Saturn's aurorae

The aurora at Saturn represents a direct manifestation of the interaction between the planet’s surrounding space environment and its upper atmosphere. Our understanding of this interaction has greatly improved over the past decade, as a result of both in-situ and remote sensing of the aurora by Cassini, as well as through Earth-based observations.

On Earth, the interaction is dominated by the connection between the magnetosphere and the Solar Wind, with opening and closing of magnetic field lines leading to sporadic aurora that are strongly controlled by changes in the solar wind. On Jupiter, internal plasma sources combine with a rotationally-dominated magnetosphere to produce intense currents associated with the breakdown in co-rotation in the magnetosphere, producing powerful and continuous aurora.

Investigations into Saturn’s aurora have shown that the overall morphology changes dramatically with the arrival of compressions in the solar wind, suggesting a strong interaction with the solar wind at Earth. However, the varying rotation rate of Saturn’s magnetosphere, first identified by measurements of Saturn’s radio emission, can also be measured in many aspects of the auroral emission. This in turn suggests a degree of rotational control within the aurora. As such, the better we understanding the relative strength of these influences on the aurora of Saturn, the more we can understand how the magnetosphere interacts the planet and how, in turn, the planet drives changes in the magnetosphere.

Here, we will present observations of the auroral emission directly produced by particles precipitating into Saturn’s atmosphere (radio emission), the resultant atmospheric auroral excitation this produces within of atomic hydrogen (UV and visible emission) and molecular hydrogen (UV emission), as well as thermal emission from both ions produced through auroral ionization and neutral species heated within the auroral region (IR emission). These observations show a wide variety of different auroral features ranging from the rotational pole, though the main auroral emission and down to latitudes where Saturn’s atmosphere interacts with Enceladus.

Observations at these different wavelength, when compared and contrasted, reveal details about the particle precipitation process that drive them, as well as the affect these currents have on the surrounding neutral atmosphere. In-situ measurements by Cassini of the particles and magnetic field above the polar region allow us to measure and understand the field-aligned currents that produce the aurora. In comparing these currents with the auroral emission at the foot of these field lines, it is possible to understand the magnetospheric origin for Saturn’s auroral emission, as well as understanding the two-way interaction between the atmosphere and magnetosphere that is driven though the currents that produce this aurora.