Secular Variation on Jupiter and Stochastic Modelling of its Magnetosphere

From the earliest interplanetary missions investigating magnetism, comparisons with features of Earth’s magnetic field have been of the utmost interest. In particular, the ability of a planetary field to change with time has only ever been conclusively observed on Earth and its detection on other bodies would be vital for the understanding of planetary dynamos. With the Juno spacecraft in orbit about Jupiter since 2016, a continuous time series of magnetic measurements now exists over an extended period. This provides a suitable foundation by which to study any potential secular variation and determine its validity.

Some of the first indications of secular variation have been attributed to the advection of Jupiter’s magnetic field by flow in the form of zonal winds. This was first postulated with respect to variations between the measurements of Juno and earlier satellites such as Pioneers 10 and 11 and Voyager 2. Subsequent work has been carried out using select data from the Juno spacecraft, that has proposed these observations can account for differential rotation in the planet’s deep interior and a time varying zonal flow (the latter suggesting the detection of Alfvén waves as a means of describing the behaviour).

Although such deductions lend credence to the detection of secular variation, they must be subject to continued analysis. This is tackled through the investigation of the null hypothesis, in which Jupiter’s field is taken to be static and subsequently analysed to see if its behaviour can be adequately described. Such models are constructed through regularised inversion and then subject to residual analysis. Through these means, we find conclusions relating Juno data to earlier spacecraft, difficult to unambiguously determine.

In addition to this, the effect of the magnetosphere of Jupiter on satellite measurements is not concretely understood. A problem with this lies in the potential for unmodelled phenomena to influence subsequent internal field models. The majority of models take the external field to be uniform with its source in the magnetodisc – this describes a ring current, tilted with respect to Jupiter’s equatorial plane. The effect of this is the production of a uniform field adequately described by an order l = 1 spherical harmonic expansion. To supplement this uniform field, a stochastic treatment is considered in which averaging over randomly oriented currents contributes to large scale correlations. The resulting model describes a uniform magnetodisc as well as the remaining non-uniform currents distributed about Jupiter. Such a treatment is yet to be applied to the case of a planetary magnetosphere but has shown its power in the treatment of seamounts and crustal magnetisation on Earth.