Europa's far ultraviolet oxygen aurora from a comprehensive set of HST observations

Abstract

We analyze a large set of far ultraviolet oxygen aurora images of Europa's atmosphere taken by Hubble's Space Telescope Imaging Spectrograph (HST/STIS) in 1999 and on 19 occasions between 2012 and 2015. We find that both brightness and aurora morphology undergo systematic variations correlated to the periodically changing plasma environment. The time variable morphology seems to be strongly affected by Europa's interaction with the magnetospheric plasma. The brightest emissions are often found in the polar region where the ambient Jovian magnetic field line is normal to Europa's disk. Near the equator, where bright spots are found at Io, Europa's aurora is faint suggesting a general difference in how the plasma interaction shapes the aurora at Io and Europa. The dusk side is consistently brighter than the dawnside with only few exceptions, which cannot be readily explained by obvious plasma physical or known atmospheric effects. Brightness ratios of the near‐surface OI] 1356 Å to OI 1304 Å emissions between 1.5 and 2.8 with a mean ratio of 2.0 are measured, confirming that Europa's bound atmosphere is dominated by O2. The 1356/1304 ratio decreases with increasing altitude in agreement with a more extended atomic O corona, but O2 prevails at least up to altitudes of ∼900 km. Differing 1356/1304 line ratios on the plasma upstream and downstream hemispheres are explained by a differing O mixing ratio in the near‐surface O2 atmosphere of ∼5% (upstream) and ≲1% (downstream), respectively. During several eclipse observations, the aurora does not reveal any signs of systematic changes compared to the sunlit images suggesting no or only weak influence of sunlight on the aurora and an optically thin atmosphere.

DETECTION OF A HYDROGEN CORONA IN HST Lyα IMAGES OF EUROPA IN TRANSIT OF JUPITER

“…At higher altitudes, the relative abundance of the lighter species (O, H 2 O, OH, H 2 , H) increases compared to O 2 (Shematovich et al 2005;Smyth & Marconi 2006), which was observationally confirmed for atomic oxygen O (Hansen et al 2006;Roth et al 2016). The Monte Carlo simulations by Shematovich et al (2005) and Smyth & Marconi (2006) further suggest that the escape of hydrogen, in particular H 2 , is higher than the oxygen escape and that hydrogen is, therefore, the main species forming the neutral torus that was detected near Europa (Lagg et al 2003;Mauk et al 2003).…”

“…The cross-section of the primary atmospheric constituent, O 2 , reaches the highest values at the observed FUV wavelengths near 1244.4 and 1205.4 Å, and at the SchumannRunge continuum above ∼1300 Å (Ogawa & Ogawa 1975;Lu et al 2010). At 1216 Å (Lyα) the cross-section assumes a lower value of ∼1×10 −20 cm 2 (Ogawa 1968 (Hall et al 1998;Roth et al 2016), which corresponds to a maximum line-of-sight column density of , or a transmittance of >99.9%. Hence, although O 2 is the dominant species, there is no absorption by it at the Lyα.…”

Detection of a hydrogen corona at Callisto

“…The higher sensitivity of the HST Cosmic Origins Spectrograph (COS) later allowed the detection of faint oxygen emission of a few rayleighs [ Cunningham et al , ], suggesting an O 2 column density of ∼4 × 10 15 cm −2 on the leading/Jupiter‐facing hemisphere. This column density is 5 times larger than the CO 2 column density and approximately 1 order of magnitude larger than the O 2 column densities at Ganymede and Europa [ Hall et al , ; Roth et al , ].…”

Probing for Evidence of Plumes on Europa With HST/Stis

“…Though the exosphere of Europa arises from material escaping from the surface, its basic properties such as areal coverage, density, thermal structure, and spatial and temporal variability are poorly understood (McGrath et al, 2009;Roth et al, 2015).…”