sketch of the magnetosphere of Saturn, from Bagenal (1992).
Saturn's magnetosphere is large and fast (10-hr) rotating like that of Jupiter. It also has internal plasma and neutral sources in the icy satellites and the rings, both with ephemeral atmospheres, and also Titan, which has a dense atmosphere. However, unlike Io, Titan orbits in the outskirts of the system, sometimes inside of it and sometimes "outside" of it and directly exposed to the solar wind. The Kronian and Jovian magnetospheres therefore have dominant or at least substantial internal mass and rotational energy sources against an external solar wind control, in contrast to the case of the Earth. In the 1980-1981 Voyager encounters, Saturn's magnetic field was found to be surprisingly aligned with the spin axis, and presents a simpler system than Jupiter's because the field non-dipolar terms are much smaller. Saturn's aurora was also positively identified in UV emission and seemed to emanate from high latitude ovals thought to map to the magnetopause, although, as for Jupiter, the models and mapping are uncertain. Saturn' system may thus be controlled mainly by the solar wind interaction, rather than by internal forces. However, Voyager also revealed kilometric radio emissions (SKR) consisting of sychrotron radiation by precipitating auroral electrons. The SKR and some ring spokes showed some enhancements at a given magnetic longitude, thus indicating that longitudinal (and thus corotational) effects are at play as well. HST imaging will be key for untangling the behavior of Saturn's aurora and magnetosphere.
The figure on the left was the first ever obtained of Saturn in the far-ultraviolet (~1200-2100 Ang) in 1994 with the WFPC2 camera on HST. They were published by John Trauger et al. (1998). Saturn's disk and rings are seen in reflected sunlight (longwards of ~1600 Ang). The auroral emissions are easy to distinguish in the polar regions because those regions are dark in the UV due to UV-absorbing polar hydrocarbon hazes formed in turn by the auroral processes. Both emissions in H2 and H Lyman alpha are detected in these images. A strong auroral event was taking place in Saturn's morning aurora, as shown in the right figure.
The figure on the right, from an HST press release also by Trauger et al., was obtained with the STIS instrument on HST. The STIS far-UV imaging mode has a about 10 times better sensitivity than a typical WFPC2 exposure, and about 4 times the spatial resolution (eg, the Cassini division of the rings is now clearly visible). Details of the aurora can now be better studied. The STIS far-UV MAMA detector covers the 1200-1800 Angstroms, so there is also less reflected sunlight as compared to the WFPC2 images. By 1997 the northern hemisphere of Saturn was barely observable. Brighter morning emission is seen on the outh east limb, while a more diffuse emission is seen in the afternoon. The auroral double structure seen on the south east limb is an artifact since the image is a summation of two STIS exposures. The double structure in turn shows that the aurora has temporal and/or longitudinal variations. The latter explanation implies that Saturn's magnetic field is not at dipolar as previously believed. The emissions also show a not too smooth oval (towards the central portion of the Earth-facing side of the auroral oval).
The figure on the left is a composite of WFPC2 sub-images of Saturn's north aurora reported by Trauger et al. (1998) . They show temporal variations in Saturn's northern aurora. The top 4 panels show data obtained within about 5 hours on 9 October 1994 during the very first imaging of Saturn in the far-UV (of which the full first exposure is depicted on the left figure). Saturn was undergoing an auroral event, where bright emission remained fixed at 8 AM local time throughout the ~5 hours observing period. The auroral storm was dimming with time. Two observations taken on two separate dates in 1995 revealed very little auroral emission.
Saturn's aurora was known to be bursty from both Voyager and contemporaneous IUE observations, and we think that Voyager may have observed on such morning auroral event. Such events seem to be typical of fast-rotating magnetospheres. (On Jupiter the events are fixed at dawn rather than at 8 AM in magnetic local time.) More information on the temporal behavior of Saturn's aurora can be obtained from analysis of the newer STIS data, because these data was obtained in the STIS time-tag mode. Coordinated observations should be made in the years to come with the Cassini mission in-situ solar wind and magnetospheric measurements.
Because the rotational period of Saturn is highly uncertain, we do not know if the bursts of UV aurora enhancements are constrained in magnetic longitude. With HST imaging observations, made over extended periods and combined with Cassini in-situ data, we may be able to resolve this issue. The STIS images can also be used to resolve corotationa