Global MHD simulations of Mercury's magnetosphere with coupled planetary interior: Induction effect of the planetary conducting core on the global interaction

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.

A modelling approach to infer the solar wind dynamic pressure from magnetic field observations inside Mercury’s magnetosphere

“…These time-varying changes in electric currents and their associated magnetic fields generate induced currents at the boundaries between the conductive core and the relatively resistive mantle of Mercury to cancel out magnetic field perturbations inside the core. Outside the conductive core, however, the induced currents temporarily increase magnetic fields in Mercury's magnetosphere, from a few nano-Tesla to hundreds of nano-Tesla, depending on the solar wind dynamic pressure variations (Jia et al 2015). It has been theoretically suggested that the induced magnetic fields keep the magnetopause at ∼1.1-1.2 R M (Hood & Schubert 1979;Suess & Goldstein 1979;Jia et al 2015), and thus the solar wind would not easily access the surface of Mercury even during large dynamic pressure changes.…”

“…Outside the conductive core, however, the induced currents temporarily increase magnetic fields in Mercury's magnetosphere, from a few nano-Tesla to hundreds of nano-Tesla, depending on the solar wind dynamic pressure variations (Jia et al 2015). It has been theoretically suggested that the induced magnetic fields keep the magnetopause at ∼1.1-1.2 R M (Hood & Schubert 1979;Suess & Goldstein 1979;Jia et al 2015), and thus the solar wind would not easily access the surface of Mercury even during large dynamic pressure changes. Jia et al (2015), using a global MHD simulation, have studied the electromagnetic response of Mercury's interior for large solar wind dynamic pressure changes.…”

“…Large increases in the solar wind dynamic pressure considerably move the magnetopause and magnetotail current sheets closer to Mercury and intensify their current densities (Slavin et al 2014;Jia et al 2015). These time-varying changes in electric currents and their associated magnetic fields generate induced currents at the boundaries between the conductive core and the relatively resistive mantle of Mercury to cancel out magnetic field perturbations inside the core.…”

“…It has been theoretically suggested that the induced magnetic fields keep the magnetopause at ∼1.1-1.2 R M (Hood & Schubert 1979;Suess & Goldstein 1979;Jia et al 2015), and thus the solar wind would not easily access the surface of Mercury even during large dynamic pressure changes. Jia et al (2015), using a global MHD simulation, have studied the electromagnetic response of Mercury's interior for large solar wind dynamic pressure changes. According to their simulations, a solar wind dynamic pressure change from ∼11 nPa to ∼66 nPa can induce ∼200 nT fields at the surface of Mercury, which is comparable to or even larger than the strength of the surface field generated by the intrinsic magnetic dipole moment.…”

Estimating a planetary magnetic field with time-dependent global MHD simulations using an adjoint approach

“…As a consequence, the magnetic field of the electric currents of the interaction is not negligible, even in the immediate proximity of the planet (e.g., Glassmeier, 2000). Furthermore, electromagnetic induction effects within the planet might be important (e.g., Grosser et al, 2004;Jia et al, 2015). To estimate the planetary magnetic field precisely, the time-dependent interaction needs to be determined.…”