The Effects of a Time Varying Magnetosphere on Internal Jovian Field Reconstruction

Owing to the strength of its internal, dynamo-driven magnetic field, Jupiter supports the largest planetary magnetosphere in the solar system. This structure houses vast current systems that are known to significantly affect measurements made by visiting spacecraft. These pervasive currents are supplied primarily by the ionisation of neutral gases lost at Jupiter’s moon Io, as well as originating from external sources such as the solar wind. Although the supply and transport of such magnetospheric currents is not entirely understood, it is known that such processes can lead to changes over wide ranges of timescales from hours to months [1].

The Juno spacecraft has collected data about Jupiter over nine years (2016-2025), providing a valuable opportunity to explore how these current systems vary over time. By first testing the null hypothesis, in which a static field model is used to describe Juno data, we find that that some consideration of time variation is likely necessary to accurately reconstruct global fields. The majority of Jovian field models, however, do not consider such time variation to be associated with its magnetosphere. Rather, changes to the Jovian field have been entirely attributed to internally sourced secular variation, despite it occurring over significantly longer timeframes of years [2].

Here, we aim to quantify the effect of time varying magnetospheric current systems on global field reconstructions. At Earth, the impact of such currents can be mitigated through strict data selection criteria and by using high degree, deterministic magnetosphere models. Given the lesser availability of data at Jupiter, both of these methods are deemed infeasible. Instead, we resort to a stochastic description of magnetospheric currents. Such descriptions have proven useful by revealing correlations between measurements that would otherwise be treated as independent. Originally proposed in the context of seamount magnetisation, this machinery was later developed to describe the influence of crustal fields and static planetary magnetospheres [3,4,5]. These methods are also able to describe time varying currents by scaling magnetosphere parameters in line with observations. In the case of Jupiter’s magnetosphere, we relate these parameters to timeseries of activity at Io and solar wind at Jupiter.

By doing so, we intend to determine how much of the temporal variation of Jupiter’s magnetic field can be attributed to changes in its magnetospheric current systems. Such knowledge also places limits on the extent of secular variation that can be inferred from Juno data.

References:

[1] M. Vogt et al. (2019) JGR: Space Physics, 124

[2] J. Bloxham et al. (2022) JGR: Planets, 127

[3] R. Parker (1988) JGR: Solid Earth, 93, 3105

[4] A. Jackson (1990) GJI, 103, 657

[5] M. Loncar & A. Jackson (2026) JGR: Planets [in review]