Person:

Bloxham, Jeremy

We investigate the determination of the rotation period of Saturn's interior using magnetic field measurements from Cassini. First, we vary the rotation period and search for the period that yields the smallest rms misfit of a magnetic field model to the data. Second, we search for the period that yields the most power in the non-axisymmetric components of a magnetic field model. Neither method enables us to determine the rotation period. However, we are able to place a bound on the magnitude of the non-axisymmetric component of Saturn's magnetic field finding it to be no greater than 4–5% of the axisymmetric component.

Determination of the time dependency (secular variation) of a planet’s magnetic field provides a window into understanding the dynamo responsible for generating its field. However, of the six Solar System planets with active dynamos, secular variation has been firmly established only for Earth. Here, we compare magnetic field observations of Jupiter from the Pioneer 10 and 11, Voyager 1 and Ulysses spacecraft (acquired 1973–1992) with a new Juno reference model (JRM09)1. We find a consistent, systematic change in Jupiter’s field over this 45-year time span, which cannot be explained by changes in the magnetospheric field or by changing the assumed rotation rate of Jupiter. Through a simplified forward model, we find that the inferred change in the field is consistent with advection of the field by Jupiter’s zonal winds, projected down to 93–95% of Jupiter’s radius (where the electrical conductivity of the hydrogen envelope becomes sufficient to advect the field). This result demonstrates that zonal wind interactions with Jupiter’s magnetic field are important and lends independent support to atmospheric and gravitational-field determinations of the profile of Jupiter’s deep winds.