Roth, Lorenz (2012) Aurorae of Io and Europa: Observations and Modeling. PhD thesis, Universität zu Köln.
In the present dissertation we study the auroral emissions emanating from the tenuous atmospheres of Jupiter's satellites Io and Europa. The satellites are embedded in a dense magnetospheric plasma environment. Due to Jupiter's fast rotation the corotating magnetospheric plasma particles constantly flow past Io and Europa causing a complex interaction and triggering auroral emission in the atmospheres. Therefore, aurora observations are a useful tool to explore both the magnetospheric environment and the neutral gas clouds of the satellites. For our analysis, images of Io's and Europa's ultraviolet (UV) emissions are extracted from a large data set of observations by the Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope (HST). Additionally, high-resolution images taken by the Long-Range Reconnaissance Imager (LORRI) of the New Horizons spacecraft of Io's visible aurora and a simultaneous observation by the HST Advanced Camera for Surveys (ACS) are examined. Io's aurora is characterized by bright emissions on the sub-Jovian and anti-Jovian flanks close to the equator and a fainter limb glow around the polar regions. Analyzing the STIS images we demonstrate that the variations of Io's UV aurora observed over a period of five years can be attributed to changes in the magnetospheric environment as well as to the varying viewing perspective. Based on these findings, an analytical model for the three-dimensional distribution of the UV emission around Io is developed. By fitting the parameters of this phenomenological model to the STIS observations, we are able to derive universal, quantitative properties of the emission distribution. Thereby, we find that the aurora above the sunlit part of Io's surface is brighter than on the night side or during an eclipse event, when Io moves through Jupiter's shadows. By comparing the LORRI and ACS observations of Io's aurora in eclipse to results from a three-dimensional two-fluid plasma simulation model, we show that the reduced auroral brightness originates from a lowered atmospheric density. Our results are a strong indication, that Io's atmosphere is driven by sublimation of SO2 frost rather than direct volcanic outgassing. The ultimate source for Io's atmospheric gas is widely debated for many years. We also investigate the observed variation or rocking of the bright auroral spots around Io's equator. The location of the spots has been shown to be correlated to the Jovian magnetic field orientation at Io. The exact correlation is, however, not 1:1, but is presumably affected by local perturbations of the magnetic field. Therefore, we analyze the influence of the magnetic field perturbations due to the plasma interaction as well as due to induced fields from Io's interior on the expected variations of the aurora spots. According to our calculations, the observed rocking of the aurora is not consistent with a conductive magma ocean below Io's surface. A rough estimation of the plasma interaction effects on the auroral spots does not yield conclusive results. Furthermore, we examine the morphology and brightness of oxygen emissions in the STIS observations of Europa's UV aurora. We find that most emission is observed on the disk of Europa rather than around the limb like in comparable observations of Io's aurora. We show that an increasing O2 density towards the sub-solar point possibly explains the observed morphology as well as previous observations. While the OI] 1356 Å emission pattern appears to vary periodically in correlation with the changing magnetospheric environment, the OI 1304 Å morphology is clearly dominated by a very bright locally confined emission in the northern, anti-Jovian quadrant of Europa's disk. The location of this anomaly coincides exactly with the longitude, where a peak in water vapor production is predicted due to increased shear heating at the surface cracks. Estimating the emission brightnesses expected for a local water plume, we find that the observed UV emission intensities are principally consistent with a locally confined abundance of water vapor. However, due to observational uncertainties and since we have neglected the effects of the plasma interaction for the approximation of the H2O abundance, our results can not be seen as prove for the existence of water plumes on Europa. To accurately determine the effects of an asymmetric O2 atmosphere and the influence of a local water plume, the plasma interaction has to be simulated.
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