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

Several observations using the Hubble Space Telescope reported that the brightness morphology of the oxygen O-I] 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 "slowdown 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 similar to 5). The results indicate that additional model scenarios are required to fully explain the north-south asymmetry of Europa's oxygen aurora morphology. Plain Language Summary Europa is one of Jupiter's icy moons and possesses a tenuous oxygen atmosphere. A ultraviolet oxygen aurora at 135.6 nm is generated due to the collision between atmospheric oxygen molecules and magnetospheric electrons in Europa's atmosphere, and the aurora has a time-variable north-south asymmetric structure. We simulated electron motion near Europa and calculated the aurora brightness to investigate how the spatial distribution of Europa's oxygen aurora becomes north-south asymmetric. Previous studies qualitatively explain that the strong deceleration of the magnetospheric plasma flux tube results in the unequal electron flux into the atmosphere to generate the north-south asymmetric aurora structure, but this explanation has never been quantitatively evaluated for the case of Europa. Based on this "slow-down effect" scenario, we successfully reproduced the time-variation of the north-south asymmetry of Europa's oxygen aurora. However, our model estimates a larger north-to-south brightness ratio on the leading side, whereas previous auroral observations show that the trailing hemisphere has more pronounced north-south asymmetry. This result indicates that the "slow-down effect" cannot fully explain the structure of Europa's oxygen aurora and that additional model scenarios are required to understand the auroral north-south asymmetry.