No abstract

Mercury's comparatively weak intrinsic magnetic field and its close proximity to the Sun lead to a magnetosphere that undergoes more direct space‐weathering interactions than other planets. A unique aspect of Mercury's interaction system arises from the large ratio of the scale of the planet to the scale of the magnetosphere and the presence of a large‐size core composed of highly conducting material. Consequently, there is strong feedback between the planetary interior and the magnetosphere, especially under conditions of strong external forcing. Understanding the coupled solar wind‐magnetosphere‐interior interaction at Mercury requires not only analysis of observations but also a modeling framework that is both comprehensive and inclusive. We have developed a new global MHD model for Mercury in which the planetary interior is modeled as layers of different electrical conductivities that electromagnetically couple to the surrounding plasma environment. This new modeling capability allows us to characterize the dynamical response of Mercury to time‐varying external conditions in a self‐consistent manner. Comparison of our model results with observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft shows that the model provides a reasonably good representation of the global magnetosphere. To demonstrate the capability to model induction effects, we have performed idealized simulations in which Mercury's magnetosphere is impacted by a solar wind pressure enhancement. Our results show that due to the induction effect, Mercury's core exerts strong global influences on the way Mercury responds to changes in the external environment, including modifying the global magnetospheric structure and affecting the extent to which the solar wind directly impacts the surface. The global MHD model presented here represents a crucial step toward establishing a modeling framework that enables self‐consistent characterization of Mercury's tightly coupled planetary interior‐magnetosphere system.

No abstract

The Jovian moon, Europa, hosts a thin neutral gas atmosphere, which is tightly coupled to Jupiter's magnetosphere. Magnetospheric ions impacting the surface sputter off neutral atoms, which, upon ionization, carry currents that modify the magnetic field around the moon. The magnetic field in the plasma is also affected by Europa's induced magnetic field. In this paper we investigate the environment of Europa using our multifluid MHD model and focus on the effects introduced by both the magnetospheric and the pickup ion populations. The model self‐consistently derives the electron temperature that governs the electron impact ionization process, which is the major source of ionization in this environment. The resulting magnetic field is compared to measurements performed by the Galileo magnetometer, the bulk properties of the modeled thermal plasma population is compared to the Galileo Plasma Subsystem observations, and the modeled surface precipitation fluxes are compared to Galileo Ultraviolet Spectrometer observations. The model shows good agreement with the measured magnetic field and reproduces the basic features of the plasma interaction observed at the moon for both the E4 and the E26 flybys of the Galileo spacecraft. The simulation also produces perturbations asymmetric about the flow direction that account for observed asymmetries.