Saturn’s Radio Emissions and their Relation to Magnetospheric Dynamics (invited)

With the arrival of the Cassini spacecraft at Saturn in July 2004, there have been quasi-continuous observations of Saturn Kilometric Radiation (SKR) emissions. In this paper we review the response of these emissions to dynamics in Saturn’s magnetosphere, driven by factors internal and external to the system. We begin by reviewing solar wind data upstream of Saturn and discuss the link between solar wind compressions and dynamics in Saturn’s magnetosphere, evidenced by intensifications and occasional phase changes in the SKR emission. We then review the link between magnetotail reconnection and planetary radio emissions. We begin in the well-sampled magnetotail of Earth and then move to Saturn where exploration of the nightside magnetosphere has revealed evidence of plasmoid-like magnetic structures and other phenomena indicative of the kronian equivalent of terrestrial substorms. In general, there is a good correlation between the timing of reconnection events and enhancements in the SKR emission, coupled with extension of the emission to lower frequencies. We interpret this as growth of the radio source region to higher altitudes along the field lines, stimulated by increased precipitation of energetic electrons into the auroral zones following reconnection. We also comment on the observation that the majority of reconnection events occur at SKR phases where the SKR power would be expected to be rising with time, indicating that reconnection is most likely to occur at a preferred phase. We conclude with a summary of the current knowledge of the link between Saturn’s magnetospheric dynamics and SKR emissions, and list a number of open questions to be addressed in the future.

Analysis of Latitudinal Dependence of Saturnian Radio Emissions (abstract)

Intense saturnian radio emission has been observed since June 2004, and until today, by the Radio and Plasma Waves Experiment (RPWS) on board Cassini spacecraft. During this long period of about six years the spacecraft was orbiting principally in the planetary equatorial plane. However in 2007 and 2008 orbits reach latitudes higher than 50? which lead us to investigate sub-auroral saturnian radio emissions. In this study we examine the spectral distinctions between the saturnian radio emissions observed in the equatorial plane and those in latitudes close to the southern and northern auroral regions. We consider the three components reported by Galopeau et al. [2007, J. Geophys. Res., 112, A11213] principally dominated by the so-called saturnian kilometric radiation (80 kHz - 900 kHz). We analyze the occurrence probability of these components taking into consideration different geometric configurations between the spacecraft, the planet and the Sun. We discuss the spectral alteration and change, from the equatorial plane to the high latitudes, of Saturnian radio emissions. Also we examine the spectral distinction between the radio emissions emitted from the northern and southern hemispheres, in particular in the case of the saturnian kilometric radiation (SKR). Our results are discussed and compared with those already reported in the literature dealing with the Voyager and the Cassini missions.

Observations of Chorus at Saturn by Cassini (abstract)

The Location of the High-Density Boundary in Saturn’s Inner Magnetosphere (extended abstract)

Electron density measurements from the Cassini RPWS Langmuir Probe instrument have identified a sharply-defined region of low plasma densities in Saturn’s magnetosphere outside a dipole L-shell of ~15. Gradients in the density profiles define a boundary identified as the plasmapause [Gurnett et al., 2010] that separates the region of higher plasma density from the region of very low plasma density. During seven consecutive high-latitude passes in the northern hemisphere from September through December 2006, Cassini followed a series of trajectories that skimmed along high-latitude magnetic field lines for several days. The orientation of these trajectories made it possible for the RPWS to detect modulations in the high-latitude auroral hiss emissions at a 10.6 hour rotational modulation rate [Gurnett et al., 2009] and for the RPWS Langmuir Probe instrument to detect modulations in the electron density profiles that were anti-correlated with the hiss emissions [Gurnett et al., 2010]. The strong and periodic modulations in the density profiles indicate that Cassini is passing in and out of a plasma region of higher densities. One example during this seven-orbit time interval is shown in Figure 1. The periodic modulations in the density profile are shown in the bottom panel and are clearly anti-correlated with the periodic occurrence of intense auroral hiss emissions shown in the upper panel. The highest densities in this high-latitude region are 0.1 cm-3, two orders of magnitude greater than the lowest densities in this part of Saturn’s magnetosphere but well below the densities seen inside Saturn’s plasma disk [Morooka et al., 2009;Persoon et al., 2009].

Density and Temperature of the Electron Core in the Inner Magnetosphere of Saturn from Cassini/RPWS Antennas (abstract)

Overview of Saturn Lightning Observations

The lightning activity in Saturn's atmosphere has been monitored by Cassini for more than six years. The continuous observations of the radio signatures called SEDs (Saturn Electrostatic Discharges) combine favorably with imaging observations of related cloud features as well as direct observations of flash-illuminated cloud tops. The Cassini RPWS (Radio and Plasma Wave Science) instrument and ISS (Imaging Science Subsystem) in orbit around Saturn also received ground-based support: The intense SED radio waves were also detected by the giant UTR-2 radio telescope, and committed amateurs observed SED-related white spots with their backyard optical telescopes. Furthermore, the Cassini VIMS (Visual and Infrared Mapping Spectrometer) and CIRS (Composite Infrared Spectrometer) instruments have provided some information on chemical constituents possibly created by the lightning discharges and transported upward to Saturn's upper atmosphere by vertical convection. In this paper we summarize the main results on Saturn lightning provided by this multi-instrumental approach and compare Saturn lightning to lightning on Jupiter and Earth.

Ground-Based Study of Saturn Lightning

Radio signatures of lightning discharges on Saturn have first been discovered by the Voyager spacecraft in 1980/81. After the Voyager flybys, the next sets of measurements only became available in 2004, when the Cassini spacecraft approached Saturn. Since then, Cassini provides continuous monitoring of Saturn’s lightning activity. In 2006, ground-based observations became available as a complementary source of information. Using a new broadband receiver at the UTR-2 radio telescope (Ukraine), Saturn lightning was observed over the whole spectral range of the instrument (10-30 MHz). This allows study of the temporal fine structure of the emission with a much finer temporal resolution than that of the routine satellite observations. More recently, Saturn lightning was also observed by two further ground-based radiotelescopes, namely WSRT (the Netherlands) and LOFAR (Europe). We present first results of recent ground-based observations of Saturn lightning performed with the radiotelescopes UTR-2, WSRT and LOFAR, and we describe the aims of future observations using these instruments.

Electric Field Transients Observed by the HUYGENS Probe in the Atmosphere of Titan: Atmospheric Electricity Phenomena or Artefacts? (abstract)

During the first 35 close Titan flybys the Radio and Plasma Wave Science instrument (RPWS) aboard the CASSINI orbiter did not observe radio signals possibly associated with lightning in the atmosphere of Titan [Fischer et al., 2007, Geophys. Res. Lett., 34, L22104). The electric field sensors of the HUYGENS PWA instrument (permittivity, waves and altimetry) observed smooth variations as well as impulsive events varying with altitude during the descent of the probe in the atmosphere of Titan. While a part of the low frequency signals was explained as externally driven Schumann resonances, there is still a debate on the origin of the impulsive events. In order to differentiate natural atmospheric discharges from sources on the parachute or the probe the HUYGENS electric field data have been re-evaluated und combined with probe attitude and velocity. The correlation results indicate that atmospheric electricity phemonena are present in the atmosphere of Titan.

New Type of Periodic Bursts of Non-Io Jovian Decametric Radio Emission

Analyzing the data acquired by STEREO/WAVES,Wind/WAVES and Cassini/ RPWS instruments during the time interval between the years 2002-2010 we have revealed unusual periodic radio bursts of the non-Io controlled component of the Jovian decametric radiation (non-Io DAM). These non-Io bursts are typically observed in a frequency range from ~ 5 MHz up to ~ 10–16 MHz and recur during several Jovian days with a surprisingly new period of ~ 10.07 hours. This period is 1.5% longer than the rotation period of the inner Jovian magnetosphere (System III, 9.925 hour). The occurrence probability of these periodic bursts has been found to be significantly higher in the sector of Jovian Central Meridian Longitude between 300? and 60? (via 360?), corresponding to the region of non-Io-C sources. Stereoscopic observations performed by STEREO/WAVES as well as Wind/WAVES and Cassini/RPWS suggest that the sources of the periodic bursts sub-corotate with Jupiter. The relations between the occurrence of the periodic bursts and solar wind activity have been analyzed.

Jupiter’s Decametric and Hectometric Radio Emissions Observed by Cassini RPWS and Voyager PRA

The relationship between Jupiter’s decametric (DAM) and hectometric (HOM) radio emissions is important to help understand the emission mechanism that both of them have in common, but it has remained an elusive enigma. We have investigated Jovian DAM and HOM emissions observed by the Cassini, Voyager 1 and Voyager 2 spacecraft. We made a statistical comparison of Cassini and combined Voyager 1 and 2 data for occurrence probability histograms in Central Meridian Longitude (CML) and in Io phase from 2 to 16 MHz, and a statistical analysis of Jovian HOM polarization plotted as a function of Jovian magnetic latitude and frequency below 3 MHz based on only the Cassini data. We found that (1) the position of Source B shows shifts in longitude from 10 to 16 MHz as seen in both Cassini and combined Voyager 1 and 2 data, (2) the effect of Io can be seen down to 4 MHz, (3) the occurrence probability of HOM emissions are separated into right- and lefthand polarization senses, and (4) attenuation bands make a large contribution to intensify the HOM emissions around the attenuated regions.