Signatures of magnetospheric injections in Saturn's aurora

Abstract

We report on the evolution of auroral structures based on two 3 h sequences of Saturn's northern hemisphere obtained with the UVIS instrument on board Cassini, and we discuss their possible association with injections in the magnetosphere of Saturn. Simultaneously with the UV auroral structures, we observe Energetic Neutral Atom (ENA) enhancements which are indicative of a rotating heated plasma region possibly related to magnetospheric injections. We examine the possibility that the UV auroral structures reported here are triggered by energetic particle injections by investigating (a) the evolution of the longitudinal extent of an injection and (b) its energy density, properties that change with time due to dispersion and ion/electron losses. We simulate the auroral counterpart of an injection considering that the precipitating energy flux could be provided to the ionosphere by pitch angle diffusion and electron scattering by whistler-mode waves. We compare the brightness and size evolution of the simulated ionospheric signature with the observed values, and we demonstrate that the UV auroral structure behaves as an auroral signature of an injection. This comparative study defines characteristics of the injections such as the spectral index and the electron energy range as well as the magnetospheric corotation rate. Additionally, based on the simultaneous ENA-UV emissions, we discuss the possibility that pitch angle diffusion and electron scattering may not be the only mechanism responsible for the observed auroral emissions. Field-aligned currents driven by the pressure gradients along the boundaries of the injected hot plasma cloud could be also considered to play a role on how injections create auroral emissions at Saturn. Key Points Observational description of auroral features (UV-ENA) related to injectionsSimulations show that the UV feature behaves as auroral signature of injectionWe discuss two mechanisms that magnetospheric injections create aurora ©2013. American Geophysical Union. All Rights Reserved.