Magnetospheric Physics at Saturn: Is It Earth-Like, Jupiter-Like, or Even Like It Was 25 Years Ago? II
Presiding: M K Dougherty, Space and Atmospheric Physics, Imperial College London; M E Hill, Department of Physics, University of Maryland at College Park
SM12A-01 INVITED 10:30h
No, Yes, and Most of the Time
Those are my answers to the questions raised in the session title. Saturn's magnetosphere is not Earth-like because, although affected by solar wind properties, it is not dynamically driven by solar-wind coupling. It is Jupiter-like in that both are powered by planetary rotation. In both cases, rotational energy is extracted from the planet by dominantly internal sources of plasma that serves to populate the magnetosphere as well as tap the rotational energy reservoir. The number, location, and even the nature of the internal plasma sources are quite different at Saturn than Jupiter but, when these differences are taken into account, there is every reason to expect that the same basic physical paradigm will apply. I will touch on the confirming evidence already provided by Cassini during its first few orbits, and on outstanding questions that we hope to address in the remainder of the mission. The third question is somewhat of a trick question, as I will explain.
SM12A-02 10:45h
Energetic ion acceleration in Saturn's magnetotail: Substorms on Saturn?
The Magnetospheric Imaging Instrument (MIMI) Ion and Neutral Camera (INCA) on the Cassini spacecraft has recorded abrupt increases in energetic neutral atom flux coming from the general direction of Saturn's magnetotail. These bursts of ion activity in the tail seem to be related to solar wind magnetic field direction as well as the passage of compression regions, and are well correlated with enhancements in the Saturn kilometric radiation. Given the similarities between these events and substorm activity on Earth, including their dependence on interplanetary conditions, we conclude that Earth-like substorms occur within Saturn's magnetosphere. Unlike Earth, the hot plasma is quickly convected about Saturn at close to corotation speed.
SM12A-03 11:00h
Interchange events in Saturn's magnetosphere: Frequency, spatial distribution, and implications for plasma transport
During the Cassini spacecraft's first orbits around Saturn, numerous transient events were observed in the planet's inner magnetosphere. These events are characterized by an abrupt increase in the magnetic field strength, a simultaneous disappearance of the low energy ion and electron populations and the appearance of a hot electron population with energies between one hundred and a few thousand eV. In some cases, these events are associated with time-dispersed energetic particle events. These characteristics are consistent with the rapid interchange of flux tubes and the injection of hot, tenuous plasma from the outer magnetosphere. This process, also observed by the Galileo spacecraft at Jupiter, is associated with the radial transport of plasma and the centrifugally-driven Raylegh-Taylor instability. We present a statistical analysis of these events, describing their average properties, frequency of occurrence, and spatial distribution. We identify the regions where these events are most frequent, with the regions where radial transport is most vigorous.
SM12A-04 INVITED 11:15h
Radio and Plasma Waves in the Magnetosphere of Saturn: Similarities to Earth and Jupiter
With a few notable exceptions, most of the radio and plasma waves observed in the magnetosphere of Saturn are remarkably similar to those observed in the magnetospheres of Earth and Jupiter. For example, Saturn kilometric radiation, terrestrial kilometric radiation, and Jovian decametric radiation have many characteristics in common and are all generated by the same basic plasma mechanism, namely the cyclotron maser instability. Similar statements can be made about a broad range of other radio and plasma wave phenomena, for example, electrostatic emissions at the upper hybrid frequency, electrostatic waves near odd half-integral harmonics of the electron cyclotron frequency, and various whistler-mode electromagnetic emissions. What is different at these various planets are the plasma parameters and the types of interactions that lead to the plasma instabilities. Thus, the frequencies of the cyclotron maser radiation at Saturn, Earth and Jupiter are all different because the magnetic field strengths are different. And, there is no terrestrial analog of the Io-controlled Jovian decametric radiation, since there is no moon orbiting in the inner region of the terrestrial magnetosphere. In this talk, we will review the radio and plasma wave observations obtained by the Cassini Radio and Plasma Wave Science (RPWS) instrument in the vicinity of Saturn, and compare these to similar observations at Earth and Jupiter with the objective of contrasting and understanding the physical processes involved.
SM12A-05 11:30h
Narrowband Electromagnetic Emissions at Saturn Revisited: Cassini Observations
Voyager observations revealed the existence of a complex set of narrowband electromagnetic emissions at Saturn in the frequency range of 3 to 30 kHz. These were found to be generated by mode conversion from electrostatic upper hybrid waves into ordinary mode electromagnetic waves which could freely propagate from the source. The frequency spacings of the narrowband emissions near the electron cyclotron frequency at Tethys, Dione, and Rhea suggested that these bands may be produced near these moons as part of their interaction with Saturn's magnetosphere. Cassini has now been in orbit at Saturn for seven months and has had an opportunity to further explore the low-frequency narrowband emissions. Similar emissions found over the ring system have been shown to be propagating in the Z-mode [Farrell et al., submitted, J. Geophys. Res., 2005], hence, would not propagate far from Saturn. However, other narrowband emissions observed by Cassini appear to be similar to those discovered with Voyager. Most of these, however, are found in the solar wind upstream of Saturn's bow shock. These emissions typically consist of a single band in the frequency range of ~6 to 8 kHz but sometimes are seen outside this range. More complex sets of bands are also seen, although not as often. In this paper we examine the new observations in an attempt to further understand the source of these emissions.
SM12A-06 11:45h
HST UV Imaging of Saturn's Southern Aurora during Simultaneous Cassini Imaging of the Northern Aurora
On 17 Feb. 2005, one week after the AGU abstract deadline, Hubble Space Telescope (HST) observations are scheduled with the Advanced Camera for Surverys (ACS) to image Saturn's UV aurora for a period of 5 HST orbits, or 8 hours, corresponding to 0.7 of one Saturn rotation. In the present epoch, observations from the Earth can observe nearly the entire southern auroral oval in sunlight, but none of the northern oval, due to the tilt of Saturn's axis. Over the same period, a Cassini imaging sequence will scan across the northern nightside auroral emission region from the nightside of the planet. This presents a unique opportunity to determine the relationship between the conjugate points in the auroral emission regions, along with charged particle and magnetic field measurements in the nightside Saturn magnetosphere. This campaign of observations will be presented in this paper, with a concentration on the HST images, along with scientific conclusions as appropriate.