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

Presentation #501.05 in the session Icy Galilean Satellites: Magnetosphere and Exosphere (Oral Presentation)

Published onOct 23, 2023
Interpretation of the North-South Asymmetric Oxygen Aurora Morphology on Europa Using Test Particle Simulation

Europa has a tenuous atmosphere composed mostly of molecular oxygen. Observations using the Hubble Space Telescope (HST) reported that the oxygen OI] 135.6 nm auroral emissions in Europa’s atmosphere has a north-south asymmetric brightness morphology which changes with the position of Europa with respect to the Jovian magnetospheric plasma sheet (Roth et al., 2016). Similar north-south asymmetry of Io’s auroral limb glow has been observed and explained by higher electron flux into the atmosphere on the hemisphere that faces the plasma sheet center (Retherford et al., 2003). This explanation, however, has not yet been evaluated for the case of Europa quantitatively.

In this study, we conducted a test particle simulation for the Jovian magnetospheric electrons to estimate the brightness of the 135.6 nm oxygen aurora in Europa’s atmosphere and to investigate the cause of the north-south asymmetry using the previously suggested idea, in which the strong deceleration of the magnetospheric plasma flux tube results in the inhomogeneous electron precipitation into the atmosphere (the “slow-down effect”). We successfully reproduced the systematic change of the north-south asymmetry of Europa’s oxygen aurora at 135.6 nm with the “slow-down effect” scenario. The maximum north-to-south brightness ratio is estimated at 2.56 on the leading (plasma-downstream) side and at 2.17 on the trailing (upstream) side with the “slow-down effect” of deceleration to 10% of the background plasma bulk flow. However, the observed brightness ratio is actually larger on the trailing side (Roth et al., 2016). Besides, the “slow-down effect” model requires a strong deceleration of the entire flux tube at Europa’s location to recreate the observed north-to-south brightness ratio, while our calculation suggests that the 40% slowdown, derived from the Galileo/PLS data at its E12 flyby in Huybrighs (2019), is not sufficient. Harris et al. (2021) conducted the MHD simulations to show that the spatial scale of the flux tube deceleration is limited near Europa. Our conclusion therefore is that the “slow-down effect” does not fully explain the north-south asymmetry of Europa’s oxygen aurora and that additional investigations with both observational and modeling approaches are required to understand the generation of Europa’s asymmetric aurora morphology.

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