Jupiter's polar ionospheric flows: High resolution mapping of spectral intensity and line-of-sight velocity of H-3(+) ions

We present a detailed study of the H3þ auroral emission at Jupiter, which uses data taken on 31 December 2012 with the long-slit echelle spectrometer Cryogenic Infrared Echelle Spectrograph (European Southern Observatory’s Very Large Telescope). The entire northern auroral region was observed using significantly more slit positions than previous studies, providing a highly detailed view of ionospheric flows,which were mapped onto polar projections. Previous observations of ionospheric flows in Jupiter’s northern auroral ionosphere, using the long-slit echelle spectrometer CSHELL (NASA Infrared Telescope Facility) to measure the Doppler-shifted H3þν2Q(1,0 ) line at 3.953μm, showed a strongly subrotating region that was nearly stationary in the inertial magnetic frame of reference, suggesting an interaction with the solar wind. In this work, we observe this stationary region coincident with a polar region with very weak infrared emission,typically described as the dark region in UV observations. Although our observations cannot determine the exact mechanisms of this coupling, the coincidence between solar wind controlled ionospheric flows and a region with very low auroral brightness may provide new insights into the nature of the solar wind coupling.We also detected a superrotating ionospheric flow measured both at and equatorward of the narrow bright portion of the main auroral emission. The origin of this flow remains uncertain. Additionally, we detect a strong velocity shear poleward of the peak in brightness of the main auroral emission. This is in agreement with past models which predict that conductivity, as well as velocity shear, plays an important role in generating the main auroral emission

Complex structure within Saturn’s infrared aurora

The majority of planetary aurorae are produced by electrical currents flowing between the ionosphere and the magnetosphere which accelerate energetic charged particles that hit the upper atmosphere. At Saturn, these processes collisionally excite hydrogen, causing ultraviolet emission, and ionize the hydrogen, leading to H3+ infrared emission. Although the morphology of these aurorae is affected by changes in the solar wind, the source of the currents which produce them is a matter of debate. Recent models predict only weak emission away from the main auroral oval. Here we report images that show emission both poleward and equatorward of the main oval (separated by a region of low emission). The extensive polar emission is highly variable with time, and disappears when the main oval has a spiral morphology; this suggests that although the polar emission may be associated with minor increases in the dynamic pressure from the solar wind, it is not directly linked to strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn's magnetosphere. The equatorward arc of emission exists only on the nightside of the planet, and arises from internal magnetospheric processes that are currently unknown

Saturn's auroral/polar H3+infrared emission: the effect of solar wind compression

Previous investigations into Saturn's aurora have shown that they are strongly controlled by the solar wind. Here, for the first time, we use a combination of ground-based infrared observations of the aurora and in situ measurements of the magnetosphere in order to better understand this association. We show that large-scale variability in both the intensity and ion flow velocities can be directly correlated with the solar wind dynamic pressure, with, in particular, the arrival of solar wind compressions. Large compressions in the solar wind trigger the same morphological changes in the auroral structure as have previously been seen in UV images, and these are accompanied by the loss of the open field line corotation region seen in the velocity measurements. This region has been explained as an “old core” of magnetic field lines open to the solar wind, protected from reconnection due to the twisting in the magnetotail, and therefore requires that this region be removed only by major compressions in the solar wind; thus, our observations here generally agree with this model. In addition, we have observed a >8 h delay between the arrival of a major compression and the resulting effect upon the aurora, suggesting that reconnection must either occur well into the tail or that there are other processes in the chain of events that lead to the major dawn brightening seen in both these observations and previously studied UV images

The auroral footprint of Enceladus on Saturn

Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus(1-9) could lead to electrodynamic coupling between Enceladus and Saturn like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io-Jupiter coupling, for example, create an auroral footprint in Jupiter's ionosphere(10,11). Auroral ultraviolet emission associated with Enceladus-Saturn coupling is anticipated to be just a few tenths of a kilorayleigh (ref. 12), about an order of magnitude dimmer than Io's footprint and below the observable threshold, consistent with its non-detection(13). Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon's footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters-and as such is probably indicative of variable plume activity