Interpretation of the North‐South Asymmetric Oxygen Aurora Morphology on Europa Using Test Particle Simulation

Abstract

Several observations using the Hubble Space Telescope reported that the brightness morphology of the oxygen OI] 135.6 nm emissions on Europa's atmosphere has a north‐south asymmetry which changes with the position of Europa with respect to the Jovian magnetospheric plasma sheet. Similar north‐south asymmetry of Io's auroral limb glow has been explained by higher electron flux into the atmosphere on the hemisphere that faces the plasma sheet center. This explanation, however, has not yet been evaluated for the case of Europa quantitatively. In this study, we used a test particle simulation for the Jovian magnetospheric electrons to estimate the brightness of the 135.6 nm aurora in Europa's atmosphere and evaluate the cause of the north‐south asymmetry with the previously suggested idea, in which the strong deceleration of the magnetospheric flux tube results in the inhomogeneous electron flux into Europa's atmosphere (the “slow‐down effect”). Our simulation successfully recreates the systematically changing north‐south asymmetry of Europa's oxygen aurora brightness using the “slow‐down effect.” With deceleration into 10% of the background plasma flow, the maximum north‐to‐south brightness ratio is estimated at 2.17 and 2.56 on the trailing (plasma‐upstream) and leading (downstream) side, respectively. However, the previously observed brightness ratio is larger on the trailing side (up to ∼5). The results indicate that additional model scenarios are required to fully explain the north‐south asymmetry of Europa's oxygen aurora morphology.