Modulations of Saturn's UV Auroral Oval Location by Planetary Period Oscillations

S.W.H Cowley,A Bader,G Provan,S.V Badman,W Pryor,J Kinrade

doi:10.1029/2018ja026117

Abstract

AbstractIt is well known that Saturn's magnetospheric dynamics are greatly influenced by the so‐called planetary period oscillations (PPOs). Based on Cassini Ultraviolet Imaging Spectrograph (UVIS) imagery, it has been shown previously that the UV auroral intensity is clearly modulated in phase with rotating field‐aligned current (FAC) systems associated with the PPOs. Here we expand upon this investigation by using the same data set to examine the PPO‐induced spatial modulation of the main auroral oval. We present a robust algorithm used for determining the location of the main emission in Cassini‐UVIS images. The location markers obtained are then used to calculate the statistical location of the auroral oval and its periodic displacement due to the PPO FACs and the related ionospheric flows. We find that the largest equatorward displacement of the main arc lags behind the PPO‐dependent statistical brightening of the UV aurora by roughly 45–90° in both hemispheres and is not colocated with it as the present model based on magnetometer observations suggests. We furthermore find the center of the auroral oval by fitting circles to the main emission and analyze its elliptic motion as the entire oval is displaced in phase with the PPO phases. It is demonstrated that the periodic displacements of both the auroral oval arc and its center are larger when the two PPO systems rotate in relative antiphase than when they are in phase, clearly indicating that interhemispheric PPO FAC closure modulates not only the intensity but also the location of the main UV auroral emission.

Highlights

Even though Saturn’s magnetic dipole tilt relative to its rotational axis is negligibly small [Burton et al, 2010; Dougherty et al, 2018], the surrounding magnetosphere is permeated with periodic phenomena occurring mostly at periods close to the planetary rotation period of ∼ 10.5 h
It is demonstrated that the periodic displacements of both the auroral oval arc and its center are larger when the two planetary period oscillations (PPOs) systems rotate in relative antiphase than when they are in phase, clearly indicating that interhemispheric PPO fieldaligned current (FAC) closure modulates the intensity and the location of the main UV auroral emission
In continuation of the recent investigation of PPO-induced UV auroral intensity modulations based on all available Cassini Ultraviolet Imaging Spectrograph (UVIS) data with exception of the mid-2013 to mid-2014 coalescence period [Bader et al, 2018], we used a subset of the same dataset to determine the spatial modulation of the main auroral emission

Summary

Introduction

Even though Saturn’s magnetic dipole tilt relative to its rotational axis is negligibly small [Burton et al, 2010; Dougherty et al, 2018], the surrounding magnetosphere is permeated with periodic phenomena occurring mostly at periods close to the planetary rotation period of ∼ 10.5 h. The main emission is generated by flow shears between different plasma populations in the outer magnetosphere - subcorotating with the planet at different angular speeds, this sets up a system of field-aligned currents (FACs) causing electrons to precipitate into the polar upper atmosphere [e.g., Badman et al, 2015; Belenkaya et al, 2014; Hunt et al, 2014] Superimposed on this local time-fixed FAC system are the two rotating FAC systems associated with one PPO system each [e.g., Andrews et al, 2010b; Provan et al, 2016]. For a more detailed overview describing the instrumentation and the projection and integration methods we refer the reader to Bader et al [2018]

Determining the location of the auroral oval

Circle fitting

Dataset selection

PPO phases

Statistical auroral oval location

Seasonal and solar cycle variation of the auroral oval location

Statistical oval displacement with PPO phase

Oval center motion

Summary