Abstract
AbstractSaturn's auroral emissions are a good measure of field‐aligned current (FAC) systems in the planet's magnetospheric environment. Previous studies based on magnetic field data have identified current systems rotating with the planetary period oscillations (PPOs) in both hemispheres, superimposed onto the local time‐invariant current system producing the main auroral emission. In this study we analyze the statistical behavior of Saturn's ultraviolet auroral emissions over the full Cassini mission using all suitable Cassini‐UVIS images acquired between 2007 and 2017. We examine auroral intensities by organizing the data by the two PPO coordinate systems. Strong statistical intensifications are observed close to the expected locations of upward FACs in both hemispheres, clearly supporting the main assumptions of the present theoretical model. We furthermore find clear signatures of modulation due to interhemispheric current closure from the PPO system in the opposite hemisphere, although with a weaker modulation amplitude. The auroral intensity in the northern hemisphere is shown to be modulated by a superposition of the FACs associated with both PPO systems, as the modulation phase and amplitude varies as expected for different relative orientations (beat phases) of the two PPO systems.
Highlights
Saturn’s ring of main auroral emission is located approximately at the open-closed field line boundary [e.g., Cowley et al, 2004]. This region maps magnetically to the outer magnetosphere where flow shears between hot plasma populations, subcorotating at different angular speeds with Saturn’s planetary rotation, are largest [e.g., Hunt et al, 2014; Belenkaya et al, 2014]. These flow shears are thought to set up a system of fieldaligned currents (FACs), of which the upward component is carried by downward electrons precipitating onto the upper atmosphere
In order to avoid introducing a statistical bias due to the phase-lock of the northern and southern planetary period oscillations (PPOs) systems in near-antiphase between mid-2013 and mid-2014, no data from this time window is included. This left us with 2777 UVIS images which were used to qualitatively and quantitatively investigate modulations of the auroral intensity, boundary locations and the auroral oval’s center position due to the two PPO systems
The auroral intensity was found to be strongly modulated by the primary PPO system
Summary
Saturn’s ring of main auroral emission is located approximately at the open-closed field line boundary [e.g., Cowley et al, 2004]. An earlier study by Carbary [2013] used Cassini UVIS imagery from 2006-2009 to investigate rotational modulation of Saturn’s auroral intensities with respect to SKR phase, but observed clear dependencies only for the southern hemisphere.
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