Georg Fischer

Saturn Electrostatic Discharges or SEDs (first detected by the Voyager spacecraft) are electromagnetic radio emissions caused by lightning discharges in Saturn's atmosphere. Previous studies suggested that the SEDs originated from the equatorial region where the rotation period of the atmosphere could explain the periodicity observed in SED activity. However, there were no convective clouds detected around the equator by the optical camera onboard Voyager 1. Recently, the SED radio observations of the Cassini spacecraft from 2004 to 2017 showed that the SEDs are produced by convective storms in Saturn's atmosphere, and the polarization of SEDs is directly linked to the hemisphere of the storm (left-handed SEDs propagated to Cassini from the North, right-handed from the South). Here, we apply a new calibration method to the Voyager radio polarization data, for which we expected to see predominantly left-handed polarization, matching the characteristics of radiation coming from convective storms in the northern hemisphere. We propose that the storm is located at 35° North, where the convective clouds were seen from the Voyager optical cameras. The shorter periodicity of SED occurrence in the Voyager data is the result of the relative motion between Saturn's atmosphere and the spacecraft, and the additional effect of radio waves being bent over the horizon in Saturn's ionosphere. This new feature of SED propagation was not known during the Voyager era, but it was observed many times by the orbiting Cassini spacecraft that the over-the-horizon effect can mimic a shorter (or longer) periodicity by shifting the sub-spacecraft western longitude range when the SEDs are observed. In this presentation, we show our new results and interpretations on the SEDs during the Voyager era.References:Imai, M., Fischer, G., Taubenschuss, U., Kolmašová, I., Santolík, O., & Píša, D. (2026). Polarization measurements and source locations of Saturn electrostatic discharges during the Voyager era. Journal of Geophysical Research: Planets, 131, e2025JE009079.https://doi.org/10.1029/2025JE009079

A Saturn Electrostatic Discharge or SED is an intense electromagnetic impulse induced by Saturn lightning, which escapes into space through the magnetoplasma of the ionosphere. Since previous SED polarizations from Voyager data are inconclusive, this polarization study is the first which properly considers the characteristics of the Voyager Planetary Radio Astronomy (PRA) instrument and the change of the antenna response as a function of spacecraft attitude. SED radio observations of the Cassini spacecraft from 2004 to 2017 showed a hemispheric dependence of their circular polarization sense at frequencies below 2 MHz, which can be explained by left‐hand ordinary mode emissions. Here we reinvestigated the SED polarizations of the Voyager radio observations at frequencies below 1.3 MHz. As a result, we found mixed polarizations and a dominance of right‐handed polarization, which is difficult to interpret in view of the Cassini result. However, we found that SEDs incident at large angles to the antenna plane (elevation angle ) showed a dominance of the expected left‐handed polarization. We show detailed results of the SED polarization analysis and also compare them with model simulations of the Voyager PRA signal processing of short SED‐like pulses. An equatorial SED source was suggested by its occurrence periodicity for both Voyagers, but imaging observations clearly detected convective clouds at 35N latitude. We demonstrate with Cassini data that the so‐called over‐the‐horizon effect can mimic an equatorial periodicity, even when the single SED source is at 35N latitude. For Voyager 2 also equatorial clouds were imaged.

Auroral kilometric radiation (AKR), Earth’s strongest radio emission, has long been associated with discrete auroras and electrons near a few kilo–electron volt (keV) range. However, auroras also occur in diffuse forms with broader electron energies, raising the question of why AKR has not been observed above diffuse auroras or linked to electrons outside the kilo–electron volt population. Comprehensive AKR source distributions have remained elusive because of observational limitations, and their local-time coverage remains largely unknown. Using spacecraft measurements, we identify a “radio oval” above the optical auroral oval, spanning the full local-time range, where AKR is emitted over both discrete and diffuse auroras. The AKR source electrons display diverse precipitation features, including monoenergetic (peak flux at 3.82 kilo–electron volts), broadband (1.34 kilo–electron volts), low-energy (0.47 kilo–electron volts), and diffuse types (>1 kilo–electron volt). These results reveal that the cyclotron maser instability—the mechanism driving AKR—can arise in diverse plasma environments, broadening our understanding of both AKR generation and auroral complexity.

Saturn kilometric radiation (SKR) is mostly a fully circularly polarized radio emission from Saturn's auroral region. However, Fischer et al. (2009, doi:10.1029/2009JA014176) found that SKR can show a linear component and be elliptically polarized, and this SKR property is typically found above observational latitudes of about 30 degrees (in both hemispheres). Using all available RPWS (Radio and Plasma Wave Science) data throughout the Cassini mission, we calculated mean polarization properties (linear, circular, total) of SKR for each hour in the frequency range from 100 to 1200 kHz. This revealed transitional latitudes from 20 to 40 degrees in which the linear polarization degree of SKR rises from around 0.1 (which is the usual error for the polarization measurement) up to 0.6. Furthermore, we found that SKR shows a lower total polarization degree at the transitional latitudes. We will try to give a reason for this unexpected behavior. We will also show comprehensive meridional plots of SKR circular, linear, and total polarization to understand the polarization properties of this important Saturnian radio emission.

Measurements from the Cassini Radio and Plasma Wave Science (RPWS) experiment obtained during the entire orbital phase of the Cassini mission around Saturn (13.2 years) are processed into a meridional map of plasma densities, comprising the innermost region of the ring ionosphere, the Enceladus plasma torus, and the outer magnetosphere, up to a dipole L‐shell of 30. We combine data from RPWS wave observations, such as whistler‐mode waves and upper hybrid electrostatic emissions, and from the RPWS Langmuir probe when operated in the proxy mode, providing an estimate for the spacecraft potential. In the region between dipole L‐shells of 2.4 and 30, observed electron densities are described by an analytic model that fits two functions, one for the water group ions and one for the protons, to observed densities across latitude on each magnetic field line. The derived electron density profiles are then augmented by a model for the cold core electron temperature as a function of L‐shell to obtain a meridional map of the electrostatic potential of the ambipolar electric field. The potential is extrapolated to the inner region of the rings, i.e., to below , to solve for the distribution of electron density in the ring ionosphere. A solution is based on a diffusive equilibrium model for the electrons and two ion species, and on observations from Cassini along the Saturn Orbit Insertion trajectory. A combination of analytic and diffusive equilibrium results finally yields an average global picture for the distribution of electron density in Saturn's magnetosphere.

Based on the 13 years‐long observation of Saturn kilometric radiation (SKR) by the Cassini/Radio and Plasma Wave Science instrument, this study derives statistics for left‐hand (L‐O) mode SKR emissions. These emissions span a frequency range from below 10 kHz to approximately 1,000 kHz, with spectral flux densities ranging between 10⁻²⁴ and 10⁻¹⁸ W/(m² Hz) consistent with, although fainter than, right‐hand mode SKR. Predominantly observed within 20 Saturn radii, the occurrence rates of L‐O mode SKR exhibit a preference below 20° in latitude, and a local time asymmetry with a higher occurrence rate on the dayside. Occurrence maps indicate that low‐frequency (100 kHz) emissions can pass through unhindered. Additionally, a substantial proportion of high‐latitude L‐O mode emissions exhibit strong elliptical polarization, contrasting with predominantly circular polarization observed at low latitudes.

Auroral Kilometric Radiation (AKR), the dominant radio emission from Earth, has been extensively studied, though previous analyses were constrained by limited spacecraft coverage. This study utilizes long‐term observations from Polar, Wind, and Arase spacecraft to generate comprehensive global AKR occurrence rate maps, revealing a high‐latitude and nightside preference. A detailed investigation of the equatorial shadow region confirms that the dense plasmasphere blocks AKR emissions across all wave frequencies. Low‐frequency emissions (100 kHz) propagate via plasmaspheric ducting and leakage, filling the equatorial region immediately outside the plasmasphere. Ray‐tracing simulations identify low‐density ducts within the plasmasphere as crucial channels that enable AKR to penetrate the dense plasmasphere, particularly at higher frequencies. These results align with meridional AKR observations, offering new insights into AKR propagation patterns.

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 South latitude display a strong right–hand sense of circular polarization, whereas SEDs from storms located at and 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.

The Radio and Plasma Wave Science (RPWS) experiment onboard the Cassini spacecraft collected electron density data in Saturn's magnetosphere during the entire orbital phase of the mission, comprising over 13 years of data. Electron densities have been derived from observations of electromagnetic and plasma waves, like whistler-mode hiss and electrostatic upper hybrid emissions, and also from a Langmuir probe, which measures the spacecraft potential as a proxy to the electron density in a thin plasma region. The method based on wave observations has a good coverage of high densities exceeding 1 cm-3 in the Enceladus plasma torus, whereas Langmuir probe proxy data cover regions with lower densities well. Both datasets are combined in the present study and fit by an analytic model that establishes charge neutrality between the electrons and two ion species in the form of hydrogen (protons) and water group ions. The fitting is based on bin-averages across all local times and both latitudinal hemispheres, and it spans between dipole L-shells of 2.4 and 30. Electron densities in the ring ionosphere at L < 2.4 are reconstructed from the modeled potential of the ambipolar electric field and a simplified diffusive equilibrium between electrons and a single ion species.

The radio waves with Saturn lightning origin have been studied since the first detection by Voyager 1, but their wave polarization has rarely been explored. Fischer et al. (2007, JGR 112, A12308) examined lightning from a storm located at 35° south latitude and found its radio emissions below 2 MHz to be highly polarized (80%) in a right-handed circular sense with respect to the wave propagation direction. They explained this by absorption of the extraordinary mode in Saturn's ionosphere and the dominance of the ordinary mode emission, as the radio waves are propagating against a direction of the magnetic field when coming from a source in the southern hemisphere. A limited examination of Saturn lightning from the so-called Great White Spot at 35° north latitude by Fischer et al. (2011, Nature 475, 75-77) revealed radio wave polarization in the left-handed sense. In this presentation we will show the radio wave polarization of lightning from various other storms in Saturn's atmosphere, which have not been examined until today. In this way we want to corroborate the hypothesis that the sense of the circular radio wave polarization of Saturn lightning depends on the hemispherical location of the storm.