The far ultraviolet aurora of Ganymede

Abstract

The far ultraviolet (FUV) aurora on Jupiter’s largest moon, Ganymede, is characterized by two distinct ovals in the northern and southern hemisphere, which have been investigated by several campaigns of the Hubble Space Telescope (HST) in the past two decades (e.g., Hall et al. (1998), Feldman et al. (2000) and McGrath et al. (2013)). The aurora is generated by electron-impact dissociative excitation of atomic and molecular oxygen in Ganymede’s tenuous atmosphere. The most likely acceleration mechanism for the high energetic electrons triggering the auroral emission are field aligned electric currents (FAC) accelerating electrons along the open-closed magnetic field lines boundary (OCFB) of Jupiter’s and Ganymede’s magnetic field towards the moon’s atmosphere (Eviatar et al. 2001a). This acceleration mechanism is consistent with the locations of the observed ovals being close to the intersection lines of the OCFB, predicted by numerical modeling, with Ganymede’s atmosphere (Feldman et al. 2000; Eviatar et al. 2001a; McGrath et al. 2013). The compression of Ganymede’s mini-magnetosphere due to the impinging Jovian magnetospheric plasma flow on the upstream side shifts the OCFB and accordingly the auroral ovals to elevated planetographic latitudes (between 40 and 55 ) on the trailing side (Neubauer 1998; Feldman et al. 2000; McGrath et al. 2013). On the downstream side, the mini-magnetosphere is stretched which shifts the OCFB to lower latitudes (between 10 and 30 ) on the leading side. Furthermore, the aurora on Ganymede is expected to be time-variable since the moon is exposed to the time-periodic plasma and magnetic field of Jupiter’s magnetosphere. The influence of periodically changing local plasma conditions on the morphology and brightness of Ganymede’s aurora has not been analyzed yet. In this thesis we systematically analyze the spatial structure and the temporal variability of Ganymede’s FUV auroral ovals as a function of its time-variable magnetospheric environment. We analyze spectral images obtained between 1998 and 2011 by the Space Telescope Imaging Spectrograph (STIS) on-board of HST. The observations cover the satellite at eastern and western elongation, observing Ganymede’s leading and trailing side. The observations also cover various magnetic latitudes of Ganymede within the Jovian plasma sheet. As a result of our study, we find both, asymmetries in the spatial distribution of auroral brightness on the observed moon disk and temporal variations correlated to Ganymede’s changing position relative to the Jovian current sheet. We find a hemispheric dichotomy of the total disk averaged brightness between the leading side (95.4 ± 2.1 R) and the trailing side (67.2 ± 2.9 R), i.e., the plasma downstream side is significantly brighter than the plasma upstream side. Furthermore, the Jupiter-facing side of the moon disk is brighter than the anti-Jovian side by a factor of 1.81 ± 0.06 on the leading side and by a factor of 1.41 ± 0.14 on the trailing side, indicating local inhomogeneities in the current systems associated with the generation of the aurora. We demonstrate, that the auroral brightness is clearly correlated to Ganymede’s position relative the to Jovian current sheet, as we see an increased brightness on the leading side and a decrease of brightness on the trailing side, when Ganymede is inside the current sheet compared to elevated magnetic latitudes. At the same time, the auroral ovals shift on the leading side towards Ganymede’s planetographic equator by an average of 4.1° ± 0.7° latitude, and on the trailing side towards the poles by an average of 2.9° ± 1.5° latitude when Ganymede is at the center of the current sheet. The brightness variations and the ovals’ movements are a response to the changing local plasma conditions inside the current sheet as Ganymede’s mini-magnetosphere is exposed to a stronger interaction with the Jovian magnetospheric plasma. By calculating the center between the northern and southern oval we are able to derive further constraints on the orientation of Ganymede’s magnetic equator. We find that Ganymede’s dipole magnetic moment is oriented further westward at approximately 47° (+58°/-43°) planetographic west-longitude compared to previous estimates. Finally, by analyzing the amount, the size and structure, and the longitudinal distribution of bright auroral spots along the ovals, we find that the occurrence of the spots is rather randomly than systematically ordered, which might be due to the intermittent magnetic reconnection at Ganymede’s upstream side (Eviatar et al. 2001a).