Cassini nightside observations of the oscillatory motion of Saturn's northern auroral oval

Abstract

In recent years we have benefitted greatly from the first in-orbit multi-wavelength images of Saturn's polar atmosphere from the Cassini spacecraft. Specifically, images obtained from the Cassini UltraViolet Imaging Spectrograph (UVIS) provide an excellent view of the planet's auroral emissions, which in turn give an account of the large-scale magnetosphere-ionosphere coupling and dynamics within the system. However, obtaining near-simultaneous views of the auroral regions with in situ measurements of magnetic field and plasma populations at high latitudes is more difficult to routinely achieve. Here we present an unusual case, during Revolution 99 in January 2009, where UVIS observes the entire northern UV auroral oval during a 2 h interval while Cassini traverses the magnetic flux tubes connecting to the auroral regions near 21 LT, sampling the related magnetic field, particle, and radio and plasma wave signatures. The motion of the auroral oval evident from the UVIS images requires a careful interpretation of the associated latitudinally "oscillating" magnetic field and auroral field-aligned current signatures, whereas previous interpretations have assumed a static current system. Concurrent observations of the auroral hiss (typically generated in regions of downward directed field-aligned current) support this revised interpretation of an oscillating current system. The nature of the motion of the auroral oval evident in the UVIS image sequence, and the simultaneous measured motion of the field-aligned currents (and related plasma boundary) in this interval, is shown to be related to the northern hemisphere magnetosphere oscillation phase. This is in agreement with previous observations of the auroral oval oscillatory motion.

Recurrent pulsations in Saturn’s high latitude magnetosphere

“…The observed particles and waves are consistent with similar observations discussed in Mitchell et al (2009a) that have previously been associated with auroral activity on both open and closed field line configurations (as determined by energetic electron anisotropies). The pulsations seen in the in situ data are similar to those discussed in Mitchell et al (2009a), Badman et al (2012), Jasinski et al (2014), and Bunce et al (2014), although in this event we do not have the required data to make a direct inference regarding the field topology.…”

“…Radioti et al (2013) showed two ''rebrightenings'' of the bifurcations of the main emissions, with an interval of 1 h, but no other mention has been made of the repeated brightening at about a 1 h period in the auroral data. There has been no association made between these repeated auroral events and the 1 h pulsations observed regularly in the radio wave and particle data, for example in Mitchell et al (2009a), Jasinski et al (2014), Bunce et al (2014). Badman et al (2012) showed the correspondence of similar 1 h pulsations in the in situ data with the presence of conjugate auroral arcs, but did not have the temporal resolution to observe any intensity variations in the auroral emissions.…”

Field‐aligned currents in Saturn's southern nightside magnetosphere: Subcorotation and planetary period oscillation components

“…Nevertheless, on the basis of our results we would expect the main upward current layer, sheet 2, to be associated with the main UV oval emissions, and that a secondary narrower dimmer emission associated with sheet 4 should generally be present ~2° equatorward of it. Although the structure of nightside emissions has yet to be the subject of large‐scale study at this level of detail, examples of such multiple nightside arcs are known in the literature [ Melin et al ., ; Bunce et al ., ]. With regard to modulation by the PPO oscillation, we note that the total current and the current density in upward current sheet 2 are modulated by a factor of more than ~2 over the PPO cycle, which should lead to strong variations in the emission intensity in proportion to the square of the current density, while the related modulations in sheet 4 are significantly smaller.…”

Field‐aligned currents in Saturn's magnetosphere: Local time dependence of southern summer currents in the dawn sector between midnight and noon

“…These data were discussed both individually and statistically by Talboys et al [, ] and Bunce et al [] and more recently by Hunt et al [, ]. Later in the second high‐latitude interval, extending into 2009, the periapsis distance was again beyond ~10 R s , such that traversals of the auroral field lines were slower and partial [ Bunce et al , ], and thus less suitable for the present study.…”