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5 - The Mysterious Periodicities of Saturn
- Edited by Kevin H. Baines, University of Wisconsin, Madison, F. Michael Flasar, NASA-Goddard Space Flight Center, Norbert Krupp, Tom Stallard, University of Leicester
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- Book:
- Saturn in the 21st Century
- Published online:
- 13 December 2018
- Print publication:
- 06 December 2018, pp 97-125
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Summary
The rotation rate of a planet is a fundamental parameter, no less than its mass or composition, and planetary investigators require this rate to assess various other phenomena such as planetary wind speeds, internal and atmospheric models, ring dynamics and so forth. Saturn presents a conundrum, however, because none of its various planetary periods indicates the “true” rotation of the planet. Thus, although the planet displays an abundance of periodicities near 10.7 hours, the exact rotation period of Saturn is unknown. In the magnetosphere, “planetary-period oscillations” (PPOs) appear in charged particles, magnetic fields, energetic neutral atoms, radio emissions and motions of the plasma sheet and magnetopause. In Saturn’s rings, the spoke phenomenon can exhibit periodicities near 10.7 hours, and ring phenomena themselves may be related to the interior rotation of the planet. In the high-latitude ionosphere, modulations near this period appear in auroral motions and intensities. In the upper atmosphere, some cloud features rotate near this period, although wind speeds are generally faster, and the well-known polar hexagon rotates with a period close to 10.7 hours. Some of the magnetospheric/ionospheric oscillations differ in the northern and southern hemispheres and their periods do not remain constant, sometimes varying on long time scales of a year or longer and sometimes on much shorter time scales. These variations in the period argue against a cause related to changes interior to Saturn, and because the magnetic and spin axes of Saturn are reported to be axisymmetric (unlike those of any other known planet), Saturn’s periodicities cannot be explained as “wobble” caused by a geometric tilt or by a nondipolar magnetic anomaly. Several models have been proposed to account for the observed periodicities, including rotating atmospheric vortices, periodic plasma releases and a flapping magnetodisk, but none can satisfactorily explain all of Saturn’s periodicities nor their common origin, and none can determine the exact rotation rate of the planet. This chapter reviews Saturn’s periodicities, theories thereof, and how they might be used to determine the elusive rotation rate of the planet.
7 - Saturn’s Aurorae
- Edited by Kevin H. Baines, University of Wisconsin, Madison, F. Michael Flasar, NASA-Goddard Space Flight Center, Norbert Krupp, Tom Stallard, University of Leicester
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- Book:
- Saturn in the 21st Century
- Published online:
- 13 December 2018
- Print publication:
- 06 December 2018, pp 166-195
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Summary
The aurorae of each planet are produced as a direct interaction between the upper atmosphere and magnetosphere of that planet. Energetic particles from the magnetosphere are driven into the top of the atmosphere, depositing energy there, and ultimately resulting in an electromagnetic emission. As a result, aurorae are related to conditions within the planetary magnetospheres so an understanding of the auroral emission provides a view of both the magnetospheric structure and how that magnetosphere is coupled with the underlying ionosphere. In the past, Saturn’s magnetosphere, and thus its aurorae, have been seen as something of a hybrid between the solar-wind-driven interaction at Earth and the rotationally dominated system at Jupiter. However, observations across a wide wavelength range by both the Cassini spacecraft and supporting Earth-based telescopes have revealed Saturn’s aurorae to be highly complex. We now recognize that Saturn’s aurorae are driven by the dynamic magnetic field interactions between the atmosphere, the solar wind and plasma trapped within the magnetosphere, all strongly affected by the rapid rotation of the planet.
In this chapter, we highlight the broad variety of auroral features observed at Saturn, and discuss how these are generated by energetic particles moving within current systems that link to solar wind interactions (Section 7.2), interactions with plasma generated within the magnetosphere (Section 7.3) and with current systems that vary periodically, including those linked to weather systems within Saturn’s upper atmosphere (Section 7.4). Finally, we conclude with a discussion of the major questions that remain about Saturn’s aurorae, and summarize the upcoming observations that will help us answer them. We begin with a discussion of how the auroral emission is generated and the characteristics of aurorae observed at Saturn. In particular, we highlight the most recent auroral research, following on from the overview of Saturn’s auroral processes presented in past reviews of the subject (for example, Kurth et al. 2009).