title: Jovimagnetic secular variation
Creators: name: Ridley, Victoria Anne
Publisher: name: University of Liverpool Repository
Year: 2013

description: Planetary dynamos, resulting from fluid flow in electrically conductive parts of their interior, are thought to be highly time dependent. Currently, our understanding of temporal variation of these fields is limited because we only have observations for one example, the Earth. To overcome this, data acquired by 6 NASA space missions between 1973-2003 are used to investigate possible time variation (secular variation) of Jupiter's magnetic field. Previous attempts to constrain jovimagnetic secular variation have been inconclusive or ineffective for various reasons. We attempt to resolve these issues in a number of ways. All data available within 12 Jovian radii are considered and modelling of the external field takes place for each individual orbit. Whilst we find that non-uniqueness limits resolution of Jupiter's magnetodisk configuration, it does not prevent the resulting field from being constrained: we find this field does not vary greatly with time and conclude that solar activity is not a strong control on its generation. Of particular significance is our regularised minimum norm approach to modelling the planetary field. This approach allows construction of numerically stable models with small-scale (high spherical harmonic degree) structure that directly fit the observations. Two models of Jupiter's magnetic field are presented: the first time-averaged over the whole dataset, whilst the second allows for linear time variation of the field. With the inclusion of secular variation, we find an improved fit with fewer additional parameters, suggesting that changes to the field can be resolved. Our favoured solution indicates a 0.012%/year increase in Jupiter's dipole magnetic moment over the investigated time period; this value is roughly a factor of four less then that currently observed at Earth. Relating field changes to internal dynamics, we use our models of secular variation to infer the motion of material at the top of the dynamo source. Velocities on the order of 200km/year are found, approximately an order of magnitude faster then at Earth, but with similarities in flow configuration. Further analysis shows that our optimal solution may be too conservative and that we are able to approximate a polar configuration previously only attained by models employing the additional constraint of auroral footprint position. In doing so, the models favour reversed polar flux, which has important dynamical implications for the interior. The longitude of data used in these models are defined relative to the System III 1965.0 rotation period, itself defined by the magnetic field. Thus, some of the secular variation could result from inaccuracies in the determination of this reference frame. We find that such an affect cannot explain all the observed secular variation. The constraint of our magnetic models on changes to planetary rotation rate, allows a bound to be placed on angular momentum transfer between the atmosphere and deep interior, analogous with variations in Earth's observed length-of-day. This provides strong observational evidence against models directly linking surface winds to deep Jovian convection.