Giampieri G, Dougherty M K, Smith E J, Russell C T
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA.
Nature. 2006 May 4;441(7089):62-4. doi: 10.1038/nature04750.
The rotation rate of a planet is one of its fundamental properties. Saturn's rotation, however, is difficult to determine because there is no solid surface from which to time it, and the alternative 'clock'--the magnetic field--is nearly symmetrically aligned with the rotation axis. Radio emissions, thought to provide a proxy measure of the rotation of the magnetic field, have yielded estimates of the rotation period between 10 h 39 min 22 s and 10 h 45 min 45 s (refs 8-10). Because the period determined from radio measurements exhibits large time variations, even on timescales of months, it has been uncertain whether the radio-emission periodicity coincides with the inner rotation rate of the planet. Here we report magnetic field measurements that revealed a time-stationary magnetic signal with a period of 10 h 47 min 6 s +/- 40 s. The signal appears to be stable in period, amplitude and phase over 14 months of observations, pointing to a close connection with the conductive region inside the planet, although its interpretation as the 'true' inner rotation period is still uncertain.
行星的自转速率是其基本属性之一。然而，土星的自转很难确定，因为没有可供计时的固体表面，而另一种“时钟”——磁场——几乎与自转轴对称排列。人们认为，无线电辐射可作为磁场自转的替代测量方法，据此得出的自转周期估计值在10小时39分22秒至10小时45分45秒之间（参考文献8 - 10）。由于通过无线电测量确定的周期即使在数月的时间尺度上也表现出很大的时间变化，所以无线电辐射的周期性是否与行星的内部自转速率一致一直不确定。在此，我们报告的磁场测量结果显示出一个周期为10小时47分6秒±40秒的时间稳定的磁信号。在14个月的观测中，该信号在周期、振幅和相位上似乎都很稳定，这表明它与行星内部的导电区域有密切联系，尽管将其解释为“真正的”内部自转周期仍不确定。
