We report data from the Cassini radio and plasma wave instrument during the approach and first orbit at Saturn. During the approach, radio emissions from Saturn showed that the radio rotation period is now 10 hours 45 minutes 45 +/- 36 seconds, about 6 minutes longer than measured by Voyager in 1980 to 1981. In addition, many intense impulsive radio signals were detected from Saturn lightning during the approach and first orbit. Some of these have been linked to storm systems observed by the Cassini imaging instrument. Within the magnetosphere, whistler-mode auroral hiss emissions were observed near the rings, suggesting that a strong electrodynamic interaction is occurring in or near the rings.
[1] We report 5 kHz narrowband Z mode emissions observed by the Cassini Radio and Plasma Waves Science (RPWS) instrument during high latitude perikrone passes. The narrowband emissions observed below the local electron cyclotron frequency ( f ce ) are >20 dB more intense than the usual L-O mode narrowband emissions observed above local f ce . Polarization measurements show that the narrowband emissions observed below f ce are oppositely polarized to those above f ce , which identifies the emissions below f ce with Z mode. We propose that the L-O mode narrowband emissions observed at 5 kHz are mode converted from the Z mode waves at a density gradient or density irregularity. The Z mode to L-O mode conversion via scattering off of density irregularities can also account for the direction finding results of 5 kHz narrowband emissions which point to a source in the auroral zone. We also present the first magnetic field measurements of Saturn narrowband emissions validating their electromagnetic nature.
[1] Since Cassini's arrival at Saturn in 2004, the Radio and Plasma Wave Science instrument has detected numerous narrowband (NB) radio emission events. These emissions, mostly detected around 5 and 20 kHz, usually occur periodically for several days after intensifications of Saturn kilometric radiation. We present calculations based on an electron density profile of Saturn's plasma torus and a dipole magnetic field model showing that the NB emissions originate from the northern and southern edges of Saturn's plasma torus at L shells $ 8 to 10 for 5-kHz NB and L $ 4 to 7 for 20-kHz NB. In many cases, Cassini passes through the source region of the 20-kHz NB, as indicated by intense electrostatic upper hybrid (ESUH) waves in close proximity to electromagnetic emissions on spectrograms. The positions of the spacecraft when intense ESUH waves are observed agree with the model predictions of the NB source locations. Source locations determined by goniopolarimetric (also known as direction-finding) analysis of the NB emissions also support the above results, although sometimes the directions of arrival point toward the region interior to Saturn's plasma torus. A polarization reversal technique is applied to localize the NB emissions observed during spacecraft rotation, on the basis of the fact that the source is within the antenna plane when the apparent circular polarization degree switches sign. The NB emissions are found to be L-O mode polarized, which is consistent with the prediction of linear/nonlinear mode conversion theory. It is also found that sometimes right-hand polarized NB emissions are generated at second harmonic frequencies of the 20-kHz NB; in which case, wave-wave interactions between oppositely propagating ESUH waves may play an important role in the mode conversion process.
Lightning discharges in Saturn's atmosphere emit radio waves with intensities about 10,000 times stronger than those of their terrestrial counterparts. These radio waves are the characteristic features of lightning from thunderstorms on Saturn, which last for days to months. Convective storms about 2,000 kilometres in size have been observed in recent years at planetocentric latitude 35° south (corresponding to a planetographic latitude of 41° south). Here we report observations of a giant thunderstorm at planetocentric latitude 35° north that reached a latitudinal extension of 10,000 kilometres-comparable in size to a 'Great White Spot'-about three weeks after it started in early December 2010. The visible plume consists of high-altitude clouds that overshoot the outermost ammonia cloud layer owing to strong vertical convection, as is typical for thunderstorms. The flash rates of this storm are about an order of magnitude higher than previous ones, and peak rates larger than ten per second were recorded. This main storm developed an elongated eastward tail with additional but weaker storm cells that wrapped around the whole planet by February 2011. Unlike storms on Earth, the total power of this storm is comparable to Saturn's total emitted power. The appearance of such storms in the northern hemisphere could be related to the change of seasons, given that Saturn experienced vernal equinox in August 2009.
Saturn has been known for over thirty years to emit an intense radio emission at kilometer wavelengths called Saturn Kilometric Radiation (SKR) that is modulated by the rotation of the planet. Although the period of this modulation was initially thought to represent the rotation period of the planet, it is now known that the radiation has two distinctly different rotational modulation periods that vary by on the order of one percent on times scales of years. One component originates primarily from the northern auroral region, and the other originates primarily from the southern auroral region. The differences in the modulation periods are believed to be due to latitudinal variations in the slippage of the magnetosphere relative to the interior of planet, apparently controlled by the seasonal variation in the tilt of Saturn's rotational axis. Since other magnetospheric phenomena display similar complicated rotational modulation effects, there is a need to define north and south longitude systems based on the variable SKR modulation periods in the two hemispheres. Because the SKR signal received by the spacecraft often includes both components it is sometimes difficult to separate the phases of the two components. In this paper we describe a method of determining the two phases based on a tracking filter approach that can separately track the modulation waveforms of the two components. The phases of the two waveforms can then be used to define a new longitude system for the northern and southern components that we call the SLS4 longitude system. This is an extension of the previous SLS2 and SLS3 longitude systems, which only described phase variations of the southern component.
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