Planetary Period Oscillations in Saturn's Magnetosphere: Comparison of Magnetic and SKR Modulation Periods and Phases During Northern Summer to the End of the Cassini Mission

E. J. Bunce,S. W. H. Cowley,G. Provan,L. Lamy

doi:10.1029/2018ja026079

Abstract

AbstractWe compare periods and phases of Saturn planetary period oscillations determined from Cassini magnetic field and Saturn kilometric radiation (SKR) data from the beginning of 2016 to the end of mission in mid‐September 2017, encompassing northern summer solstice in May 2017. Both data sets show that the periods are almost unchanging, varying by only ~ ±0.01 hr about 10.79 hr for the northern system and 10.68 hr for the southern system, close to values attained by mid‐2015 after period coalescence between mid‐2013 and mid‐2014. The mean absolute differences between the magnetic and SKR periods are ~0.0036 hr (~13 s), consistent with estimated magnetic measurement uncertainties, while the overall mean difference is less than 0.001 hr (~2–3 s), at the limit of resolution. The relative phasing between magnetic and SKR modulations is correspondingly near constant and such that the equatorial planetary period oscillation fields of the northern/southern systems point radially outward near‐oppositely at ~14.3/2.5 hr local time at corresponding SKR maxima, with upward planetary period oscillation currents located ~2 hr postdawn for both systems, consistent with previous intervals having dawnside spacecraft apoapsides. Southern SKR emissions are found to be significantly dual modulated at both southern and northern periods in data limited to lie well within the southern shadow zone of the northern sources. These northern period modulations are shown to be approximately in phase with those in the northern emissions, consistent with a recent suggestion that bidirectional auroral electron acceleration may generate in phase SKR emissions in both hemispheres.

Highlights

Despite the perfect axisymmetry of Saturn's internally generated external magnetic field, within measurement uncertainty (Burton et al, 2010; Dougherty et al, 2018), modulations at approximations to Saturn's rotation period, termed “planetary period oscillations” (PPOs), have been found to be ubiquitously present in magnetospheric plasma properties, energetic neutral atom emissions, plasma waves, magnetic fields, current systems and boundaries, as well as in auroral ultraviolet (UV), infrared, and radio emissions (see review by Carbary and Mitchell (2013), and references therein)
Cassini magnetic field and Saturn kilometric radiation (SKR) data from the beginning of 2016 to the end of mission in mid‐September 2017, encompassing northern summer solstice in May 2017. Both data sets show that the periods are almost unchanging, varying by only ~ ±0.01 hr about 10.79 hr for the northern system and 10.68 hr for the southern system, close to values attained by mid‐2015 after period coalescence between mid‐2013 and mid‐2014
Lamy (2017) reported that the southern SKR periodogram displays a secondary peak at the northern period after late 2016, spanning northern summer solstice, this mirroring the situation observed during southern summer conditions in 2005–2007 when a secondary peak occurred in the northern periodogram at the southern period

Summary

Introduction

Despite the perfect axisymmetry of Saturn's internally generated external magnetic field, within measurement uncertainty (Burton et al, 2010; Dougherty et al, 2018), modulations at approximations to Saturn's rotation period, termed “planetary period oscillations” (PPOs), have been found to be ubiquitously present in magnetospheric plasma properties, energetic neutral atom emissions, plasma waves, magnetic fields, current systems and boundaries, as well as in auroral ultraviolet (UV), infrared, and radio emissions (see review by Carbary and Mitchell (2013), and references therein) Analyses of these modulations have pointed to the PPO sources being located in the two polar thermospheres/ionospheres, associated with rotating twin‐vortex flows (Cowley et al, 2016; Hunt et al, 2014, 2015; Jia et al, 2012; Jia & Kivelson, 2012; Smith, 2006; Smith et al, 2016). Determination of the PPO properties in these regions requires secure knowledge of the period and phase of the PPO oscillations derived from measurements obtained at larger distances on these orbits, as investigated here

Nature of the PPO Current System

Data Interval

Magnetic Field Data Analysis Procedures

SKR Data Analysis Procedures

Comparison of Magnetic Field and SKR PPO Periods

Dual Modulation of SKR Emissions

Summary and Conclusions