What Makes Saturn's Magnetosphere Beat with One-hour Periodicity?

During its 13-year presence at Saturn, the Cassini spacecraft frequently detected 1-hour quasi-periodic pulsations in magnetic field and particle measurements in Saturn's outer magnetosphere (Roussos, 2016; Palmaerts, 2016). The events are typically 4-6 hours long. The oscillations are in the direction transverse to the magnetic field with amplitudes between of 0.1 and 1 nT. They frequently recur at the period of Saturnian kilometric radiation (SKR). In this study, we analyze newly calibrated Cassini magnetometer (MAG) and the Cassini Plasma Spectrometer (CAPS) data from the Cassini's orbit insertion in 2004 to the end of each instrument's availability (MAG up to the Grand Finale in 2017, and CAPS up to 2012), and map the events along magnetic field lines using a magnetic field model for Saturn with nominal IMF conditions (Khurana, 1997). We used a software algorithm to identify over 500 oscillation packets of 4 hours or more in the magnetometer (MAG) and when available in the thermal plasma (CAPS) data. Most of these events occur on field lines that map close to the magnetospheric boundary and map to the poles between 75 ^o and 85 ^o latitude in System III (S3C) coordinates. Using metadata derived from these events, we evaluate whether the events could be generated internally at Saturn, showing a Saturnian longitude (SLS) or an invariant latitude organization, or are externally driven by the solar wind compressions on the magnetosphere (Viall, 2010). For the latter case, we search for and identify possible periodic density structures advecting in the solar wind in Cassini and Voyager data near Saturn. The frequency of such advecting structures, as well as the amplitude and periodicity variations of the oscillations throughout the magnetosphere help distinguish between internal or external origins. A standing wave model along the closed field lines (Yates, 2016) explains the Alfvénic nature of the waves. Our results clarify the driver of these standing waves.