus be used to determine the hemispherical origin of the causative lightning storm. The polarization of the SEDs is fixed in the plasma of Saturn's ionosphere, in which the SEDs are split into a left–hand ordinary (L‐O) mode wave and a right–hand extraordinary (R‐X) mode wave. We confirm the theory of Fischer, Gurnett, et al. (2007, JGR 112, A12308, https://doi.org/10.1029/2007JA012592) that the SEDs display the polarization characteristic of an L‐O mode wave since the R‐X mode is absorbed in Saturn's ionosphere below 2 MHz. Key Points: The polarization of Saturn lightning radio emissions from all thunderstorms in the Cassini era are investigatedThe radio waves are highly circular and are left–handed for storms in the northern and right–handed for storms in the southern hemisphereBelow 2 MHz Saturn lightning emissions are in L–O mode since the R–X mode is absorbed in Saturn's ionosphere [ABSTRACT FROM AUTHOR]
ISSN:21699380
DOI:10.1029/2024JA033560