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The electromagnetic interaction between Europa and the plasma sheet in the Jovian magnetosphere generates Alfvén waves, ultimately generating auroral footprints in Jupiter's atmosphere. The position of Europa's auroral footprint is a proxy for travel time of the Alfvén waves. We measured Europa's footprint position using the far‐ultraviolet images of Jupiter obtained by the Hubble Space Telescope in two observing campaigns in 2014 and 2022. The measured footprint position indicates a longer Alfvén travel time in the 2022 campaign. We retrieved the plasma sheet parameters at Europa's orbit from the footprint position by tracing the Alfvén waves launched at Europa and found an increase of both mass density and temperature in the plasma sheet in 2022. The Poynting flux generated at Europa is calculated with the retrieved plasma sheet parameters, which suggests the total energy transfer from Europa to its auroral footprint is similar to the case of Io. Plain Language Summary: Europa is an obstacle to the plasma corotating with Jupiter's magnetosphere. Through the interaction between Europa and the magnetospheric plasma flow, Alfvén waves are launched at Europa. The Alfvén waves propagate along the field line and ultimately generate auroral emissions at locations distant from the instantaneous magnetic footprint of Europa. The position of Europa's auroral footprint is a proxy for the travel time of the Alfvén waves. We measured the position of Europa's auroral footprint using the far‐ultraviolet images of Jupiter obtained by the Hubble Space Telescope in two observing campaigns in 2014 and 2022. We found large deviations of the footprint position between the two observing campaigns. By tracing the Alfvén waves launched at Europa, we retrieved plasma mass density and temperature at Europa's orbit from the measured footprint position. It is revealed that time variation in the plasma mass density and temperature caused the deviations in the footprint position. We also calculated the Poynting flux generated at Europa using the retrieved plasma parameters and found that the total energy transfer from Europa to its auroral footprint is similar to the case of Io. Key Points: We measured the equatorial lead angle of Europa's auroral footprint in Jupiter's atmosphere with the HST data taken in 2014 and 2022Plasma conditions at Europa's orbit are retrieved from the measured lead angle by tracing the Europa‐originated Alfvén wavesChanges in the plasma conditions at Europa's orbit can account for the variation of the footprint lead angle [ABSTRACT FROM AUTHOR]

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. 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. Key Points: Morphology of Europa's oxygen 135.6 nm aurora was reproduced by test particle method to investigate the north‐south aurora asymmetryWe confirmed that a flux tube decelerated to 10% of the background plasma flow creates the periodic changes of the north‐south asymmetryBut the slow‐down scenario does not explain the larger north‐south asymmetry observed on the trailing hemisphere [ABSTRACT FROM AUTHOR]