The Variable Rotation Period of the Inner Region of Saturn's Plasma Disk D. A. Gurnett, et al. Science 316, 442 (2007); DOI: 10.1126/science.1138562 The following resources related to this article are available online at www.sciencemag.org (this information is current as of October 20, 2008 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/cgi/content/full/316/5823/442 Supporting Online Material can be found at: http://www.sciencemag.org/cgi/content/full/1138562/DC1 A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/cgi/content/full/316/5823/442#related-content This article cites 26 articles, 9 of which can be accessed for free: http://www.sciencemag.org/cgi/content/full/316/5823/442#otherarticles on October 20, 2008 This article has been cited by 20 article(s) on the ISI Web of Science. This article has been cited by 3 articles hosted by HighWire Press; see: http://www.sciencemag.org/cgi/content/full/316/5823/442#otherarticles This article appears in the following subject collections: Planetary Science http://www.sciencemag.org/cgi/collection/planet_sci www.sciencemag.org Information about obtaining reprints of this article or about obtaining permission to reproduce this article in whole or in part can be found at: http://www.sciencemag.org/about/permissions.dtl Downloaded from Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2007 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS. REPORTS netometer (MAG) instruments on the Cassini The Variable Rotation Period spacecraft, which is in orbit around Saturn. In an earlier study (16) using electron density mea- of the Inner Region of Saturn’s surements from the RPWS, we noticed that in the vicinity of Enceladus’ orbit, from about 3 to 5 RS, the electron density is often quite different on the Plasma Disk inbound and outbound portions of the same pass D. A. Gurnett,1 A. M. Persoon,1 W. S. Kurth,1 J. B. Groene,1 T. F. Averkamp,1 (Fig. 1). This variability was initially thought to M. K. Dougherty,2 D. J. Southwood2,3 be attributable to the spacecraft position relative to Enceladus, but further study showed that it was We show that the plasma and magnetic fields in the inner region of Saturn’s plasma disk rotate not so. Instead, using the time-variable SKR in synchronism with the time-variable modulation period of Saturn’s kilometric radio emission. longitude system introduced by Kurth et al.(17), This relation suggests that the radio modulation has its origins in the inner region of the plasma we found (Fig. 2) that the electron density in the disk, most likely from a centrifugally driven convective instability and an associated plasma outflow inner region of the plasma disk has a nearly that slowly slips in phase relative to Saturn’s internal rotation. The slippage rate is determined sinusoidal variation with SKR longitude and by the electrodynamic coupling of the plasma disk to Saturn and by the drag force exerted by its that this variation is in phase with a similar interaction with the Enceladus neutral gas torus. nearly sinusoidal variation in the azimuthal, Bϕ, component of the magnetic field in the ecause Saturn is a giant gaseous planet, magnetosphere from near the outer edge of the plasma disk. This relation strongly suggests its internal rotation period cannot be Aring(11, 12). The co-rotation is caused by that the plasma density and magnetic field in Baccurately determined from visual obser- electromagnetic forces imposed by currents that the inner region of the plasma disk have a vations. Although it has been long accepted that flow along the highly conducting magnetic field rotational control that is directly linked to the the magnetic dipole axis of Saturn is aligned lines between the plasma disk and the upper time-variable period of the SKR modulation. almost exactly with its rotational axis (1, 2), atmosphere of Saturn. Most of the plasma in the By itself, a sinusoidal longitude variation does various magnetospheric phenomena display ro- plasma disk is believed to originate from not provide proof of rotational control, even tational modulation effects near its nominal ionization of a torus of neutral gas that is present though the longitude is measured relative to a 3–5 rotation period ( ). Of these, the most near the orbit of Enceladus. This torus originates rotating reference. To demonstrate rotational on October 20, 2008 thoroughly studied is Saturn kilometric radiation from geyserlike plumes of water on Enceladus control, it is essential that measurements be made (SKR), which is an intense radio emission that inject water vapor and ice particles into orbit over a range of local times in order to confirm that discovered during the 1980–1981 Voyager flybys around Saturn (13–15). Because of the rapid the longitude is the controlling variable. Fortu- of Saturn (3). The SKR modulation period was rotation of the plasma disk, the centrifugal force nately, the orbit of Cassini provides good local- determined by Voyager to be 10 hours, 39 min, at these radial distances is substantially larger time coverage and longitude coverage of both the 24 ± 7s (6). This value is now the internationally than Saturn’s gravitational force. Interchange magnetic field and the electron density (figs. S1 accepted rotation period of Saturn (7). More motions driven by the centrifugal force then act andS2).Asimpletestcanthenbeperformedto recently, radio measurements from the Ulysses to transport the plasma outward into the magne- confirm rotational control. This test consists of 8–10 and Cassini spacecraft ( ) have shown that tosphere. Because the plasma particles are con- plotting the measured quantities as a function of www.sciencemag.org the SKR modulation period varies by as much as strained by electromagnetic forces to move the longitude relative to some fixed direction, such 1% on time scales of years. Because of its large approximately along magnetic field lines, the as the Sun. This test shows that the amplitude of inertia, the internal rotation period cannot possi- centrifugal force also acts to concentrate the the modulation decreases and the spread of the bly have changed by such a large amount. How plasma near the equatorial plane, thereby forming data points relative to the best fit increases then is the SKR period related to the rotation the disklike structure. significantly (compare Fig. 2, B and C, with fig. period of the interior of Saturn? In this paper we ThedatausedinthisstudyarefromtheRadio S4, B and C), thereby providing convincing show that the SKR modulation is directly linked and Plasma Wave Science (RPWS) and Mag- evidence that both the magnetic field and the to the rotational modulation of plasma and Downloaded from magnetic fields in the inner region of Saturn’s Fig. 1. The electron density plasma disk, near the moon Enceladus, which ne obtained from Cassini orbits Saturn at 3.95 Saturn radii (RS)(1Saturn RPWS upper hybrid reso- radius = 60,268 km). We propose that the nance measurements (20) rotational modulation is caused by a centrifugally for 14 equatorial orbits from driven two-cell convective instability in the plas- 1 July 2004 to 1 July 2006. ma disk that originates from its interaction with Thedashedlineisapower- the neutral gas torus produced by Enceladus. law fit to the region beyond R k 4 −3 This instability causes a rotating plasma outflow 5 S,and =5.5×10 cm R 4.14 is the constant in the that imposes rotational control on other processes S power law. Only equatorial farther out in the magnetosphere, such as the orbits with a north-south dis- generation of SKR. tance z from the equatorial Saturn’s plasma disk, sometimes also called plane less than 0.1 RS were the plasma sheet or plasmasphere, is a dense co- used in this study. There are rotating plasma with a north-south thickness of large variations from the R about 1 to 2 S that extends outward into the power-law fit in the region from 3 to 5 RS. By following the colored line for a given 1Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242, USA. 2Blackett Laboratory, Imperial orbit, one can see that the electron densities for the inbound and outbound portions of the same pass College, London SW7 2AZ, UK. 3European Space Agency, are often quite different. This hysteresis-like dependence on radial distance strongly suggests a 75738 Paris, France. longitudinal control. 442 20 APRIL 2007 VOL 316 SCIENCE www.sciencemag.org REPORTS plasma density in the inner region of the plasma discovered rotational density modulation in the the plasma disk to the planet. Because the ejection disk rotate at a rate that is synchronous with the inner region of the plasma disk acts as the cam that of mass from Enceladus probably has long-term time-variable SKR modulation. For other factors drives other rotationally modulated effects farther variations that affect the density of the neutral gas that could affect this interpretation, such as aliasing out in the magnetosphere. The main questions that torus, this model provides a ready explanation for due to data gaps, and for an analysis of the slow remain are how are these rotational effects the somewhat irregular long-term variations in the long-term time variations in the rotation rate, see produced in the inner region of the magnetosphere, SKR modulation period (9).