The Spatiotemporal Structure of Induced Magnetic Fields in Callisto's Plasma Environment Due to Their Propagation With MHD Modes

Abstract

We investigate how the spatiotemporal structure of induced magnetic fields outside of Callisto is affected by their propagation with the magnetohydrodynamic (MHD) modes. At moons that are surrounded by dense magnetized plasmas like the Galilean moons, low-frequency induced magnetic fields cannot propagate with the ordinary electromagnetic mode as is implicitly used by standard analytical expressions. Instead, the induced magnetic fields propagate with the MHD modes, which exhibit anisotropic propagation properties and have finite velocities. Using an MHD framework, we model the spatiotemporal effects of the transport on the induced signals and analyze their contribution to Galileo's C03 and C09 flyby observations. We find that the induced magnetic field in Callisto's plasma environment is asymmetric with a pronounced upstream/downstream asymmetry. By neglecting the transport effects, the amplitude of the induced magnetic field is under- or overestimated by up to tens of percent, respectively. Additionally, we find that MHD wave and convection velocities are strongly reduced in Callisto's local plasma environment, resulting in an additional temporal delay between the emergence of the induced field at the surface of Callisto or the top of its ionosphere and the measurements at spacecraft location. The associated phase shift depends on the location of the observer and can reach values of several to tens of degrees of the phase of the primary inducing frequency. Transport effects impact the observed induction signals and are consistent with the C03 and C09 magnetic field measurements.