Comment on “ A new approach to Saturn ’ s periodicities ” by J . F . Carbary

The phenomenon of “planetary period oscillations” in Saturn’s magnetosphere was first observed by the Voyager spacecraft via modulations near the planetary rotation period in the intensity of radio emissions at kilometer wavelengths [Warwick et al., 1981; Desch and Kaiser, 1981], together with related oscillations in particles and magnetic fields in situ within the magnetosphere [Carbary and Krimigis, 1982; Espinosa and Dougherty, 2000]. Subsequent remote observations of Saturn kilometric radiation (SKR) by the Ulysses spacecraft showed that the period can change by of order ~1% over yearly intervals, such that the modulations cannot be connected directly to the rotation of the body of the planet [Galopeau and Lecacheux, 2000]. A more recent Cassini discovery has been that two such variable modulations are generally present in SKR data, with periods of ~10.6 and ~10.8 h during Saturn southern summer conditions early in themission [Kurth et al., 2008; Gurnett et al., 2009a], though later becoming closer together, more variable, and with intervals of only one detectable period postequinox [Provan et al., 2014; Fischer et al., 2014; Cowley and Provan, 2015]. As will be indicated below, in subsequent discussions these two periods have generally been taken to correspond to planetary period oscillation (PPO) phenomena associated with the Northern and Southern Hemispheres of the planet. In a recent paper, however, Carbary [2015] has suggested a “new approach” to these findings, pointing out the simple fact via a number of elementary demonstrations that if a single sinusoidal oscillation is modulated in amplitude, frequency, or phase, its frequency spectrum is no longer monochromatic andmay become split into a number of peaks near that of the principal period, suggested to correspond to the observed dual PPO modulations. The simplest illustrative example is that of a signal S(t) consisting of a “carrier” of frequency ω2 whose amplitude is modulated by a sinusoid of frequency ω1, for which it can be shown by elementary trigonometry that