Statistical Planetary Period Oscillation Signatures in Saturn's UV Auroral Intensity

Abstract

Saturn'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.

The Dynamics of Saturn's Main Aurorae

“…While this modulation should theoretically be of comparable strength at all LTs (Hunt et al, ), it is here barely discernible at dusk. This difference in modulation amplitude agrees with statistical findings (Bader et al, ) and might be related to a seemingly larger spread of the PPO currents at dusk than at dawn (Andrews et al, ).…”

“…The mean and median UV power distributions (Figure c) are in close agreement between noon and midnight but clearly differ near dawn—the mean maximizing here, while the median minimizes. The mean auroral power agrees very well with intensity averages of previous observations which all showed a distinct peak between 6 and 9 LT (e.g., Bader et al, ; Badman et al, ; Carbary, ; Kinrade et al, ; Lamy et al, , ; Nichols et al, )—but, as seen here, the mean UV intensity/power is obviously not a good representation of the typical state of the aurora. The median directly shows that in more cases than not, the dawn aurora is dimmer than the dusk aurora and not brighter; it is the few transient high‐power events subcorotating through dawn which skew the mean power to unrepresentative high values at these LTs.…”

“…This agrees with field line mapping of the main aurorae which places the main upward FAC sheet at an equatorial distance beyond 10 R S , outward from the middle ring current (e.g., Belenkaya et al, ; Bradley et al, ; Talboys et al, ). The flow of the solar wind and the associated Dungey cycle activity (e.g., Cowley, Bunce & Prangé ; Jackman & Cowley, ) seem to have little to no impact on this system, since no significant LT asymmetries in auroral FACs (Hunt et al, ) and auroral brightness are observed, contrary to previous findings (e.g., Bader et al, ; Badman et al, ; Carbary, ; Kinrade et al, ; Lamy et al, , ; Nichols et al, ), where observed asymmetries were likely an artifact of small data sets and averaging procedures unsuitable for determining the full variability of Saturn's auroral dynamics. This is supported by earlier studies estimating the Dungey cycle contribution to magnetic flux transport to be roughly an order of magnitude lower than the contribution arising from rotational flows in quiet solar wind conditions such that no asymmetry in auroral brightness is expected (e.g., Badman & Cowley, ; Badman et al, ).…”

“…Previous studies using auroral imagery obtained by the Hubble Space Telescope in the ultraviolet (UV) wavelength band (e.g., Kinrade et al, ; Lamy et al, , ; Nichols et al, ) and by the Cassini spacecraft at infrared (IR) and UV wavelengths (e.g., Bader et al, ; Badman et al, ; Carbary, ) have statistically identified such a brightness asymmetry, seemingly confirming that Saturn's main aurorae are indeed significantly solar wind driven. However, most of these studies used rather small sets of single exposures lacking context and/or short observation series without good time resolution to obtain statistical averages, hence not taking into account the complicated dynamics of Saturn's aurora which had already been observed by the Voyager spacecraft (Sandel & Broadfoot, ; Sandel et al, ).…”

“…Their rotation at roughly the planetary period generates periodic signatures in all plasma properties and processes in Saturn's environment, the two systems exhibiting close but distinct periods that vary with time (e.g., Provan et al, , ). Each PPO system is usually dominant in one hemisphere, but its associated system of FACs partly closes in the opposite hemisphere such that each hemisphere experiences a double modulation of, for example, auroral FACs by both the northern and southern PPO systems (e.g., Bader et al, ; Bradley et al, ; Hunt et al, ; Provan et al, ).…”