Are Saturn's Interchange Injections Organized by Rotational Longitude?

S W H Cowley,G B Hospodarsky,N Sergis,X Jia,S.‐Y Ye,G Provan,A R Azari,M F Thomsen,C Paranicas,D G Mitchell,A M Rymer,M W Liemohn

doi:10.1029/2018ja026196

Abstract

AbstractSaturn's magnetosphere has been extensively studied over the past 13 years with the now retired Cassini mission. Periodic modulations in a variety of magnetospheric phenomena have been observed at periods close to those associated with the emission intensity of Saturn kilometric radiation (SKR). Resulting from Rayleigh‐Taylor like plasma instabilities, interchange is believed to be the main plasma transport process in Saturn's inner to middle magnetosphere. Here we examine the organization of equatorially observed interchange events identified based on high‐energy (3–22 keV) H+ intensifications by several longitude systems that have been derived from different types of measurements. The main question of interest here is as follows: Do interchange injections undergo periodicities similar to the Saturn kilometric radiation or other magnetospheric phenomena? We find that interchange shows enhanced occurrence rates in the northern longitude systems between 30° and 120°, particularly between 7 and 9 Saturn Radii. However, this modulation is small compared to the organization by local time. Additionally, this organization is weak and inconsistent with previous findings based on data with a limited time span.

Highlights

The rotation rates of gas giant planets, such as Jupiter or Saturn, cannot be determined by tracking surface features; instead, periodic variations in the emission strength within the radio frequency band are used
Periodic modulations in a variety of magnetospheric phenomena have been observed at periods close to those associated with the emission intensity of Saturn kilometric radiation (SKR)
The main question of interest here is as follows: Do interchange injections undergo periodicities similar to the Saturn kilometric radiation or other magnetospheric phenomena? We find that interchange shows enhanced occurrence rates in the northern longitude systems between 30° and 120°, between 7 and 9 Saturn Radii

Summary

Introduction

The rotation rates of gas giant planets, such as Jupiter or Saturn, cannot be determined by tracking surface features; instead, periodic variations in the emission strength within the radio frequency band are used. Estimations of the period of Saturn's kilometric radiation (SKR) have shifted significantly from the Voyager era (10 hr 39 min 24 s ± 7 s), to Ulysses (10 hr 42 min 34.2 s), and to Cassini (10 hr 45 min 45 ± 36 s; Desch & Kaiser, 1981; Galopeau & Lecacheux, 2000; Gurnett et al, 2005). The observed shifts in the SKR derived period are too large to represent changes in the rotation rate of the planetary body itself and must be produced by some other process yet undetermined (e.g., Cecconi & Zarka, 2005; Galopeau & Lecacheux, 2000; Stevenson, 2006). A model that incorporates flow vortices in the upper atmosphere/ionosphere has been demonstrated to be able to account for many of the observed periodic phenomena with quantitative fidelity (e.g., Jia & Kivelson, 2012; Jia et al, 2012)

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