11:00 AM - 11:15 AM
[PPS01-07] Interpretation of the North-South Asymmetry of the Oxygen Aurora Morphology on Europa Using a Test Particle Simulation
Keywords:Europa, Jupiter, Oxygen aurora emission
Europa has a tenuous atmosphere composed mostly of molecular oxygen. Roth et al. (2016) found that brightness of the oxygen OI] 135.6 nm emission, which is generated due to the electron-impact dissociative excitation of O2, has north-south asymmetric morphology and changes with Europa’s position relative to the equatorial plasma sheet in the Jovian magnetosphere. They suggested that the aurora asymmetry results from inequality of electron flux into Europa’s atmosphere.
The idea of unequal electron flux was originally proposed to explain similar morphology of oxygen aurora found in Io’s atmosphere by Retherford et al. (2003). They explained that when the corotating plasma flow slows down due to the moon-plasma interaction, most electrons in an intersecting flux tube collide with Io. The electrons above the moon precipitate into the northern hemisphere, and those below the moon precipitate into the southern hemisphere. If the flux tube sufficiently slows down and split at the location of the moon into two northern and southern parts with unequal electron contents, the electron flux will be inhomogeneous between the northern and southern hemispheres. This creates asymmetric electron energy flux into the atmosphere when the moon is far from the plasma sheet center. However, the idea of the “slow-down effect” has never been evaluated for the case of Europa quantitatively.
We conduct a test particle simulation to trace electron trajectories near Europa. The electron flux to Europa’s surface and the volume emission rate of the OI] 135.6 nm emission in the atmosphere are calculated during tracing electron motion. We use a tilted dipole model as Jupiter’s primary magnetic field and another small dipole moment located at Europa as the induced field (Liuzzo et al., 2016; Breer et al., 2019). Field perturbation due to the moon-plasma interaction is not considered, but we artificially slow down the entire flux tube into 10% of the corotation velocity to demonstrate how the “slow-down effect” brings out the north-south asymmetry of Europa’s oxygen aurora brightness.
The local electron flux to the surface is estimated on the order of ~ 1010 cm-2 s-1. We found that spatial distribution of the surface electron flux corresponds to the field line angle with respect to the local surface normal. The equatorial region receives smaller electron flux than the polar regions because the magnetic field lines are tangential to the surface normal so that the fast electrons (e.g., the velocity is ~ 6 × 103 km/s at 10 eV) just pass over the surface.
The calculated volume emission rate at 135.6 nm was convolved with the point spread function of the Hubble Space Telescope (HST) and the statistical photon noise on the detector, and then we generated model spectrum images of the OI] 135.6 nm aurora on Europa’s atmosphere. With the “slow-down effect” for deceleration into 10% of the background plasma flow, the systematic changes of the north-south asymmetric aurora in Europa’s atmosphere have been reproduced in our model images. The model images also recreate the observed patchy, concentrated brightening found by several observations (e.g., McGrath et al., 2004; McGrath et al., 2009; Roth et al., 2016) rather than a uniform limb glow expected in a previous modeling (McGrath et al., 2004).
However, the “slow-down effect” needs quite strong deceleration of the entire flux tube at Europa’s location. Huybrighs (2019) reported that the plasma flow slows down into 40% of the corotation, analyzing the Galileo/PLS data at its E12 flyby. 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 explain the north-south asymmetry of Europa’s oxygen aurora and that there needs to be additional investigation with both observational and modeling approach to fully understand the generation of Europa’s asymmetric aurora morphology.
The idea of unequal electron flux was originally proposed to explain similar morphology of oxygen aurora found in Io’s atmosphere by Retherford et al. (2003). They explained that when the corotating plasma flow slows down due to the moon-plasma interaction, most electrons in an intersecting flux tube collide with Io. The electrons above the moon precipitate into the northern hemisphere, and those below the moon precipitate into the southern hemisphere. If the flux tube sufficiently slows down and split at the location of the moon into two northern and southern parts with unequal electron contents, the electron flux will be inhomogeneous between the northern and southern hemispheres. This creates asymmetric electron energy flux into the atmosphere when the moon is far from the plasma sheet center. However, the idea of the “slow-down effect” has never been evaluated for the case of Europa quantitatively.
We conduct a test particle simulation to trace electron trajectories near Europa. The electron flux to Europa’s surface and the volume emission rate of the OI] 135.6 nm emission in the atmosphere are calculated during tracing electron motion. We use a tilted dipole model as Jupiter’s primary magnetic field and another small dipole moment located at Europa as the induced field (Liuzzo et al., 2016; Breer et al., 2019). Field perturbation due to the moon-plasma interaction is not considered, but we artificially slow down the entire flux tube into 10% of the corotation velocity to demonstrate how the “slow-down effect” brings out the north-south asymmetry of Europa’s oxygen aurora brightness.
The local electron flux to the surface is estimated on the order of ~ 1010 cm-2 s-1. We found that spatial distribution of the surface electron flux corresponds to the field line angle with respect to the local surface normal. The equatorial region receives smaller electron flux than the polar regions because the magnetic field lines are tangential to the surface normal so that the fast electrons (e.g., the velocity is ~ 6 × 103 km/s at 10 eV) just pass over the surface.
The calculated volume emission rate at 135.6 nm was convolved with the point spread function of the Hubble Space Telescope (HST) and the statistical photon noise on the detector, and then we generated model spectrum images of the OI] 135.6 nm aurora on Europa’s atmosphere. With the “slow-down effect” for deceleration into 10% of the background plasma flow, the systematic changes of the north-south asymmetric aurora in Europa’s atmosphere have been reproduced in our model images. The model images also recreate the observed patchy, concentrated brightening found by several observations (e.g., McGrath et al., 2004; McGrath et al., 2009; Roth et al., 2016) rather than a uniform limb glow expected in a previous modeling (McGrath et al., 2004).
However, the “slow-down effect” needs quite strong deceleration of the entire flux tube at Europa’s location. Huybrighs (2019) reported that the plasma flow slows down into 40% of the corotation, analyzing the Galileo/PLS data at its E12 flyby. 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 explain the north-south asymmetry of Europa’s oxygen aurora and that there needs to be additional investigation with both observational and modeling approach to fully understand the generation of Europa’s asymmetric aurora morphology.