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We report results of a Hubble Space Telescope (HST) campaign with theAdvanced Camera for Surveys to observe Europa at eastern elongation, i.e.Europa's leading side, on 2008 June 29. With five consecutive HST orbits, weconstrain Europa's atmospheric \ion{O}{1} 1304 \A and \ion{O}{1} 1356 \Aemissions using the prism PR130L. The total emissions of both oxygen multipletsrange between 132 $\pm$ 14 and 226 $\pm$ 14 Rayleigh. An additional systematicerror with values on the same order as the statistical errors may be due touncertainties in modelling the reflected light from Europa's surface. The totalemission also shows a clear dependence of Europa's position with respect toJupiter's magnetospheric plasma sheet. We derive a lower limit for the O$_2$column density of 6 $\times$ 10$^{18}$ m$^{-2}$. Previous observations ofEuropa's atmosphere with STIS in 1999 of Europa's trailing side show anenigmatic surplus of radiation on the anti-Jovian side within the disk ofEuropa. With emission from a radially symmetric atmosphere as a reference, wesearched for an anti-Jovian vs sub-Jovian asymmetry with respect to the centralmeridian on the leading side, and found none. Likewise, we searched fordepartures from a radially symmetric atmospheric emission and found an emissionsurplus centered around 90 degree west longitude, for which plausiblemechanisms exist. Previous work about the possibility of plumes on Europa dueto tidally-driven shear heating found longitudes with strongest local strainrates which might be consistent with the longitudes of maximum UV emissions.Alternatively, asymmetries in Europa's UV emission can also be caused byinhomogeneous surface properties, inhomogeneous solar illuminations, and/or byEuropa's complex plasma interaction with Jupiter's magnetosphere.

HST/ACS Observations of Europa's Atmospheric UV Emission at Eastern Elongation