An Overview of the Rotational Modulation of Three Types of Saturnian Radio Emissions

Details An Overview of the Rotational Modulation of Three Types of Saturnian Radio Emissions An Overview of the Rotational Modulation of Three Types of Saturnian Radio Emissions

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p23c..07g&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P23C-07

Other

2772 Plasma Waves And Instabilities (2471), 5719 Interactions With Particles And Fields

Scientific paper

In this paper we analyze and compare the rotational modulation of three types of Saturnian radio emissions detected by Cassini. These are (1) Saturn kilometric radiation (SKR), which is an intense radio emission generated by the cyclotron maser mechanism along the high latitude auroral magnetic field lines at typical frequencies ranging from about 50 to 500 kHz; (2) narrowband Saturn myriametric radiation (nSMR), which is a narrowband radio emission generated in the inner region of the magnetosphere by mode conversion from electrostatic upper-hybrid emissions at frequencies ranging from about 5 to 20 kHz; and (3) auroral hiss, which is a whistler-mode emission generated at frequencies below about 100 Hz by electron beams associated with the auroral field-aligned current system. Spectrum analyses of the rotational modulation of the SKR shows that this radiation is made up of at least two components. The first component, which is believed to originate from a pulsing clock-like source on the dayside of Saturn, has a modulation period that has varied slowly from about 10.76 to 10.81 hours over the roughly three-year period since Cassini arrived at Saturn. This period is substantially greater than the rotation period of 10.543 hours recently reported by Anderson and Schubert (2007) for the interior of Saturn, implying a significant slippage relative to the rotation of the internal magnetic field of Saturn. The second component of the SKR, which has only recently been clearly identified from passes at high latitudes, has a much less variable period ranging from about 10.54 to 10.60 hours. This period is very close to the internal rotation period reported by Anderson and Schubert. The close proximity of this period to the internal rotation period suggests that the source may be associated with the rotating features commonly observed in auroral images, possibly indicating the presence of a rotating anomaly associated with the internal magnetic field at high latitudes. Another possibility is that the second component may be generated by a periodic phase modulation of the first slowly varying (clock-like) component by long period variations in the solar wind velocity, as has been suggested previously by Cecconi and Zarka (2005). Because the nSMR tends to occur in association with magnetospheric storms and is not continuously present, the modulation period of this radio emission cannot be determined as accurately as for the SKR. To within the available accuracy the modulation period of the nSMR is consistent with the period of the first (clock-like) component of the SKR, which is known to be locked to the rotation rate of the magnetic field (and possibly of the plasma) in the inner region of Saturn's magnetosphere. This implies that the source of the nSMR is slipping relative to the internal rotation of Saturn at a rate that is very similar to the first component of the SKR. Because of the limited amount of data that is available in the high latitude regions where the auroral hiss is observed, the modulation period of the auroral hiss is even more difficult to determine with useful accuracy. However, our preliminary measurements show that the modulation period of the auroral hiss is close to the internal rotation period reported by Anderson and Schubert, which would be consistent with a rotating auroral source.

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