The aurorae of each planet are produced as a direct interaction between the upper atmosphere and magnetosphere of that planet. Energetic particles from the magnetosphere are driven into the top of the atmosphere, depositing energy there, and ultimately resulting in an electromagnetic emission. As a result, aurorae are related to conditions within the planetary magnetospheres so an understanding of the auroral emission provides a view of both the magnetospheric structure and how that magnetosphere is coupled with the underlying ionosphere. In the past, Saturn’s magnetosphere, and thus its aurorae, have been seen as something of a hybrid between the solar-wind-driven interaction at Earth and the rotationally dominated system at Jupiter. However, observations across a wide wavelength range by both the Cassini spacecraft and supporting Earth-based telescopes have revealed Saturn’s aurorae to be highly complex. We now recognize that Saturn’s aurorae are driven by the dynamic magnetic field interactions between the atmosphere, the solar wind and plasma trapped within the magnetosphere, all strongly affected by the rapid rotation of the planet.

In this chapter, we highlight the broad variety of auroral features observed at Saturn, and discuss how these are generated by energetic particles moving within current systems that link to solar wind interactions (Section 7.2), interactions with plasma generated within the magnetosphere (Section 7.3) and with current systems that vary periodically, including those linked to weather systems within Saturn’s upper atmosphere (Section 7.4). Finally, we conclude with a discussion of the major questions that remain about Saturn’s aurorae, and summarize the upcoming observations that will help us answer them. We begin with a discussion of how the auroral emission is generated and the characteristics of aurorae observed at Saturn. In particular, we highlight the most recent auroral research, following on from the overview of Saturn’s auroral processes presented in past reviews of the subject (for example, Kurth et al. 2009).

The magnetometer measurements taken by Cassini have confirmed the unusual character of Saturn’s internal magnetic field known from previous flybys and have revealed additional properties that suggest a rather unique dynamo in this planet. Within measurement uncertainty, the internal magnetic field is completely symmetric with respect to Saturn’s spin axis. The upper limit on the tilt of the magnetic dipole could be reduced from 1 to 0.06 degree. Moreover, only axisymmetric quadrupole and octupole moments are needed to fit the data. The lack of non-axisymmetric field components prevents a reliable determination of the bulk rotation rate of Saturn’s deep interior. Using data from Cassini’s closest approach to Saturn during orbit insertion, the magnetic moments of degrees four and five have been determined. The spatial power spectrum shows a zig-zag pattern with high power in odd spherical harmonic degrees and low power in even degrees. Compared to a simple dipole field, this corresponds to a concentration of magnetic flux towards the rotation poles. The flux concentration becomes progressively more pronounced when the field is continued into the interior. Comparison of the Cassini field model with that based on the Pioneer 11 and Voyager 1 and 2 measurements taken roughly 30 years earlier suggests that the secular variation of Saturn’s field is at least one order of magnitude slower than that of the Earth. A viable explanation for most of the unusual field properties is that a stably stratified and electrically conducting layer, formed by a partial demixing of helium from metallic hydrogen, exists on top of a “standard” dynamo in Saturn’s deep interior. This dynamo, driven by thermal and compositional convection, generates a magnetic field that is moderately asymmetric and time dependent. Rapid time variations and non-axisymmetric field components are filtered out in the stable layer by a skin effect. This model also implies that the top of the active dynamo may be located rather deep in Saturn’s interior and the geometric drop-off of the dipole strength with the radius cubed could explain the unexpectedly low field strength at Saturn’s surface. The stable layer model does not provide an explanation for the magnetic flux concentration towards the poles. Strong differential rotation in the dynamo region can have this effect, but a physical mechanism for such a flow state remains to be explored. From magnetic measurements to be taken during the very close approaches in the Grand Finale of the Cassini mission, we can expect to characterize Saturn’s magnetic field up to at least spherical harmonic degree nine and possibly to detect weak non-axisymmetric field components, which would enable an accurate determination of Saturn’s rotation period.

In December 2010, a major storm erupted in Saturn’s northern hemisphere near 37° planetographic latitude. This rather surprising event, occurring at an unexpected latitude and time, is the sixth “Great White Spot” (GWS) storm observed over the last century and a half. Such GWS events are extraordinary, planetary-scale atmospheric phenomena that dramatically change the typically bland appearance of the planet. Occurring while the Cassini mission was on orbit at Saturn, the Great Storm of 2010–2011 was well suited for intense scrutiny by the suite of sophisticated instruments onboard the Cassini spacecraft as well by modern instrumentation on ground-based telescopes and onboard the Hubble Space Telescope. This GWS erupted on 5 December close to the peak of a westward jet and generated a major dynamical disturbance that affected the whole latitude band from 25° to 48°N. At the upper cloud level, following the rapid growth of the bright outbreak spot, a blunt aerodynamic-shaped head formed due to interaction of the spot with the westward zonal jet, with the winds reaching velocities of 160 m s−1 along the periphery of the arc. Eastward of the head, the disturbance progressed in the following months forming a turbulent wake or tail with growing vortices, one of them a major enduring anticyclone (called AV) with a size of ~11,000 km. Lightning events were prominent and detected as outbursts and flashes at the head and along the disturbance at both optical and radio wavelengths. The activity of the head ceased after about seven months when AV reached it, leaving the cloud structure and ambient winds perturbed. The tops of the optically dense clouds of the head reached the 300-mbar altitude level (~50 km below tropopause), where a mixture of ices was detected, including (1) a component of water ice lofted over 200 km altitude from its 10-bar condensation level, (2) ammonia ice as the predominant component and (3) a component that might be ammonium hydrogen sulfide ice. The energetics of the frequency and power of lightning, as well as the estimated power generated by the latent heat released in the water-based convection to create the observed dynamical three-dimensional flows, both indicate that the power released for much of the 7-month lifetime of the storm (~1017 Watts) was a significant fraction of Saturn’s total radiated power (~2.2 1017 W). A post-storm depletion of ammonia vapour was also measured in the upper troposphere. The effects of the storm propagated into the stratosphere, forming two warm air masses at the ~0.5- to 5-mbar pressure level altitude that later merged into a so-called “beacon” because of its 80 K temperature excess relative to its surroundings. Related to the stratospheric disturbance, hydrocarbon composition excesses were found, in particular for ethylene (C2H4), in the high stratosphere at the ~0.1- to 0.5-mbar altitude level. Numerical models of the storm dynamics explain the major observed features that essentially result from two processes: (1) a huge and sustained, moist, convective storm at the water clouds (altitude level 10–12 bar, or ~250–275 km below the tropopause) and (2) the interaction of the updraft columns with the ambient winds that generates the turbulent wake consisting of vortices and waves. Model simulations of the GWS require a low vertical shear of the zonal winds and low static stability across the weather layer where the disturbance develops. Its upward propagation into the stratosphere involves Rossby waves and their breaking and energy deposition to form the beacon and induce chemical changes.

The decades-long interval between storms is probably related to the insolation cycle and the long radiative time constant of Saturn’s atmosphere, and several theories for temporarily storing energy have been proposed.

This chapter reviews the state of our knowledge about Saturn’s polar atmosphere that has been revealed through Earth- and space-based observation as well as theoretical and numerical modeling. In particular, the Cassini mission to Saturn, which has been in orbit around the ringed planet since 2004, has revolutionized our understanding of the planet. The current review updates a previous review by Del Genio et al. (2009), written after Cassini’s primary mission phase that ended in 2008, by focusing on the north polar region of Saturn and comparing it to the southern high latitudes. Two prominent features in the northern high latitudes are the northern hexagon and the north polar vortex; we extensively review observational and theoretical investigations to date of both features. We also review the seasonal evolution of the polar regions using the observational data accumulated during the Cassini mission since 2004 (shortly after the northern winter solstice in 2002), through the equinox in 2009, and approaching the next solstice in 2017. We conclude the current review by listing unanswered questions and describing the observations of the polar regions planned for the Grand Finale phase of the Cassini mission between 2016 and 2017.