PlanetMag manuscript

Spacecraft investigation of planets within the solar system have provided a rich context for understanding their dynamic magnetic environments. Magnetic field measurements are able to tell us much about the planet’s interior dynamics, composition, and evolutionary timeline. Magnetic fields also serve as the source for passive magnetic sounding of their moons. The time-varying magnetic fields experienced by their moons, due to relative planetary motion, interact electrically with conductive layers within these bodies (including salty subsurface oceans) to produce induced magnetic fields that are measurable by nearby, magnetometer-equipped spacecraft. Many factors influence the character of the induced field, including the precise amplitude and phase of the time-varying field, known as the excitation or driving field and represented by excitation moments. In this work, we present an open-source software package named PlanetMag that features calculation of planetary magnetic field models available in the literature at arbitrary positions and times. The implemented models enable a simultaneous inversion of the excitation moments across a range of oscillation frequencies using a linear least-squares method and ephemeris data with the SPICE toolkit. Here we summarize the available magnetic field models and their associated coordinate systems. Precisely-determined excitation moments are a critical input to forward models of induced planetary magnetic fields. Our results serve as a prerequisite to any precise comparison to spacecraft data for magnetic sounding investigation of giant planet moons—connecting the induced magnetic field to a moon’s interior requires an accurate representation of the oscillating components of the excitation field. We calculate complex excitation moments relative to the J2000 epoch and share the results as ASCII tables compatible with MoonMag or other software packages intended for induction response calculations.